The Anesthesia Gas Machine
Michael P. Dosch CRNA PhD, Darin Tharp CRNA MS
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
This site is http://healthprofessions.udmercy.edu/programs/crna/agm/.
Revised April 2019
ANESTHESIA GAS MACHINE> COMPONENTS & SYSTEMS> DISPOSAL> SCAVENGING, WASTE GASES
Definition Scavenging is the collection and removal of vented anesthetic gases from the OR. Since the amount of anesthetic gas supplied usually far exceeds the amount necessary for the patient, OR pollution is decreased by scavenging. If a fresh gas flow-sized volume enters the breathing circuit each minute, the same flow must leave it, or barotrauma will result. Scavenger and operating room ventilation efficiency are the two most important factors in reduction of waste anesthetic gases (WAGs).
Scavenging may be active (suction applied) or passive (waste gases proceed passively down corrugated tubing through the room ventilation exhaust grill of the OR). Active systems require a means to protect the patient's airway from the application of suction, or buildup of positive pressure. Passive systems require that the patient be protected from positive pressure buildup only.
Another important distinction is that scavenger interfaces may be open (to the atmosphere) or closed (gases within the interface may communicate with the atmosphere only through valves; the older type). The different types of interface have clinical implications. Clearly, open interfaces are safer for the patient. From being relatively unknown 10 years ago, they are becoming almost universal on new equipment, so patient (and anesthetists') safety demands user's attention to the distinctions.
Open interfaces are found on most newer gas machines (e.g. Aisys, Apollo, Fabius GS). Aestiva may have an open or closed interface.
The Joint Commission requires scavenging. AANA published recommendations in 2018, available at AANA.
NIOSH recommendation to OSHA: Workers should not be exposed to an eight hour time-weighted average of > 2 ppm halogenated agents (not > 0.5 ppm if nitrous oxide is in use) or > 25 ppm nitrous oxide.
The scavenger interface is the most important component. It protects the breathing circuit from excess positive or negative pressure. Positive-pressure relief is mandatory to vent excess gas in case of occlusion distal to a closed interface. If active disposal system, must have negative pressure relief as well. Reservoir highly desirable with active systems.
Dräger open scavenging interface Click on the thumbnail, or on the underlined text, to see the larger version.
Dräger open scavenging interface Click on the thumbnail, or on the underlined text, to see the larger version (37 KB).
Diagram of the closed scavenging interface (Ohmeda). Inset: external appearance of the interface. Click on the thumbnail, or on the underlined text, to see the larger version (42 KB).
Interfaces can be open or closed types. Open interface has no valves, and is open to atmosphere (allows both negative and positive pressure relief). Should be used only with active systems. Keep the suction indicator between the white etched lines. Remember that slight hissing from an open interface is normal- there is no audible indication of waste gas leaks.
While safer for the patient (no hazard of positive or negative pressure being applied to the airway as a result of scavenger failure), the risk of occupational exposure for providers ignorant of their proper use is higher with the open interface (Anesth Analg 1992;75:1073).
Closed interface communicates with atmosphere only through valves. Should adjust vacuum so that reservoir bag neither flat not over-distended.
Effectiveness: Unscavenged operating rooms show 10-70 ppm halothane, and 400-3000 ppm N2O. Minimal scavenging brings these levels down to 1 and 60 ppm respectively; adding careful attention to leaks and technique can yield levels as low as 0.005 and 1 ppm.
Avoiding waste gas exposure: Evidence of harm to anesthesia personnel from waste gases is suggestive but unproved (strongest relationship is N2O and reproductive difficulties). There are definite hazards to patients when scavenging systems fail- so consider the scavenger part of the breathing system and check it each day. The smell of gas during a case is abnormal and the cause should be sought. Good technique will also help lessen exposure: