Introduction to scavenging

It is desirable to vent out the exhaled anaesthetic vapours and maintain a vapour-free operating theatre environment.

Although not universally agreed upon, the recommended maximum accepted concentrations in the UK, over an 8-hour time-weighted average, are:

 100 particles per million (ppm) for nitrous oxide

 50 ppm for enflurane

 50 ppm for isoflurane

 10 ppm for halothane

In other countries, the maximum accepted concentrations are 25 ppm of nitrous oxide and 2 ppm of any halogenated agent.

All operating rooms and areas in which inhalation anaesthesia is carried out should be equipped with a scavenging system in order to reduce the exposure of operating room personnel to waste anaesthetic agents vented from the breathing system.

Breathing system outlet

Excess gas is usually (except in the case of Mapleson E and F systems) vented from the breathing system via an adjustable pressure-limiting (or pop-off) valve.
 


The valve disc is held in place by a weak spring, the tension on which can be adjusted by a screw mechanism, thereby adjusting the pressure required to open the valve.

The outlet of the breathing system must be connected to the scavenging system and not simply vented into the operating room. Modern adjustable pressure relief valves incorporate a 19 mm outlet for this purpose:
 

Old expiratory valve with no scavenging outlet.   



Modern expiratory valve with 19 mm scavenging outlet.