Waste Anesthetic Gases and Scavenging Systems

Trace Concentrations of Anesthetic Gases

Concern over trace concentrations of anesthetic gases dates back to 1967, when Vaisman reported findings of a survey of 354 anesthesiologists in Russia. All worked in poorly ventilated operating rooms and used nitrous oxide (N 2 O), halothane, and ether. Of the total, 303 responded to the survey; and of these, 110 were female. Female responders reported 31 pregnancies, 18 of which ended in spontaneous abortion. One pregnancy resulted in a congenitally abnormal child. Vaisman concluded that these problems in pregnancy—as well as other reported effects, such as nausea, irritability, and fatigue—were due to a combination of long-term inhalation of anesthetic vapors, emotional strain, and excessive workload. Although uncontrolled and largely anecdotal, this study drew attention to the possibility that trace concentrations of anesthetics may be harmful. The matter was taken up by investigators in Europe and in the United States. Their results, and those from animal studies, gave cause for further inquiry.

In 1970, the U.S. Congress passed the Occupational Safety and Health Act, the purpose of which was to ensure “safe and healthful working conditions for all men and women in the nation.” The act established the National Institute for Occupational Safety and Health (NIOSH), which was given the responsibility to conduct and fund research in exposure hazards and to recommend safety standards. The act also established the Occupational Safety and Health Administration (OSHA), which, after due procedure, would enact into law and then enforce NIOSH recommended standards. NIOSH funded a number of studies, one of which was the National Survey of Occupational Disease Among Operating Room Workers, conducted in conjunction with the American Society of Anesthesiologists (ASA) Ad Hoc Committee on Waste Anesthetic Gases. This study surveyed 49,000 people who were potentially exposed—members of the ASA, American Association of Nurse Anesthetists (AANA), Association of Operating Room Nurses (AORN), Association of Operating Room Technicians (AORT)—and 26,000 unexposed personnel, members of the American Academy of Pediatrics and American Nurses Association. The results, published in 1974, showed an increased reported incidence among women of spontaneous abortion, liver and kidney disease, and cancer, and a higher incidence of congenital abnormalities in the offspring of exposed women. Among the exposed men, the incidence of cancer did not increase, but that of hepatic disease did. NIOSH sponsored further related studies that included investigation of methods for reducing exposure to waste gases. The organization planned to introduce scavenging of waste anesthetic gases and repeat this survey to determine whether scavenging did indeed reduce these adverse health effects and show an association between trace anesthetic gases and disease.

In March 1977, before a repeat survey was commenced, NIOSH published Criteria for a Recommended Standard; Exposure to Waste Anesthetic Gases and Vapors . This report estimated that 214,000 workers were potentially exposed to trace concentrations of anesthetic gases on a day-to-day basis. The document reviewed all the available data and found that, although not definitive, the evidence suggested a relationship between health hazards and trace concentrations of anesthetic gases. No cause-and-effect relationship was established, and no safe exposure levels could be identified. However, the document recommended that risks be minimized as much as possible by maintaining “exposures as low as is technically feasible.” The document also recommended measures to reduce exposure and to monitor exposure levels, and it advocated extensive recordkeeping regarding the health of operating room (OR) personnel.

NIOSH recommended environmental limits for the upper boundary of exposure: “Occupational exposure to halogenated anesthetic agents shall be controlled so that no worker is exposed at concentrations greater than 2 ppm [parts per million] of any halogenated anesthetic agent. When such agents are used in combination with N 2 O, levels of the halogenated agent well below 2 ppm are achievable. In most situations, control of N 2 O to a time-weighted average (TWA) concentration of 25 ppm during the anesthetic administration period will result in levels of approximately 0.5 ppm of the halogenated agent. Occupational exposure to N 2 O, when used as the sole anesthetic agent, shall be controlled so that no worker is exposed at TWA concentrations greater than 25 ppm during anesthetic administration. Available data indicate that with current control technology, exposure levels of 50 ppm, and less for N 2 O, are attainable in dental offices.”

These recommended exposure limits were based on two studies. First, Whitcher and colleagues showed that these levels were readily attainable in the OR when certain precautionary measures were taken. Second, Bruce and Bach found no decrement in the psychomotor capacities of volunteers exposed for 4 hours at the recommended levels. The newer volatile agents, such as sevoflurane and desflurane, were not available at this time, so exposure limits for these agents have not yet been assessed.

To provide a perspective on how small 1 ppm is, consider that the presence of 25 ppm of N 2 O in the atmosphere represents a concentration of one four-hundredth of 1% (100% N 2 O is 1 million ppm [by volume]):

100% N 2 O = 10 6 ppm
1% N 2 O = 10,000 ppm
(1/100) × 1% N 2 O = 0.01% N 2 O = 100 ppm
(1/400) × 1% N 2 O = 0.0025% N 2 O = 25 ppm

Similarly, 2 ppm and 0.5 ppm of halothane represent a concentration of one five-thousandth and one twenty-thousandth of 1% halothane:

100% halothane = 10 6 ppm
1% halothane = 10,000 ppm
(1/5000) × 1% halothane = 0.0002% = 2 ppm
(1/20,000) × 1% halothane = 0.00005% = 0.5 ppm

To express this in terms of anesthetic potency, divide the value expressed in parts per million by the minimum alveolar concentration (MAC) of halothane (0.76%) at 1 atmosphere pressure:

What levels of trace anesthetics may be found in the OR? When no attempt has been made to reduce leakage or to scavenge waste gases, trace gas levels of 400 to 600 ppm of N 2 O and from 5 to 10 ppm of halogenated agents may be detected. Effective scavenging alone can reduce these levels more than 10-fold.

The volume of N 2 O that must be released into an OR to reach the NIOSH maximum recommended limit of 25 ppm can be calculated by the following equation. Assume the size of an OR is 5 m × 4 m. The volume of the OR is therefore given by:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='500cm×500cm×400cm=100×106mL=100,000L’>500cm×500cm×400cm=100×106mL=100,000L500cm×500cm×400cm=100×106mL=100,000L
500 cm × 500 cm × 400 cm = 100 × 10 6 mL = 100 , 000 L
Therefore the NIOSH limit is 25/10 6 . If the OR volume is 100 × 10 6 mL, this limit is reached by release of 2500 mL (25 × 100) or 2.5 L of N 2 O.

This calculation assumes uniform mixing of all gases and no ventilation or air conditioning of the OR. If it is assumed that the OR ventilation system produces 12 air changes per hour, in the OR described above, a leakage rate of 2.5 L/5 min or 0.5 L/min N 2 O would be necessary to maintain the ambient air N 2 O level at 25 ppm.

In 2000, OSHA revised its recommendations on waste anesthetic gases in the light of current knowledge. The revised recommendations are published on the Internet for informational purposes only and are regularly updated as information becomes available. The document is not published in the standard OSHA manual on occupational hazards, however. The recommendations are advisory and have not been promulgated as a standard; rather, they are to be seen as guidelines. OSHA recommends scavenging of waste anesthetic gases in all anesthetizing locations and advocates work practices to reduce trace levels of anesthetic gases in the ambient air. A documented maintenance program should be in place for all anesthetic delivery machines, and an ongoing education program for all personnel to inform them of these recommendations must exist. OSHA recommends a program for monitoring trace anesthetic gases and also recommends a preemployment medical examination for all employees. Each institution also should have a mechanism in place for employees to report any work-related health problems.

Contamination with trace concentrations of anesthetic gases also may occur in the corridors of an OR suite, in the anesthesia workroom, and in the N 2 O storage area. Poorly ventilated postanesthesia care units (PACUs) also may be contaminated with exhaled anesthetic agents.

Sources of Anesthetic Gas Contamination

Potential sources of anesthetic gas contamination are the adjustable pressure-limiting (APL) or “pop-off” valve, the high- and low-pressure systems of the anesthesia machine, the anesthesia ventilator, cryosurgery units, and other miscellaneous sources.

Adjustable Pressure-Limiting Valve

The APL valve of the anesthesia breathing circuit is the outlet for waste anesthetic gases during spontaneous or assisted ventilation. Depending on the inflow rate of fresh gas, more than 5 L of gas can exit the circuit through this valve every minute. The effect of such spillage on the level of anesthetic contamination in the OR is given by the following equation:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='C=(L×60×106)/(NV)’>C=(L×60×106)/(NV)C=(L×60×106)/(NV)
C = ( L × 60 × 10 6 ) / ( NV )
where C is the OR pollutant level in parts per million, L is the pollutant spillage in liters per minute, V is the OR total air volume in liters, and N is the number of air exchanges per hour. For example, if L = 3 L/min, V = 100,000 L, and N = 10 exchanges per hour, then
<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='C=(3×60×106)/(12×100,000)=150ppm’>C=(3×60×106)/(12×100,000)=150ppmC=(3×60×106)/(12×100,000)=150ppm
C = ( 3 × 60 × 10 6 ) / ( 12 × 100 , 000 ) = 150 ppm

Very large volumes of anesthetic gas are discharged through the relief valve of nonrebreathing systems, such as the Bain circuit (Mapleson D) or Jackson-Rees (Mapleson E). Without scavenging and proper room ventilation, N 2 O levels as high as 2000 ppm have been found in the breathing zone of anesthesiologists.

High- and Intermediate-Pressure Systems of the Anesthesia Workstation

The high- and intermediate-pressure systems of the anesthesia workstation include the N 2 O central supply pipeline and reserve tanks and the internal piping of the machine that leads to the N 2 O flowmeter ( Fig. 5-1 ). The pressure in this system can be from 26 to 750 psig; leaks therefore are likely to contribute significantly to the contamination of the OR. Common sources of leaks are defective connectors in the N 2 O central supply line “quick connects” (see Fig. 5-1 ) and defective yokes for the N 2 O reserve tanks. In one OR suite, major high-pressure leaks were detected in 50% of the anesthesia machines. When the OR is not being used, background N 2 O contamination is primarily caused by high-pressure leaks.

Aug 12, 2019 | Posted by in ANESTHESIA | Comments Off on Waste Anesthetic Gases and Scavenging Systems
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