RUBIN EALTY 1 LIBR STATE HEALTH ho TECHNICAL INFORMATION Development and Evaluation of Methods for the Elimination of Waste Anesthetic Gases and Vapors / In Hospitals Cons SpE Pere hii =r DEVELOPYENT AND EVALUATION OF ETHODS FOR THE ELIMINATION OF WASTE ANESTHETIC GASES AND VAPORS IN HOSPITALS Charles Whitcher, M.D. Robert Piziali, Ph.D. Rudolph Sher, Ph.D. Robert J. Moffat, Ph.D. Stanford University Schools of Medicine and Engineering Stanford, California 94305 Contract Number HSM 99-73-73 U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE Public Health Service Center for Disease Control National Institute for Occupational Safety and Health Division of Field Studies and Clinical Investigations Cincinnati, Ohio 45202 May 1975 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 N10 The contents of this report are reproduced herein as received from the contractor except for minor changes in the title page. The opinions, findings, and conclusions expressed are those of the authors and not necessarily those of the National Institute for Occupational Safety and Health (NIOSH). It is recognized that equipment and services other than those cited in this report are available. Mention of company or product names, therefore, is not to be considered as an endorsement by NIOSH. NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH PROJECT OFFICERS John M. Dement Philip J. Bierbaum HEW Publication No. (NIOSH) 75-137 ABSTRACT A health hazard affecting operating room personnel is probably related to chronic exposure to trace concentrations of the inhalation anesthetics. The sources of such agents are discussed as well as their distribution in the operating room. Effective control requires a systematic approach that includes collection of gases and vapors at the anesthetic breathing systems and disposal procedures (scavenging) ’ "low leakage'' practices by the anesthetist to minimize gas concentrations in the operating room ’ equipment maintenance specifically intended to reduce gas leakage . air monitoring program to indicate the effectiveness of the control measures Careful application of these measures will yield less than 30 ppm nitrous oxide and less than 0.5 ppm halothane in the operating room air. These concentrations represent approximately 0.005 percent of the anesthetics administered to the patient and act as a guide to reasonable achievable concentrations of other anesthetic agents. | Section VI. CONTENTS INTRODUCTION . THE HEALTH HAZARD IN THE OPERATING ROOM . LEAKAGE OF WASTE ANESTHETIC GASES IN THE OPERATING ROOM Ir © Tm m OO OO Ww > ce ee ee ea Diffusion The Anesthesia Machine with co, Absorber Ventilator with Scavenging . Other Breathing Systems . Scavenging Components Miscellaneous Equipment and Fittings Work Practices and Anesthetic Gas Leakage . Postanesthesia (Recovery) Room EXPOSURE LEVELS AND GAS DISTRIBUTION IN THE OPERATING ROOM A. B Cc. D E. Reported Gas Concentrations The Standard Operating Room Concentrations Obtained in the Present Studies Distribution of Anesthetic Gases in the Operating Room Air Air-Conditioning Systems SCAVENGING SYSTEMS FOR WASTE ANESTHETIC GASES . A. D. Collection of Waste Gases from the Breathing Systems . Disposal of Waste Anesthetic Gases Alternatives to the Proposed Scavenging Techniques Initiating a Scavenging Program EQUIPMENT MAINTENANCE . A. B. Maintenance Schedules and Leak Tolerances for Anesthesia Machines and Related Equipment . Procedures for Leak Testing Anesthesia Equipment. 03725 RO 82 47 PUBL 11-1 H-1 H-1 Hi-1 1-8 1-8 11-8 Hi-12 I-14 11-15 Iv-1 1v-1 1v-1 1v-1 IV-4 IV-5 V-1 Y-7 V-22 V-22 Vi-1 Vi-1 Vi-2 CONTENTS (cont) ‘Cc. Tests for Leakage in Anesthesia Machines and N,0 Supply Hoses D. Tests To Determine Leakage in Miscellaneous Equipment +s wo» VII. AIR MONITORING IN THE OPERATING ROOM . A. Samples Obtained in Syringes B. Sampling in Bags C. Samples Obtained in Proximity to Personnel D. Continuous Monitoring E. Discussion Vill. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS A. Reasonable Concentrations of Inhalation Anesthetics . B. Techniques of the Anesthetist . C. Waste Gas Scavenging . D. Anesthesia Equipment . D. Air Monitoring . Appendix A. FUNDAMENTALS OF PHYSIOLOGY, ANESTHETIC TECHNIQUES AND AGENTS, BREATHING SYSTEMS, AND TEST LUNG c + # 4 w® x = Appendix B. DIFFUSION LEAKAGE Appendix C. DISTRIBUTION OF WASTE ANESTHETIC GASES IN THE OPERATING ROOM AIR Appendix D. DIAGRAMS AND PRESSURE FLOW RELATIONSHIPS OF SCAVENGING COMPONENTS . Appendix E. GAS ANALYSIS AND CALIBRATION TECHNIQUES . REFERENCES vi Vii-1 Vii-2 Vii-5 vVii-8 VIii-10 Vii-11 Vilti-1 VARNES! ARRES Viii-1 Viti-2 VIilti-2 ASA C cc CFM cm CO, E ECD Eq. FID eo fil ft3/min gm Hg H,0 HP hr ID in. IR L L/min min mm N20 NFPA NIOSH 0D PVC ppb ppm psig ABBREVIATIONS AND SYMBOLS American Society of Anesthesiologists centigrade cubic centimeter or centimeters cubic foot or feet per minute centimeter or centimeters carbon dioxide east electron capture detector equation flame ionization detector foot or feet cubic foot or feet cubic foot or feet per minute gram or grams mercury water horsepower hour or hours inside diameter inch or inches infrared liter or liters liter or liters per minute meter or meters micron or microns minute or minutes millimeter or millimeters nitrogen nitrous oxide National Fire Protection Association National Institute for Occupational Safety and Health oxygen outside diameter polyvinyl chloride parts per billion (by volume) parts per million (by volume) pounds per square inch, gauge pressure vii Sc3H scav S.E. sec vac wk tritiated scandium scavenging standard error second or seconds vacuum west week or weeks ACKNOWLEDGEMENT Equipment for leak detection and measurements of trace con- centrations of inhalation anesthetics present in the operating room air was loaned by the General Electric Company; Inficon, Incorporated; Sensors, Incorporated; and Wilks Scientific Corporation. The engineering expertise of Mr. A. Reid Chappell and Mr. Charles Drace of the Stanford University Hospital Depart- ment of Engineering and Mr. Glenn Brown of Summit Services, Los Gatos, California is acknowledged as is the consultation on gas analysis provided by Dr. James Trudell of Stanford Univer- sity. The participation of graduate students at Stanford University is appreciated, including Mr. James Eastwood, Mr. Donald Harter, and Mr. Thomas Yackle. The technical assis- tance of Mrs. Jeanne Daney and Mr. Daniel Chin is acknowledged. rw - ee. ahem ro allEA no AO tnd - HLT Sin lle cea V oo - te A ti - - . a ee ren aegis I. INTRODUCTION The motivation of this study is the recent recognition of health hazards affecting operating room personnel and its possible relationship to the anesthetic gases to which these people are exposed. The objective is to present the information necessary to establish and maintain low anesthetic gas concentrations in the operating suite by using feasible and, in most cases, commercially available control measures without major alterations in established safe anesthetic practices. The anesthetic agents primarily considered are nitrous oxide (N,0) and halothane. These agents are the most frequently used and, although quite different in physical properties, display similar air distribution patterns in the operating room. It is assumed, therefore, that the patterns determined for N20 and halothane may be applicable to other inhalation anesthetics. Halothane in air is properly referred to as a vapor but, because it behaves like a gas, will be regarded as such. The technologies for scavenging (collecting waste anesthetic gases at the anesthetic breathing systems and the disposal procedures), equipment maintenance, air monitoring,and minor modifications in the techniques of administering anesthesia presented in this monograph are intended primarily for the hospital environment. Inhalation anesthesia is administered elsewhere, and scavenging should be considered in all anesthetizing locations. For example, the veterinary clinic may be scavenged, employing the techniques described. The unique technology required for the dentist's office is being investigated by the National Institute for Occupational Safety and Health (NIOSH) under Contract CDC-210-75-0007. The hospital in which the present studies were conducted is a private teaching institution with 618 beds. The operating suite includes 14 rooms in which 13,836 surgical procedures were completed during 1973-1974. The Department of Anesthesia was comprised of approximately 12 anesthesiologists from three private practice groups who provided anesthesia services for the private practice surgeons regularly occupying eight operating rooms. In addition, 12 Stanford University faculty anesthesiologists and residents were assigned to six operating rooms. The inhalation anesthetic techniques employed the high flow rates of anesthetic gases presently in widespread use. Il. THE HEALTH HAZARD IN THE OPERATING ROOM Animal experiments have provided evidence that anesthetic gases are a health hazard. Epidemiological studies suggest that a similar hazard existsin exposedl personnel. The teratogenic effects of anesthetic concentrations of inhalation agents administered to pregnant animals at critical periods of gestation have been known for some time.! More recently, animal experiments have demonstrated the embryotoxicity of trace concentrations of nitrous oxide? and a decreased survival rate on exposure to trace concentrations of fluroxene.® These effects are dose related. Another dimension in the toxicology of anesthetics is: the reported impairment of cognitive and motor skills in anesthetists exposed to trace concentrations of N,0 and halothane" and enflurane.® A number of epidemiological surveys have been conducted. The first was a report on the health of 303 Russian anesthesiologists.® In this group, an unusually high incidence of headaches, fatigue, and irritability was noted. Of 110 women, an increased incidence of spontaneous abortion and a high incidence of abnormal pregnancies also were reported. Halogenated anesthetics were not employed. Nitrous oxide and ether were the principal agents and, in the absence of air-conditioning and scavenging, the operating room air probably contained high concentrations of these anesthetics. Additional surveys have confirmed and extended these findings. [In one study, exposed female anesthetists were compared to a group of unexposed female pediatricians, and an increased abortion rate was noted in the exposed women.’ A similar study in the United Kingdom reported the same result plus a trend toward an increased incidence of congenital malformations.?® In Michigan, an increased rate of cancer’ and birth defects!® in nurse anesthetists was suggested. The most comprehensive study'! was initiated in 1972 under the combined auspices of an ad hoc committee on the Effects of Trace Anesthetics on the Health of Operating Room Personnel of the American Society of Anesthesiologists (ASA) and NIOSH. The data obtained from 40,044 respondents exposed to the operating room were compared to data obtained from unexposed control groups. The American Association of Nurse Anesthetists, Association of Operating Room Nurses, American Academy of Pediatrics, and a segment of the American Nursing Association also participated in this study. In addition to confirming an increased occurrence of spontaneous miscarriage and birth defects in the offspring of exposed personnel, an unexpected finding was noted when comparing the congenital abnormality rate of female pediatricians to that of the unexposed wives of practicing anesthesiologists. Although not occupationally exposed to anesthetics, these wives demonstrated a birth defect rate of 5.4 percent in comparison to 4.2 percent for the female pediatricians, almost a 31 percent higher incidence of defective exposed personnel are defined as those who regularly administer anesthetics or work in the operating room. 1-1 offspring. This suggests that the exposed male transmits the defect to his unexposed wife. Another significant finding was the high incidence of liver disease; ASA females exposed in the operating room presented an incidence of 4.9 percent in comparison to 2.9 percent in the female pedi- atricians, a 69 percent higher incidence. Males were similarly affected. These epidemiological studies indicated an increased incidence of embryo- toxicity, mutagenesis, carcinogenesis, and liver disease among female personnel working in the operating room. The inhalation anesthetics present in the air are the most likely offending agents although a cause-effect relationship has not been established. The evidence for such a relationship, however, is further weighted by the experimental animal data. The ASA ad hoc committee considered the experimental and epidemiological evidence sufficiently strong to urge the prompt application of scavenging techniques for waste anesthetic gases. 11-2 111. LEAKAGE OF WASTE ANESTHETIC GASES IN THE OPERATING ROOM The principal source of waste anesthetic gases in the operating room is leakage associated with the anesthesia equipment and with the work practices of the anesthetist. Leakage from the equipment has been reported as one significant source of anesthetic gases in the operating room air, accounting for 7 to 87 percent of the total concentrations.'? In the present studies, leakage was considered in terms of smaller components (breathing bags, face masks, and tubing) in addition to such intact major assemblies as the carbon dioxide (CO,) absorption system connected to the anesthesia machine. Ventilators used with both the anesthesia machine and the CO, absorber also may be a leak source, and even the scavenging equipment itself was investigated. Methods for measuring leakage varied according to the equipment being examined. Breathing bags and hoses were tested by means of simple methods-- pressurization, immersion in water, and observation of any bubbling. Other equipment, such as ventilators, were evaluated under simulated clinical conditions in an operating room by determining the increase in concentrations of anesthetics in the air. The reader who is unfamiliar with physiology, anesthetic techniques, and related equipment will find a review of these subjects in Appendix A. A. Diffusion The diffusion of anesthetic gases through rubber and plastic material (breathing tubes, bags, scavenging lines) presents a possible source of leakage in the operating room air. Experiments were designed to measure diffusion under controlled conditions in a test box (Appendix B). These studies indicated that concentrations caused by diffusion are negligible in comparison to other larger leak sources. For example, a 2-m length of the most permeable breathing tube tested contributed only 172 ppb (parts per billion) N,0 and 0.354 ppb halothane at gas flows of 2.5 L/min N,0/0, with 1 percent halothane added. B. The Anesthesia Machine with CO, Absorber Ts Without Scavenging The anesthesia machine is usually adjusted to deliver more anesthetic gases than the patient can absorb, as discussed in Appendix A. In the absence of scavenging, these excess gases escape from the popoff valve into the operating room, generally at a rate of 0.5 to 5 L/min. Studies completed (Appendix C) indicate that, for any known leak rate of gases from a source, such as an anesthesia machine, a close correlation exists in the average mixed concentrations of N,0 and halothane present in * Corrugated wire-reinforced vinyl tubing, Medicon Plastic Company, Upland, California. 1-1 the operating-room air, whether measured by gas analysis or by calculation. This was determined by measuring continuously the concentrations of N,0 and halothane at the air-conditioning exhaust grille where an average mixed sample was obtainable. Nitrous oxide and halothane were determined by infrared analysis and gas chromatography (analytical methods are detailed in Appendix E). It was found that, if 3 L/min N20 escape into the air in an average operating room (volume 4000 ft, nonrecirculating air-conditioning flow rate 667 ft3/min, or 10 air exchanges/hr), the average concentration of N20 is 159 ppm (parts per million). This calculation is shown by co L x 60 x 10° (3.1) NV where C = concentration of gas in room (ppm) V = volume of room in liters (L) N = nonrecirculated room air changes per hour L = leak rate of gas (L/min) Example: operating room = 20 x 20 x 10 = 4000 ft? therefore, V = 4000 x 28.3 = 113000 N = 10 changes (667 ft®/min, 4000 ft® room) L=31 6 3 x 60 x 10 [ B srwe— 159 ppm 10 x 113000 A simplified expression applicable to a 4000 ft® room with an air- conditioning system providing 10 air exchanges per hour with thorough mixing is leak rate N20 100 cc/min » 5.3 ppm room concentration 1-2 2. With Scavenging a. Without Special Leakproofing After an efficient scavenging system has been installed, major leakage is no longer associated with the popoff valve; however, a variety of potential leak sources still remains and high room concentrations may prevail. The design of the equipment may be faulty, or quality control and maintenance could be inadequate; breathing hoses and bags can be poorly repaired, gaskets might be missing, or tapered fittings deformed. Small gas leaks are found frequently at compression fittings in the plumbing. Fifteen anesthesia machines (Table 111.1) employed in routine clinical service were studied in terms of their contribution to anesthetic gas concentrations in the operating room air. A nonleaking breathing bag and tubing set with a Y-piece were attached to the machines, in addition to a scavenging system that included a popoff valve vented to the exhaust grille of the nonrecirculating air-conditioning system. A test lung (Appendix A) was fitted to the Y-piece. Anesthetic gas concentrations present in the operating room were determined under conditions simulating both spontaneous and controlled breathing. In this experimental method, a distinction could be made between high and low pressure leaks. High pressure leaks occur between the flowmeter control valves and the wall gas supply connectors; low pressure leaks occur downstream from the flowmeter control valves, in- cluding the face mask or endotracheal tube. Table III.1 ANESTHESIA MACHINES Machine Type Serial No. Absorber 1 Foregger L.I.C.H. BC-6108 Jumbo 2 Foregger Texas RC-1883 Jumbo 3 Foregger Texas RC-1473 Jumbo 4 Foregger Texas RC-2074 Jumbo 5 Foregger Montreal RC-11630 Jumbo 6 Foregger Texas RC-1882 Jumbo 7 Ohio Kinet-o-Meter 1548 Model 18 8 Ohio Kinet-o-Meter 246 Model 18 9 Ohio Kinet-o-Meter 794 Model 18 10 Ohio Kinet-o-Meter 2471 Model 18 11 Ohio Kinet-o-Meter mt Model 18 12 Ohio Kinet-o-Meter 6543 Model 18 13 Ohio Kinet-o-Meter 3958 Model 21 14 Ohio Kinet-o-Meter 3956 Model 21 15 Ohio Kinet-o-Meter 790 Model 18 11-3 Control measurements of gas concentrations present in the air were obtained first with the anesthesia machine out of the room; it was then moved into the room and the high pressure hoses, scavenging system, and test lung were attached. Under these conditions, the increase in gas concentrations was the result of leakage in the high pressure system. Gas flows of 3/2 L/min N20/02 with 1 percent halothane were then established through the flowmeters, and the popoff valve was fully opened. Any increase in room con- centrations of anesthetic gases then indicated leakage in the low pressure system. These conditions simulated the use of the machine during spontaneous breathing. The breathing system was then pressurized to 10 cm H,0 by partially closing the popoff valve, which aggravated any low pressure leak and simulated leakage present during assisted or controlled breathing. The results of these studies are illustrated in Fig. Ill.1. The height of each bar represents the total concentration of anesthesia in the air for each machine studied. The high pressure leak (machine connected to the high pressure hoses, flowmeters off) and the low pressure leak (at atmospheric pressure, with the flowmeters on and the popoff valve open) were determined only where indicated. Variations are evident in the total gas concentrations and in the high and low pressure components. For example, No. 11 presented a total leak rate sufficient to raise the room concentration of N20 to 34 ppm and halothane to 0.47 ppm, whereas the values for No. 2 were only 2.9 ppm N20 and 0.030 halothane. High pressure leakage is notable with Nos. 8, 11, and 12; low pressure leakage is prominent with Nos. 3, 10, and 11. With all machines tested, high pressure leakage began as soon as the gas supply hoses were attached and usually was caused by leaky or cracked compression fittings. Low pressure leakage was often present in or near the absorber and frequently was caused by defective or poorly seated gaskets, valve covers, drain cocks, damaged metal-to-metal taper fittings, or inadequately sealed 0, analyzers. A dissociation of leak rates was evident; that is, a machine showing a high leak rate for one gas did not necessarily show a high leak rate for another. For example, No. 5 indicates a high leak rate for halothane but not for N20, and No. 8 indicates the reverse. As a result, the fixed ratio of N20 to halothane (60:1) delivered by the gas machine was not maintained in the air during the majority of tests. Opportunities for a dissociation of leak rates are evident in Fig. A.2. Relatively high halothane concentrations would be anticipated as a result of leakage from vaporizers of the ''copper kettle'' type and associ- ated components because this vaporizer is located apart from the stream of N20. Relatively high N20 concentrations would result from any leakage upstream from the copper kettle vaporizer. I1-4 [XXX XXXII LRERRIERLRN 2 NNN > - - ~ Ez - Loca Qo 20:1) indicate less effi- ciency. On the basis of these studies, it was predicted that the devices tested could scavenge efficiently during quiet spontaneous breathing and with ventilators such as the Ohio-Monaghan (where the exhaust flow rate is low) but that, with manual ventilation, all devices could leak. The Boehringer was more resistant to such leakage than the others. (The efficiency of all devices could be improved by adding a 1 to 3 L scavenging reservoir bag in line between the popoff valve and the interface, but all require high ratios of scavenging suction to anesthetic flows. A suction flowmeter is also desirable.) 3. Liquid Sealed Interface System The liquid sealed interface system (Fig. V.11) did not require evaluation by the above relatively complicated methods. Diffusion studies indicated negligible losses of N,0 and halothane, and any other leakage from this interface was apparent by inspection. F. Miscellaneous Equipment and Fittings It is impossible to list every conceivable source of leakage; however, the following additional sources should be noted. Vaporizers of the flow and pressure compensated type (Fluotec) are potentially gas tight, but loosened screws and missing or defective seals and gaskets cause leakage that can be easily corrected. 1-12 Table III.S SCAVENGING SUCTION/ANESTHETIC GAS RATIOS FOR TUBE-WITHIN-A-TUBE INTERFACE SYSTEMS Anesthetic Gas Flow¥ at Which Leakage Began (L/min) a . 8 Avenging Ventilator Forced Hand Breathing Suction Interface : : Flow Scavenging Scavenging (L/min) Anesthetic Suction’ Anesthetic Suction’ Gas Flow Anesthetic Gas Flow Anesthetic Gas Ratio Gas Ratio 20 10 2.0 <1l.0 >20 Foregger 10 4.5 2.2 <1.0 >10 5 1.5 3.3 <1.0 > 5.0 20 15 1.3 1.0 20 Ohio 10 8.0 1.2 1.0 10 5 3.0 1.7 1.0 5.0 20 12.0 1.7 6.5 3.1 Boehringer 10 8.0 1.3 4.0 2.5 5 3.0 1.7 1.0 5.0 T Low ratio of scavenging suction (through interface) to anesthetic gas (from anesthesia machine) indicates efficient operation. Anesthetic gas flow indicates threshold rate at which leakage began. Fittings for the attachment of oxygen analyzers to the CO, absorber often leak. Correction in some cases was difficult, requiring a redesign of the attachment. . Leakage may occur in metal-to-metal slip fittings. Such fittings were replaced when possible with plastic- to-metal or plastic-to-plastic connections, or new metal parts were installed. The joints at leaky fittings were often sealed by covering them with a sleeve of rubber or silastic tubing. 11-13 G. Work Practices and Anesthetic Gas Leakage Given the use of low leak equipment for anesthesia and scavenging systems, the work practices of the anesthetist are the principal contri- butors to anesthetic gas levels into the operating room. Scavenging tubes are not always properly connected, face masks may fit poorly, cuffs on the endotracheal tubes might leak, and anesthetic liquids could be spilled during the filling of vaporizers. It is possible to estimate the work practice contribution to nitrous oxide in the air by subtracting the contribution of the anesthesia machines from the total gas concentrations observed. With the use of the face mask, spontaneous breathing would be the most frequently occurring mode, and the mean pressure was estimated at 2.5 cm H,0. Referring to Table 111.3, the total leak rate of the machines after leakproofing was 100 cc/min. Assuming laminar flow during leakage as evidenced in the data presented in Table I11.2, the leak rate falls linearly with the pressure; at a pressure of 5 cm H,0, it would be 17 cc/min and, at 2.5 cm H,0, it would be 8.4 cc/min. Referring to Eq. (3.1), these leak rates, converted to concentrations, would be 0.90 ppm for 17 cc/min, and 0.45 ppm for 8.3 cc/min. The experimentally observed concentrations of N,0 presented in Section VI were 36 ppm with the face mask and 15 ppm with the endotracheal tube. The concentrations included in Table 111.6 show that, in subtracting the machine component, the work practice contribution to N20 in the air is estimated at 99 percent with the face mask technique and 94 percent with the endotracheal tube. No estimate was made with the ventilator-endotracheal tube group because many ventilators had not been fitted with leakproof scavenging apparatus. Table III.6 WORK PRACTICE CONTRIBUTION TO N,0 IN AIR Concentration (ppm) Technique Percent . Work Total Machine . Practice Face mask 36 + 6.71 | 0.45 + 0.95 | 35 + 6.8 99 Endotracheal tube 15 £ 2.4 0.90 £ 0.19 14 + 2.6 94 t +s.E. I-14 i. Postanesthesia (Recovery) Room Anesthetic gases are eliminated by exhalation. The major portion ibsorbed is exhaled before the patient arrives in the recovery area, but he remaining traces are a potential source of personnel exposure.!3 1% \t the Stanford University Hospital, the recovery room is air conditioned it a high dilution rate, and preliminary studies indicated that both N,0 ind halothane are found in low concentrations. Any comprehensive investi- jation of personnel exposure would include the recovery area. 111-15 T= ed an ele oe emi SA) eich ta ene dt ne el IV. EXPOSURE LEVELS AND GAS DISTRIBUTION IN THE OPERATING ROOM A. Reported Gas Concentrations Anesthetic agents are detectable in the operating room air whenever they are administered. Reported waste gas concentrations vary markedly because of the variability in measuring conditions. Reported values obtained in the absence of scavenging indicate a range from 10 to 85 ppm halothane and from 130 to 9700 ppm nitrous oxide. ' “1° B. The Standard Operating Room The ''standard operating room'' concept was developed to reduce the variables for determining anesthetic gas concentrations. This concept relates the concentration and distribution of waste gases measured in operating rooms of different sizes and air-conditioning flow rates to a hypothetical operating room with a volume of 4000 ft® and an air- conditioning flow rate of ten nonrecirculating air exchanges per hour. In studies of gas concentrations, the physical size of the operating room and the air-conditioning flow rate were measured, and the results were normalized to the above standard operating room values.T Table 1V.1 lists these values. C. Concentrations Obtained in the Present Studies Minimum Achievable Concentrations Under optimal conditions during clinical anesthesia, the average anesthetic mixed gas concentrations at the exhaust grille were kept below 1 ppm for N,0 and 0.025 ppm for halothane. Samples obtained in the region around the anesthetist indicated similar concentrations. These values were considered minimum and not readily attainable during routine anesthesia. The clinical conditions for such low concentrations were as follows. (a) All equipment received low leak preventive maintenance. (b) The anesthetist was instructed in the use of low leak techniques. (c) Gas concentrations were delivered by mask or endotracheal tube, using a semiclosed system with a 5 L/min total flow. + observed gas concentration x fresh air normalized exchange/hr x room volume ft?3 concentration of = anesthetic gas 10 exchanges/hr x 4000 ft? Iv-1 Table IV.1 AIR-CONDITIONING PARAMETERS (Operating Rooms) Room Inlet Air Room No. Plow Exchanges yoluge (ft°/min) (per hr) {££°) 1 325 6.73 2900 2 285 5.99 2850 3 300 5.96 3020 4 690 11.1 3730 5 605 9.74 3730 6 350 5.64 3730 7 600 9.66 3730 8 700 11.3 3730 9 950 14.5 3930 10 1100 15.9 4140 11 590 9.72 3640 12 550 8.86 3730 13 w:500" 12.6 6210 E:800 596+65 9.82%0.91 | 3770%230 T Room has two inlets. ¥ + g5.E. 1v-2 (d) Measurements were made during steady-state conditions: anesthesia in progress at least 30 min patient not disconnected from the breathing system within the preceding 15 min vaporizer not filled during immediate 15 min preceding the study. Anesthetists who were to participate in determining the lowest attainable N20 concentrations were presented with the following set of instructions for work practices that reduce gas leakage. (a) (b) (c) (d) (e) (f) (9) Confirm that the waste gas disposal lines are properly connected. Select a face mask that will ensure a tight fit with minimal pressure. When filling the vaporizer, use the funnel to avoid spillage. Switch the anesthetic vaporizer off when not in use. If it is your practice to turn on the N,0 before the induction of anesthesia, make use of a Y-piece provided with a shutoff valve; otherwise, cap the end of the Y-piece before the mask is attached and open the popoff valve. The gases will then escape via the scavenging system. Crutch tips are available for use as caps. After anesthesia has started, avoid disconnecting the patient from the breathing circuit unless flow- meters are turned off or the Y-piece is sealed. If it is necessary to empty the breathing bag, empty it into the scavenging system without dis- connecting it from the absorber or patient. When terminating the case, try to wash the anesthetic agents out of the patient with oxygen, leaving him attached to the breathing system as long as convenient. It is good patient care to give extra oxygen at this time. Concentrations Achievable in Routine Practice The average concentrations of gases found in air samples obtained at the air-conditioning exhaust grille in a large operating room while anesthesia was in progress were 28 ppm N20 and 0.45 ppm halothane. The methods for obtaining these values are detailed in Section VII. It is notable that gas concentrations in the region of the anesthetist were usually not higher than in other areas of the operating room. 1V-3 D. Distribution of Anesthetic Gases in the Operating Room Air 1. Distribution Studies Anesthetic gases not removed by the scavenging system can be widely distributed in the air. Waste gas distributions were studied in an operating room when surgery was not in progress. Relevant information on room dimensions and air-conditioning flow rates are shown in Table IV.1 (Room 11 and a delivery room). Nitrous oxide and halothane were released from an anesthesia machine, and the resultant concentrations were measured at multiple (36 to 48) sampling locations to map gas distribution patterns. A gas analysis was obtained by rapid continuous infrared spectroscopy supplemented by gas chromatography. These studies are reported in Appendix C. 2. Results and Discussion It was found that anesthetic gases were not always distributed uniformly throughout the operating room. This unequal distribution must be considered in an analysis of individual exposure levels and in the selection of sampling sites for a monitoring system. Variations in anesthetic concen- trations depended on the flow rate of the air-conditioning system, the site and rate of gas leakage, location of equipment and furniture, and the movements of personnel. At low air-conditioning flow rates (five air changes/hr), some room areas showed 10 to 15 times the average gas concentrations. Although also present in rooms with higher air change rates, these ''hot spots'' were less numerous and of lower concentrations, and varied within a given operating room. Short-term changes in gas concentrations were induced by movements of personnel. Longer term variations were related to the position of furniture, equipment, surgical drapes, and the position of air-conditioning inlets and outlets. Such hot spots may be significant when selecting a representative sampling site in a monitoring program. It is evident that the gas concentrations measured around one person do not necessarily parallel the concentrations present near others. To obtain a comprehensive picture of the exposure risk, all personnel would have to be monitored. Aside from a few hot spots, the general distribution of waste gases proved to be remarkably uniform, especially at air exchange rates of 10/hr and higher. The effects of differing diffusion rates and specific gravities were minimal for halothane and N20, each showing approximately equal concentrations at the floor and ceiling. Personnel exposure was not reduced by venting waste gases to the floor. Similar findings were reported in an earlier study of the distribution of halothane.!® These findings are consistent with the observations in unpublished studies of gas distribution completed in the Netherlands, !® but are incon- sistent with widely accepted opinions. The prevailing assumption that specific IV-4 gravity causes a''layering'' of the heavier anesthetics at floor level is significant when air exchange rates are below 10/hr. This might be of special concern if flammable liquid anesthetics such as ether were to be spilled on the floor. A search of the literature??-3° reveals no comprehensive mapping of anesthetic gas distributions during the adminis- tration of clinical anesthesia or under conditions simulating such adminis- tration, except as mentioned above. The gas concentrations measured at the air-conditioning exhaust grille were stable and representative for the operating room as a whole. This area was selected, therefore, as the most satisfactory sampling site for monitoring. This conclusion is supported by previous studies showing an even distribution of halothane, '® except that higher concentrations were found at sampling sites within 1 to 2 ft of the relatively leaky anesthesia machines and scavenging equipment then employed. Higher con- centrations also were found when scavenging was not practiced. In other operating rooms, mapping studies may prove necessary to establish a representative sampling site. As noted above, personnel sampling would be the ideal technique for estimating the exposure of operating room personnel and should be applied in the absence of thorough mixing. Personnel monitoring is mentioned in Section VII. E. Air-Conditioning Systems The air-conditioning system in the operating room is of prime importance in establishing scavenging because of its effect on the distribution of gases. It also provides a disposal pathway for the collected gases. Significant factors in the distribution of gases include the dilution rate (fresh air exchange) and the method of dispersing the air as it enters the room. The higher the air exchange rate the lower the trace gas concentrations; however, high flow rates are costly and may cause drafts, and many people are uncomfortable at air exchange rates above 15/hr. Drafts are reduced by the careful choice and location of the supply grille. Efficient diffusers, such as the aspirating and entrainment types, tend to mix the room air at uniform velocity, thereby reducing drafts and localized high concentrations of anesthetics. Air-conditioning systems induce a slightly positive pressure in the operating room with respect to the adjacent hallway, and a slight outward movement of air is maintained. Air-conditioning systems are either recirculating or nonrecirculating (Fig. 1V.1). The nonrecirculating system takes in exterior air and pro- cesses it by filtering, heating, cooling, and adjusting humidity. This processed air is circulated through the operating room and then exhausted to the atmosphere. The air exchange rate frequently varies between 6 and 25 changes per hour although the latter is a minimum requirement. With such air conditioning, waste gases collected from the anesthetic breathing system could be disposed of anywhere in the exhaust duct system. Recommended minimum air exchange rate widely quoted®!:3? is 25/hr which is greater than actually found in many operating rooms. Vv 5 REHEAT HUMIDIFY COOL PREHEAT FILTER PREFILTER FRESH - — - — y AIR : INTAKE SUPPLY FAN w} FILTER 2 Os QL ~ 3 2 : Sc! j=. nd : om -- 1 R > i tf fie Qi ! I : ' ' i 4 OPERATING ! ; EXHAUST AIR 1 ROOM ' FROM ' ' OPERATING | ' ROOMS ! } EXHAUST FAN | H— I | L bcc cccccccae= Fig. IV.1. AIR-CONDITIONING SYSTEMS. In contrast, with recirculating air conditioning, a small amount of exterior air is taken in from the atmosphere, a minimum of five air changes per hour, and a similar small amount of air is exhausted. Most of the exhaust air (20 air exchanges per hour) is shunted back into the intake and recirculated into the operating room. In a recirculating system, waste gases must be disposed of at a carefully chosen site in the exhaust system, downstream from the shunt, thereby ensuring that the waste anes- thetics will not be circulated anywhere within the building. In the past, recirculating systems were not used because of the danger of recirculating bacteria contaminated air. With the high efficiency particulate air (HEPA) filters, recirculation is now possible; however, these filters do not remove anesthetic gases. Even if the waste gases are emptied into the exhaust duct beyond the bypass, the recirculating system is less effective in the removal of waste gases than the nonrecirculating system because of the lower dilution rate. Scavenging is not completely efficient, and residual anesthetic gases are always present in the operating room. Another consideration relating to the use of air-conditioning systems for waste gas disposal is the pressure drop downstream in the exhaust duct. If waste anesthetic gases are introduced at the exhaust grille, negative pressure is low and does not interfere with the breathing systems. If such waste gases, however, are introduced at a distance downstream in the duct where the negative pressure is greater, interference with the breathing system may result. Pressure equalization then becomes necessary, as discussed in Section V. 1V-6 Laminar-flow air conditioning must be considered in relation to anes- thetic gas scavenging. These systems usually recirculate. The air enters at a velocity typically 90 ft/min spread over a large surface (approximately 90 percent or more of a wall or ceiling). A standard operating room (10 x 20 x 20 ft with a wall area of 100 ft?) would be provided with a flow rate of at least 9000 ft®/min, and the amount of fresh air is approxi- mately 5 percent of the recirculated air, or 450 ft3/min. Under these circumstances, the calculated trace gas concentrations are 32 percent higher than in a standard room provided with 667 ft®/min of non- recirculated air. A phenomenon conceivably occurring with laminar-flow systems is that, because of the lack of turbulence, any anesthetic gases leaking into the air could be carried without much dilution toward personnel and could cause heavy exposure. This emphasizes the necessity for efficient scaveng- ing and suggests the advisability of locating the anesthesia equipment "downwind'' from personnel. In disposing of waste anesthetic gases via the air-conditioning system, these gases must go directly out, without contaminating the air in other hospital areas. It may be difficult to ascertain that all such gases are actually exhausted. Errors in construction or an unfortunate choice of disposal site upwind from an air intake could result in the contamination of intake air and permit an insidious recirculation of anesthetic gases. Only a gas analysis would provide assurance that no internal contamination exists. Present air pollution, building, and fire codes do not specify the requirements for disposing of anesthetic gases via air-conditioning systems. In the future, such local, state, and federal codes will have to be met. 1v-7 V. SCAVENGING SYSTEMS FOR WASTE ANESTHETIC GASES Scavenging is defined as the collecting of waste anesthetic gases and vapors from the breathing systems at the site of overflow and the disposal of them. The purpose of such systems is to protect the personnel by preventing dispersal of anesthetics into the air of the operating room. A scavenging system consists of two major components: a collecting device (or scavenging adapter) to collect waste gases and a disposal route to carry such gases out of the room. The disposal of waste gases to the atmosphere assumes that they are exhausted directly outside without con- taminating air intakes so as to comply with local, state, and federal building, fire, and air pollution codes. Concern about the waste gas problem is expressed in an appreciable body of literature on scavenging equipment and its use.3° 3% A variety of gas collector devices®? "°° and several disposal methods °®”°% have been described. The design features frequently reflect the ingenuity of the authors by employing materials already available in the hospital. A few of the devices are compatible with widespread acceptance and meet all the criteria of ideal scavenging components: fail safe, easy to use, economical, and effective. A. Collection of Waste Gases from the Breathing Systems 1. Circle Absorber Figure V.1 is a typical collection system for the circle absorber, consisting of an air-tight enclosure surrounding the popoff valve. Selected performance specifications of two scavenging popoff valves are presented in Appendix B. The popoff valves generally require frequent adjustment during manually assisted or controlled breathing. This inconvenience is eliminated by the use of a valve that automatically disposes of excess gases at the end of each exhalation. One such valve is the Berner;"® similar devices (AGAT and Georgia¥) indicated slight leakage when pressurized and immersed. 2. Ventilator The ventilators were equipped with waste gas collecting devices; some were built into the ventilator, and others were attached accessories. TN 371 Excess Pressure Valve (prototype model), AGA Corporation, Seacaucus, NJ. ¥Georgia Valve, stock number 207-8168-800, Evacuator Kit, stock number 216-6409-880, Ohio Medical Products, Madison, Wisconsin. V-1 ( —— Fig. V.1. DUPACO SCAVENGING POPOFF VALVE FOR CIRCLE ABSORBER. When the ventilators are used during anesthetic administration, the scavenging collector device from the ventilator can be connected to a Y-piece (Fig. V.2) through which the effluent gases from the scavenging popoff valve and the ventilator are carried to the disposal system. The Y-piece eliminates the need to reattach the disposal tubing when alternating between ventilator and manual breathing. Certain of the ventilators tested (Ohio 300 series, Monaghan, Ventimeter, and Bird) required a one-way check valve located in the waste gas disposal line adjacent to the ventilator. This valve prevents waste gases from leaking from the anesthesia machine through the ventilator and into the room when the ventilator is attached to the disposal system while detached from the patient. It proved difficult to find a check valve that will not leak under any circumstance. The pressure present in many disposal systems approaches atmospheric and tends to discourage the firm seating of many of the available valves. This is not easy to recognize and may require the use of a leak detector during clinical use of the ventilator. Even the DeVilbiss valve was modified by sealing two small holes adjacent to the valve seat. New ventilators with scavenging adapters are now available, and existing units can be adapted. The efficiency of the collectors in the ventilators tested varied. Some units leaked sufficiently to cause high concentrations of gases in the operating room (see Section I11.C for details). The Bennett scavenging trap (Fig. V.3) is intended exclusively for waste gas disposal into the suction system. This is an undesirable constraint and, in addition, there is no provision for receiving the effluent from the gas machine. As a result, this device was modified by sealing the suction outlet and attaching a Y-piece at the outlet. V-2 POP wW/ MEL VENTILATOR GAS MACHINE Y—-PIECE 7/8" WASTE FROM VENTILATOR — EXHAUST Fig. V.2. USE OF A Y-PIECE TO COLLECT WASTE GAS FROM ABSORBER, VENTILATOR, AND CHECK VALVE (C) TO PREVENT LEAKAGE. y-3 manufacturer's collector device suction outlet (occluded) waste gas outlet, leading to Y-joint Fig. V.3. MODIFICATION OF GAS COLLECTOR DEVICE FOR BENNETT VENTILATOR. The scavenging adapter, manufactured by Monaghan for the Ohio and Monaghan units, was found to be gas tight. The Ohio adapter, however, tended to leak. The Air-Shields Ventimeter ventilator when provided with a specially lapped relief valve (an internal component of the ventilator) proved to be relatively gas-tight, but current production units unfortunately do not include this improvement. Recently the manufacturer indicated that in the near future all new units will include the gas-tight valves and a for existing units a field conversion kit will be available. It was noted that this ventilator added 10 to 15 L/min of driving gas (the gas employed to operate the ventilator) to the disposal line. If the operating room suction system is used for waste gas disposal, this extra capacity must be available (see Section V.B). Factory conversion of older Ventimeter units for scavenging is expensive, but a more economical method has been described.®> Unfortunately this conversion is not likely to result in low leak performance. 3. Nonrebreathing System In the nonrebreathing system evaluated in this study, with a scavenging adapter (Fig. V.4), fresh anesthetic gases enter at the breathing bag and all excess gases leave through the adapter. This valve is especially sensitive to negative pressure; the bag tended to empty unless the system was vented into a disposal pathway presenting a slight positive pressure. Performance data are cited in Appendix D. kL, T-Tube (Summers Modification) With this T-tube (Fig. V.5), fresh anesthetic gases enter the system at the side arm. All exhaust leaves the tail of the bag and, employing the collection system developed in the present study, this exhaust passes through an adapter attached to the disposal line. Accidental occlusion of the tail of the bag, by twisting on itself, is prevented by inserting a length of plastic tubing through the tail into the lumen. Accidental disassembly was prevented by tightly fitting the tubing into the tail (an assembly operation facilitated by the use of a stilet). The tubing, being resilient, allowed the pinch clamp to compress the tail of the bag together with the contained plastic tubing, thereby achieving the desired degree of occlusion required for intermittent positive pressure breathing. Recently, it was found that a tight connection between the cork and the disposal system is easily achieved by inserting a plastic adapter.’ It is emphasized that the convenience of this method is enhanced by the use of a short length of highly flexible lightweight disposal tubing, with a loop fixed to the operating table to prevent traction on the breathing bag. Selected specifications for this device are presented in Appendix D. Waste gas collector devices, ventilators, and related equipment utilized in this study are summarized in Table V.1. TBennett Respiration Products--Part No. 3038, Adapter. v-4 FLEXIBLE (— TUBING EXHAUST ADAPTER _— CAP Fig. V.4. WASTE GAS COLLECTOR FOR DUPACO NONREBREATHING VALVE. Effluent gases captured by adapter cap. INTERFACE Si py 7(8"—%= OR DISPOSAL se LA CLAMP CONNECTOR ADAPTER A — ———. PATIENT PLASTIC EN TUBING : | Fig. V.5. WASTE GAS COLLECTOR FOR T-TUBE. Effluent gases captured at tail of bag. V-5 Table V.1 GAS COLLECTOR DEVICES AND VENTILATORS System Device(s) Circle Clean OR Valve, Catalog No. 39930, Dupaco Corp., Absorption San Marcos, Calif. $66 Gas Evacuator-Relief Valve, Part No. 207-8172-800, Ohio Medical Products, Madison, Wisc. $86.50 Berner Valve, Complete Assembly, Part No. 12090, Dameca A/S, Islevdaluej 211 DK-2610, Rgdovre, Copenhagen Denmark, approximately $100 Nonrebreathing Modified positive pressure nonrebreathing valve with exhaust adapter, Part No. 24130, Dupaco Corp., San Marcos, Calif. $28 T-Tube System must be assembled Estimated cost of parts: $5/unit Ventilators Bennett Anesthesia Ventilator, Model BA-4, Part No. 1000, Gas Evacuator, Part No. 7430, Bennett Respiration Products, Inc., Santa Monica, Calif. Monaghan Anesthesia Ventilator Model M300, Part No. 13775, Gas Dump Kit, Part No. 13818, Monaghan, Littleton, Colo. Ohio Fluidic Anesthesia Ventilator, Stock No. 309-0612-800, Ohio Medical Products, Madison, Wisc. Check Valve DeVilbiss Input Check Valve, Part No. 2-1019,T The for Ventilators DeVilbiss Company, Medical Products Division, Somerset, Pa. * modification required: see text. V-6 B. Disposal of Waste Anesthetic Gases Waste anesthetic gases once collected at the anesthetic breathing system must be disposed of, usually into the atmosphere. Any applicable codes related to building, fire, and air pollution must be considered. A functionally gas-tight connection between the breathing system and the disposal system is required because the air-conditioning system does not efficiently remove waste gases escaping into the air in the operating room. With the low pressure disposal techniques (air-conditioning exhaust and special low velocity duct), the collector device may be directly connected to the disposal system. In contrast, disposal into the central vacuum system requires special components for equalizing the pressure, and a flowmeter is desirable for monitoring the suction flow. It should be emphasized that waste gas disposal must be engineered to avoid contamination of intake air or of areas where personnel are working. 1. Air-Conditioning Exhaust The simplest disposal system routes the anesthetic waste gases from the scavenging trap directly to the room's air-conditioning exhaust grille (Fig. V.6) where the sweeping effect of the air flowing into the grille carries all waste gases outside. A minor hazard to the patient may result from an accidental occlusion of the tubing that connects the scavenging collector to the grille, which may cause a temporarily over- filled breathing bag; this problem can be prevented by providing a positive pressure relief valve. No such valve was employed in the present studies, and this type of disposal system has been used at Stanford University Hospital since 1969 and at another large operating suite without significant complications.>® In the present experimental studies, the negative pressure measured at the junction of the anesthetic breathing and disposal systems was in the range of 0 to -4.4 mm H,0. When the air-conditioning exhaust grille was used for waste gas disposal, the scavenging tubing was terminated at the grille. One device (manufactured by Ohio Medical and Surgical Corp.) discharges the waste gases at right angles to the flow of the air-conditioning exhaust; however, leakage into the operating room may occur in the presence of low air- conditioning flow rates. It is safer to discharge the waste gases in line with the exhaust stream. The simple method illustrated in Fig. V.6 uses a metal plate drilled to accept a 1 in. length of 7/8 in. copper tubing for receiving the disposal line. In recirculating air-conditioning systems, the exhaust can be employed if the waste gases are introduced into the exhaust duct by entering downstream from the point of recirculation (Fig. IV.1). Negative pressure increases downstream in the exhaust system and may be sufficient to empty the breathing bag. If so, interfacing equipment is required (see Section D.h). In some operating rooms, the air-conditioning exhaust vent was separated from the anesthesia machine, thereby requiring a long length of disposal tubing. This objectionable feature was eliminated by arranging the tube to follow the same path as the anesthetic gas supply hoses. A Y~7 Fig. V.6. WASTE GAS DISPOSAL INTO AIR-CONDITIONING EXHAUST SYSTEM LOCATED IN OPERATING ROOM. wall or ceiling service panel may be connected to a permanently concealed waste gas line joined to the air-conditioning exhaust duct in the crawl space. The location of this junction can be critical. Because negative pressure increases with the proximity to the exhaust fan, a large enough pressure imbalance may exist to empty the breathing bag. In such rooms, the negative pressure was balanced by locating the junction in a concealed site close to the exhaust grille (Fig. V.7); other pressure balancing methods, described in Section B.5, also are applicable. If electrical outlets are close to the service area and flammable agents are used, this disposal route can be hazardous. 2. Low Velocity Specialized Duct System Another disposal route for the waste anesthetic gases is a separate low velocity duct system leading to the atmosphere (Fig. V.8). As with all waste gas disposal, any applicable codes related to building, fire, and air pollution must be considered. In this study, and by this technique, waste gases from two operating rooms were collected into a common duct that led to a duct main and were dis- charged at the roof. A fan provided sufficient negative pressure and air flow to ensure that cross contamination in the operating room did not occur. Selected performance specifications for this system are presented in Appendix D. [It was found that negative pressure was sufficient to open certain popoff valves (Ohio and Dupaco) and to empty the breathing bag. It should be noted, that, under the clinical conditions of this study, all the anesthetists, without special instructions, compensated for this con- dition by partially closing the popoff valves. Excessive negative pressures also may be compensated for, using pressure balancing devices (described in Section V. 15. v-8 _1 ! | --— det \) i SY ' y N Vi NN vl NN "i NIN 4 \ Vl — il SCAV. VAC. |] y A LN} 0 || H WASTE GAS a VACUUM 2 Be i N,O Fig. V.7. CONCEALED ACCESS TO AIR-CONDITIONING EXHAUST. FAN ED / (NZAR ov a ROOF) INLET FROM ROOM A — BALANCING VALVE INLET FROM ROOM B —» Fig. V.8. LOW-VELOCITY DUCT SYSTEM FOR WASTE GAS DISPOSAL. V-9 In planning the installation of a special duct system, all factors that could possibly influence its performance must be considered. For example, markedly varying positive pressures among certain operating rooms could cause reduced flow in one of the room disposal ducts and result in leakage. Leakage also could occur if the reservoir volume in the short lengths of disposal tubing is very small. The volume between gas machines must be sufficient to accept the short rapid pulses that occur when the anesthetist compresses the breathing bag. Any possibility of leakage could be reduced by inserting a scavenging reservoir bag near the breathing system. Suitable materials for the duct must be selected. Special duct systems installed during the present study employed polyvinyl chloride or stainless steel piping. Nylobraidel could have been used. A separate duct for venting waste gases directly outside without the use of a fan may be an acceptable alternative, but several limitations are apparent. A separate line would be required for each operating room to prevent the cross contamination with waste gases among the operating rooms. A safe disposal site would be necessary. The possible effects of variations in wind velocities and direction would require a means for preventing a reverse flow in the disposal system. Compliance with any ap- plicable codes, building, fire,and air pollution would be necessary. The use of a standard gravity ventilator diverter should be considered, (available through heating and air-conditioning dealers). Despite these limitations, the separate duct without the use of a fan may be ideal in older hospitals constructed with windows that are not opened and in the absence of nonrecirculating air conditioning. In the present studies, the special duct system was designed to function at approxi- mately atmospheric pressure; however, a continuum of designs is conceivable, beginning with systems operating at atmospheric pressure and extending up to standard line vacuum. One manufacturer (Ohio Medical and Surgical Corp.) has recommended a system in which all rooms are connected by a common manifold to a low vacuum pump (approximately 40 mm Hg), and each room outlet is pro- vided with a flow adjusted to approximately 20 L/min with all outlets open. With an understanding of the pressure and flow requirements of the breathing systems, several design concepts should prove satisfactory. Fire,building,and air pollution codes do not specifically provide for low velocity anesthetic waste gas disposal systems. Exhaust fans in areas intended for the storage of flammable agents, however, must be installed near the disposal site and have nonsparking wheels.2? + A flexible plastic tubing armored with molded-in plastic mesh; available from dealers in plastic tubing. 3. Central Vacuum System The central vacuum system available in operating rooms can be used for waste gas disposal. A prime consideration is to ensure that the lines and vacuum pump have the capacity required for the extra burden of scavenging. Even more important is a vacuum break (an interface or pressure balancing system, described later this section) located between the suction outlet in the operating room and the anesthetic breathing system. This interface must not leak anesthetic gases and must provide absolute assurance that the unrestricted negative pressure cannot enter the anesthetic breathing system and possibly cause atelectasis (collapse of the patient's lungs). One method of waste gas disposal by suction is to install three separate outlets that enter the same suction line, one for the surgeon, one for the anesthetist for removal of secretions, and one reserved exclusively for waste gas disposal (Fig. V.9). A completely separate suction system with its own lines and vacuum pump may be ideal and has been employed in new construction (Ohio Medical and Surgical Corp.). In the absence of the three suction outlets, .a single outlet (Fig. V.10) can be branched to provide separate lines, one each for patient secretions and for waste gas disposal. In Fig. V.10, the branch is indicated by the letter T. This system includes a suction flowmeter ‘and BALANCING VALVE 4 SURGEON —pm= ANESTHETIST — 3 WASTE GAS —3= Fig. V.9. DISPOSAL INTO WALL SUCTION -- THREE OUTLETS IN TYPICAL OPERATING ROOM. control valve, plus a shut-off valve for the line intended for secretions. This equipment is mounted on the anesthesia machine within easy reach of the anesthetist. With an efficient interfacing system requiring a rela- tively small amount of suction (Section 4), sufficient suction was available for scavenging and secretions. In an emergency, the convenience of the controls permitted dedication of all suction to the removal of secretions. The data presented in Table V.2 suggest a suction system of marginal capacity. It is questionable whether such a suction should be used for scavenging because diverting any significant portion reduces the capacity required for surgical use and for the removal of secretions. In these studies, operating rooms contained one suction bottle each for the surgeon and anesthetist (Fig. V.10). For flow measurements, the branch for secretions from the suction catheter was open, and flow rates of 0, 10, and 20 L/min were attained in the disposal system using the scavenging flow- meter. Table V.2 illustrates an unusual phenomenon in Room 1. With no flow diverted for scavenging, the remaining unrestricted flow was 35 L/min; with 20 L/min of suction diverted for waste gas disposal, the unrestricted flow actually increased to 36 L/min, accompanied by a 1 mmHg increase in negative pressure. This phenomenon was the result of variations in line pressure related to demands on the suction system elsewhere in the suite. Without simultaneous observations, it was impossible to obtain measurements under steady-state conditions. Room 3 was the most markedly affected by the diversion. Although the static pressure was 33 L/min with no diversion, the flow decreased to 51 percent of this value (17 L/min) with 10 L/min diversion and to 60 percent with 20 L/min. The corresponding pressures dropped to 37 percent (9 mmHg) of the control pressure for 10 L/min diversion SCAVENGING SUCTION FLOWMETER CONTROL VALVE — CONTROL VALVE FROM SUCTION CATHETER SCAVENGING POP VALVE SCAVENGING WASTE GAS // RESERVOIR BAG 2L 7mm 1.D. SUCTION TUBING 23 mm} SUCTION BOTTLE WALL VACUUM Fig. V.10. DISPOSAL INTO WALL SUCTION -- ONE OUTLET. Table V.2 REDUCTION OF AVAILABLE SUCTION BY SCAVENGING Suction Flow Pressure Available (L/min) for Suction Catheter Onsvslineg Diverted Available ’ for for Suction Percent mmHg Percent Scavenging Catheter 1 0.0 35 100 14 100 10 35 100 13 92 20 36 102 15 107 2 0.0 36 100 14 100 10 31 88 12 85 20 22 62 10 71 3 0.0 33 100 24 100 10 17 51 9. 37 20 20 60 6.0 25 4 0.0 33 100 13 100 10 31 93 11 84 20 23 69 8. 61 5 0.0 33: 100 24 100 10 31 93 19 79 20 22 66 10 41 0.0 34 + 0.637 [1000.0 | 18 £ 2.5 100 10 29 += 3.1 85 +8.7 13 + 1.7 75 20 25 + 2.9 72 +7.7 10 + 1.5 61 Ty Standard error. and to 25 percent (6 mmHg) at 20 L/min. At a flow rate of 20 L/min diverted for scavenging, the remaining flow in the suction catheter for all rooms tested averaged 25 L/min or 75 percent of the suction available with no diversion. To measure the pressure, the suction catheter was removed, and the pressure gauge substituted. Again, the three different scavenging flow rates were successively obtained. At an average flow rate of 20 L/min diverted for scavenging, the pressure remaining at the suction catheter was 10 mmHg, or 61 percent of the pressure available with no diversion. Because these values are too low for the effective removal of secretions (falling short of the recommended 1.0 ft®/min),>° the suction available in this suite was considered inadequate to provide as much as 10 L/min of suction for scavenging. Any use, then, of this system for waste gas disposal would, at best, require a highly efficient interface (Section 5) and a means of shutting off the scavenging flow in an emergency. Discussion of these data with the hospital engineering department led to cleaning the wall-mounted suction inlet piping and regulators, with the result that the use of 20 L/min of suction for scavenging caused negligible reduction of the remaining flow or pressure. A questionable objection to the use of suction for waste gas disposal relates to possible corrosive effects of the anesthetic gases on the vacuum pump. The exclusive use of suction for scavenging would maxi- mize the possibility of corrosion, but the shared use for surgical purposes would result in a massive dilution of the anesthetic mixture. The average flow rate of anesthetic gases rarely exceeds 5 L/min, and the surgical suction alone flowing at 30 L/min or more would result in a maximum increase in the concentration of oxidants (N,0 and 0) of 11 percent above room air values which would be unlikely to damage the pump. One difficulty with the normal wall suction system is that its use in the disposal of flammable agents is prohibited in Code 56A.?2° Nevertheless, there appears to be no conflict with the requirements of this code if the usual oil lubricated suction pump is replaced by a water- sealed unit. The disposal of halogenated anesthetics into a canister contain- ing activated charcoal has been recommended.’® Limited experience with this technique indicates that the concentration of halothane is reduced, but N20 is not significantly absorbed by activated charcoal. One ineffective disposal method vents the waste gases from the popoff valve to the floor in the mistaken belief that anesthetic gases, being heavier than air, would settle to the floor away from personnel. The clinical and laboratory tests (discussed in Section IV.D, VII.C. and Appendix C) indicate that the distribution of N,0 and halothane is usually essentially uniform throughout the operating room and the gases are thoroughly mixed by the movement of the personnel and by the air- conditioning system. These factors discount this ''floor venting'' method. L, Pressure Balancing or Interfacing Any marked pressure differential between the anesthetic breathing system and the disposal piping or duct must be equalized to prevent inter- ference with the breathing system, such as a collapse of the breathing bag or a collapse of the patient's lungs. If the disposal system presents a slight negative pressure, up to 5 mmHg, pressure balancing can be achieved by adjusting the scavenging popoff valve. The spring loaded Ohio valve, for example, can be regulated by turn- ing the adjustment screw; the disk-type Dupaco valve can be adjusted by substituting a metal disk for the lightweight plastic disk usually provided. Appropriately weighted disks are available from the manufacturer, and installation requires disassembly of the valve. It has been noted that the nonrebreathing system is particularly sensitive to slightly negative pressures. Such pressures can be controlled by the popoff valve on the absorber (Fig. V.11). The bag outlet and the inhalation valve were connected, and the fresh gas inlet was closed. The effluent from the nonrebreathing valve was then introduced into the exhalation valve and passed through the popoff valve to the disposal system. This method was particularly satisfactory with the Ohio device because it is easily adjusted. Negative pressure greater than 5 mmHg requires special interfacing equipment for additional pressure equalization. In these studies, the equipment requiring the least suction flow was the liquid-sealed interface, shown in Fig. V.11, filled with a nonflammable nonvolatile silicone liquid (Dow Corning 550 or 200). During use, the waste gases leave the anesthetic breathing system and the scavenging reservoir bag temporarily stores high peak flow rates, thereby contributing to pressure relief and the prevention of leakage. N = nonrebreathing valve = disposal tubing E = exhalation check valve = occluded fresh gas inlet D Cc I = inhalation check valve B = scavenging reservoir bag L P = scavenging popoff valve liquid-sealed interface device Fig. V.11. USE OF ABSORBER CIRCUIT FOR WASTE GAS DISPOSAL. The function of the interface in Fig. V.11 differs according to the disposal system employed. When the disposal system exhibits high negative pressure, the suction flow must be adjusted to a level slightly greater than the waste gas flow from the breathing system. The scavenging bag is then collapsed, and a small amount of room air enters the relief opening (Fig. V.10) and slowly bubbles through the liquid seal. The slight excess of suction flow (v0.5 L/min) is maintained because the precise balancing of suction and waste gas flow is impractical. When the disposal system exhibits minimal negative pressure (air-conditioning exhaust grille), no gas bubbles appear in the liquid. Selected aspects of the performance of this interface are described in Appendix D. The advantages are as follows. (a) It is applicable to all standard breathing and disposal systems. (b) Relief is ensured against both positive and negative pressures which are limited to 4 to -1 cm H50. (c) Monitoring is provided by means of gas bubbles and distension or collapse of the reservoir bag, thus minimizing the possibility of gas leaks caused by unrecognized occlusion of the distal disposal route. (d) It is efficient because it requires minimal suction flow in excess of anesthetic gas flow. Another commercially available interfacing system, designed exclusively for use with the wall suction, was evaluated (Fig. V.12). A valve is installed for negative pressure relief, and a degree of pro- tection against excessive positive pressure is supplied by the scavenging reservoir bag. Inputs to receive waste gases from the ventilator and absorber are provided. Considerable difficulty was encountered in adjust- ing the suction because resistance to flow through the relief valve can cause inappropriate emptying of the breathing bag. This problem was reduced by improving the control of suction through the addition of a flowmeter and adjusting the scavenging popoff valve. This interface is described in Section I11.E.1 and Appendix D.6. Several interfacing systems were evaluated but were considered unsuitable because of difficulty in monitoring the scavenging efficiency or because of leaky operation. For reference, such systems are illustrated in Fig. V.13. Evidence of leakage in the Ohio and Foregger tube-within-a-tube interfaces (Fig. V.13a,b) is presented in Section Ill. The small hose relief (13c) developed during these studies is not recommended because it lacks any sufficient indication of proper functioning. The custom built- in (13d) was installed in an operating room and is satisfactory but, again, there is no indication of proper functioning. 5. Tubing for the Intraoperating Room Portion of Waste Gas Disposal Systems The requirements for the tubing that connects the anesthetic breathing system to the disposal system vary according to the breathing V-16 TO SUCTION LL SCAVENGING Jj FLOWMETER VALVE SCAVENGING POP VALVE NEGATIVE RELIEF VALVE ~g } ABSORBER VENTILATOR INLET SCAVENGING BAG \ . INLET Fig. V.12. DUPACO INTERFACE, system employed and to the scavenging components. The ideal tubing should be kinkproof, free of leaks, and capable of forming a gas-tight seal at any connection. In addition, it must be easy to assemble and disassemble without accidental disconnections. When long lengths are required, those portions exposed to occlusion should be nonresilient; however, the use of long lengths was considered an expediency until appropriate permanent connections were provided. The location of the high pressure gas supply hoses and the location of the disposal line are carefully planned. The first or primary length of tubing is attached to the anesthetic breathing system. For the T-tube and nonrebreathing systems, this primary length must be lightweight and flexible. Away from these breathing systems, a less resilient material is suitable. The choice of disposal tubing for the circle absorber is not critical. The bore of the tubing should be sufficient to avoid undue back pressure in the breathing system. A 7/8 in. bore was chosen in these studies because of its availability and low resistance during exhalation; however, 1/2 in. bore is acceptable. Pressure and flow specifications for various tubing types and diameters are plotted in Fig. V.14. The bore of the tubing that eventually will be employed in waste gas disposal systems should not be interchangeable with any breathing system components. To avoid wrong connections, color coding would be desirable. a. Toa ORIFICE Pa — Foregger tube-within-a-tube ROOM AIR — RELIEF TUBING SCAVENGER POP VALVE ed 7/8" h WASTE GAS tr SCAVENGER BAG c. Small bore relief SCAVENGER NEGATIVE RELIEF POP VALVE AND RESERVOIR Faaslle aa ROOM AIR WASTE + ROOM AIR 7/8" 1.D. WASTE GAS CORRUGATED TUBING (~ 6) b. Ohio tube-within-a-tube EXHAUST CEILING 14" SHUNT GAS I MACHINE ] | d. Custom built-in Fig. V.13. INTERFACING SYSTEMS. 1 | | 1 I CORRUGATED 100[~poL YETHYLENE/® - 5/8" ID GARDEN HOSE (5/8" ID) _. 80 9 ® — z J i GARDEN HOSE (1/2 ID) ~ 60 op ® — cy oc 2 . 5 TYGON (3/8”ID) 2 40-ee ® — ® 20 v/ _—" DIMENSIONS = ID INCHES ~~ ® _e | | 1 ] 1 5 10 15 20 25 Cm HO STATIC PRESSURE TApproximate oD = 7/8 in. Fig. V.14. FLOW-PRESSURE RELATIONSHIPS FOR VARIOUS TYPES AND SIZES OF TUBING. For disposal into the suction the tubing chosen for the primary length, leading to the interface, is determined according to the anesthetic breathing system employed. After the interface, the needs are not critical and ordinary 1/4 in. suction tubing suffices. 6. Flammable Anesthetics Flammable anesthetic agents, such as ether and cyclopropane, must be considered in waste gas disposal systems because the use of such agents continues in certain hospitals. Dilution of waste gases with room air affects flammability; mixtures below approximately 1.8 percent are considered nonflammable. Initially, the concentration of anesthetic gases in the disposal system is the same as the patient's inspired gas mixture. Certain disposal systems employing an interfacing device, such as the central vaccum system, dilute the gas mixture and reduce flam- mability and corrosion (Appendix A). Massive dillution occurs with disposal into the air-conditioning exhaust. The special duct system employed in the present studies carries the undiluted anesthetic gases outside, but other duct systems have been designed to entrain room air, thus diluting the mixture. Tubing containing high concentrations of flammable agents should be kept away from electrical connections and electronic monitoring equipment. 7+ Summary of Disposal Systems Methods of waste gas disposal are summarized in Table V.3. It is reasonable to employ the simplest and least expensive methods of proven effectiveness which are compatible with all regulations (building, fire, intake air caused by the infrequent failure of a suction fan or vacuum pump should be of little significance. Insurance against more frequently occurring contamination would be provided through the air monitoring program. and air pollution). Any contamination of the Costs of various disposal systems are estimated in Table V.h. Table V.3 SUMMARY OF DISPOSAL SYSTEMS Compatibility Disposal System Recirculating Flammable Comments Air Conditioning Agents Air-conditioning exhaust At grille No Yes Economical Tubing on floor should be avoided Downstream from grille No¥ Yes Pressure balancing may be required Specialized duct Yes Yes Spark-proof fan required Central vacuum Yes No* Vacuum pump must vent to a safe disposal site Lines and vacuum pump must be adequate Negative pressure re- lief mandatory t+ All applicable codes, local, and air pollution must be considered. + Recirculating air conditioning is compatible if duct is entered downstream from point of recirculation. * Flammable agents compatible if water-sealed compressor is provided. V-20 state and federal, related to building, fire, DISPOSAL SYSTEMS : Table V.4 ESTIMATED COSTS AND SOURCES Estimated Method of Disposal Components Cost (9%) Per Room Air conditioning exhaust Tubing to reach grille: te grills e Garden hose material 3.00 * 3-M corrugated tubing 2.00 Grille adapter (custom made) 5.00 Concealed access to duct Labor and tubing 200.00+ Low-velocity special duct? Fan, Cincinnati exhauster, 15.00 1/2 HP motor Tubing system 1-1/2 or 2 in. 30.00 PVC pipe to each room Stack to roof, 4 in. PVC 20.00 pipe Labor 200.00t+ Central vacuum (including Water-sealed compressor, 1400.00t+ interface devices) piping system, labor, and materials Liquid-sealed interface 75.00 or Dupaco Vacuum Manifold 53.00 Boehringer interface 40.00 Flowmeter with control valve 20.00 Labor to mount components 50.00+ T Actual cost for the two-room system evaluated in this study: Fan, 1/4 HP motor 100 ft PVC pipe and fittings Labor $120.00 30.00 260.00 $410.00 Costs estimated on the basis of a ten-room suite. V-21 C. Alternatives to the Proposed Scavenging Techniques Thus far, the assumption has been made that inhalation anesthetics are to be used only with adequate scavenging. In the absence of scavenging, totally closed system inhalation anesthesia reduces exposure, but most anesthetists prefer not to use this technique. In addition, few ''closed system'' anesthetics are actually closed 100 percent of the time, and the semiclosed technique is usually employed, particularly at induction and recovery.’ Regional anesthesia theoretically obviates the need for waste gas control measures, but many patients prefer to be asleep. It would be ideal if the waste gases could be absorbed at the breathing system, eliminating the need for complicated scavenging hoses and plumbing. The limitations of activated charcoal filters have been considered. The high solubility of N,0 in water led to an effort to scrub the gas in an industrial water spray scrubber. This proved ineffective because the resi- dence time for N20 to go into solution would require a scrubber of gigantic proportions. D. Initiating a Scavenging Program The following set of instructions should be of value to those who are interested in setting up a scavenging program. (1) Examine all anesthetizing locations such as operating rooms, delivery suites, X-ray, outpatient clinic, emergency room, psychiatry to determine local in-house differences in options available for scavenging. For example, in the operating room, the air-conditioning exhaust may be suitable for waste gas disposal because it is nonrecirculating; however, it may be unsuitable in X-ray because it could recirculate. (2) Consider disposal system alternatives and arrange for parts and installation. Air-conditioning exhaust: assemble and install grille adapter install duct work if gases are to be introduced downstream Special duct system: duct work and fan (if required) "nevertheless, the use of the closed system could be encouraged for other reasons--economy and reduced pollution of the air outside the hospital. An additional factor is the availability of analyzers for 0, and anesthetic agents, resulting in increased safety. V-22 (3) (4) (5) (6) (7) (8) Central vacuum system: purchase and install interfacing device, flowmeter, and control valves Order scavenging popoff valve for circle breathing system. The gas-tight valves used in the present studies were Ohio: simple to adjust Dupaco: impedance adjustable by adding weights Berner: requires no critical adjustment of leak rate during assisted or controlled breathing Order scavenging nonrebreathing valve (if nonrebreathing is used). Assemble scavenging collector device for T-tube (if T-tube is used). Select interfacing equipment if central vacuum system is used. Decide whether to convert existing ventilators for scavenging or to purchase new units. The least leaky ventilators in the present studies were Bennett: scavenging adapter is modified (Fig. v.3) Ohio DO 300 and Monaghan: easily converted, using factory equipment made by Monaghan Ohio Fluidic: with built-in scavenging equipment Plan for air monitoring, considering either purchasing or leasing the equipment or contracting with an extramural company or laboratory. Consider whether halogenated anesthetics (ether and cyclopropane), as well as N,0, will be monitored. V-23 a A a i f t i pe VI. EQUIPMENT MAINTENANCE Equipment maintenance is a key factor in the prevention of anesthetic gas leaks and in the prompt correction of leaks that do occur. The objective of the present studies was to secure low leak performance (defined as a maximum leak rate of 100 cc/min at a pressure of 30 cm H,0) for all anesthesia machines. It must be emphasized that such per- formance requires rigid standards of preventive maintenance and servicing beyond that necessary for the administration of clinical anesthesia. The maintenance procedures (Section B) presented are based on the find- ings of the studies on equipment leakage discussed in Section III. A. Maintenance Schedules and Leak Tolerances for Anesthesia Machines and Related Equipment (1) Anesthesia machines receive preventive maintenance at four- month (minimum) intervals by manufacturer's service repre- sentatives or by other qualified personnel. Following such maintenance, high pressure leakage should be less than 2 ppm NO in room air. The low pressure leak rate should be less than 50 cc/min at 30 cm H,0. (2) The low pressure systems of the anesthesia machines (from the flowmeters to the breathing tubes) are leak tested at monthly (minimum) intervals and whenever the soda-lime is changed. This is readily done by hospital-based personnel. Leakage should be less than 100 cc/min at 30 cm H,0. (3) Ventilators receive preventive maintenance at four-month (minimum) intervals by service representatives or other qualified personnel. (4) Breathing hoses attached to the anesthesia machines are leak tested as part of the low pressure test. Breathing hoses associated with the T-tube, nonrebreathing system, and ventilators are tested at four-month intervals. All leaky hoses are replaced. (5) Breathing bags attached to the anesthesia machines are leak tested as a separate procedure at the time of the low pressure test. Other breathing bags associated with the T-tube, nonrebreathing system, and ventilators are tested at four-month intervals. (6) Waste gas disposal tubing is leak tested at four-month inter- vals. Leaky tubing is replaced. (7) New equipment should be leak tested by the manufacturer before being placed in service. Minimum standards should be met. Vi-1 B. Procedures for Leak Testing Anesthesia Equipment The standard leak testing and localization procedures include (1) pressurization of the equipment with air followed by observation of a gauge for pressure loss or immersion in water in search of bubbles, and (2) the application of soap solution to suspected leak sites. Leak testing and localization may also be accomplished with industrial halogen leak detectors (Section Vil) or continuous trace gas analyzers. Leakage in the high pressure components of the anesthesia machine (from the N,0 cylinder or room pipeline connector to the flowmeters) occurs frequently and is easily observed via the air-monitoring method (test 1 in Section C). This procedure can be completed within a few minutes. A less satisfactory approach is to pressurize the machine with the high pressure hoses and to test suspected leak sites (by soap solution, immersion, or halogen leak detector, where applicable), beginning at the N,0 room pipeline connector (or N,0 cylinder), tracing the circuit to the machine through the chassis to the flow- meters, and including the valve stem seals of the standby Ny0 cylinders. Leakage in the low pressure system from the flowmeters to the breathing tubes occurs frequently, and leak testing is facilitated when the machine is equipped with an absorber pressure gauge and a low range (20 to 100 cc/min) flowmeter. Nitrous oxide is suggested rather than 0, because many anesthesiologists avoid the use of low range 0; flowmeters. An abbreviated form of this test can be completed within seconds and is therefore adaptable to routine use. This test measures the total leak rate, employing only the first three steps in test 2 (Section C). Unfortunately, however, it is not sufficient for patient safety because the breathing bag must be removed from the machine. Leak detection and localization are initiated by hospital-based personnel. Repairs, however, are best accomplished by the factory- trained service technician, with the exception of simple procedures within the capabilities of the personnel, such as the replace- ment of accessible gaskets or tightening of valve covers. The tests described in the following section serve to identify all leaks in the anesthesia equipment. Exceptions include gas machine equipped with internal check valves where certain low pressure leaks may be missed. Such leakage occurs infrequently and should be checked during regular preventive maintenance. Vi-2 C. Tests for Leakage in Anesthesia Machines and N,O Supply Hoses TEST 1. LEAKAGE IN HIGH PRESSURE SYSTEM (N,0 cylinder or room pipe- line connector to flowmeters) (a) (b) TEST 2. The anesthesia machine is attached to the gas supply hoses in the operating room and the flowmeter valves are turned off. After all flowmeters have been turned off for at least 1 hr, the room concentration of N,O is determined. With nonrecirculating air conditioning, this value represents the leakage. With recirculating systems, the N,0 concen- tration of the inlet air is measured and then subtracted from the room concentration to determine leakage. LEAKAGE IN LOW PRESSURE SYSTEM (between flowmeters and breathing hoses) 1. Integral Flowmeter, Absorber Pressure-Gauge Method (a) (b) (c) (d) (e) The CO, absorber is sealed by means of breathing tubes; one length joins openings to the breathing valves and the other joins outlets for the breathing bag and popoff valve. The breathing bag must be removed to reduce compliance. Popoff valve is opened and the vaporizer control switch is turned on. Leak rate from the complete machine is first measured. Using any low range flowmeter, the flow is adjusted to maintain a steady pressure of 30 cm H,0, measured on the breathing circuit pressure gauge. (CAUTION: Pressure rapidly increases in a tight machine. When flowmeters are adjusted, the pressure gauge must be observed to avoid damage caused by over pressurizing. Professional service engineers employ a safety valve.) Leakage from the vaporizer system is excluded by turning off the vaporizer switch and again adjusting the flow- meter to compensate for any change in leak rate. Leakage from absorber and breathing tubes is then excluded by occluding gas outlet tubing, again adjusting the flow rate if necessary. In the event that a measurable flow rate remains, leakage in a seal is suspected. Vi-3 (f) Accessible seals (flowmeter tubes and valves) are conveniently inspected with a halogen leak detector (vaporizer switch turned on) or with a soap solution. Valve stem seals of the N,0 standby cylinders are leak tested along with the other seals. 2. Accessory Flowmeter: Pressure-Gauge Method In the event that the machine is not equipped with a low range flow- meter or pressure gauge, this equipment can be applied at the anesthetic gas outlet by inserting a T in the gas delivery tubing. The above steps are then completed, with the precaution that the tubing be occluded between the absorber and the T. 3. Immersion Method for Localizing Leakage in Absorber This approach may be employed to identify leakage not found by less cumbersome methods. The machine is prepared according to the first three steps in the low pressure test. Precaution must be observed concerning pressurization and the possibility of subsequent damage to the pressure gauge. In addition, the gauge must be kept dry, and screening should be installed to prevent soda-lime from entering the breathing hose connectors. D. Tests To Determine Leakage in Miscellaneous Equipment Equipment such as breathing bags, hoses, and devices with metal-to- metal connections suspected of leakage are tested by standard procedures (Section B). Leakage from ventilators is considered when trace gas concentrations increase during the use of this equipment. Following cleaning, the components are inspected and assembled with care, checking that all gaskets are in place and properly fitted. Vi-4 VII. AIR MONITORING IN THE OPERATING ROOM Proof of the success of such waste gas control measures as scavenging, the cooperation of the anesthetist, and equipment maintenance is based in the air-monitoring program. In the present study, the gas of primary interest was the widely employed inhalation anesthetic, N,0. The technique for measuring trace concentrations of N,0 by infrared spectroscopy proved technically simple and reliable. In contrast, the halogenated anesthetics were less fre- quently used and, because the concentrations are relatively lower, a highly sensitive analyzer is required. Such infrared analyzers working at high gain are subject to electronic noise and drift. An additional difficult problem was interference from alcohols employed in cleaning the operating room and in sterilizing the patient's skin for surgery. Other analytical methods were considered too expensive or cumbersome for routine use. Samples for the monitoring studies were collected at the air-conditioning exhaust grille where the trace gas concentrations representative of personnel exposure were obtainable. A gas is required for zero adjustment on the infrared analyzer. With nonrecirculating air conditioning, room air obtained at a fresh air inlet of the air-conditioning system provides a suitable source. Clean air is ensured by inserting the sampling tubing well into the inlet duct. With recirculating air conditioning and with certain heat exchangers, the "fresh air entering the room may contain N,0; if so, a convenient gas source is the central oxygen system. (Oxygen should not be taken from the anesthesia machine because of possible contamination with N50.) A difference in the zero baseline due to contaminants was regularly observed between inlet air and pure 0,. [If uncorrected, this would intro- duce an error in the N,0 concentrations equivalent to 2.5 to 5 ppm, depending on the analyzer employed. The frequency of air sampling is intended to reflect minute-to-minute changes in the average trace concentrations of inhalation anesthetics, plus long-term exposure averages. Short-term changes were observed by continuously sampling and analyzing and were found to be closely related to the anesthetist's activities. A small battery-powered analyzer detected abnormally high levels and immediately displayed the results of any corrective measures that were applied. A monitoring technique, employing samples collected in syringes, was developed for evaluating long-term changes. Such changes were also determined by collecting samples in gas-tight bags, filled over a period of hours. VIil-1 A. Samples Obtained in Syringes The feasibility of analyzing room air samples obtained in syringes depended on the availability of a trace gas analyzer capable of responding to the relatively small samples (100 cc). Two 50 cc glass syringes were rapidly filled with air obtained at the air-conditioning exhaust grille located in each operating room. Prior to analysis, these syringes were capped and then stored until all rooms had been sampled. Their contents were then injected into the infrared gas analyzer. The entire procedure for the 14-room suite was completed with- in 30 min. The first group of samples was obtained before the arrival of the anesthetists so as to determine the high pressure leakage of the gas machines. Subsequent samples were obtained to determine the concentrations of N,0 relating to the exposure of personnel during the workday and to search for low pressure leaks. To secure randomization, samples were collected at a different hour each day. The results of early morning sampling are shown in Fig. VIil.1. In 73 out of 137 samples, N,0 concentrations were 1.0 ppm or less. In five instances, the values were 25 ppm or above; one measurement (110 ppm) resulted when the N,0 flowmeter on the anesthesia machine was accidently left on. Another elevated value was caused by a high pressure leak in a N,O0 hose connection. The mean concentration was L.1 + 1.2 ppm ¢ S.E.). A%l1 results were obtained using the fresh air zero reference. 80 - = BEFORE ARRIVAL OF ANESTHETISTS = 60 - wn w ~ a = < wn w - — 4 40 > oc > wv uw 5 110 G20 110 S 32 > 29 25 0-1 2-3 4-5 6-7 8-9 10-11 12+ Fig. VII.1. RANGES OF ROOM AIR CONCENTRATIONS N70 (ppm) . VIi-2 The results obtained from 461 N,0 analyses of samples collected in syringes during a workday are presented in Fig. VII1.2; 344 samples (74 percent) contained N,0 in the range from 0 to 10 ppm, and 13 samples (2.7 percent) contained N,0 above 111 ppm. The mean concentration was 16 + 1.9 ppm. This study included samples obtained during the intervals between cases plus those taken during the induction, maintenance, and recovery phases of anesthesia. Scavenging procedures were in effect, and the anesthesia equipment, with the exception of a few ventilators, was relatively gas tight. 340-1 - o 320 - = [72] w g 300 | - z IRRESPECTIVE OF ROOM ACTIVITIES 380 |, = JA 360 / \ ) / w 17 340 /1 [+4 > 180 w 60 160 o 140 z 140 Q 400 130 — 2 120 2 120 20 120 — ] [ [] [] 1 mM MC me [ | ” — i”; ; 81— 101-110 0-10 4420 21730 31404180 51.69 61-7077 go ®' 9091 _100 m+ Fig. VI1.2. RANGES OF ROOM AIR CONCENTRATIONS N,O (ppm). These results, during the administration of anesthesia, were separated into groups, with a breakdown according to choice of the face mask, endo- tracheal tube, or ventilator technique (Table VIIl.1). Separation of these groups is not statistically achieved although the average concentration of N»0 in the syringe samples appears to be minimum (15 ppm) when the endotracheal tube was used. When the face mask and endotracheal tube with ventilator were applied, the mean concentrations were approximately twice as high (36 and 34 ppm, respectively). Figure VI1.3 shows the distribution of room concentrations found in the 208 samples taken during the administration of N,0. The anesthetic technique is not considered. It can be seen that waste gases in the 0 to 10 ppm concentration range predominate, being observed in 110 of the 208 samples. VIi-3 Table VII.1 CONCENTRATIONS OF N,0 IN ROOM AIR DURING ANESTHESIA 2 No. of Average : ; : Concentration Anesthetic Technique Syringe Samples of a? (ppm) Mask 75 A Endotracheal tube 76 15 2.4 Endotracheal tube with ventilator 57 34 + All samples 208 28 * 1 + S.E. —-— © o S100 f— —_ [7] z y 360 oa 3 J %% 340 5% 0 7 ANESTHESIA IN PROGRESS, ALL TECHNIQUES 2 4 © 2 180 E a0 140 — n 140 ie 130 o 120 uw 120 s 120 2 20— 100 — 100 [1 l | B Mr 0] 0-T0 op 2-80-80 Te BRE, Fig. VI1.3. RANGES OF ROOM AIR CONCENTRATIONS N,0 (ppm) . VII-4 Figure VII.k4 represents specific anesthetic techniques, face mask, endotracheal tube, and endotracheal tube with ventilator. All demon- strate similar distribution patterns, and the 0 to 10 ppm range again predominates. A few samples containing 100 ppm N,0 and more are note- worthy. In samples obtained when the face mask was used (upper panel), six contained over 100 ppm because of leakage under the mask. When the ventilator was employed (center panel), values in five samples were 100 ppm or greater. These were probably the result of leaky ventilators. These data are interpreted to indicate the need for greater care in the use of the face mask and for nonleaking ventilators, rather than for increased use of the endotracheal tube. B. Sampling in Bags Gas-tight bags were filled at a constant rate over a period of several hours to obtain average data on long-term exposure to the inhala- tion anesthetics. The collection system (Fig. VII.5) included a home- type aquarium pump for filling the bags and a flowmeter for monitoring the filling rate. The aquarium pump was located at the exhaust grille of the air-conditioning system and at several other locations. The bag samples were analyzed for N,0 by infrared methods; halogenated anesthe- tics were determined by gas chromatography. The operating rooms chosen for these studies were heavily scheduled for inhalation anesthesia. Analysis of the samples obtained included normalization to predict the concentrations of gases that would have been present in the bag had the anesthetic agents been administered over the entire collection period. Nitrous oxide calculations rarely required such correction because it was usually administered throughout the case. The exception occurred during local anesthesia. In contrast, halothane was less frequently used. As an example of the normalization procedure, when halothane was detected in a bag, the duration of administration was first determined by inspection of the anesthesia record. |f halothane had been administered during 50 percent of the total sampling period, the normalized value would be twice the value actually measured. Such normalization indicated the room concentration, if halothane had been administered throughout the sampling period, and corrected the bias related to the lower incidence of halothane anesthesia. The following results were obtained from the samples collected. Measured Normalized No. of Bag Concentration Concentration Samples (ppm) (ppm) Nitrous oxide 48 19 + 3.0 28 + 4.5 Halothane 27 0.24 + 0.05 0.45 + 0.11 VIil-5 1 o 7 , wn w 30 — 2 ZA FACE MASK TECHNIQUE 7, : 1 4 20} = w 2 x 360 & 200 w 180 ° 140 Ba 10 130 _| wm i 130 5 120 z nl | Heal] — ¥ 0-10 yy 20 Ll =30 51 40 41-50 g, go 61-70, oo 81-90 "101 tM y _— = y 5 / 74 7, a 7 / E 111] 2 VENTILATOR EMPLOYED 10 i 7 z 340 > 220 “ 120 oc 100 [-4 100 Ww 5 = 2 | | | 0-10 | 21-30, , 41-50 61-70 _ 81-90, ~101* © = —_— «» 40 “* viz y a. 74 7 “ A . uw 20 ENDOTRACHEAL TECHNIQUE — 2 oc > © uw o xc a = 10H — 2 z ] | | | 120 l A ri 0-10 21-30 41-50 61-70 81-90 101+ 11-20 31-40 51-60 71-80 91-100 Fig. VIl.h. RANGES OF ROOM AIR CONCENTRATIONS N,O (ppm). VI1-6 = . B - gas-tight bag BX 5 RK : pF SN (20 <> to 200 cc/min) XO | ER R - resistance RK, ] RRR ar p SES ub tight 25% Toye in tub- oe 22% L - aquarium bleed 56% valve / P - aquarium pump Fig. VII.5. BAG SAMPLING EQUIPMENT AT EXHAUST GRILLE. The concentrations of N,0 measured in the bags represent actual exposure levels and are comparable to the 467 syringe samples. The average for these syringes samples was 16 ppm which is lower than the 19 ppm determined for the bag samples. The reason for this difference may be the limited number of samples; however, the bags were placed deliberately in heavily scheduled rooms, whereas the syringes were filled in succession without regard for the activities within the rooms. The distributions of these bag samples are illustrated in Fig. VII.6 for N,0 (top panel) and for halothane (lower panel). Similar to the samples in syringes, predominating concentrations are found at the lower ranges, 0 to 20 ppm for N,0 and 0 to 0.40 ppm for halothane. The normalization procedure allows the bag samples to be compared to the syringe samples collected during the administration of anesthesia. Correlations in the average values of N,0 obtained by the two methods were 28 + 4.5 and 28 + 3.4 ppm for the bag and syringe samples, respectively. These values for N,0 (28 ppm) determined from the samples collected both in syringes and in bags (normalized) and for halothane collected in bags (normalized to 0.45 ppm) lead to the suggested maximum concentrations, 30 ppm for N50 and 0.5 ppm for halothane. These equal 0.005 percent of the administered concentrations of 60 percent N,0 and 1.0 percent halo- thane and provide a '‘rule of thumb'' for estimating reasonable concentra- tions of other inhalation agents. vii-7 N,O (48 SAMPLES) 10H — : ln. nll Mm [1 0-10 11-20 21- =30 41 40 41 50 —60 60 81 =70, _80 81-90 91-100 101 NUMBER OF BAG SAMPLES 0p HALOTHANE (27 SAMPLES) - . - — i = [1 0-.20 .41-.60 .81-1.00 1.21-1.40 1.61-1.80 .21-.40 .61-.80° 1.01-1.20 1.41-1.60 1.81-2.00 Fig. VI1.6. RANGES OF ROOM AIR CONCENTRATIONS. It is reiterated that, during the present studies, not all reasonable precautions were taken to minimize trace gas concentrations. Leaky ventilators had not been replaced, equipment maintenance procedures were not fully refined, and personnel did not always connect the scavenging equipment or secure the best possible mask fit. With the support of the anesthetists, lower concentrations in the range of 5 to 10 ppm N,0 and 0.1 to 0.2 ppm halothane were and can be regularly achieved. C. Samples Obtained in Proximity to Personnel Up to this point, the air monitoring methods presented in this report have been based on the average room concentration of N,0 sampled at the air- conditioning exhaust grille. This procedure was considered to represent personnel exposure because of the essentially uniform distribution of the leakage gases discussed in Section Ill and Appendix C. Nevertheless, the question remained whether, because of his proximity to the leak source, the anesthetist might inhale higher concentrations of N,0 than the room air average. As a result, the present study was designed to compare various sampling sites including operating room personnel during the performance of their normal duties. vVIii-8 The sampling sites included the air-conditioning exhaust grille at its center; an area near the center hinge of the door of the operating room, and selected personnel (the anesthetist, surgeon, and the circulating and scrub nurses). The sampling site for these personnel was an area immediate- ly posterior to the back of the neck, a location chosen for its accessibility during surgery and its proximity to the air actually inhaled while avoid- ing the exhaled CO, which interferes with the IR analysis. The analytical instrument was a small battery-powered continuously sampling infrared trace gas analyzer with nondispersive filtering at a wavelength of 4.48 pu and a calibrated range of 1 to 200 ppm N,0. This instrument was hand-carried into the operating rooms, and the sampling probe was first held at the air- conditioning exhaust grille. After equilibration (10 to 30 sec), the personnel were sampled, then the door, and finally a repeated measurement was taken at the grille. Following these measurements, the instrument was carried to the next room. In this manner, all available rooms were sampled in sequence with more than one set of samples obtained in each room. A total of 334 samples resulted, comprised of 54 sets of 7 samples each, except that in 44 instances a sampling site was omitted because of inaccessi- bility. These data were obtained by a single technician in 2.5 working days at approximately 4 hours per day. In reducing these data, four sets were dis- carded because the analyzer was overloaded by concentrations exceeding 100 ppm N,0. The instrument range was then extended to 200 ppm to include the suc- ceeding higher concentrations. Nineteen additional data sets were deleted because of a difference between grille measurements indicating nonsteady state conditions (> + 20 percent at concentrations greater than 10 ppm and > + 2 ppm at concentrations below 10 ppm).T The results below comprise the 202 remain- ing samples including 60 at the grille sites, and 142 near the personnel. Nurses Sampling Anes- Circu- Site Grille thetist Surgeon Scrub lating Door Grille N,O (ppm) 15 +2.7 18 +2.9 17 +5.0 13 +3.2 14 +2.8 14 +2.8 14 +2.8 (+ S.E.) - These results reveal little or no significant differences among the various sampling sites employed. Area monitoring at the grille or at the door appears to be representative of personnel exposure under the conditions prevailing in the present operating suite. In fact, sampling could be accomplished through a small hole in the door, thus avoiding the inconve- niences of entering the room. Other operating suites interested in establish- ing representative sampling sites could easily complete the necessary studies, given the speed and the convenience of the portable trace gas analyzer. The lower mean N,0 values reported above as compared to the concentrations re- ported in the gas-tight bag and syringe samples, 15 ppm versus 28 ppm, are explained by the recent results of the present study and the anticipated effect of an increased experience with the waste gas control measures. +These criteria seemed reasonable upon inspecting the data. Viil-9 D. Continuous Monitoring Specific operating rooms were selected for continuous monitoring of N,O. The instrumentation was the portable infrared monitoring unit. Initial experience indicated that the members of the operating room team were very interested in the ambient trace gas concentrations and in the rapid changes noted during anesthesia. This was particularly true of the anesthetist who could immediately observe the effects of his clinical techniques on trace gas concentrations and could modify them to achieve a further reduction in gas leakage. Inspection of the continuous recordings indicated that NO in the air could be maintained below 5 ppm throughout the period of anesthesia, both with mask and endotracheal tube; however, any momentary disconnection of the breathing system produced transient elevations. A chronic leak was reflected promptly in sustained elevated concentrations. Many of the observed elevations could not be explained by the anesthetist's techniques. Disconnections in the disposal system or leaks in the high pressure hose connections occasionally were responsible. Following correction of the problem, a reduction in the air concentrations of N,0 rapidly (within 30 sec) become apparent. If such reduction did not occur, the leaks were located through more careful search. When moving the monitoring station about the surgical suite, many unexpected events were encountered. The following are examples of such occurrences. (1) A hot spot, with N,0 at 100 ppm, was found in a hallway, leading to the discovery of a leaky connection in the room control valve system that probably was present since its original installation. (2) On entering the operating room during the early morning rounds, high concentrations were discovered because flow- meters were left on overnight. (3) A cylinder with a leaky valve was found to be flooding the storeroom with N50. (4) A machine developed a high pressure leak during an opera- tion, and the air concentration increased to above 120 ppm. (5) An accidental dislodgement of an endotracheal tube during a cleft palate repair was suspected as the cause of a sudden increase in N,0 concentration. The mobile N,0 monitoring unit is considered a valuable tool. It not only identifies leaks in the equipment, but it can inform the anesthesia staff on the impact of their techniques and provide a method for immediately observing the results of their improved or modified work practices. VII-10 E. Discussion The air monitoring program should be executed by an interested and qualified person, preferably an anesthetist, nurse, technician, or environmental engineer. A major factor in determining the type of personnel is whether the hospital chooses to operate its own monitoring program or to depend on an outside contractor. Ideally, the gases to be monitored include all inhalation agents employed in an operating suite. Meeting such a standard may be difficult, and a decision to monitor only selected agents could depend on the frequency of their use, availability of an appropriate analytical method, and cost of instrumentation. Nitrous oxide is the most important anesthetic in a monitoring program because it is the most widely administered. It is easily moni- tored and is found in many components of the anesthesia equipment. Because, in the anesthesia machine, many parts are subject only to leakage of N,0 and because more suitable analyzers are available for monitoring N,O than for halothane, assessment of N,0 leakage alone is often suffi- cient. The components subject solely tC leakage of halothane are few and largely confined to the ''copper kettle'' type of vaporizers and associated plumbing. Because these vaporization parts are localized to a small area and can be leak tested and serviced easily, the need for their moni- toring is reduced. Despite the above considerations, a case can be made for monitoring halothane and other agents such as enflurane, ether, and cyclopropane in operating suites frequently using these agents. During the present studies, it was found (Section Il1) that air contamination with halothane regularly occurs when the vaporizers are filled; furthermore, leakage of halothane did not always occur in the same proportions as delivered with N,O, indicating independent leakage rates of the two agents. At present, the only generally available proven program for monitoring gases other than N_O is the collection of samples for analysis in an outside laboratory. Air samples for the monitoring program may be obtained from a location close to the inspired air of one or more persons in the operating room (person- nel sampling) or from other sites in the general area occupied by personnel (area sampling). Personnel sampling is usually considered the preferable method®0 but experience with monitoring in the operating room during the pre- sent studies indicates that scrubbed personnel may be disturbed by the risk of accidental contact with the sampling equipment. For this reason it is best if possible to rely on area sampling. This requires the establishment of a rea- sonable correlation between area and personnel samples, a test which is easily accomplished with the portable, battery powered N50 analyzer. If such tests are not feasible it may be reasonable to sample in close proximity to a single person, such as the anesthetist, or a single area in the exhaust stream of the air-conditioning system such as the grille. Area sampling is convenient, inexpensive, and easily accomplished. The methods include (1) measurement of multiple samples or (2) continuous monitoring by means of the infrared analyzers or sampling pumps and gas- tight bags (Fig. VIl.5). Either method can yield representative Vii-11 time-weighted exposure patterns. Battery-powered pumps are the most suitable for personnel monitoring because the movements of the staff in the operating room are not restricted. The cost is high, however, in comparison to aquarium pumps. Aquarium pumps are suitable for area monitoring when equipped with a bleed valve, resistance, and a flowmeter. Their usefulness can be extended by the addition of a sampling probe. This modification can be made simply by sealing the case from room air except for a length of sampling tubing which opens into the case. Some models are so small, lightweight, and quiet that they can be easily carried by personnel. Regardless of the type of pump employed, means must be provided for the temporary storage of samples. The minute capillary tubes filled with activated charcoal are convenient for the storage of halogenated anesthetics; unfortunately, however, they cannot store N,0, and bags must be employed. The size of the bag is determined by the requirements of the analyzer; certain IR units with a cell volume of 5 L require a sample larger than 15 L. A careful technique and a tactful approach are important factors in attaching the larger bags in such a way that movements will not be restricted, thereby minimizing objections of personnel to such paraphernalia. The significance of the time period over which an air sample is obtained should be emphasized. Unless the air is thoroughly mixed, a air sample from a single rapidly filled syringe may be unrepresentative. Uneven data can be smoothed out if a number of samples are averaged. Gas-tight bags filled at a constant rate over a period of hours could produce the most representative averages. Analzyers suitable for N,0 monitoring are available (Table V1.2) as well as instruments capable of measuring trace concentrations of the halogenated anesthetics, ether and cyclopropane. Halothane analysis is less satisfactory than N,0 analysis in the operating room because of interfering substances such as alcohols, freons, and halogenated cleaning solutions. Solid-state ionizing halogen leak detectors adapted for continuous monitoring are under development, and a preliminary evaluation of a prototype model is promising in that it revealed minimal sensitivity to interfering substances. Other problems, particularly zero drift, are being investigated by the manufacturer. The advantages of a monitoring program that makes use of a continuously measuring gas analyzer should be considered. The program in operation at Stanford University Hospital includes area monitoring during anesthesia at one-week intervals and the measurement of early morning samples (before the arrival of the anesthetists) at one-month intervals. The advantages of this method include the development of the desired record of trace gas concentrations and the resultant incidental but ongoing public-relations program to encourage cleaner techniques. In rooms selected for continuous monitoring, the anesthetist can appreciate the responsiveness of the room air concentrations of N,0 to the usual changes in leakage rates resulting from his techniques. An additional use of the continuous monitor is the analysis of bag samples obtained from peripheral anesthetizing locations. VIil-12 Table VII.2 GAS SAMPLING AND ANALYTICAL EQUIPMENT Equipment Capabilities Manufacturer/ Approximate Cost AIR MONITORING Miran I Gas Analyzer (infrared) Miran 101 Specific Vapor Analyzer (infrared) Sensors NpO Monitor (prototype model) (infrared) Halogen Leak Detector With single filter and 1 m sam- pling cell, for N,0. Variable filter models and longer sam- pling cells are available with increasing versatility and sensitivity. With single filter and 5.5 m sampling cell, for N,O, and/or halogenated anesthetics With single filter and 5 in. sampling cell; volume 30 cc for N,0, including 100 cc samples Modified for continuous monitor- ing (under development). Con- tinuous monitoring for halo- genated anesthetics Wilks Scientific Corp., S. Norwalk, Conn. $3050 Wilks Scientific Corp., S. Norwalk, Conn. $2950 Sensors, Inc., Ann Arbor, Mich. $1500 Inficon, Inc., E. Syracuse, N.Y. $1500 SAMPLING IN GAS-TIGHT BAGS Snout-Type Sample Bags Aquarium Pump (Hush 1) Bleed Valve Flowmeter (Dwyer Series VF, 0.06 to 0.5 L/min) Various capacities,2 to 44 L Economical method for filling sampling bags Standard hardware item. Choose one with an O-ring seal Calibrated Instruments, Ardsley, N.Y. $6-314 Inc., Metaframe Aquarium Products, Maywood, N.J. $5 Available in pet shops $1 Dwyer Instruments, Inc., Michigan City, Ind. $18 SYRINGE SAMPLING INCLUDING TRACE GAS ANALYSIS Complete Kit Includes sampling syringe and mailing box. Single analysis, includes all gases Boehringer Labs., Wynnewood, Penn. $35 LEAK DETECTORS Ferret Industrial Leak Detector (ionizing) Halogen Leak Detector (HLD-1) (ionizing) NyO Leak Detector (ionizing) For halogenated anesthetics For halogenated anesthetics For N50 (under development) General Electric Co., Lynne, Mass. $1250 Inficon, Inc., E. Syracuse, N.Y. $1500 Inficon, Inc., E. Syracuse, N.Y. $1500 VIii-13 Despite these advantages, many hospitals, especially those with less than ten operating rooms, may decide to monitor less frequently and to depend on an extramural laboratory for trace gas analysis. If a four- month interval between sampling procedures is the decision, it may be reasonable to monitor at the time of preventive maintenance of the anes- thesia machines, using equipment supplied by the maintenance company. F. Procedures for Air Monitoring 1. Trace Gas Analyzer Available (a) Nitrous oxide leakage in high pressure components of each anesthesia machine is determined over a period of at least | min in each operating room at a sampling site related to the average room air concentration (usually the air-conditioning exhaust grille). This test is begun when the N,0 has been off for at least one hour. Throughout this time, the high pressure hoses are attached. Nitrous oxide in the air should be less than 2 ppm. In the present program,this test is repeated monthly, but longer intervals might be considered, up to four months. (b) Total N,0 leakage is determined over a period of at least 1 min in each operating room during anesthesia. The sampling site is usually the air-conditioning exhaust grille. This test is begun at a different hour to secure randomization. Nitrous oxide concen- tration should be less than 30 ppm. This test is repeated weekly in the present program, but intervals up to four months might be appropriate. 2. Trace Gas Analyzer Not Available The sampling system requires a sampling pump and a gas- tight bag (Fig. VII.5); a flowmeter is desirable. One such assembly is placed in each operating room, usually at the air-conditioning exhaust grille. The bags are allowed to fill over a period of hours, after which they may be sent to the laboratory for analysis. Results of the analysis should be less than 30 ppm N,0. This test is repeated at four-month intervals or more frequently in each operating room. VII-14 VIII. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS It is assumed that the presence of inhalation anesthetics in the operating room, even in trace concentrations, are potentially toxic. No ''safe" concentration can be defined, but the adverse effects appear dose-related. The most reasonable course, therefore, is to limit personnel exposure. Control measures include informing operating room personnel as to the sources of leakage and instituting programs for anesthetic gas scavenging, equipment maintenance, and air monitoring. To an important extent, the effectiveness of these control measures will depend on the cooperation of the anesthetist. A. Reasonable Concentrations of Inhalation Anesthetics Under favorable conditions, concentrations of 5 to 10 ppm N;0 can be obtained, with only transient increases at such times as induction, recovery, and suctioning. Achievable concentrations during the routine use of the inhalation agents are 30 ppm nitrous oxide and 0.5 ppm halothane with nonrecirculating air conditioning. These values represent average con- centrations mixed with room air, and samples are collected at a representa- tive site such as the air-conditioning exhaust grille. Maintaining these concentrations with recirculating air conditioning demands a greater effort because, under these circumstances, the room air dilution rate is lower. B. Techniques of the Anesthetist The most obvious cause of room air contamination is the work practice of the anesthetist. Spillage of liquid agents can be reduced by the use of a funnel when filling the vaporizer. A poorly chosen or improperly applied face mask may also result in higher sustained concentrations; the well-fitted mask is always desirable. The anesthetist should be reassured that the adoption of scavenging and low-leak work practice requires no major modification in his customary procedures. The low-leak anesthesia equipment is simple to use, and the scavenging apparatus imposes no serious burden. £. Waste Gas Scavenging Waste gas scavenging is a significant factor in obtaining clean air in the operating room. To be effective, gas-tight collection devices must be provided for all breathing systems, The simplest disposal system makes use of the air-conditioning exhaust duct although it is used primarily with the nonrecirculating system. It can be used with the recirculating system, however, if the disposal tubing enters the duct downstream from the point of recircula- tion. Other disposal pathways are available, such as the special low velocity duct and the high velocity suction systems. Viii-1 D. Anesthesia Equipment Newly purchased anesthesia equipment should be retested for leakage before being placed in service. Preventive maintenance should be regarded as a continuum with servicing at the time leakage is discovered. In the present program, all equipment subject to leakage receive pre- ventive maintenance at four-month intervals, the work being performed by qualified technicians. This procedure makes use of halogen leak detectors and facilities for the immersion of the pressurized absorber unit as well as the standard pressure, flow, and soap-bubble tests. The anesthesia machines are leak tested at the time of preventive mainte- nance. Additional testing of low pressure components is necessary because of their high susceptibility to leakage. This is performed at monthly intervals and also whenever the soda-lime is changed. Such leakage may be held to less than 20 cc/min, with a suggested maximum of 50 cc/min following preventive maintenance and 100 cc/min at other times. Leakage in the high pressure com- ponents of the machines is best determined as a part of the air-monitoring program as described below. E. Air Monitoring In the present program, samples are obtained either at the air-conditioning exhaust grille or at the door of the operating room, and these samplings yield N20 concentrations representative of personnel exposure. Nitrous oxide is the gas primarily considered because it is the most widely employed inhalation anesthetic. Trace concentrations of N,0 are easily analyzed with existing instrumentation. Most important, N,0 leakage frequently occurs and is often recognized only through monitoring. Leakage of halogenated anesthetics can occur independently, and the future development of appropriate analyzers may lead to the adoption of multigas monitoring. Equipment for the air-monitoring program is capable of surveying a large operating suite in a few minutes. Such surveys are suggested as a means of identifying leakage in the high pressure components of the anesthesia machines. This test is initiated in the operating rooms after the N,0 flowmeters have been turned off for at least one hour and the high pressure gas supply hoses are attached. The results should show less than 2 ppm N20 in the room. Although this test is repeated at least monthly in the present program, longer intervals, up to four months, might be considered. Additional room surveys completed during operating hours and each beginning at a different hour achieves randomization and ensures a reliable representation of the prevailing N,0 concentrations. Many rooms show less than 5 ppm N,0. A suggested maximum average is 30 ppm N,0. In the present program these surveys are completed at weekly intervals, but some institutions might consider longer intervals, even up to four months. A smaller operating suite, perhaps less than ten rooms, may be unable to justify a trace gas analyzer. Such a suite could rely on air sampling and analysis at the time of preventive maintenance every four months. Vili-2 Appendix A FUNDAMENTALS OF PHYSIOLOGY, ANESTHETIC TECHNIQUES AND AGENTS, BREATHING SYSTEMS, AND TEST LUNG This appendix provides background information on physiology and anesthetic techniques and agents that may be of value to the hospital engineer, administrator, or architect who is considering a scavenging program. 1. Physiological Considerations The adult patient breathing spontaneously under light surgical anesthesia inspires approximately 500 cc of gas mixture each breath at a rate of 10 to 15 breaths per minute. Inspiration requires an active muscular effort, and expiration occurs passively as a result of elastic recoil of the lung-thorax system. Oxygen is absorbed at approximately 240 cc/min, and an approximately equal amount of CO, is excreted. The peak flow rate in the trachea ranges from 10 to 25 L/min during quiet breathing but may reach 400 L/min under forced breathing. With the initiation of anesthesia, anesthetic agents present in the inspired gas mixture are absorbed rapidly into the body until equilibrium is approached (for a relatively insoluble agent such as N;0, this occurs within a few minutes). The more soluble agents such as halothane do not reach equilibrium because absorption from the breathing system persists for many hours. During anesthesia, the patient may breathe ''spontaneously' (on his own) without assistance from the anesthetist, or breathing may be "assisted" by the anesthetist who squeezes the breathing bag near the end of exhalation to augment the amount of inspired air. Breathing can also be ''controlled'" wherein the patient makes no respiratory effort and the anesthetist ''takes over'' the process of inspiration. This form of breathing is frequently used during abdominal and chest surgery where muscle relaxant drugs curtail the patient's ability to breathe. Controlled ventilation is provided by intermittent manual compression of the breathing bag or automatically by means of the ventilator. 2. Anesthetic Techniques and Agents Surgical procedures are performed under local, regional, or general anesthesia. Local anesthesia is achieved by the injection of an anesthetic agent directly into the affected tissues. Regional anesthesia results from the injection of a local anesthetic into the tissues adjacent to the nerve supply of a specific region, thus anesthetizing the area of surgery. For major surgery, patients and their physicians frequently favor the unconscious state, and general anesthesia is requested. Sleep is commonly induced through the intravenous injection of barbiturates such as thiopental (Pentothal).+ Anesthesia is then secured with the more potent inhalation anesthetics (N,0 and/or halothane) or nonvolatile drugs (meperidine or a point of confusion is that all drugs are known by at least two terms, the nonproprietary or official name and the manufacturer's trade name. A-1 morphine). The nonvolatile agents do not require scavenging, but their use is apt to present the disadvantage of a prolonged period of sleep in the postanesthetic period. As a result, there may be an increased risk of complications such as atelectasis and pneumonia.in the postoperative period. The most frequently employed anesthetic gas is nitrous oxide (N20). Being inadequately potent if administered as the sole agent, N20 is supplemented with nonvolatile drugs or with such vapors as halothane or enflurane. Because such inhalation anesthetics are eliminated from the body primarily via the lungs, with a minimum of metabolic degradation, prompt recovery is more easily secured. It is concluded that the preferences of the patient and pharmacological principles suggest the continuing widespread administration of the inhalation anesthetics, with the concomitant necessity for scavenging. In a recent analysis, 2-6 January, 1975, inhalation anesthesia was administered in 120 of the 192 surgical procedures performed in the operating room. Of these inhalation anesthetics, 99 percent included N,0; 25 included halothane and 17 included enflurane, for a 35 percent incidence of anesthesia with halogenated agents. The ''balanced technique," employing N20 with a narcotic such as meperidine, was recorded in 63 percent of the inhalation anesthetics. The flammable agents such as ether and cyclopropane are no longer used in all operating rooms, but some anesthetists remain convinced of their advantages in selected cases. Because the occasional use of such agents will probably continue for several years, the technology for scavenging must be available. The limits of flammability of ether and cyclopropane, in relation to the inhaled volumes and concentrations, are of interest in estimating the dilution necessary in achieving nonflammability. Both agents are non- flammable at concentrations below approximately 2 percent. Ether anesthesia is maintained at flow rates of 50 to 250 cc/min in concentrations of 3 to 5 percent; cyclopropane anesthesia is maintained at flow rates of 50 to 500 cc/min at concentrations of 12 to 25 percent, with both agents briefly administered during induction up to twice the concentrations and flow rates mentioned. With the maximum concentrations employed at induction, ether must be diluted 5:1, cyclopropane 25:1, so as to enter the nonflammable range. The anesthetic breathing systems (to be described) must be coupled to the patient either by a face mask or an endotracheal tube. a. Face Mask Although most general anesthetics are initiated in adult patients by means of sodium thiopental (Pentothal),the continuing use of the inhala- tion anesthetics requires the face mask, at least for the induction of anesthesia in preparation for the endotracheal tube. Many anesthetics are satisfactorily initiated and completed with the face mask alone. The fit of the face mask is critical. A relatively gas-tight fit must be secured to effectively deliver the anesthetic to the patient without leakage,but excessive pressure against the face must be avoided to prevent injury. In rare cases, a satisfactory fit is impossible because the shape of the face is incompatible with the available masks. [If this occurs, an endotracheal tube is substituted. In the event of a small leak, the anesthetic can be satisfactorily completed although the exposure of personnel to the anesthetics may be relatively heavy. With the increasing concern for the exposure of personnel to trace concentrations of anesthetic agents, however, the anesthetist should increase his efforts to eliminate even the smallest leaks. The face mask is unsuitable for certain types of anesthesia. For example, the mask may be contraindicated during facial operations because of interference with the surgical field or during assisted and controlled breathing where the presence of high positive pressure in the patient's mouth could inflate the stomach. Under these circumstances, the anesthetic begins with intravenous barbiturates, and the mask is then applied for oxygenation in preparation for intubation of the trachea. Following intu- bation, anesthesia is continued via the endotracheal tube. b. Endotracheal Tube The endotracheal tube consists of a 10 in. length of flexible tubing usually fitted at the distal end with an inflatable rubber balloon (referred to as a ''cuff''). The purpose of the cuff is to provide a gas- tight seal in the trachea. It is good anesthetic practice to intubate the trachea only when specifically indicated because the use of the tube is occasionally accom- panied with side effects. For example, insertion often requires the use of the laryngoscope, an instrument capable of damaging the teeth or the soft tissues in the throat. The tube itself may cause inflammation of the trachea. 3. Anesthesia Machine The anesthesia machine (Fig. A.1) is designed to deliver accurately measured concentrations of anesthetic gases, N,0 and oxygen, and vapors such as halothane or enflurane into any breathing system. The normal source of the gases is the central pipeline system or gas cylinders. These gases are administered in controlled concentrations by means of rotameters cali- brated in cubic centimeters or liters per minute, and the flow rate is controlled by valves. The anesthesia machine is usually equipped with a vaporizer designed to convert a volatile liquid anesthetic agent into a vapor and to administer a controllable volume or concentration. This vapor is mixed with the other gases and then delivered to the breathing system. The vaporizer system (Fig. A.1) is an out-of-breathing-circuit ''copper kettle' design. A sintered bronze diffuser ensures the perfusion of the liquid anesthetic with minute bubbles of carrier oxygen, and the effluent achieves saturation. The flow of carrier oxygen thus accurately controls the volume of saturated vapor and, by knowing the flow rate of the diluent gases, the concentration of the delivered gases and vapor can be determined. Nitrous oxide and oxygen are normally administered together at 0.5 to 5.0 L/min each, with halothane at 0.5 to 75 ml/min. The anesthesia (or gas) machine, strictly defined, includes no breathing system. The carbon dioxide (C0,) absorption system, however, is regularly attached to the basic machine so that the combined units are loosely considered as the .''anesthesia'' or ''gas'' machine. |t is this device (Fig. A.1) that delivers the vast majority of anesthetics. ei. POPOFF VALVE WITH SCAVENGING ADAPTER EXHALATION VALVENS™ = MASK - INHALATION VALVE J {PATIENT} LIQUID LPM AGENT I SINTERED ~~ N,0 BRONZE ( GAS SOURCES ; - 0,— i VAPORIZER SYSTEM | \ irvennnnu menerasa stn sserse ss ; J BAG “ A) ANESTHESIA MACHINE i \J RIVING GAS C) VENTILATOR B) CO, ABSORPTION SYSTEM Fig. A.l. ANESTHESIA MACHINE WITH co, ABSORBER AND VENTILATOR. L. The Anesthetic Breathing Systems a. CO, Absorption System The CO, absorption unit (Fig. A.1) is an anesthetic breathing system that includes a canister designed to contain soda-lime for the absorption of CO,. The soda-lime is a mixture of the hydroxides of calcium and sodium in granular form (mesh size 4 to 8) in a volume of approximately 2 L. This breathing system also includes a reservoir (or breathing) bag made of thin flexible conductive rubber or plastic sheeting. Its volume is 2 to 5 L for adult patients. The purpose of this bag is to provide compliance in the breathing system for absorbing the rapidly inspired and expired volumes produced by the patient. The bag may also be compressed intermittently to inflate the lungs when spontaneous breathing is inadequate. Other essentials in this breathing system include the check valves that ensure unidirectional gas flow and prevent any rebreathing of gases not cleared of CO, . A pair of flexible corrugated breathing tubes, terminating in a Y-piece at the face mask or endotracheal tube, completes the system. The popoff valve (Fig. A.1) is of prime importance in waste gas scavenging because it is the chief source of gas leakage. With the use of the CO, absorption system, the anesthetic mixture is usually administered in volumes greater than the patient's metabolic requirements. This excess is vented out of the breathing system (to prevent overfilling) via the popoff valve. The newest popoff valves are equipped with a means for capturing these waste gases to prevent room air contamination. The flow rate of gases issuing from the valve depends on the type of breathing. During spontaneous breathing, gases leave the valve during each exhalation; the breathing bag first fills, and then any excess of gases passes through the popoff valve. The flow rate, therefore, is the sum of the fresh gas flow rate plus that of the exhaled gases. The velocity of the escaping gas is in the range of 0.5 to 25 L/min, and the average volume approximately equals the fresh gas flow rate because oxygen and CO, enter or leave the patient in equivalent volumes. With spontaneous breathing, the popoff valve is usually operated fully open so as to avoid interference with exhalation. In contrast, during manually assisted or controlled breathing, the patient's lungs are forcibly expanded, and expiratory flow rates are more rapid (10 to 50 L/min) than normal. These increased flow rates must be handled by the scavenging system without leakage into the operating room and without excessive back pressure. With this type of breathing, the popoff valve during inhalation must be partially closed; otherwise, the gases would be fcrced through the valve without providing the desired ventilation. The adjustment of the popoff valve during assisted and manually controlled breathing is critical. If the valve is closed too tightly, the bag overfills and exhalation is impeded; if the valve is opened too wide, the patient's lungs cannot be expanded sufficiently. In practice, the system is rarely in balance, and this can be compensated for only through frequent adjustments. This balancing problem is present in the vast majority of scavenging popoff valves in use today; however, scavenging popoff valves are available that open automatically to vent any excess gases at the end of each exhalation (Berner valve, Table V.1). A-5 When the ventilator is employed during controlled breathing, the popoff valve is completely closed, and excess gases are automatically vented by the ventilator. The use of the CO, absorption system can be considered in terms of the fresh gas flow rate entering the breathing system from the gas machine. This system can be used in the ''closed' mode in which the gases are supplied to the patient in the amount actually absorbed, or in the "'semiclosed'' mode with an excess of gases provided. The engineer might question the need for an excess of gases. The reason is that the anesthetist must know the approximate composition of the gas mixture in the breathing system. Unfortunately, this is difficult to predict because, for any given patient, the rate of uptake of 0, and the anesthetic agents is not precisely known. As a result, the closed system requires meticulous attention to the patient's vital signs; otherwise, the patient becomes too lightly anes- thetized or excessively depressed. The closed mode is so difficult to master that it is not taught in many residency training programs, and its universal application is unadvisable. The use of the semiclosed mode is far more frequent, where an excess of gases is provided in the range of 1 to 7 L/min. At this flow rate, the absorption of gases from the breathing system is inconsequential in terms of the total flow provided, and the composition of the gas mixture in the absorber is then accurately controllable through the adjustment of the flowmeters. Again, the necessity for scavenging is emphasized. b. Ventilator The CO, absorber is frequently used with the anesthetic ventilator. This device is a pneumatically operated mechanical pump inserted into the breathing system in place of the rebreathing bag (Fig. A.1). The lungs are automatically ventilated, thereby freeing the anesthetist's hands for other important tasks. The ventilator is usually driven by oxygen or air pressure. This driving gas is not mixed with the breathing gas mixture. Because the popoff valve is closed when the ventilator is used, a waste gas collector must be attached to the ventilator and a disposal system should be provided. Because many anesthetists switch back and forth between hand breathing and automatic ventilation, a means must be provided for simultaneously scavenging from both the breathing system and ventilator. The Y-piece method (Fig. V.2) is such a means without any need to change connections. Ce Nonrebreathing and T-Tube Systems The circle absorption system is considered by many anesthetists to be unsuitable for pediatric anesthesia because of resistance to breathing imposed by the soda-lime and the long lengths of low pressure tubing. The nonrebreathing or T-tube technique may then be employed. These systems receive fresh gases from the anesthesia machine (Fig. A.1). The nonrebreathing system (Fig. D.3) includes two one-way valves to prevent rebreathing. All fresh gases are provided by the gas machine and A-6 enter the patient's lungs from the reservoir bag, and all exhaled gases leave the system through the exhalation valve. This system must be sup- plied with a slight excess of gases beyond the total amount respired each minute, generally 5 to 15 L/min, so that the bag remains properly filled. The scavenging collector applied to this system is closed to room air and therefore cannot leak. In pediatric anesthesia, many anesthetists avoid the use of valves and prefer the T-tube instead (Fig. D.4). In this system, fresh gases enter the patient's lungs primarily from the fresh gas source and only a small amount from the reservoir bag, thus preventing excessive rebreathing. All exhaled gases and any excess of fresh gases leave the system through the tail of the bag. Relatively high flow rates (5 to 15 L/min) are required to avoid significant rebreathing of the gas mixture. This gas collection system is also inherently gas-tight. d. Other Breathing Systems Other breathing systems encounter unsolved scavenging problems. The open drop technique (rarely used at present) employs a wire mesh face mask. A gauze material several layers deep covers the mesh, serving to vaporize potent liquid agents. The insufflation system (also rarely used) delivers anesthetic gases directly into the patient's mouth without the use of a reservoir bag. Dentists often use a system in which the gas machine delivers a constant high flow of gases (5 to 10 L/min) to the nasal mask. This breathing system consists of a length of 7/8 in. tubing, including a reservoir bag that terminates at the mask. Inhaled gases must pass through the nose. Exhalation occurs through a one-way valve located in or near the mask and also through the mouth, and the remainder leaks around the nasal mask. The patient may be completely anesthetized and unconscious but, more frequently, a semiawake state of analgesia is secured, with some ability to communicate. The protective cough reflex is retained. If the patient is unconscious, the ''throat pack' consisting of a wad of wet gauze is inserted in the back of the throat, and the airway is thus protected from the aspiration of foreign material. The positioning of the pack is critical because breathing must continue behind the pack; however, it must fit close enough to protect the airway. The throat pack also serves to diminish the dilution of the gas mixture with room air. 5. The Test Lung The anesthesia machine and ventilator were tested for leaks under conditions simulating spontaneous and controlled breathing. A compliant reservoir, similar to a breathing bag, was attached to the breathing hoses at the Y-piece to simulate the patient's lungs. This test lung ts com- mercially available (Ohio Medical Products; Bennett). Appendix B DIFFUSION LEAKAGE Anesthetic agents such as N;0 and halothane diffuse through rubber and plastic tubing, bags, and other anesthesia equipment, and contribute to the trace gas concentrations in the operating room air. The present studies were conducted to quantify such diffusion leakage. The method was to place the sample to be tested inside an air-tight box (Fig. B.1) and to pass an anesthetic gas mixture through the sample. Because the sample was sealed from the interior of the box, any anesthetic gas detected within must have entered only by diffusion. A carrier gas perfusing the box is equilibrated with the diffused gases, thereby producing the samples for determining the diffusion rate. These samples were analyzed by gas chromatography (Appendix E). The box with a capacity of 92 L was made of acrylic and lined with 10 mil mylar, a material resistant to the diffusion of anesthetic gases. A fan ensured thorough mixing of the contents. Temperature changes were mini- mized by mounting the fan motor outside the box. Any possibility of gas leakage was excluded by pressurization and observation for pressure loss, and gross leakage from the samples was excluded by pressurization, immer- sion, and observation for bubbles. After the equipment to be studied (Table B.1) had been installed in the box, anesthetic gas flows were provided by a standard anesthesia machine, at 2.5 L/min of N,0 and oxygen with 1 percent halothane. This gas mixture was vented through the air-conditioning exhaust to the atmosphere. The carrier gas perfusing the box consisted of compressed air introduced at a flow rate of 1 L/min. FAN [pH re Cm =) CARRIER AIR AIR + DIFFUSED GASES —_— —— N20/0,/halo A) WASTE » TUBING UNDER TEST Fig. B.1. BOX TEST FOR DIFFUSION STUDIES (N,0/0,/halothane). B-1 z-9 Table B.1 SAMPLES AND EQUIPMENT Sample Description Distributor Eva-Flex Disposable Tubing Length of sample: 3.66 m Total surface area which permits diffusion: 0.347 m Anesthesia Tubing (Bain Breathing Circuit) Length of sample: 2.18 m Total surface area which permits diffusion: 0.274 m2 Dameca Tubing--""Anti-statik" Length of sample: 1.0 m Total surface area which permits diffusion: 0.154 m2 Medi-Pak Disposable Tubing Therapy Product Length of sample: 1.18 m Total surface area which permits diffusion: 0.113 m2 Med-Econ clear vinyl flex tubing with wire reinforcement inside tube Length of sample: 1.15 m Total surface area which permits diffusion: 0.083 m2 Dupaco Conductive Neoprene Breathing Bag: 3L Air Products and Chemicals, Inc. Respiratory Care, Inc. Dameca A/S, Copenhagen, Denmark Aerosol, Inc. Medicon Plastic Co. Dupaco Corp. The resulting carrier gas concentrations of anesthetic gases were normalized to the greater volume of the operating room and the nonrecirculating flow rate of the air-conditioning system using transfer coefficients. It should be noted that the concentration levels referred to in this appendix are in terms of mass ratios rather than volume ratios. This is consistent with diffusion analysis,and the conversion is direct. Knowing that the diffusion constant remains the same in the box or in a room, where u; = diffusing constant at specified conditions for gas j (gr/hr m2) J = H for halothane, j = N for N,0 py = density of the gas outside the tube, assumed equal to density of carrier gas (gr/L) Q(room) = flow rate of carrier gas (air conditioning) through the room (L/hr) Q(box) = flow rate of carrier gas through the box (L/hr) M, = mass concentration of gas j in environment k (gr /gr i) j = H for halothane, j = N for N,0 k = 1 for inside tube k = 2 for outside tube A, (box) = surface area of tube in box test (mn?) A, (room) = surface area of tube in box test ti) In practice, H, 1M. 2>>1 and the term 1 can be dropped from the parenthesis. ’ ’ If the gas mixture inside the tube is the same both in the box test and during surgery, then B-3 A, (room) Q (box) M. 2 = M. 5 (box) J A, (box) Q (room) I» For any tube, Ai is proportional to length, Lys for any room volume, Q(room) is proportional to the number of room changes per hour (N). Consequently, by knowing Q(box), L, (box), and M; , (box), the mass concentration in the room can be predicted by M. room) = C, —— 3.2% JN where J is a transfer coefficient. The values presented in Table B.2 and Fig. B.2 are for a 4000 ft 2.5 L/min N,O, 2.5 L/min 0,, and 1 percent halothane and are expressed room and for in-tube gas mixtures of in parts per million. The diffusion coefficients for N,0 and halothane are listed in Table B.2 for the seven standard hoses and the breathing bag described in Table B.1. To determine room concentrations for these data, the transfer coefficients in Table B.2 are plotted as a function of L/N in Fig. B.2a for N,0 and in Fig. B.2b for halothane. These values can be applied easily to a 4000 fe room. For example, in a 4000 fed room with 10 air changes/hr and 10 m of hose, L,/N = 1; for hose A, the mass concentration in the room of N,0 is 1.6 x 107“ ppm and is 4.62 x 107° L/N = 1, the values of y in Table B.2 equal the mass concentrations in ppm for halothane. In fact, for parts per million in the room. These data can be applied to rooms of arbitrary volume V, via 4000 L, M. = C, —— — j,2 Joy N To illustrate, if V = 3000 ft3 L, = 7m N = 15 changes/hr tube = D B-4 Table B.2 DIFFUSION AND TRANSFER COEFFICIENTS FOR ANESTHETIC TUBING AND BREATHING BAG (gas in tube: N,0 = 2.5 L/min; 0, = 2.5 L/min; halothane, 1 percent) Transfer Coefficients Diffusion Coefficients (gm/hr m2) gm (anesthetic gas) X room changes/hr Sample 6 10° gm (carrier gas) For N,O For Halothane Xm of tubing (wy (Ug) For 50 For Halothane (Cy (Cy -2 3 - ~3 A 5.8 X 10 6.5 X 10 1.6 X 10 2 4.62 X 10 B 10.5 X 102 8.4 X 107° 3.9 X 102 7.79 X 107° Cc 2.8% 102] 20.6 x 107° 1.6 Xx 1072 1.55 x 10~2 -2 -3 -2 -4 D 24.5 X 10 54.0 X 10 8.6 X 10 1.77 X 10 -2 -— — - E 11.9 X 10 4.8 X 10 3 3.1 X 10 2 2.51 X 10 3 _at _a¥ _o%k WE F 5.4 X 10 4 2.97 X 10 3 2.43 X 10 2 2.97 X 10 4 t For Cy and Cy: room = 4000 ft3. + For breathing bag, units are gm/hr bag. * For breathing bag, set tube length L, = 1.0. B-5 2 S 30} < i D Z 20} w OQ : 0 Q ' 10 = E 2 Cus AC— => = 1 1 1 | 0 3 (a) 2 o k 0007} x E0006 4.0005 8 S.0004 YW 0003 <{ ££ .0002 S =.000! IT £ 0 = L{/N METERS/ROOM CHANGES/HOUR My = C L3/N L+= LENGTH OF TUBE IN METERS N= ROOM CHANGES/HOUR OF AIR COND. My, = ROOM CONC. OF HALOTHENE PPM (b) Fig. B.2. EXTRAPOLATION OF RESULTS OF DIFFUSION STUDIES TO 4000 FT® ROOM. B-6 then _ -2 4000 7 _ My, 2 = 8.6 x 10 3000 15 = 0.00535 ppm N,0 _ -4 4000 r My 2 = 1.77 x 10 3000 ic = 0.00011 ppm halothane It is apparent that, for N,0 and halothane, diffusion leakage is extremely low (in the parts per billion range). These values are negligible con- sidering the 0.2 to 1 ppm range of N,0 and the 0.01 to 0.025 ppm range of halothane observed when the gas machine and ventilator are in the operating room. Appendix C DISTRIBUTION OF WASTE ANESTHETIC GASES IN THE OPERATING ROOM AIR 1. Waste Gas Distribution Studies Effective control measures intended to minimize the exposure of personnel to trace concentrations of waste anesthetic gases depend on an understanding of the distribution of such waste gases in the operating room air. The present studies were designed to determine the waste gas distribution with reference to (a) the role of the air-conditioning system, especially the flow rate and use of recirculation, (b) the determination of an air sampling site to represent the average exposure of personnel for the air monitoring program,and (c) the effectiveness of venting the waste gases to the floor as a control measure. The study was conducted in an obstetric delivery room and in an operating room. Each room contained the normal furniture plus measuring equipment. Patient care was not in progress. Anesthetic gases were introduced into the room by means of a standard gas machine, with the outlet tube located 92 cm or 2 cm above the floor. Gas sampling stations were spaced 106 cm apart to cover the floor area of the rooms and at three planes located approximately 2, 140, and 220 cm above the floor. This resulted in a total of 36 sampling sites in the delivery room and 48 sites in the operating room. The gas samples were obtained at each station over a period of time sufficient for stabilization of the gas analyzer (v30 sec). The instrumentation included infrared gas analyzers with a reading accuracy of 10 percent. The time constant of the N20 analyzer (calibrated from 5 to 850 ppm) is 1.4 sec and that of the halothane analyzer (calibrated from 0.3 to 20 ppm) is 22 sec. Halogen leak detectors, with time constants of approximately 1 sec, were used to detect the relative changes in halothane concentrations. All data were continuously recorded on a multichannel chart recorder. The air flow into the room was measured by laminar flowmeters (Balcone) with an accuracy of +20 percent. A hot wire anemometer measured the flow velocity across the outlet of the laminar flow cone and, although the average of these readings was consistent with the Balcone reading, the large variations in flow velocity made numerical integration of point values inaccurate. The gas machine flowmeters were compared to an accu- tately calibrated flowmeter and were found to be within a reading accuracy of +5 percent. Any comparison of expected and measured concentrations therefore involves analyzer errors, air-conditioning flow rate errors, and anesthetic gas flow errors, resulting in an overall accuracy of approximately *25 percent. In the delivery room (Fig. C.la), a high velocity ceiling and wall jet produced considerable entrainment and a major eddy. Nitrous oxide and halothane concentrations were measured at the normal air-conditioning a. delivery room \/] Hy FN b. Operating room Fig. C.1. AIR DISTRIBUTION PATTERNS. c-2 flow rate of 1000 ft®/min (21.3 room changes/hr) and at a reduced flow rate of 450 ft/min (10.4 room changes/hr). In addition, the anesthetic gas mixture was released at two different levels, 92 and 2 cm above the floor. In the operating room (Fig. C.1.b), an X-ray positioning beam obstructed normal flow and produced a jet down the center of the room 50 to 80 cm below the ceiling, with no observable major eddies. Nitrous oxide and halothane concentrations were measured at the normal flow of 1200 ft3/min (19.5 room changes/hr). The parameters of these four tests are tabulated in Table C.1. ~ An earlier set of experiments also measured concentrations of N,0 and halothane with air-conditioning flows of 240 ft*/min (5.1 room changes/ hr) in the delivery room and 625 ft®/min (10.2 room changes/hr) in the operating room. At each flow rate, gas was released at 92 cm above the floor and at floor level. In these experiments, some of the N,0 analyses were quantitatively uncertain, but the halothane results and qualitative data were valid. Inspection of a chart recording (Fig. C.2) identifies a certain sampling site as a ""hot spot' area where the concentration of N,0 varies rapidly and reaches 10 to 15 times the room average (data from the delivery room, at an air-conditioning rate of 5.5 room changes/hr). The halothane concentrations observed in these areas were similar to the N,0 variations. Because such hot spots depend strongly on the air-conditioning flow pattern, positions of the anesthetic gas source and furniture, and the movement of personnel, their location and magnitude are not predictable. In the delivery room (Fig. C.la), they were located near the wall containing the air-conditioning inlet in the far right-hand corner of the room at heights of 140 and 220 cm in tests 1 and 3 and at a height of 2 cm in test 2. In the operating room in test 4, hot spots were located throughout 1,000 500 rrp rrr Lelia ly | 1 0 05 1.0 TIME (min) N,O CONCENTRATION (ppm) Fig. C.2. CONTINUOUS CHART RECORDING OF N,0 CONCENTRA- TION, SHOWING HOT SPOT. C-3 1-9 Table C.1 AIR CONDITIONING AND ANESTHETIC GAS PARAMETERS . A heti Air-Conditioning Flow | Height of nesihistle tio of Expected Average 3 Gas Flow Ratio o Concentrations Test Room Anesthetic ) Flowrates No. °° ft/min Boon Gas Source (L/min) € (ppm) / Changes /hr N,0/Halothane (cm) NoO | Halothane N,0 | Halothane 1 Delivery 1000 21.3 92 3.0 0.050 60 106 1.77 2 Delivery 1000 21.3 2 3.0 0.050 60 106 1.77 3 Delivery 450 10.4 92 3.0 0.050 60 238 3.93 4 Operating 1200 19.5 92 3.0 0.050 60 88 1.47 the far half of the room between the gas machine and the wall carrying the air-conditioning inlet, but only at a height of 140 cm. In an earlier series of tests (results not shown), they shifted to the front half of the room near the floor when the air-conditioning flow had been reduced. Observation of these hot spots, well away from the anesthetic gas machine, indicates their importance in the exposure of all operating room personnel. The distribution of anesthetic gases throughout the room is a function of air current mixing, diffusion, and buoyancy. Molecular diffusion processes have been considered and were found to be far too slow to con- tribute to the observed distributions. Because N,0 and halothane are heavier than air (molecular weight 44 and 119, respectively), buoyancy effects can be observed by the distribution of both gases or by their relative distribution. The data in Table C.2 reveal no significantly higher concentrations going down from 220 to 2 cm. Tests 3 and 4 do not have significant floor-to-ceiling eddies; thus, ceiling levels are somewhat reduced by the mixing patterns, rather than buoyancy, because levels are generally higher at 140 cm than at 2 cm. For example, in the operating room where the air currents are predominant at midlevel (140 cm), concen- trations of N,0 and halothane are higher at 140 cm than at either 2 or 220 cm. The ratio of N,0 to halothane does not decrease with decreased sampling height and, therefore, buoyancy effects are not sufficient to cause N20-halothane separation. These results verify that air currents are the primary factor in the distribution of anesthetic gases in an operating room. Nitrous oxide and halothane are found throughout the room in the same ratio as they are introduced. One method considered for reducing the exposure of personnel is to exhaust the anesthetic gases at floor level. Comparing tests 1 and 2 in Table C.2, no reduction in concentrations at the 140 cm level was observed. The earlier series of tests collected five sets of comparative data on anesthetic gas sources at 92 cm and at floor level. The halothane data were inconsistent but, on the average, the concentrations decreased by 13 and 25 percent at 220 and 140 cm and increased by 7 percent at 2 cm. This series covered a wide range of air flow patterns and velocities. The results indicated that venting anesthetic gases at the floor produces slight, if any, reduction in exposure of personnel. The distribution of anesthetic gases is generally uniform if there are few hot spots. This can be observed in Tables C.2 and C.3, particularly in the latter where the maximum standard deviation is 27 percent of the mean. The average concentration without hot spots is normally within 10 percent of those predicted for anesthetic gas and air-conditioning flow rates; however, the data from test 3 are within only 50 percent of the predicted concentration. This phenomenon was observed repeatedly. As the flow rate decreases, mixing decreases and a high concentration zone exists between the anesthetic gas source and the air-conditioning exhaust grille. This phenomenon was observed by random sampling but is not well reflected in the sampling site data. With surgery in progress, however, the presence of flow obstructions and the movements of personnel would increase mixing, and a more uniform and higher concentration would result. In addition, a decrease in mixing was associated with increased maximum concentrations at the 140 cm level (Table C.3). C-5 9-2 Table C.2 OBSERVED AVERAGE CONCENTRATIONS OF ANESTHETIC GASES IN OPERATING AND DELIVERY ROOMS (Standard Deviations and Concentration Ratios at Three Height Levels are Included) Height (220 cm) Height (140 cm) Height (2 cm) Test No. N,0 Halothane N,0/ N,0 Halothane N,0/ N,0 Halothane N,0/ (ppm) (ppm) Halothane (ppm) (ppm) Halothane (ppm) (ppm) Halothane 1 113 + 49 1.80 10.72 63 + 3 119 + 22 1.88 0.41 64 + 2 116 +25 | 1.76 £0.35 66 + 2 (100 +21) | (1.60 £0.32) (115 +19) | (1.80 £0.32) 2 100 + 13 1.76 £0.27 57 + 4 106 + 18 1.94 +£0.35 58 + 2 110+14 | 2.00 £0.25 55 + 1 (97 + 6) (1.6910.07) (103 +14) | (1.87 £0.28) 3 104 + 46 1.54 10.40 62 + 4 114 + 66 2.11 +£0.99 53 + 4 130 +24 | 2.06 £0.41 64 t+ 4 (92 +20) (1.46 £0.30) (8915) | (1.72 +£0.25) 4 71% 11 1.08+0.14 65 + 4 143 +107 | 2.53 £1.81 56 + 3 97 £20 (1.66+0.34| 59 % 3. (74 +18) | (1.37 £0.32) t pata in parenthesis do not include hot spots. L-) Table C.3 OBSERVED AVERAGE CONCENTRATIONS OF ANESTHETIC GASES (Standard Deviations and Concentration Ratios Included) i Maximum Con- Average Average Concentrations at Exhaust centrations at Test Concentration Concentration Ny0/ Gem) Height of 140 cm No. N,0 Halothane Halothane (ppm) (ppm) (ppm) N50 Halothane N,0/Halothane N,0 Halothane 1 116 + 33 (110 * 22) 1.81 *£ 0.51 (1.72 * 0.34) 64 + 3 134 £ 9 2.27 £ 0.11 60 * 3 400 4.0 2 105 * 15 (103 * 12) 1.89 £ 0.31 (1.85 * 0.25) 56 * 3 123 + 4 2.22 * 0.05 55 + 1 - 250 2.9 3 116 * 49 (104 * 27) 1.90 * 0.70 (1.75 * 0.41) 59 + 6 159 2.96 54 wt 7.2 4 103 + 69 (81 * 20) 1.76 * 1.2 (1.37 * 0.37) 60 £5 90 * 4 1.47 * 0.06 61 £5 of ot T pata in parenthesis do not include "hot spots." Indicates concentrations beyond instrument ranges. To reduce energy costs, many new hospitals are considering recirculating air-conditioning systems. A typical recirculating system consists of five fresh air changes per hour and 20 filtered recirculated changes per hour. Although the HEPA filters remove bacteria, they do not remove anesthetic gases, and these gases are returned into the room. At air-conditioning flow rates on the order of 20 room changes/hr, the anesthetic gas distri- bution was found to be uniform and consistent with measured flow rates. Assuming uniform distribution and equating anesthetic gas inflow and outflow, the concentration level (C) in parts per million is 60 x L x 108 c= —— (c.1) N(1-r)V where L = rate at which anesthetic gases are introduced (not including recirculated) in L/min N = number of room air changes per hour r = fraction of air changes recirculated V = volume of room in liters Using this equation, the effects of various air-conditioning systems can be compared. For example, given a constant leakrate (L) and 25 air changes/hr and if 20 changes are recirculating, the concentration of anesthetic gases is five times that of the concentration when changes are nonrecirculating. Even if the nonrecirculating changes are decreased to 15/hr, the concen- tration levels are only 0.33 those of the above recirculating system. A review of air flow patterns and anesthetic gas distribution indicates that high flow rates and a major eddy pattern (tests 1 and 2, Table C.] and Fig. C.la) produce the best mixing and have reduced hot spot effects when compared to low flow rates (test 3, Table C.1) or to a central jet with no major eddies (test 4, Table C.1 and Fig. C.1b). It should be remembered that the position and magnitude of hot spots are a function of other variables as well, and the effects of such areas on personnel exposure levels have not been determined. An extensive study of anesthetic gas concentrations is facilitated when a single sampling site adequately represents the average exposure level. These studies indicate that measurements at the exhaust grille are highly representative at high air-conditioning flow rates (approximately 15 changes/ hr); however, at low air-conditioning flow rates (less than 10 changes/hr), the concentration varies across the grille and multiple measurements must be obtained. c-8 2. Relationship of the Waste Gas Distribution Studies to the Explosion Hazard in the Operating Room Explosion hazards are implied in the distribution of anesthetic gases in the operating room. The current National Fire Protection Association (NFPA) Code 56A has defined a hazardous zone,’ thus suggesting distribution studies. Early editions of this code identify the hazardous zone as the entire room in which combustible anesthetics are used or stored and also include an area outside the room. This zone is reduced to 7 ft above the floor and is confined to the room if adequate ventilation is present.’ This edition also requires the use of expensive explosion-proof lights; however, to date, no explosions have ever been attributed to lighting fixtures. By 1949, NFPA reduced the hazardous zone to a height of 5 ft above the floor. Their Appendix justifies this reduction on the following basis. "Available data and recent investigations (1948 to 1949) indicate that under customary operating procedures explosive anesthesia mixtures are diluted by air in the operating room to a nonflammable range before reaching a point two feet distant from a point of leak involving the quantities of anesthetics used in anesthesia procedures. However, the liquid character of ether permits its spillage and ether fumes may collect close to the floor as well as being released from the anesthesia system. 20 In the Appendix of the current 1973 NFPA Code, the justification is based on the following statement. "Available data and recent investigations indicate that under customary operating procedures, flammable anesthetic mixtures are diluted by air in the anesthetizing area to a nonflammable range before reaching a vertical height of one foot from any source of leakage or spillage involving quantities of anesthetics used in anesthesia procedures." Neither Appendix cites specific references. A review of the literature revealed limited research on the distribu- tion of anesthetic gases in operating rooms. Coste and Chaplin 3 reported that, after one hour into surgery, a maximum concentration of 0.057 percent ether was found in air samples obtained at 7, 8, and 28 in. horizontally from the mask and at mask and floor levels. (The minimum explosive concen- tration of ether in air is approximately 2 percent.) The technique for the administration of ether is not mentioned and ventilation is described as ''by extraction fan.'' Their data indicate slight (approximately 2 to 1) layering. In additional tests, the ether concentration (''open'' adminis- tration) was found to be 1.87 and 0.27 percent at 2 and 3 in. above the mask, respectively. In an extensive study of explosive anesthetics, Jones et al 2% did not report any distribution data. Personal correspondence with one of the co- authors (G. J. Thomas), however, indicates that such studies were conducted Yearly editions of NFPA Code #65 were not available. This quotation is taken from discussions of this code. £9 and that ''vapors were found 1 to 12 in. above the floor and spread 4.5 to 5 ft horizontally. Samples taken at the face mask could be found 12 to 16 in. from the nose, mouth, and exhaust port of the face mask, before mi- grating toward the floor.' No concentration levels or ventilation conditions were reported. In a recent study, Vickers?S observed that concentrations never exceeded 5.5 percent of the minimum explosive concentration when ether was released in an unventilated operating room. The vapor was released at 8 L/min, and its concentration varied from 6 to 15 percent. Measurements were made at points on the wall approximately 225 cm from the source, at points on a false wall 15 cm from the operating table, and on the floor beneath the head of the table. Layering occurred only at the false wall. Within 2 min after the ventilation was turned on, all concentrations fell below the range of the gas analyzer, except on the floor at the head of the table. In two other experiments in an operating room (no surgery in progress), ether was released in a concentration of 15 percent at 8 L/min, and cyclo- propane also was released in a concentration of 50 percent at 1.0 L/min. Samples were measured at 17 sites near the release point and, in both experi- ments, no explosive concentrations were found. A single measurement of 90 percent of the explosive concentration was obtained at 10 cm from the gas source. No layering was observed because the higher concentrations were located closest to the source. These experiments also revealed that the presence of oxygen in the anesthetic gas has a negligible effect on the oxygen level in the air beyond 5 cm from the source. The concentration of vapor above a pool of ether has been investigated. Coste and Chaplin’? stated that, above a contained pool of ether, the lower limit of an explosion hazard would be less than 2 ft in still air. They also emptied a 4 oz bottle of ether onto the floor and found that the explosive zone extended to only a few inches above the floor. Zabetakis et al?® conducted similar tests in a quiescent atmosphere. In ether spillages of 15, 50, 150, and 350 cc, they found no flammable concentrations above 2.5 in. from the floor, even in the center of the pool. When ether was spilled from a height of 3 ft, flammable mixtures persisted for a short time in the region through which the vapor was spilled. In atmospheres where air moves at 50 ft/min, flammable mixtures extended vertically and less horizontally (no quantitative data given). Various statements have been made concerning the hazardous zone. Jones et al?" stated that little vapor exists above a height of 7 ft. Guest et al?”7 states that explosion-proof plugs should be used below 5 ft, as does the NFPA Code 56A, although neither source cites references in support of this limit. Cole?8 also states that electrical outlets should be elevated but, again, no references are offered. Vickers?® observed that the hazardous zone accepted in England, (4.5 ft above the floor level for a horizontal radius of 4 ft) is too extensive and he recommends a hazardous zone of 25 cm around and below any anesthetic apparatus. In a report of an ASA committee in 1941, Greene?® presented a study of 230 clinical fires and explosions. Not one was related to light fixtures, and no mention was made of electrical wall sockets or switches. c-10 Air conditioning has been considered in reference to explosion hazards. Phillips30 recommends nonrecirculating systems, and NFPA Code 56A%0 asserts that recirculating systems do not increase the hazard of fires and explosions. The present study agrees that an increase in anesthetic gas concentration resulting from recirculation should be small relative to the concentration levels required for flammability. The above discussion does not argue for or against the current definition of the '"hazardous zone.'!' It is a review of relevant data concerning the role of anesthetic gas distribution in explosion hazards. From the present study and from a review of the literature, it is believed that the recommended hazardous zone is too extensive under standard procedures; however, if spillage of liquid ether is a possibility, a 5 ft hazardous zone represents a conservative, but not unreasonable, safety factor. 3. Conclusions (a) Nitrous oxide and halothane are not separated by buoyancy effects and are present throughout the room at a constant concentration ratio. (b) Venting waste gases at the floor reduces inhaled concentration levels only slightly, if at all. (c) Hot spots (areas of highly concentrated anesthetics) were observed and are related to air-conditioning flow rates and to other variables. Such hot spots may surround the head of any person in the operating room with high concentrations of anesthetic gases and may be critical in the exposure of personnel. (d) Reducing the air-conditioning flow rate increases the average concentration of the anesthetic gases and the concentrations present in "hot spot'' areas. (e) Monitoring the average concentration of anesthetic gases in an operating room is best accomplished at the air-conditioning exhaust grille. (f) Recirculating air exchanges do not reduce the average concentrations of anesthetic gases. Consequently, if a recirculating system providing five fresh air exchanges per hour is compared to a non- recirculating system providing 15 fresh air exchanges per hour, the recirculating system would tolerate only 1/3 the leak rate of anesthetic gases if the objective is to expose personnel to equivalent concentrations of waste gases with both systems. In other words, with the recirculating system, the equipment must be three times as tight and the techniques of the anesthetist three times as leak free. Swe Ee — Tre rm Kio. Appendix D DIAGRAMS AND PRESSURE FLOW RELATIONSHIPS OF SCAVENGING COMPONENTS This appendix describes the characteristics and performance of several devices and systems used in the scavenging procedures of this study. The range of pressure flow measurements is extended to the higher flow rates typical of such procedures as opening the oxygen flush valve. The differential pressure readings were obtained with one side of the water manometer attached to the device or system at a specified location and the other side open to room air. The instrumentation included standard Thorpe tube flowmeters (Fisher-Porter) and manometers (Dwyer). The results obtained from one specimen of each component tested are described; when evaluated, the variations between specimens were found to be minor. The following series of tests provides pressure and flow data that could be useful in designing complete anesthetic waste gas scavenging systems; however, the limited nature of these data and the many variations suggest the inclusion of certain adjustable components. In many cases, system per- formance is indicated by the degree of fullness in the breathing bag, which is a function of gas flow, stiffness of the bag wall, and pressure. The filling process has a long time constant, and the conditions for 50 per- cent filling under manual ventilation in comparison to spontaneous breath- ing are different. There are also significant pressure variations as the bag unfolds. As a result, any data obtained can only be approximate. The results from a typical 3 L neoprene bag are shown below. Pressure at Bag Full (cm H20) (2) 0.10 30 0.25 50 0.30 75 0.52 85 1.10 75 2.40 100 D-1 1. Ohio Gas Evacuator Relief Valve (a scavenging popoff valve) Function: Waste gas collection for co, absorption breathing systems. Location: The exhalation limb of the circle absorber. Rt I = inlet from breathing RINGS system 1234 CLOSED 0 = outlet to disposal Wi svsten M = manometer site for S measurement Rt = positive/negative pres- sure relief flap valve S = spring Fig. D.1. OHIO GAS EVACUATOR RELIEF VALVE. Special Features: Spring S adjusts spring pressure on Rt which, in turn, controls the pressure in the breathing system. The valve is normally closed. Rings machined into the adjustment knob indicate the amount of spring compression and closure pressure. This valve easily adjusts the impedance required by many interface systems and protects the patient against negative pressure in the disposal system. Test Conditions (Table D.1): Gas flow was introduced at | at a specific flow rate. The pressure drop from | to 0 was measured. The pressure flow data were collected at various spring settings. Performance: |f the adjustment knob is closed too much, with a significant vacuum at 0, the system can oscillate between the open and closed positions and can cause considerable noise. This can be corrected by opening the valve or by reducing the vacuum. The flap valve Rt is actually seated at all times; thus, any negative pressure seals the valve. D-2 Table D.1 PRESSURE FLOW RELATIONSHIPS: OHIO GAS EVACUATOR RELIEF VALVE Gauge Pressure at M Gas Flow (cm H,0) t I g Position of Control (L/min) 57 4 3 2 1 1 0.5 6.0 20 35 50 2 1.0 6.0 20 35 50 5 2.0 7.0 21 36 51 10 2.0 8.0 22 36 51 20 2.5 10 23 36 51 30 3.0 12 25 38 52 t Number of rings remaining visible on control knob. 2. Dupaco Clean OR Valve (a scavenging popoff valve) Function: Waste gas collection for co, absorption breathing system. Location: The exhalation limb of the circle absorber. OPEN 25% CLOSED 50% CLOSED 75% CLOSED vee VARIABLE ORIFICE R+ | | an fe I = inlet from breathing M = manometer site for measurement system 0 = outlet to disposal R+ = positive pressure relief system Fig. D.2. DUPACO CLEAN OR VALVE. Special Features: The variable orifice provides variable flow resistance and is the standard adjustment in routine use. This valve may be used to adjust impedance required by many interface systems; however, disassembly is necessary to add weighted check valve disks at R+. Test Conditions (Table D.2): Gas flow was introduced at | at a specific flow rate, and 0 was open to room air. The manometer measured the pres- sure drop from | to 0. The pressure flow data were collected for various orifice openings and disk weights. Performance: Generally, the variable resistance can supply sufficient resistance to keep the bag properly filled. [If the vacuum is too great, however, the weighted check valve can provide compensation by producing the required impedance for bag filling without creating a large pressure drop at high flows. D-4 Table D.2 PRESSURE FLOW RELATIONSHIPS: DUPACO CLEAN OR VALVE Weight Gauge oss at Mt Flow of Check 2 (L/min) Valve Percent Closed (e 0 25 50 75 1 0.2 0.0 0.0 0.0 0.0 2 0.2 0.0 0.03 0.08 0.25 5 0.2 0.11 0.18 0.40 1. 10 0.2 0.64 1.0 2.3 6. 20 0.2 1.5 2.5 7.0 24 50 0.2 9.0 18 40 70 1 1.6 0.41 0.43 0.45 0.48 2 1.6 0.64 0.64 0.69 0.90 5 1.6 0.76 0.90 1.0 2.0 10 1.6 1.0 1.4 2.8 6.9 20 1.6 2.0 2.5 7.0 18 50 1.6 10 18 42 71 1 4.5 1.3 1.3 1.3 1.4 2 4.5 1.0 1.1 1.1 1.4 5 4.5 1.5 1.6 2.0 2.8 10 4.5 1.5 2.8 5.1 6.0 20 4.5 3.0 4.0 7.0 18 50 4.5 9.0 19 43 74 Minimum readable pressure was 0.02 cm 0. Alinearity in these pressure data was observed several specimens of this value. D-5 3. Dupaco Modified Positive Pressure Nonrebreathing Valve with Exhaust Adapter (a scavenging nonrebreathing valve) Function: Waste gas collection for nonrebreathing breath'ng systems. Location: Fits over the exhalation valve R+. G | = inlet from breathing system M = manometer site for measurements 0 = outlet to disposal system R+ = positive pressure relief PT = inlet/outlet to/from B = bag (not part of collector device) patient G = fresh gas inlet into breathing system Fig. D.3. DUPACO MODIFIED POSITIVE PRESSURE NONREBREATHING VALVE WITH EXHAUST ADAPTER. Special Features: The exhaust adapter prevents a positive pressure at 0 from reaching the nonrebreathing system. Test Conditions (Table D.3): PT was sealed, 0 was open to room air, and flow was introduced at G. The scavenging adapter is actually from | to 0; however, unit construction required the pressure measurement to be made at M. The pressure flow data, therefore, are for the entire valve and exhaust adapter unit from M to O. D-6 Performance: The system is highly sensitive to negative pressure at 0, which quickly empties the bag. adjusted for proper filling. Table D.3. An impedance valve at 0 can be PRESSURE FLOW RELATIONSHIPS: DUPACO MODIFIED POSITIVE PRESSURE NONREBREATHING VALVE WITH EXHAUST ADAPTER Anesthetic Bag Gauge Gas Flow Filled Pressure at M (L/min) (%) (cm i, 0)" 2 0 0.0 5 50 0.0 10 75 0.64 20 90 1.0 t Minimum readable pressure was 0.02 cm HO. 2 D=7 k, Stanford Waste Gas Collector for T-Tube (Summers modification) Function: Waste gas collection for T-tube breathing systems. Location: At tail of the breathing bag. —| 2cmie I P 0 | = inlet from breathing system M = manometer site for measurements 0 = outlet to disposal system B = bag (not part of disposal system) PT = inlet/outlet to/from G = fresh gas inlet to breathing patient system Fig. D.4. STANFORD WASTE GAS COLLECTOR FOR T-TUBE. Special Features: Clamp CL permits adjustment of outflow to maintain a properly filled breathing bag. Tube T prevents accidental occlusion of the tail of the bag and should be as large in diameter as possible to minimize resistance to outflow from the bag and to reduce the possibility of accidental disassembly. The tube material and wall thickness should be selected so that twisting of the bag does not cause collapse but that direct pressure will obstruct flow. Insertion of T into the bag is facil- itated by the use of a stilet and by lubrication with a volatile lubricant such as water or alcohol. This device is not commercially available but is easily assembled. Test Conditions (Table D.4): PT was sealed, and flow was introduced at G. With the collecting system off, the tail of the bag was open to room air and the manometer was located at M; with the system on, 0 was open to room air and the manometer was again located at M. As a result, the difference between the two sets of pressure flow data represents the pressure flow data for the collector from | to O. Performance: The vacuum at 0 and/or the resistance at CL must be adjusted to keep the bag properly filled. Table D.4 PRESSURE -FLOW RELATIONSHIPS: STANFORD WASTE GAS COLLECTOR FOR T-TUBE Gauge Pressure at M Flow (cm H,0) (L/min) Collecting System On off 2 0.18 0.05 5 0.76 0.20 10 2.3 0.81 20 8.1 3.0 D-9 5. Stanford Pressure Limiting Liquid Sealed Interface Function: Limits pressure in any disposal system. Location: Downstream from the scavenging collector device, such as the popoff valve. I = inlet from breathing M system 2 0 = outlet to disposal system 72 M = manometer site for measurements Rt = positive/negative pressure relief opening — zm fe L = liquid level 4 7777 U rrrz B = scavenging reservoir bag \ /] | I 0 T = pressure limiting tube B | T I | ange MM A L ZIT TTT TI IIIT. Fig. D.5. STANFORD LIQUID SEALED PRESSURE LIMITING INTERFACE. Special Features: This valve limits both positive and negative pressures. If there is an excessive vacuum at M, air flows from Rt to M and reduces the vacuum. If there is an excessive positive pressure at M, air flows from M to Rt. Rt is open to room air, and tle system design and liquid level determine pressures at which air passes through the valve. The construction material is plastic and the liquid is Dow-Corning 550 (or 220) which is nonvolatile and nonflammable. This interface is applicable to any disposal system. When the central vacuum is used, B is included for compliance. The system functions efficiently and requires a minimum of disposal suction. The baffling prevents spillage of liquid through the relief opening or into the disposal system at high flow rates. Test Conditions (Table D.5): R+* was open to room air, a bag was attached at B, and the liquid level was 0.2 cm above the bottom of T. For negative pressure relief, | was sealed and flow passed out of the valve at 0; for positive pressure relief, 0 was sealed and flow passed into the valve at I. In both cases, the pressure was measured at M, providing pressure drop data for flow between R* and M. Performance: The interface limits positive and negative pressure, but a variable impedance device (Dupaco clean OR valve) may be necessary to maintain proper bag filling. Although the baffling is effective, flows over 100 L/min may cause liquid to spill out at Rt. Table D.5 PRESSURE FLOW RELATIONSHIPS : STANFORD LIQUID-SEALED PRESSURE-LIMITING INTERFACE Gauge Pressure at M Flow (cm H,0) (L/min) - Positive Negative 1 1.3 0.33 2 1.3 0.33 5 1.3 0.38 10 1.3 0.51 20 2.1 0.76 50 5.1 2.3 D-11 6. Dupaco Vacuum Manifold (an interfacing device) Function: Relieve negative pressure when disposal enters the central vacuum system. Location: Downstream from a scavenging collector device such as a popoff valve. I = inlet from breathing system 0 = outlet to disposal system M = manometer site for measurements R- = negative pressure relief (flexible flap-type) valve B = scavenging reservoir bag er] 2cm Fig. D.6. DUPACO VACUUM MANIFOLD. Special Features: This device limits negative pressure only and is solely applicable to disposal into the central vacuum system. B is included for compliance. Test Conditions (Table D.6): | was sealed,R- was open to room air, and negative pressure was introduced at 0 to produce the desired flow rates. The manometer measured the pressure drop from R- to M. Performance: Impedance is often necessary at | to keep the anesthetist's bag properly filled. D-12 Table D.6 PRESSURE FLOW RELATIONSHIPS: DUPACO VACUUM MANIFOLD on) Pre M (cm H,0) 1 0.25 2 0.25 5 0.50 10 1.3 20 2.3 50 3.3 D-13 7 Low Velocity Special Duct System for Waste Gas Disposal Function: Waste gas disposal system. Location: Downstream from collecting and interfacing devices. 175M IB —» 272mm y 1A = from operating room A 0 = outlet to disposal system (on roof, via fan) 1B = from operating room 8 M = manometer site for measurements Fig. D.7. LOW VELOCITY DUCT SYSTEM FOR WASTE GAS DISPOSAL. Special Features: The fan, located at 0 is a Cincinnati Fan and Ventilator Company Model PB8, size 8. To reduce the system vacuum, the blower is powered by a 1/4 HP 1725 rpm Westinghouse electric motor rather than a standard 3450 rpm motor. The pressure flow characteristics of the motor blower are approximately: Static Pressure Flow (in H20) (CFM) 0 190 0.5 140 1.0 54 1.5 0 The normal 4 in. inlet has been reduced to 1.5. in. to match the ducting. The flow rate through the system (with both inlets open to room air) is 36 L/min. This blower could serve additional rooms. The system provides waste gas disposal independent of the air-conditioning system. Test Conditions (Table D.7): The atmospheric pressure in room A was 0.02 cm H20 greater than in room B. The manometer was located in room A. Results were obtained for the following four test conditions: (1) 1B sealed, flow introduced at 1A (2) 1A sealed, flow introduced at 1B (3) 1B open to room air, flow introduced at 1A (no flow from room A to B) (4) equal flows introduced at 1A and 1B Performance: The balance valves were not in place during the test series but should be included for uniform performance. The blower flow should be sufficient to prevent cross flow from room to room under all operating conditions. The pressure in the anesthetic system can be adjusted by the fan flow or by the pressure-limiting or impedance-producing devices dis- cussed in Section V. Table D.7 PRESSURE FLOW RELATIONSHIPS: LOW VELOCITY SPECIAL DUCT SYSTEM FOR WASTE GAS DISPOSAL Negative Pressure at M Flow under Test Conditions (L/min) (cm H,0) 1 2 3 4 0 1.8 1.8 -— -—— 1 1.8 1.8 1.5 1.8 2 1.8 1.8 1.4 1.8 5 1.8 1.8 1.4 1.7 10 1.8 1.8 1.4 1.5 20 1.6 1.5 1.1 1.0 50 1.0 1.0 0.76 +0.13 Appendix E GAS ANALYSIS AND CALIBRATION TECHNIQUES This appendix is a brief review of trace gas analysis and calibration techniques. The major instruments in the present studies included the ionizing halogen leak detector employed in the refrigeration industry, the infrared absorption analyzer, and the gas chromatograph. 1. lonizing Leak Detector Although rapid response ionizing leak detectors were designed to detect minute leaks of fluorinated refrigerants, they are also sensitive to halogenated anesthetics. The gas sample is continuously aspirated through a flexible sampling probe and ionized by a heated filament. The presence of a halogen is indicated either as a voltage display or as an audible signal. These instruments are sensitive enough to detect a leak rate of 9 x 1077 cc/sec. In the present studies, the ionizing detectors were used to leak test the components exposed to halogenated anesthetics; however, leaks difficult to locate frequently occurred in equipment normally used only with nitrous oxide. Such equipment was leak tested with the halogen leak detectors, employing a tracer technique. Freons or other halogenated compounds were introduced into leaky components not generally used with halogens. Interpretation of leak detector results requires considerable skill and experience. The technician may be misled and fail to realize that gases can flow long distances along the surfaces to lower levels remote from the actual leak site. Confusion also may result from the identifica- tion of insignificant diffusion leaks detectable through gaskets and tubing. Nevertheless, scientists familiar with leak detectors have expressed the opinion that the anesthesia machines could be leak tested with greater speed and accuracy with such devices, in comparison to the more conventional methods. The applicability of these instruments should be extended with the availability of a N,0 detector (Table VII.3). Leak detectors sensitive to both N,0 and halogens appear to be feasible and could prove to be useful in maintaining the anesthesia equipment. The ionizing leak detectors offer limited application for quantitative analysis and continuous monitoring. Although not designed for this purpose, one such analyzer was modified to reduce drift (Table VII.3). Their prin- cipal use in the present study was to indicate trends in gas concentrations to be quantitated later by the slower infrared analyzers. Improved quanti- tative halogen leak detectors, if available, could fulfill the need for a practical and relatively inexpensive instrument suitable for monitoring the halogen concentrations present in the operating room air (Section Vit). Ultrasonic leak detectors working on the doppler principle are available. They are inexpensive and nonspecific but, in the present studies, sensitivity proved to be too limited. Fel 2. Infrared Analyzer The IR absorption analyzer is a useful compromise between the more cumbersome but highly specific gas chromatograph and the extremely sensitive but unstable ionizing leak detector. The portable IR analyzer is relatively easy to operate. The most sensitive analyzer employed was designed with variable interference filters and a single infrared beam. With a 20 m infrared pathlength, the lowest detectable limit of halothane was 0.04 ppm and it was 0.2 ppm for N,0. The time constant of the 20 m sampling cell approaches 30 sec; the 1 m cell approaches 2 sec at the sampling flow rate of 15 L/min. The long path cell is reduced to manageable dimensions by mirrors that reflect the infrared beam. While IR analyzers are in use, the sampling pump continuously samples through a catheter pointed at the desired sampling site. Interference caused by the presence of CO, and water vapor in the sample must be considered; expired air obviously contains both elements. Such samples usually were collected in storage bags and processed before analysis, first passing through soda-lime and then through calcium sulfate. Gas measurements in the operating room do not present such difficulties because water vapor and CO, remain constant. The choice of a suitable gas for zero determination was discussed in Section VII. Most gas analyzers intended for air monitoring also may be employed as leak detectors. The effectiveness, however, of the more sensitive units may be limited by their slow response time and the large volume of the sampling cell. A high volume cell also precludes the convenience of analyzing small gas samples collected in syringes. 3. Gas Chromatograph The gas chromatograph is a versatile laboratory instrument, readily applicable to the analysis of small samples. The analyzer employed in the present studies (Varian 2100) provided dual channel analyses of N20 and halothane at room temperature. The electron capture detector (ECD, ScH foil) has a useful dynamic range of 0.2 to 1500 ppm of nitrous oxide. Halothane was monitored by a flame ionization detector (FID) with a dynamic range from 11 ppb to 2 percent. Both N,0 and halothane were calibrated by volume dilution. a. Conditions for N20 Detection A gas sampling valve with a 5 cc sample loop injected samples into a3 mx 3 mm ID silanized glass column packed with 100 to 120 mesh Poropak QS. Conditions included an ECD at 180°C, column temperature at 25°C, injector at 50°C, and a carrier gas flow N; at 70 cc/min (scrubbed for 0, and dried over CaSO, and 5x molecular sieves). The columns optimized for N,0 quantitation included 2 m x 1.5 mm 1D, 100 to 120 mesh Poropak QS with the ECD at 300°C and 70 cc/min of N2. Ten foot (3 m) 3 mm ID columns proved best for N,0 resolution at room temperature. £-2 All glass columns were acid washed, silanized, packed, and conditioned at 200°C for 48 hr, with a 10 cc/min flow of Ns. b. Conditions for Halothane Detection A gas sampling valve with a 1 cc sample loop injected samples into a 3.1mx 3 mm ID silanized glass column packed with 3 percent 0V-101 on Chrom W, 100 to 120 mesh. Conditions included a column temperature at 25°¢, injector at 50°C, flame ionization detector at 180°C, and 30 cc/min of high purity nitrogen as the carrier gas. Other halogenated hydrocarbons resolved on this column were detected by ECD or FID. The columns optimized for halothane quantitation included Tmx 1.5mm ID, 100 to 120 mesh Poropak QS, carrier flow at 30 cc/min N, with ECD temperature at 300°C, and a column temperature at 190° to 200°C. cs Electron Capture Detector The ECD will detect halogenated compounds as well as N20. Sensi- tivity limits and response ranges are as follows. At 180°C, it can detect 0.2 to 1300 ppm N20 with 55 percent standing current and is sensitive to 0.11 ppb to 3 ppm halothane with 55 percent standing current. Sensitivity to halothane is slightly improved at 300°C. Tempera- ture, however, is critical in the detection of N,0, and a lack of repro- ducibility occurs at 300°C. In practice, 180°C proved to be reasonable for both halothane and N30. d. Injection Systems Gas sampling valves are a reproducible method for introducing gas samples into the chromatograph although gas-tight syringes may be used. The Varian six-port gas sampling valve presents a low dead volume (less than 0.2 ml), and sample loops are accurate to 1 percent of the rated volumes. These valves have Teflon seats and rotors, and care should be taken to introduce clean air samples to prevent particles from lodging in the Teflon (seven micropore filters prevent particulate matter from entering the injector). e. Closed-Loop Method of Calibrating Gas Analyzers The calibration of gas analyzers is essential on initial instal- lation and at regular intervals thereafter. Standard gases in cylinders are available, but their accuracy and stability are uncertain. The use of the "copper kettle' type of vaporizer may prove satisfactory for calibration of the anesthetic liquids but, for precise or low level measurements, these vaporizers also require calibration. E-3 The method presented (Fig. E.1) provides an absolute standard for calibration. It depends on the recirculation of small measured samples of pure gases in an air-tight box of relatively large fixed volume. The box is lined with mylar to reduce anesthetic gas absorption. The recircu- lation feature allows unlimited time for the stabilization of slowly responding analyzers. A wide range of gas concentrations is easily pre- pared. Using nitrous oxide as well as halothane and other liquid agents, this method has been applied to the calibration of a gas chromatograph and to infrared and ultraviolet absorption analyzers. The volume of the box is sufficient to permit the appropriate dilution of samples while making use of the accurate ranges of the sampling syringes. A 100 L volume covers most requirements. One end of the box is removable to permit servicing and to introduce materials for leakage and diffusion tests. A fan ensures thorough mixing of the contents and is installed with the motor mounted outside the box to facilitate heat dissipation. Several ports provide for the injection and withdrawal of samples and for air flushing to reduce the contained concentrations and to establish zero lines. Other ports provide for the closed-loop system in calibrating analyzers of the continuous flow-through type. The loop includes an air- tight pump to keep the diluted samples recirculating, thus ensuring steady- state conditions. ANALYZERS UNDER CALIBRATION <€&—— OPTIONS AIR 1 A) a 2 = 2 > o PUMP =F 8 0 > — ® “— »>— — — — > —GASSAMPLE — — — >» — — Q — + GAS SAMPLING VALVE Fig. E.1. CLOSED-LOOP CALIBRATION METHOD. E-4 10. 11. 12. REFERENCES Fink, B.R., Editor. Toxicity of Anesthetics. Baltimore, The Williams and Wilkins Company, 1968. Part Four, Teratogenic Effects. p. 259-323. Corbett, T. H., Cornell, R. G., Endres, J. L., Millard, R. I. Effects of Low Concentrations of Nitrous Oxide on Rat Pregnancy. Anesthesiology 39:299-301, 1973. Stevens, W.C., Eger, E. I., Il, White, A., Halsey, M. J., Munger, W., Gibbons, R. 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Lecture 126 in 1974 Annual Refresher Course Lectures, American Society of Anesthesiologists Annual Meeting, Washington Hilton Hotel, Washington, D.C.- October 12-16, 1974. Schnelle, N. and Nelson, D. A New Device Collecting and Disposing of Exhaust Gases from the Anesthesia Machine. Anes. & Analg. 48:744-747, 1969. Marrese, R.A. A Safe Method for Discharging Anesthetic Gases. Anesthesiology 31:371-372, 1969. 41. 42. 43. Ll. 4s. L6. 47. 48. Lo. 50. 51. 52. 53. 5h. 55. Corbett, T. H. The Gas Trap: A Device To Minimize Chronic Exposure to Anesthetic Gases. Anesthesiology 31: L64, 1969. Yeakel, A. E. A Device for Eliminating Overflow Anesthetic Gases from Anesthetizing Locations. Anesthesiology 32: 280, 1970. Cameron, H. Pollution Control in the Operating Room: A Simple Device for the Removal of Expired Anaesthesia Vapours. Canad. Anaesth. Soc. J. 17: 535-539, 1970. Price, M. and McKeever, R. Anaesthetic Antipollution Device. Canad. Anaesth. Soc. J. 17: 540, 1970. Best, D. W. S. A Simple Inexpensive System for the Removal of Excess Anaesthetic Vapours. Canad. Anaesth. Soc. J. 18 : 333-338, 1971. Berner, 0. A Volume and Pressure Controlling Spill Valve Equipped for Removal of Excess Anaesthetic Gas. Acta Anaesth. Scandinav. 16:252-258, 1972. Evans-Prosser, C. D. G. A Circuit To Reduce the Inhalation of Gases by Anaesthetists (Correspondence). Brit. J. Anaesth. Lh:412, 1972. Boyd, C. H. Do-It-Yourself Venting Appliance for Use with a Popular Expiratory Valve (Correspondence). Brit. J. Anaesth. 44.992, 1972. Enderby, G. E. H. Gas Exhaust Valve. Anaesthesia. 27: 334-337, 1972. Sniper, W. and Murchison, A. G. A Simple Anaesthetic Expiration Flue and Its Functional Analysis (Correspondence). Brit. J. Anaesth. L44:1222, 1972. Steward, D. J. An Anti-Pollution Device for Use with the Jackson Rees Modification of Ayre's T-Piece. Canad. Anaesth. Soc. J. 19: 670-671, 1972. Pitt, E. M. Reduction of Theatre Pollution (Correspondence) . Brit. J. Anaesth. A44:1335, 1972. Cullen, B. F. An Anesthetic Gas Scavenging Device for Use with the Modified Ayre's T-Piece. Anesth. Rev. 1:19, 1974. Rutledge, R. R. A Safe Pressure-Relief Valve and Scavenging System. Anesth. & Analg. 52: 870-871, 1973. Bruce, D. L. Venting Overflow Gases from the Air-Shields (Ventimeter, Ventilator. Anesthesiology. 41:292, 1974. 56. Vaughan, R. S., Mapleson, W. W. and Mushin, W. W. Prevention of Pollution of Operating Theatres with Halothane Vapour by Adsorption with Activated Charcoal. Brit. Med. J. pp. 727-729, 1973. 57. Jorgensen, S. Scavenging Systems on Anaesthetic Machines (Correspondence). p. 672-173. Lancet, 1973. 58. Bruce, D. L. A Simple Way To Vent Anesthetic Gases. Anesth. & Analg. 52: 595-598, 1973. 59. Standard for Medical-Surgical Vacuum Systems in Hospitals. Pamphlet P-2.1. Published by the Compressed Gas Association, Inc.,, New York, New York. Second Edition 1967. p. 4. (Vacuum Requirements). 60. Linch, A. L. Evaluation of Ambient Air Quality by Personnel Monitoring. CRC Press, Cleveland, Ohio, 1974. Yc U.S. GOVERNMENT PRINTING OFFICE: 1975-659-639/41 Region No. 5-11 . i lA i eR - = ec TRE a Nl - Darien bee ed Lee alk a EY i } i ! : | f . i i | i { : ; 3 : : i A 3 I 1 i 3 i i B 1 1 ] i HEW Publication No. (NIOSH) 75-137 Us. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE Public Health Service Center for Disease Control National Institute for Occupational Safety and Health TECHNICAL INFORMATION C0289kL2212