WASTE GAS SCAVENGERS
Waste-gas scavengers dispose of gases that have been vented from the breathing circuit by the APL valve and ventilator spill valve. Pollution of the operating room environment with anesthetic gases may pose a health hazard to sur-gical personnel. Although it is difficult to define safe levels of exposure, the National Institute for Occupational Safety and Health (NIOSH) recom-mends limiting the room concentration of nitrous oxide to 25 ppm and halogenated agents to 2 ppm (0.5 ppm if nitrous oxide is also being used) in time-integrated samples. Reduction to these trace levels is possible only with properly functioning waste-gas scavenging systems.
To avoid the buildup of pressure, excess gas volume is vented through the APL valve in the breathing circuit and the ventilator spill valve. Both valves should be connected to hoses (transfer tub-ing) leading to the scavenging interface, which may be inside the machine or an external attachment (Figure 4–29). The pressure immediately down-stream to the interface should be kept between 0.5 and +3.5 cm H2O during normal operating condi-tions. The scavenging interface may be described as either open or closed.
An open interface is open to the outside atmosphere and usually requires no pressure relief valves. In contrast, a closed interface is closed to the outside atmosphere and requires negative-and positive-pressure relief valves that protect the patient from the negative pressure of the vacuum system and positive pressure from an obstruction in the disposal tubing, respectively. The outlet of the scavenging system may be a direct line to the outside via a ventilation duct beyond any point of recirculation (passive scavenging) or a con-nection to the hospital’s vacuum system (active scavenging). A chamber or reservoir bag accepts waste-gas overflow when the capacity of the vac-uum is exceeded. The vacuum control valve on an active system should be adjusted to allow the evacuation of 10–15 L of waste gas per minute. This rate is adequate for periods of high fresh gas flow (ie, induction and emergence) yet minimizes the risk of transmitting negative pressure to the breathing circuit during lower flow conditions
(maintenance). Unless used correctly the risk of occupational exposure for health care providers is higher with an open interface. Some machines may come with both active and passive scavenger systems.
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