Expiratory valves

Expiratory valve events during spontaneous breathing are controlled by two basic design strategies: the valve is seated by a pressure equal to the reference pressure throughout inspiration (open design), or it is closed coincidently with initiation of inspiration and opened when the end of inspiration is detected by the electronic logic (closed design). Since the patient's demand for gas is unpredictable, a 'trial-and-error' algorithmic control strategy must be employed. If the inspiratory valve opens too slowly, the duration of disparity between actual and reference pressure may be unacceptable. If it opens too rapidly, gas flow delivery may exceed the patient's requirements, producing a circuit pressure greater than reference pressure in closed design systems, i.e. a pressure overshoot or inadvertent pressure support. The presence of inadvertent pressure support may be undesirable, and should be considered when evaluating the patient's breathing efficiency and interpreting the results of bench and clinical studies comparing the work of breathing among various mechanical ventilators. In contrast, in an open-design system, gas delivered in excess of patient demand is vented to ambient, thus preventing inadvertent pressure support. Thus, from a design standpoint, the strategy for control of inspiratory and expiratory valve events involves an engineering compromise.

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