Know Your Equipment—Waste Anesthesia Gas Disposal

Verghese T. Cherian, MBBS, MD, FFARCSI

Professor of Anesthesiology & Perioperative Medicine Milton S. Hershey Medical Center Pennsylvania State University College of Medicine

Introduction Waste anesthetic gases (WAG) contaminate the air in the operating room. Historically, chronic exposure to these agents has been associated with adverse health effects. [1,2,3,4] However, these claims have not been substantiated with currently used anesthetic agents in concentrations found in an operating room with an effective scavenging system. Despite the controversy regarding these harmful effects, it seems prudent to remove these waste anesthetic gases from the room air. [1,4,5,6,7] The air exchanges by the air-conditioning unit and an effective scavenging system are the two methods used to control the air quality in the operating room. Anesthetic gases also have a significant effect on the environment. [8]

Current recommendations The recommended exposure limit (REL) of WAG, in the workspace, is described as a time-weighted average (TWA), which is the average concentration reached over an 8h period. In 1977, the National Institute for Occupational Safety and Health (NIOSH) recommended a TWA of 25 parts per million (ppm) of N2O, 2 ppm of halogenated agents when used alone, and 0.5 ppm when used in combination with N2O. [1,4,6] However, these recommendations were based on levels that could be achieved at that time rather than on exposure that could produce adverse effects. [3, 4] Moreover, these RELs were for agents used at that time (chloroform, trichloroethylene, halothane, methoxyflurane, and enflurane) and do not include isoflurane, sevoflurane, and desflurane, which are currently the commonly used inhalational anesthetics. [1,4,5] The health authorities of different countries have suggested varied RELs. The British government, in 1995, suggested a REL of 100 ppm (TWA) for N2O, 50 ppm for enflurane and isoflurane, and 10 ppm for halothane. The Center for Disease Control (CDC) and the NIOSH in their joint publication recommend installation of ventilation and scavenging systems and monitoring the air sample for these gases. [9]

Chronic exposure to anesthetic agents Various adverse effects have been historically attributed to chronic exposure to anesthetic agents. These include spontaneous abortion, infertility, birth defects, malignancy, alterations in immune response, hepatic, and hematological diseases. [2,3,4] Despite numerous animal and human volunteer studies and epidemiological data, there is no conclusive evidence to suggest a causative relationship between WAG and adverse reproductive health. [4,5,6,7]

Perhaps the first study that drew the attention of the scientific community to the risks associated with exposure to WAG came from Soviet Union in 1967. This study demonstrated fatigue, headache, and irritability among 198 men and 110 women anesthesiologists, exposed primarily to diethyl ether, N2O, and halothane. There were also 18 cases of spontaneous abortion in 31 pregnancies in this group of female anesthesiologists. [10]

The earlier studies that demonstrated a significant health risk were based on data from dental hygienists administering high concentrations (70%) of N2O to their patients, using unscavenged delivery systems in rooms without adequate air circulation. Metaanalysis of epidemiological studies on adverse effects of halogenated agents have shown that the incidence of infertility among female anesthesiologist is no greater than in other physicians, and the incidence

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of spontaneous abortion and congenital abnormality was unrelated to occupation of the mother, hours of exposure to operating room environment, or use of scavenging. [1, 6] The task force on waste anesthetic gases of the ASA occupational health committee, in their letter to the CDC, also state that a study, sponsored by the ASA, comparing mortality among anesthesiologists and internists, showed no difference in death rate due to cancer. However, the incidence of suicide, cerebrovascular, HIV, and drug related deaths were higher among the anesthesiologists, possibly related to lifestyle. [6] In their guidelines on management of waste anesthesia gases in the operating room, they recommend use of a scavenging system but not routine monitoring of gases levels.[11] In studies on reproductive problems in women chronically exposed to anesthetic agents, factors such as mental and physical stress, need for constant alertness, irregular routine, inconvenient hours interfering with domestic life, exposure to transmissible infection, solvents, propellants, cleaning solutions, methylmethacrylate, and radiation, could act as confounders. [6] It is also important to emphasize that most of these studies were performed before the scavenging systems were being used and with anesthetic agents that are no longer in use, except for N2O. [10] Therefore, the potential effects of chronic exposure to inhaled anesthetics regarding reproductive health might be even weaker.

In studies that have suggested that volunteers exposed to trace gas concentration show reduced alertness and impaired performance, the concentrations tested were significantly higher than that found in scavenged operating room. [11] An important focus of recent studies is the potential of inhaled anesthetics to induce damage to chromosomes and DNA (genotoxicity and mutagenicity). [10] There have been studies that have shown chromosomal damage in subjects exposed to inhalational agents, while others have failed to do so. Oxidative stress causes damage to macromolecules, such as DNA, lipids, and proteins. There is evidence to suggest that prolonged exposure to high concentration of anesthetic gases may induce oxidative stress and damage the genome, while lower concentration does not. [8, 10]

N2O oxidizes the cobalt ion present in cobalamin (vitamin B12), leading to the inhibition of methionine synthetase with reduced production of methionine and tetrahydrofolate and its byproducts thymidine and nucleic acids (including DNA). This can lead to megaloblastic anemia, agranulocytosis, spinal cord subacute combined degeneration, and neurobehavioral disorders in individuals exposed for a long time to elevated concentrations of N2O. [12]

In summary, the conflicting data with regards to the health risks of exposure to inhaled anesthetics limits the ability to define safety levels or appropriate exposure policies. However, the need to use an effective scavenging system and adequate air handling of the operating room is unchallenged.

Anesthetic gases and environment Chemically, the inhalational anesthetic agents are closely related to the chloro-fluorocarbons (CFC) which play a significant role in ozone depletion. The ozone depletion potential (ODP) of anesthetic gases depends on its molecular weight, the proportion and type of halogen atoms, and its half-life in the atmosphere. [8] The half-life of the commonly used anesthetic gases in the atmosphere are: N2O-114 years, desflurane-10 years, halothane-7 years, sevoflurane-5 years, and isoflurane-3 years. The global warming potential (GWP) of halogenated anesthetics is reported to range from 1230 (isoflurane) to 3714 (desflurane) times the GWP of carbon dioxide (CO2). N2O captures the thermal radiation from the Earth’s surface and contributes to global warming, creating the ‘‘greenhouse effect’’. The GWP of N2O is approximately 300 times that of CO2. [13] N2O, CO2 and methane are the most important greenhouse gases. N2O is also produced from nitrogenbased fertilizers, fossil fuels, as well as by microbial action in moist tropical forests. The N2O concentration is reported to be steadily increasing at a rate of 0.7 to 0.8 parts per billion (ppb) per year over the last decade. Ten percent of N2O is converted into nitrogen oxides (NO and NO2), both of which destroy ozone. The use of inhaled anesthetics for 1 h at 1 MAC and a FGF of 1 L/min has the CO2 equivalency as a car trip of 6.5 km for sevoflurane, 14 km for isoflurane, 95 km for nitrous oxide, and 320 km for desflurane.[13]

In their article ‘Greening the operating room’, the ASA task force on Environmental Sustainability Committee on Equipment and Facilities recommended reducing the use of high impact gases such as desflurane and N2O and implementing techniques to capture and reuse anesthetic gases. [14]

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Mechanism of contamination Although the exhaled anesthetic gases are expelled out of the expiratory port of the anesthetic circuit or the ventilator, there are many other routes by which the gases escape into the operating room. [9,12] • The greatest advantage of using breathing circuits that adsorb CO2, such as the ‘circle system’, as compared to the open ones, as classified by Mapleson, is the ability of a low fresh gas flow to avoid rebreathing.

However, to displace the large volume (8L) of air within the circle system with the anesthetic gas mixture, a high gas flow during initiation of anesthesia is needed to reduce the time constant for this process. The same principle applies during ‘wake up’ to wash away the anesthetic gases in the circuit. This high flow, especially during induction using a facemask, contributes to contamination of the room.

• The anesthetic gases could leak out of the breathing circuit from loose connections, disconnections, damaged corrugated tubes, and around uncuffed endotracheal tubes, laryngeal mask airways, or ill-fitting facemasks.

• During intubation, when the facemask is off the patient’s face, it is important to turn off the fresh gas flow, as merely shutting off the vaporizer will not prevent the fresh gas from displacing the volatile anesthetic within the circuit into the atmosphere. • Trace amounts of anesthetics continue to be exhaled by the patient even after recovery from the anesthetic.

This can occur for couple of hours, depending on the anesthetic agent and the duration of anesthetic. • Although the use of the keyed-filler has significantly reduced the spillage and escape of vapor during the filling of the vaporizer, the vapor of the anesthetic agent can escape the bottle during the time the bottle is uncapped to attach the keyed-filler adaptor. • Another factor that affects the level of contamination is the potency or minimum alveolar concentration (MAC) of the agent, as a larger volume percent of a less potent anesthetic is needed to achieve the same depth of anesthesia. As an example, Desflurane, with a MAC of about 6 times compared to isoflurane, has been shown to cause more contamination. [8]

Inhaled anesthetics are also being used in dental offices, cardiac catheterization units, and interventional radiology and endoscopy suites, where the infrastructure to eliminate the WAG may be less than in an operating room. The scavenging system and the air exchanges achieved by the heating, ventilating, and air conditioning (HVAC) unit are the two ways that the WAG in the operating room can be removed. The construction of the operating room is such that the cooled, filtered air enters the room from the ceiling and exits through ports located on the wall near the floor. This ensures adequate movement and mixing of air. The recommended air exchanges in an operating room is a minimum of 15 air changes per hour, including 3 air changes of fresh air. Apart from the WAG exiting the expiratory port of the breathing circuit or the ventilator, the elimination of all other gas spillage into the operating room is achieved by the HVAC system.

Scavenging system The gases expelled from the expiratory port of the breathing circuit or the ventilator can be collected and removed to a location outside the hospital building. Depending on how the vapor containing air is expelled, there are two types of ‘scavenging’ systems. 1. Active – A dedicated central vacuum system, is used to remove the gases that are collected. 2. Passive – The collected exhaled gases are channeled to the exterior passively. Most modern surgical suites use an active system of WAGD. The scavenging system can essentially be divided into three sections.

1. Interface is the ‘container’ or space where the WAG is collected before being expelled out of the room 2. A connecting channel that brings WAG from the breathing circuit to the interface 3. A tube that channels the gases from the interface to the outside

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The interface can be a closed bag (4-5L) or a container that is open at one end. The idea of a closed bag is to contain the waste gases. However, if there was a failure with the vacuum system, the pressure within the bag would increase and it would be transmitted back to the breathing circuit. Alternatively, if the suction were too high and the bag is collapsed, the negative pressure in the system could be transmitted back to the breathing circuit. To prevent these pressure fluctuations from affecting the breathing circuit, the closed interface has a positive pressure relief valve and a negative pressure intake valve.

In an open interface, if the volume of WAG in the interface were less than what the vacuum system can expel, the air from the room would get sucked into the interface, and if it were more than what the vacuum system can handle, then the WAG could spill over into the room. Therefore, the pressure (high or low) within an ‘open’ interface is not transmitted to the breathing circuit or the patient, but there is a potential for contamination of the room. Most modern anesthesia machines have an open interface, that is attached to the side of the anesthesia workstation, and uses an active system, which is usually a dedicated vacuum system.

A short tube is used to transfer the WAG, exiting the expiratory port of the breathing circuit or the ventilator, to the interface. This tube should be wide enough to carry a high flow of gas and its connection to the interface should be accessible so that it can be disconnected in the event of malfunction of the scavenging system. The American Society of Testing and Materials (ASTM) specifies that the fittings at either end of this tubing should be 30mm in diameter, to avoid any chance of it being wrongly connected to the breathing circuit which has 15/22 mm connections. [1,4]

The tube that carries the WAG from the interface to the gas disposal system is a kink-resistant, color coded (purple) hose, with a quick coupler that connects it to the dedicated ‘waste anesthesia gas disposal’ (WAGD) wall port. The suction pressure of the WAGD is similar to the hospital vacuum at 18-20 inches of Hg and it can transport 30-50 L/min. A control knob at the junction of the vacuum hose with the interface is used to adjust the suction pressure to the appropriate level, as displayed by the indicator.

Limiting WAG The ways to limit the level of WAG in the operating room are by limiting the use of inhaled anesthetics or ensuring effective removal. • A closed circuit with low fresh gas flow can reduce the use of anesthetic gases. • Avoid high impact inhaled anesthetics, such as

Desflurane and Nitrous Oxide. • Total intravenous anesthesia (TIVA) can eliminate the use of anesthetic gases. There should also be an awareness of the effect of anesthetic gases on the environment. [8]

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Future direction • CO2 absorption using calcium hydroxide, as opposed to sodium hydroxide, minimizes compound A production, thus permitting use of sevoflurane with low fresh gas flow without the concern for compound

A toxicity. • Anesthesia Conserving Devices’ (ACD): This device,

‘AnaConDa’, is essentially a combination of a small disposable anesthetic vaporizer and a heat and moisture exchange (HME) filter, which can be attached between the y-piece connector of the circle system and the endotracheal tube. It contains a carbon filter that adsorbs the exhaled anesthetic agent and release it back during the next inspiration, which is similar to what an HME filter does with moisture. [15] (https:// www.sedanamedical.com/) This reflection reduces the total amount of anesthetic needed and also limits the

WAG.

• As part of the disposal unit of the scavenging system, activated charcoal can be used to trap the halogenated

WAG. Silica zeolite crystals (Deltazite®) can effectively adsorb isoflurane and can later be desorbed to yield liquid agent, which can potentially be reprocessed and reused. This is not currently approved by the FDA. [16]

Cold trap condensation or cooling the gas below its dew point, is another way to capture the agent in liquid form.

• There are three ways to destroy N2O, namely oxidation, reduction, or catalytic splitting. Oxidation produces NO, which also impacts the environment negatively. Reduction, on the other hand, removes oxygen to produce Nitrogen (N) plus an oxidized molecule. The catalytic splitting of N2O generates N and O2 and appears to be a logical method to reduce

N2O. [16]

Conclusion Anesthetic gases have detrimental effects on humans and environment. Although the evidence on the adverse effect of WAG on humans is inconclusive, it seems prudent to use a functioning scavenging system and a HVAC system to limit its exposure. Efforts to curb the use of inhalational anesthetics by adopting low-flow anesthesia, TIVA, or anesthesia conserving devices and limiting the contamination of the atmosphere with use of WAG adsorption methods should be encouraged. References

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WAG-Treatment-and-CO2-Absorbers-New-Technologies Assessed

April 25, 2021