Section Navigation. Facebook Twitter LinkedIn Syndicate. Minus Related Pages. Anesthetic gases in medical settings Here you will learn what we found regarding waste anesthetic gases and best practices for minimizing exposure. Overview One of the principal goals of general anesthesia is to prevent patients from feeling pain during surgery.
Acute exposure to halogenated anesthetics can cause 1 : Headache Drowsiness Difficulties with judgement and coordination Chronic halogenated anesthetics exposure has been linked adverse reproductive effects and cancer 2 , although some studies report no adverse health effects form long-term exposure to low concentrations.
Use anesthesia machines with scavenging systems; be aware that older machines may not be equipped with such systems. Use closed-system or low flow anesthesia instead of high flow anesthesia when administering anesthetic gases to patients, when practicable.
The Occupational Safety and Health Act requires employers to comply with hazard-specific safety and health standards. In addition, employers must provide their employees with a workplace free from recognized hazards likely to cause death or serious physical harm under Section 5 a 1 , the General Duty Clause of the Act.
Employers can be cited for violating the General Duty Clause if there is a recognized hazard and they do not take steps to prevent or abate the hazard.
However, failure to implement these guidelines is not, in itself, a violation of the General Duty Clause. Citations can only be based on standards, regulations, and the General Duty Clause. This document provides general information and guidance about anesthetic gases and workplace exposures. Workplace exposures to anesthetic gases occur in hospital-based and stand-alone operating rooms, recovery rooms, dental operatories, and veterinary facilities. Engineering, work practice, and administrative controls that help reduce these exposures in all anesthetizing locations, are identified and discussed.
Sources of leaks in anesthesia equipment systems, components, and accessories are identified and appropriate methods are described that limit excessive leaks. Inhaled anesthetic agents include two different classes of chemicals: nitrous oxide and halogenated agents. That recommendation remains in effect. Bruce, who conducted the study upon which the REL was based, said in letters to the editor published in Anesthesia Analgesia and Anesthesiology that he no longer believes his conclusions to be valid and that the"NIOSH standards should be revised.
NIOSH also recommended that no worker should be exposed at ceiling concentrations greater than 2 ppm of any halogenated anesthetic agent over a sampling period not to exceed one hour. It is the intention of this document to provide helpful information on protecting the health and safety of anesthesiologists, nurse anesthetists and operating and recovery room personnel working around the administration of anesthetic gases.
Sections that discuss general workplace controls, location-specific workplace controls, monitoring, a suggested medical surveillance program, hazard communication and training, and the management of spills and leaks and their appropriate disposal are designed to reduce workers' exposure to, and related health risks from, inadequately controlled waste anesthetic gases. These guidelines are not a new standard or regulation. They are advisory in nature, informational in content, and intended for use by employers in providing a safe and healthful workplace through effective prevention programs adapted to the needs and resources of each place of employment.
In addition, it is recognized that the patient's welfare, safety, and rights of privacy are paramount. The recommendations presented in this document should in no way preclude proper patient care and safety, particularly if patient needs arise that require deviation from these guidelines. The guidelines are not meant to compromise safe anesthetic practices.
Surgical inhalation anesthesia was first used in the United States when diethyl ether was administered to a patient in Since then, many chemical compounds have been used to anesthetize patients to keep them free from pain during surgical procedures. Many anesthetic agents such as diethyl ether, divinyl ether, cyclopropane, and ethylene, were effective in their intended use but posed a fire and explosion risk in the presence of a sufficient oxygen supply and an ignition source such as a spark from static electricity or electrical equipment.
In the s, developments in chlorofluorocarbon chemistry produced halogenated, nonflammable, volatile agents that replaced the explosive agents. More than 20 years ago the Joint Commission on Accreditation of Hospitals JCAH in its"Accreditation Manual for Hospitals" prohibited the use of flammable anesthetic agents in all anesthetizing locations. Table 1 lists inhaled anesthetic agents that have been used in the past and those that are currently in use. It is estimated that more than , health care professionals --including anesthesiologists, nurse anesthetists, surgical and obstetric nurses, operating room OR technicians, nurses aides, surgeons, anesthesia technicians, postanesthesia care nurses, dentists, dental assistants, dental hygienists, veterinarians and their assistants, emergency room staff, and radiology department personnel --are potentially exposed to waste anesthetic gases and are at risk of occupational illness.
Over the years there have been significant improvements in the control of anesthetic gas pollution in health-care facilities. These have been accomplished through the use and improved design of scavenging systems, installation of more effective general ventilation systems, and increased attention to equipment maintenance and leak detection as well as to careful anesthetic practice.
However, occupational exposure to waste gases still occurs. Exposure measurements taken in ORs during the clinical administration of inhaled anesthetics indicate that waste gases can escape into the room air from various components of the anesthesia delivery system.
Potential leak sources include tank valves, high- and low-pressure machine connections; connections in the breathing circuit, defects in rubber and plastic tubing, hoses, reservoir bags, and ventilator bellows, and the Y-connector.
In addition, selected anesthesia techniques and improper practices such as leaving gas flow control valves open and vaporizers on after use, spillage of liquid inhaled anesthetics, and poorly fitting face masks or improperly inflated tracheal tube and laryngeal mask airway cuffs also can contribute to the escape of waste anesthetic gases into the OR atmosphere.
Studies of the effects of these agents in the health-care setting have been made more difficult due to high job turnover of affected employees.
Publications report a wide range of exposure levels in hospital, medical, dental, and veterinary facilities Askrog and Petersen ; American Society of Anesthesiologists ; Sweeney et al. Unlike the situation in the OR, health-care workers in the recovery room also known as the postanesthesia care unit or PACU encounter occupational exposure to waste anesthetic gases from the patients instead of the anesthesia delivery system. While in the OR, patients anesthetized with inhaled anesthetic agents take-up varying quantities of these agents depending on the specific agent and its solubility, the duration of anesthesia, and the physiological make-up of the patient.
In the PACU, these gases are eliminated by the patient's respiratory system into the ambient environment. In contrast to the OR, the ambient air in the PACU may contain multiple anesthetic gases, which include but are not limited to nitrous oxide, halothane, enflurane, isoflurane, desflurane, and sevoflurane. Because PACU nurses must monitor vital functions in close physical proximity to the patient, they can be exposed to measurable concentrations of waste anesthetic gases.
While random room samples may indicate relatively low levels of waste gases, the breathing zone of the nurses may contain higher levels. Consequently, air samples obtained within the breathing zone of a nurse providing bedside care are most likely to represent the gas concentrations actually inhaled. In general, the detection of halogenated anesthetic agents by their odor would indicate the existence of very high levels, as these agents do not have a strong odor at low concentrations.
For example, detection of high levels of halothane may be difficult for PACU nurses because one study Hallen et al.
In anesthetizing locations and PACUs where exposure to waste gases is known to occur, it is important for health-care workers and their employers to understand the potential risks of excess exposure to waste anesthetic gases and to implement the appropriate controls to minimize these risks. During the past 25 years multiple studies have attempted to elucidate the risk of exposure to anesthetic agents. Animal and human studies have assessed hematopoietic, central nervous system, and behavioral effects and the effects of anesthetic agents on fertility, carcinogenicity, teratogenicity, and reproduction.
Epidemiological studies have generally focused on OR and dental workers, the two occupational groups most frequently exposed to anesthetics. The following discussion highlights these findings. While mutagenicity testing of nitrous oxide N 2 O has demonstrated negative results Baden , reproductive and teratogenic studies in several animal species have raised concern about the possible effects of nitrous oxide exposure in humans.
In general, studies demonstrate reproductive and developmental abnormalities in animals exposed to high concentrations of N 2 O. In one study by Viera et al. According to NIOSH , similar concentrations of ppm have been found in operating rooms and in dental operatories not equipped with scavenging systems. Smith, Gaub, and Moya reported fetal resorption in rats exposed to nitrous oxide at high doses. Surviving fetuses from these rats demonstrated rib and vertebral defects.
Corbett and colleagues also reported an increase in fetal deaths and a smaller number of offspring in rats exposed to levels ranging from 1, to 15, ppm of nitrous oxide. There are also studies involving human subjects. A recent retrospective study Rowland et al.
For dental assistants who used scavenging systems during N 2 O administration, the probability of conception was not significantly different from that of the non-exposed assistants. The Rowland study authors suggest that "exposure to high levels of unscavenged N 2 O can impair fertility and scavenging equipment is important in protecting the reproductive health of women who work with the gas.
Rowland and colleagues examined the relationship between occupational exposure to N 2 O and spontaneous abortion in female dental assistants. Duration of exposure was a surrogate for exposure data.
Nitrous oxide exposure was divided into two separate variables: scavenged hours hours of exposure per week in the presence of scavenging equipment and unscavenged hours of exposure per week. This finding was not observed among workers in offices where scavenging equipment was in use.
The authors concluded, "Scavenging equipment can make large differences in exposure levels at moderate cost and appears to be important in protecting the reproductive health of women who work with nitrous oxide. Several summaries of the epidemiologic studies of exposure to N 2 O and reviews of the topic generally including animal and retrospective studies Purdham ; Kestenberg ; and NIOSH have been published.
They report a consistent excess of spontaneous abortion in exposed women. Other summaries of the epidemiologic studies do not establish a cause-effect relationship Buring et al. Evidence for congenital abnormalities is less strongly associated with exposure. Halogenated agents are used with and without N 2 O and have been linked to reproductive problems in women and developmental defects in their offspring.
As early as there were reports from the Soviet Union, Denmark, and the United States Vaisman ; Askrog and Petersen ; Cohen, Bellville, and Brown that exposure to anesthetic agents including halothane may cause adverse pregnancy outcomes in health-care personnel.
Several animal studies in rats, mice and hamsters showed embryolethal and teratogenic effects and supported the findings in humans Basford and Fink ; Wharton et al. One Popova et al.
A number of human epidemiologic studies have been performed since the early s to assess the potential harm to reproductive health that exposure to anesthetics might cause.
Generally, these were mailed questionnaire surveys completed by persons usually anesthesiologists and nurses identified through registries. As such, the studies were retrospective and inquired about previous reproductive outcomes for which validation was not available.
In addition, no exposure data were available and many of the early studies predated the use of scavenging systems. Studies documenting a statistically significant excess of spontaneous abortions in exposed female anesthesiologists include those of Cohen and colleagues , Knill-Jones and colleagues , ASA , and Pharoah and colleagues Studies also documented increases in spontaneous abortion among nonphysician female OR personnel Cohen et al.
Also of interest, one study documented increased incidence rates of spontaneous abortion among wives of exposed males ASA In some exposed populations, studies failed to show that exposure to anesthetic agents caused increased risk of spontaneous abortion Rosenberg and Vanttinnen ; Axelsson and Rylander ; Tannenbaum and Goldberg ; Buring et al.
The evidence for an association between anesthetic exposure and congenital anomalies is less consistent. Only a few studies in some subpopulations of exposed workers found a positive association Corbett et al. Other studies reported no association with congenital anomalies Axelsson and Rylander ; Lauwerys et.
The retrospective study by Cohen and colleagues reported that female dental chairside assistants who had experienced heavy exposure defined as more than eight hours per week to waste anesthetic gases reported a significant increase in the rate of spontaneous abortions For the wives of dentists who had also experienced heavy exposure, a significant increase in the rate of spontaneous abortions The non-exposed group was restricted to those who did not report anesthetic exposure in any of the years before conception and including the year of conception.
Another study of reproductive outcomes associated with exposure to anesthetic gases also a questionnaire survey, conducted between and documented both a statistically significantly increased odds ratio for spontaneous abortion in exposed females odds ratio 1.
Duration of exposure as estimated by a hygiene investigation was used as an exposure surrogate. These findings of a positive association were surprising because scavenging systems were thought to have been more likely in use during the study period compared to many of the previously cited papers, almost a decade older. These volunteers exhibited decrements in performance following exposures at: ppm N 2 O in air; ppm N 2 O plus 15 ppm halothane in air; and ppm N 2 O plus 15 ppm enflurane in air.
However, studies that attempted to replicate the results of the human performance studies that showed decrements failed to confirm these findings Smith and Shirley Potential harmful effects due to desflurane exposure have been addressed in a few recent studies, including those of Holmes and colleagues , an animal study; and Weiskopf and colleagues , a study conducted with human volunteers.
However, desflurane's potential as a hazard to health-care personnel has not been thoroughly evaluated. The levels of risk for isoflurane, desflurane, and sevoflurane have not been established. Since there are limited data, occupational exposure limits for these agents have not been determined. Therefore, until more information is available, it is prudent to attempt to minimize occupational exposure to these as with all anesthetic agents. Unlike N 2 O, there is evidence that halothane is mutagenic in certain in vitro test systems Garro and Phillips and that halothane is metabolized to reactive intermediates that covalently bind to cellular macromolecules, suggesting potential mechanisms of toxicity Gandolfi et al.
Despite questions about design issues or selection bias in some studies, the weight of the evidence regarding potential health risks from exposure to anesthetic agents in unscavenged environments suggests that clinicians need to be concerned. Moreover, there is biological plausibility that adds to the concern that high levels of unscavenged waste anesthetic gases present a potential for adverse neurological effects or reproductive risk to exposed workers or developmental anomalies in their offspring Cohen et al.
While the use of prospective studies and carefully designed research protocols is encouraged to elucidate areas of controversy, a responsible approach to worker health and safety dictates that any exposure to waste and trace gases should be kept to the lowest practical level. An anesthesia machine is an assembly of various components and devices that include medical gas cylinders in machine hanger yokes, pressure regulating and measuring devices, valves, flow controllers, flow meters, vaporizers, CO 2 absorber canisters, and breathing circuit assembly.
The basic two-gas anesthesia machine has more than individual components. It allows the anesthesia provider to select and mix measured flows of gases, to vaporize controlled amounts of liquid anesthetic agents, and thereby to administer safely controlled concentrations of oxygen and anesthetic gases and vapors to the patient via a breathing circuit.
The anesthesia machine also provides a working surface for placement of drugs and devices for immediate access and drawers for storage of small equipment, drugs, supplies, and equipment instruction manuals. Finally, the machine serves as a frame and source of pneumatic and electric power for various accessories such as a ventilator, and monitors that observe or record vital patient functions or that are critical to the safe administration of anesthesia.
The internal piping of a basic two-gas anesthesia machine is shown in Figure 1. The machine has many connections and potential sites for leaks. Both oxygen and N 2 O may be supplied from two sources Figure 2 : a pipeline supply source central piping system from bulk storage and a compressed gas cylinder supply source. In hospitals, the pipeline supply source is the primary gas source for the anesthesia machine.
Pipeline supplied gases are delivered through wall outlets at a pressure of psig through diameter indexed safety system DISS fittings or through quick-connect couplings that are gas-specific within each manufacturer's patented system.
Because pipeline systems can fail and because the machines may be used in locations where piped gases are not available, anesthesia machines are fitted with reserve cylinders of oxygen and N 2 O.
The oxygen cylinder source is regulated from approximately 2, psig in the tanks to approximately 45 psig in the machine high-pressure system, and the N 2 O cylinder source is regulated from psig in the tanks to approximately 45 psig in the machine high-pressure system.
Compressed gas cylinders of oxygen, N 2 O, and other medical gases are attached to the anesthesia machine through the hanger yoke assembly. Each hanger yoke is equipped with the pin index safety system, a safeguard introduced to eliminate cylinder interchanging and the possibility of accidentally placing the incorrect gas tank in a yoke designed for another gas tank. Figure 3 shows the oxygen pathway through the flowmeter, the agent vaporizer, and the machine piping, and into the breathing circuit.
Oxygen from the wall outlet or cylinder pressurizes the anesthesia delivery system. Compressed oxygen provides the needed energy for a pneumatically powered ventilator, if used, and it supplies the oxygen flush valve used to supplement oxygen flow to the breathing circuit. Oxygen also"powers" an in-line pressure-sensor shutoff valve "fail-safe" valve for other gases to prevent their administration if the O 2 supply pressure in the O 2 high pressure system falls below a threshold value.
Once the flows of oxygen, N 2 O, and other medical gases if used are turned on at their flow control valves, the gas mixture flows into the common manifold and through a concentration-calibrated agent-specific vaporizer where a potent inhaled volatile anesthetic agent is added.
The mixture of gases and vaporized anesthetic agent then exits the anesthesia machine low pressure system through the common gas outlet and flows to the breathing system. The circle system shown in Figure 4 is the breathing system most commonly used in operating rooms ORs.
It is so named because its components are arranged in a circular manner. The essential components of a circle breathing system Figure 5 include a site for inflow of fresh gas common [fresh] gas inlet , a carbon dioxide absorber canister containing soda lime or barium hydroxide lime where exhaled carbon dioxide is absorbed; a reservoir bag; inspiratory and.
Once inside the breathing system, the mixture of gases and vapors flows to the breathing system's inspiratory unidirectional valve, then on toward the patient. Exhaled gases pass through the expiratory unidirectional valve and enter the reservoir bag. When the bag is full, excess gas flows through the APL or pop-off valve and into the scavenging system that removes the waste gases.
On the next inspiration, gas from the reservoir bag passes through the carbon dioxide absorber prior to joining the fresh gas from the machine on its way to the patient. The general use of fresh gas flow rates into anesthetic systems in excess of those required to compensate for uptake, metabolism, leaks, or removal of exhaled carbon dioxide results in variable volumes of anesthetic gases and vapors exiting the breathing system through the APL valve.
When an anesthesia ventilator is used, the ventilator bellows functionally replaces the circle system reservoir bag and becomes a part of the breathing circuit. The ventilator incorporates a pressure-relief valve, that permits release of excess anesthetic gases from the circuit at end-exhalation.
These gases should also be scavenged. No anesthesia machine system is totally leak-free Emergency Care Research Institute Leakage may originate from both the high-pressure and low-pressure systems of the anesthesia or analgesia machine.
The high-pressure system consists of all piping and parts of the machine that receive gas at cylinder or pipeline supply pressure. It extends from the high-pressure gas supply i. Leaks may occur from the high-pressure connections where the supply hose connects to the wall outlet or gas cylinder and where it connects to the machine inlet.
Therefore, gas-supply hoses should be positioned to prevent strain on the fittings ASTM Standard F; Dorsch and Dorsch and constructed from supply-hose materials designed for high-pressure gas flow and minimal kinking Bowie and Huffman High-pressure leakage may also occur within the anesthesia machine itself. Other potential sources of leaks include quick-connect fittings, cylinder valves, absent or worn gaskets, missing or worn yoke plugs in a dual yoke assembly, and worn hoses.
The low-pressure system of the anesthesia machine in which the pressure is slightly above atmospheric consists of components downstream of the flow-control valves. It therefore includes the flow meter tubes, vaporizers, common gas outlet and breathing circuit, i. Low-pressure system leaks may occur from the connections and components anywhere between the anesthesia gas flow control valves and the airway.
This leakage may occur from loose-fitting connections, defective and worn seals and gaskets, worn or defective breathing bags, hoses, and tubing, loosely assembled or deformed slip joints and threaded connections, and the moisture drainage port of the CO 2 absorber, which may be in the"open" position. Low-pressure system leaks also may occur at the gas analysis sensor i.
Inappropriate installation of a calibrated vaporizer s or misalignment of a vaporizer on its manifold ECRI can also contribute to anesthetic gas leakage. Minute absorbent particles that may have been spilled on the rubber seal around the absorber canister s may also prevent a gas-tight seal when the canister s in the carbon dioxide absorber is are reassembled Eichhorn All parts of the machine should be in good working order with all accessory equipment and necessary supplies on hand.
The waste gas disposal system should be connected, hoses visually inspected for obstructions or kinks, and proper operation determined. Similarly, the anesthesia breathing system should be tested to verify that it can maintain positive pressure. Leaks should be identified and corrected before the system is used Bowie and Huffman ; Food and Drug Administration ; Dorsch and Dorsch The ability of the anesthesia system to maintain constant pressure is tested not only for the safety of the patient dependent on a generated positive pressure ventilation but also to test for leaks and escape of anesthetic gases, which may expose health-care personnel to waste anesthetic gases.
Several check-out procedures exist. This checkout serves only as a generic guideline because the designs of different machines and monitors vary considerably. The guideline encourages users to modify the recommendations to accommodate differences in equipment design, modifications, and variations in local clinical practice. The user must refer to the machine manufacturer's operator's manual for the manufacturer's specific procedures or precautions.
Occupational exposures can be controlled by the application of a number of well-known principles including engineering and work practice controls, administrative controls, personal protective equipment, and monitoring. These principles may be applied at or near the hazard source, to the general workplace environment, or at the point of occupational exposure to individuals. Controls applied at the source of the hazard, including engineering and work practice controls, are generally the preferred and most effective means of control.
In anesthetizing locations and PACUs, where employees are at risk of exposure to waste anesthetic gases, exposure may be controlled by some or all of the following: 1 effective anesthetic gas scavenging systems that remove excess anesthetic gas at the point of origin; 2 effective general or dilution ventilation; 3 good work practices on the part of the health-care workers, including the proper use of controls; 4 proper maintenance of equipment to prevent leaks; and 5 periodic personnel exposure and environmental monitoring to determine the effectiveness of the overall waste anesthetic gas control program.
The following is a general discussion of engineering controls, work practices, administrative controls, and personal protective equipment that can reduce worker exposure to waste anesthetic gases. However, not every control listed in this section may be feasible in all settings.
Additional location-specific controls and appropriate exceptions are addressed in Section F. The collection and disposal of waste anesthetic gases in operating rooms and non-operating room settings is essential for reducing occupational exposures.
Engineering controls such as an appropriate anesthetic gas scavenging system are the first line of defense and the preferred method of control to protect employees from exposure to anesthetic gases.
An effective anesthetic gas scavenging system traps waste gases at the site of overflow from the breathing circuit and disposes of these gases to the outside atmosphere. The heating, ventilating, and air conditioning HVAC system also contributes to the dilution and removal of waste gases not collected by the scavenging system or from other sources such as leaks in the anesthetic apparatus or improper work practices.
The exhalation of residual gases by patients in the PACU may result in significant levels of waste anesthetic gases when appropriate work practices are not used at the conclusion of the anesthetic or inadequate ventilation exists in the PACU. A nonrecirculating ventilation system can reduce waste gas levels in this area. Waste gas emissions to the outside atmosphere must meet local, state, and Environmental Protection Agency EPA regulatory requirements.
In general, a machine-specific interface must be integrated with a facility's system for gas removal. The interface permits excess gas to be collected in a reservoir bag or canister and limits the pressure within the bag or canister.
A facility's gas disposal system receives waste anesthetic gases from the interface and should vent the waste gases outside the building and away from any return air ducts or open windows, thus preventing the return of the waste gases back into the facility. Refer to Appendix 3 for a more detailed description of how the scavenging interface works. Removal of excess anesthetic gases from the anesthesia circuit can be accomplished by either active or passive scavenging.
When a vacuum or source of negative pressure is connected to the scavenging interface, the system is described as an active system. When a vacuum or negative pressure is not used, the system is described as a passive system.
With an active system there will be a negative pressure in the gas disposal tubing. With a passive system, this pressure will be increased above atmospheric positive by the patient exhaling passively, or manual compression of the breathing system reservoir bag.
Use of a central vacuum system is an example of an active system: The waste anesthetic gases are moved along by negative pressure. Venting waste anesthetic gas via the exhaust grille or exhaust duct of a nonrecirculating ventilation system is an example of a passive system: The anesthetic gas is initially moved along by the positive pressure from the breathing circuit until it reaches the gas disposal assembly.
Excess anesthetic gases may be removed by a central vacuum system servicing the ORs in general or an exhaust system dedicated to the disposal of excess gases. When the waste anesthetic gas scavenging system is connected to the central vacuum system which is shared by other users, e. The central vacuum system must be specifically designed to handle the large volumes of continuous suction from OR scavenging units.
Hospitals are better now at preventing anesthetic gases from leaking into operating rooms during surgery, which reduces the exposure of workers. If you work with children or animals, it may be difficult to control the leaking from the mask because the patient may move around a lot. Who is exposed to anesthetic gases? Anyone working in an operating room or recovery room with an anesthetized patient human or animal might be exposed to anesthetic gases.
This includes anesthesiologists, dentists, veterinarians, nurse anesthetists, operating-room nurses, operating-room technicians, other operating-room personnel, recovery-room nurses, other recovery-room personnel, and surgeons. Workers are most likely to be exposed to waste anesthetic gases in operating rooms with no automatic ventilation or scavenging systems, operating rooms where these systems are in poor condition, or recovery rooms where gases exhaled by recovering patients are not properly vented or scavenged.
What is not known? Try to reduce or eliminate your exposure as much as possible. What can I do to reduce or eliminate exposure? Davis PT Collection. Murtagh Collection. About Search. Enable Autosuggest. You have successfully created a MyAccess Profile for alertsuccessName. Previous Chapter. Next Chapter. Kim B. Anesthetic Gases: Principles. Freeman B. Brian S. Freeman, and Jeffrey S. McGraw Hill; Accessed November 13, Anesthetic gases: principles.
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