When mechanical insufflation is terminated, expired gas is vented to ambient through the expiration valve. These valves are designed to permit rapid egress of gas with minimal resistance to facilitate reduction of pressure in the patient circuit to atmospheric or a selected level of PEEP. Selected PEEP is generated when the electronic logic closes the expiration valve and interrupts relief of pressure in the patient circuit according to data acquired from a pressure sensor and the control panel.
Some ventilators permit selection of an expiratory hold to determine end-expiratory lung pressure. End-expiratory lung pressure should equal baseline pressure, i.e. either atmospheric pressure or selected PEEP to avoid or increase air trapping respectively.
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