Without a doubt the most neglected safety device of the anesthesia machine is the scavenger. As you visit anesthesia personnel at work in the operating room, it is rare to see even experienced anesthesiologists adjusting the rate of suction for the scavenger device. Usually the suction rate is set and left for a number of days until someone just happens to readjust the knob during a long case. During this time the suction is either too high (creates undo negative pressure in the system unless the negative pressure relief valve opens to admit room air) or the scavenger reservoir bag is overfilling (creating a high positive pressure unless the high pressure relief valve opens to allow the anesthetic gases to escape into the room). When the reservoir bag is flat because of excessive suction, the anesthesiologist usually does not take notice of the situation. The ‘hissing’ noise of escaping anesthetic gases through the scavenger’s high pressure relief valve usually attracts attention and the suction flow rate is properly adjusted. Ideally the reservoir bag should be half-full and expanding-deflating without releasing gases to room air or allowing room air to be suctioned into the reservoir bag.
Scavenger systems were forced on anesthesia machines by the ‘recommendations’ of the National Institute for Occupational Safety and Health (NIOSH). The recommendations for room air concentrations of waste anesthetic gases is 25 ppm for nitrous oxide and 2 ppm for halogenated agents.(1) Amid a flurry of articles relating to the effects of waste anesthetic gases on operating room and dental personnel, the .recommended standards’ became effective. However, scavengers have not been shown to change the occupational hazards of waste anesthetic gases to operating room personnel. Despite controversy, scavengers are here to stay. Hospital risk managers would rather have scavengers in place than face the possibility of a NIOSH fine or a liability lawsuit from an employee.
Two Types of Scavengers Create Different Problems
Scavenging systems are either passive or active.(2) Passive systems have a large tube or canister with an inlet, an outlet, and one open end. The waste anesthetic gases from the ventilator and ‘pop-off’ valve enter the inlet. Suction is applied at the outlet. If the suction rate exceeds the rate of entry of anesthetic gases, room air is drawn into the open end. If entry of anesthetic gases exceeds the rate of suction, excess gases exit via the open end into the room.
Active systems contain negative and positive relief valves to release excess pressures. (3) Active systems have been associated with higher exposure of gases in the environment than passive systems. (4) When the reservoir bag overinflates due to an excess inflow of anesthetic gases, pressure increases until the positive pressure relief valve opens to vent excessive gases into the room. A valve malfunction will allow the pressure increase in the patient circuit to increase to the limit of the elastic properties of the particular reservoir bag. High pressures can be detected in the patient circuit by the circuit pressure gauge and/or by the high pressure gauge of the ventilator. If high pressure persists the patient may develop decreased cardiac output secondary to high intrathoracic pressure, and/or pneumothorax.
When the suction rate exceeds the anesthetic gas inflow rate, the reservoir bag deflates. Usually any excessive negative pressure is offset by room air entering the negative pressure relief valve. Some scavenger systems have a second or backup relief valve. The pressure gauge of the ventilator may detect an excessive negative pressure state. If the negative pressure relief valve system malfunctions, it could be possible for the suction to remove anesthetic gases from the patient circuit.
It behooves us to be aware of the dangers inherent in the scavenger systems of our anesthetic machines. Anesthesiologists have a multitude of distractions which are much more critical to patient safety than the status of the scavenger. Pollution of the operating room environment with ‘short bursts” of waste gases remains a consideration but should not supersede the safety of the patient. Be alert and cognizant of the relationship of a properly functioning scavenger system to the safety of the patient and to those who work in the operating room.
Dr. Petty, Professor of Anesthesiology, Uniformed Services University of Health Sciences, Bethesda, MD, lives in Salt Lake City, Utah.
- NIOSH. Occupational exposure to waste anesthetic gases and vapors. Publication No. 77-149. Cincinnati. United States Department Public Health Service, Center for Disease Control, 1977.
- Petty C: Scavenging and Pollution. The Anesthesia Machine. Churchill Livingstone, Inc., 1560 Broadway, New York, NY 10036.1987.
- Waste Anesthetic Gas. Technology for Anesthesia 12:1-4,1991. ECRI. 5200 Butler Pike, Plymouth Meeting, PA 19462.
- Are ambient anesthetic gases an occupational hazard? BioMed Safety & Standards 1992; 22:49-51.