Using Anesthetic Gases Safely

This column is for research labs that involve animal procedures or surgeries. We encounter anesthetic gas use in many settings such as medical research, veterinary schools, dental research labs and dental colleges, and biological research facilities. Surgical procedures may involve animals of all sizes, from mice to horses and everything in between. And surgical anesthesia is carried out in everything from benchtop “knockout boxes” to elaborate operating rooms. Wherever animal surgeries are performed, anesthetic agents pose a potential exposure concern. Keep reading to learn about the most commonly used agents, their health effects, signs and symptoms of exposure, how to determine if your anesthesia operations present exposure concerns and, finally, what to do to minimize or eliminate exposures.

Common agents – Health effects and exposure signs and symptoms

The main anesthetics in use currently are nitrous oxide and the class of compounds known as halogenated volatile agents. The focus of this article is the halogenated agents. These are isoflurane, enflurane, desflurane, sevoflurane and halothane.

Although OSHA has not established permissible exposure limits (PEL) for anesthetic gases, other agencies and organizations have set recommended exposure limits. ACGIH set threshold limit values (TLV) of 75 parts per million (ppm) for enflurane and 50 ppm for halothane as eight-hour time-weighted averages (TWA). In the absence of a TLV, the NIOSHrecommended exposure limit (REL) of 2 ppm as an upper limit or “ceiling” value should be used.

The NIOSH recommendation is based upon studies where halogenated anesthetic agents have been linked to reproductive effects in women and neurological effects in exposed workers. These retrospective studies have shown a statistically significant occurrence of excess spontaneous abortions in exposed female workers and spouses of exposed males.¹ Other studies have connected exposure to the halogenated agents to congenital abnormalities in children of female workers and increased incidence of hepatic disease. Chronic low-level exposures such as those encountered in operating rooms have been associated with decreases in cognitive and motor skills as well as the ability to perform complex tasks. Acute exposures can produce depression of the central nervous system functions, respiratory and cardiovascular systems and seizures.²

Typically, the halogenated agents are clear, colorless, volatile, nonflammable liquids with mild sweet or pleasant odors. However, the reported odor threshold for halothane, for example, is 33 ppm, which is very close to the TLV of 50 ppm. Therefore, do not rely on the presence of odors for adequate warning of potential exposures. The signs and symptoms of acute exposures are redness and tearing of the eyes, dizziness, headache, fatigue, slurred speech and reduced respiratory rate. Chronic exposures may include jaundice and an enlarged and tender liver in addition to the reproductive effects mentioned above. Irregularities of menstrual periods and alcohol intolerance have also been reported.

Do you have a problem? Testing and monitoring for leaks and exposures

Air monitoring is the primary tool used to evaluate potential exposures in the workplace. OSHA recommends conducting air sampling for anesthetic gases every six months to evaluate worker exposures and to check the effectiveness of control measures^.1 The three basic types of sampling are personal, area and source sampling. Personal samples are collected using small, calibrated air pumps worn by the worker with appropriate collection media placed near the worker’s breathing zone. They give the best approximation of a worker’s exposure level since they represent the actual airborne contaminant concentration during the sampling period. Personal air monitoring is the ideal method for determining a worker’s TWA exposure and should be used to assess personal exposures during anesthetic administration and during postoperative recovery. Where several workers perform the same job, one may sample a representative fraction of the employees instead of all employees. One approach often employed is to sample a number of surgeries with the highest likelihood of exposure (worst-case scenarios). If overexposures are not found during these operations, it is unlikely that they would be found during other lowerrisk events.

Area sampling is useful for evaluating overall air contaminant levels in a work area and for investigating cross-contamination with other areas in the facility. Area sampling is performed using the same equipment and media used for personal sampling. The difference is that it is placed at a specific station for the sample duration. Area sampling for some contaminants can also be done using data-logging instruments.

Source sampling is used to detect leaks in the anesthesia delivery and scavenging systems as well as ineffective capture by the scavenging system. The only way to do this is with real-time direct-reading instruments. For the halogenated anesthetic agents, the instrument of choice is a portable infrared spectrophotometer. Since instruments of this type provide continuous sampling and instantaneous readouts, sources of anesthetic gas leakage and effectiveness of control measures are immediately determined. Although these instruments are very expensive, they can be rented by the week or month from industrial hygiene equipment companies.

One effective sampling strategy is to use a progression of the various monitoring methods. First, use a direct-reading instrument to find and correct any leaks in anesthetic equipment. Then screen for the levels of contaminants at the source, in the area and in breathing zones while equipment is in use. Identify any areas or workers with levels above the action level (usually ½ REL or TLV), and then conduct full-shift personal or area monitoring in these suspect locations to establish the TWA.

Prevention is the key

Prudence dictates minimizing exposures to anesthetic agents. Peak exposures usually occur during induction and the postoperative recovery phase. The first line of defense is proper ventilation. Areas used for anesthesia should have separate ventilation systems with no recirculation. The recommended amount of ventilation is six to ten room air changes per hour. In addition, properly balance ventilation systems so anesthesia areas are slightly negative, in terms of air pressure, compared to surrounding areas. This prevents any contaminants from flowing or mixing into nearby areas.

The second key to preventing potential exposures is to set up and maintain anesthetic equipment properly. Delivery systems should have some way to scavenge waste anesthetic gases. This is usually done with either a vacuum or exhaust system or a chemical absorption filter canister. The latter is weighed at regular intervals and replaced when expended, i.e., when it has reached a certain weight. Regularly scheduled equipment maintenance is the last step. Leak check and inspect your anesthesia equipment routinely. Pay careful attention to connection points and any seals and O-rings. Don’t forget to have plenty of replacement parts on hand.

The final key to prevention is to use common sense and keep up with improvements in equipment design. Perform surgeries using nose cups and in exhaust hoods whenever possible. Train workers to take care and avoid patient-exhaled air at critical times such as induction and recovery. Rotate personnel performing surgeries if possible to further reduce any exposures.

Summary

Use of anesthetic agents is serious business. Following a few basic rules will allow you to work safely with anesthetics. Use the keys to prevention discussed above. Train your employees on proper equipment operation and maintenance and the hazards of working with these chemicals. Knowledge of exposure signs and symptoms is vital. Finally, put a monitoring strategy in place as a check to make sure everything is working smoothly