AIR ENTRAINMENT VALVE

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Also called an air inspiratory valve, this comprises a knife-edge seat, a disc and spring, and operates in an anaesthetic breathing circuit in the opposite way to a pressure relief valve, i.e. for the valve to open it requires a very small negative pressure to be exerted which causes the disc to lift allowing air to be entrained. The entrainment valve would operate if the patient was in spontaneous respiration and there was an obstruction in the fresh gas to the patient (e.g. a kink in the gas feed tube), or after the operation of an oxygen failure warning device which cuts off the the gases. Entrainment valves are incorporated in some anaesthetic machines, ventilators, and oxygen failure devices.