EXPIRATORY VALVE

Typical Number in Hospital: 200

Cost Bands: 1

References: 2

Often called just the 'pop-off', 'relief', or 'spill' valve, which can be fixed or adjustable, this is used in an anaesthetic breathing circuit to allow the expired gases and other excess gases to vent to atmosphere (usually) and then to prevent outside air from entering the breathing circuit. The most commonly used type is the Heidbrink valve or modifications of this. It is usually in a wide bore T-piece which fits in line with the breathing tube and usually as close to the patient as possible. The side arm of the T-piece contains a simple valve in the form of a light disc held against a ring by a spring. The spring tension can be adjusted by screwing the valve top. As the top is screwed in the pressure required to lift the disc becomes greater until the valve is locked in the off position with the top screwed right down. Locking the valve is useful for controlled or assisted ventilation.

If the valve opening pressure is too high there will be significant resistance to expiration. On the other hand it must not be so low that the reservoir bag will empty spontanteously. Some types include a calibrated valve spring adjuster (e.g. Magill, McKesson).

Adjustable valves are used during several types of inhalation anaesthesia: spontaneous breathing, assisted, and manually controlled ventilation. In mechanically controlled ventilation the valve is shut and gases are vented through the attached ventilator. Alternative valve designs may control exit pressure by varying the weight of the ball, by varying a magnetically controlled disc, by varying a movable piston or by varying the valve orifice.