Adjustable pressure-limiting valve

An adjustable pressure-limiting valve (commonly abbreviated to APL valve, and also referred to as an expiratory valve, relief valve or spill valve) is a type of flow control valve used in anaesthesiology as part of a breathing system. It allows excess fresh gas flow and exhaled gases to leave the system while preventing ambient air from entering.[1]

The adjustable pressure-limiting valve on a General Electric Datex-Ohmeda Aisys anaesthetic machine, with pressure gradations shown in centimetres of water.

Such valves were first described by the American dentist Jay Heidbrink, who used a thin disc that was held in place by a spring.[2] The valve is adjustable and spring-loaded, allowing the opening pressure of the valve to be controlled by screwing the valve top which modifies the pressure on the spring.[1] A very light spring is used, so that at its minimum setting the valve can be opened by the patient's breathing alone using low pressures.[3] In contemporary APL valves, three orifices or "ports" are present: one for intake of gas, one for return of gas to the patient, and an exhaust port for waste gas which can be connected to a scavenging system.[1][4]