The National Institute for Occupational Safety and Health (NIOSH), while unable to define safe levels of exposure, recommends limiting trace gas levels to:
|Anesthetic Gas ||Maximum Concentration (ppm) |
|Halogenated agent alone ||2 |
|Nitrous oxide alone ||50 |
|Combination of halogenated agent plus nitrous oxide || |
|Halogenated agent ||0.5 |
|Nitrous oxide ||25 |
In most cases, the amount to anesthetic delivered exceeds the patient’s minimal requirement. Waste gas scavenging systems help to collect and remove excess anesthetic gases that would otherwise contaminate the operating theater. Scavenging is the process by which waste anesthetic gases flowing from the patient circuit are collected, controlled, and evacuated from the workplace, to reduce ambient concentrations of agents or gases. Active scavengers use a vacuum to remove waste gases. Passive scavengers rely on the physical properties of the gases for elimination.
Anesthetic gas contamination occurs via two causes: anesthetic technique and equipment issues. Technical issues include using flows that exceed the scavenging system, poorly fitting face masks and laryngeal mask airways, flushing the circuit, leaving the anesthetic gas on after a case, filling of vaporizers, using uncuffed endotracheal tubes, and use of independent breathing circuits (ie, Jackson Rees). Equipment failures include leaks, disconnections, and malfunctioning scavenging systems.
Scavengers can fail from an obstruction. Valves help a malfunctioning scavenger by protecting from excessive pressures. Open scavenging systems are without valves. Closed systems use either positive or positive and negative pressure relief valves.
SCAVENGING SYSTEM COMPONENTS
Components of a scavenging system. (Reproduced with permission from Barash PG, Clinical Anesthesia, 7th ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2013.)
The Gas Collecting Assembly receives waste gases from either the adjustable pressure-limiting valve or ventilator relief valve.
Transfer Tubing carries the waste gases from the gas collecting assembly to the scavenging interface. The ASTM F1343-91 standard requires that the tubing be either 19 or 30 mm to distinguish it from the 22-mm breathing tubing. The tubing should be short and rigid to prevent kinking and occlusion, which can result in back pressure and ultimately barotrauma.
The Scavenging Interface protects the circuit and ventilator from positive and negative excessive pressures. Open systems are without valves, and stay open to atmospheric pressure. They require an active disposal system. Closed systems use either positive pressure valves or both positive and negative pressure valves.
Gas Disposal Tubing connects the scavenging interface to the gas disposal assembly. It should be robust as to prevent collapsing.
The Disposal Assembly is either active or passive, and eliminates the gases to the atmosphere. Active systems use a vacuum to eliminate waste, whereas passive stems rely on the heavier weight of anesthetic gases to force waste through.