FLOW RESISTANCE OF EXHALATION VALVES AND POSITIVE END-EXPIRATORY PRESSURE DEVICES USED IN MECHANICAL VENTILATION

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Abstract
The flow-impeding characteristics of the exhalation valves and PEEP [positive end-expiratory pressure] attachments commony used in mechanical ventilation was studied. To characterize these devices, the presure difference across each mechanism was measured at a series of constant flows (5-160 l/min), and resistance-related energy dissipation was measured using mechanical models of passive and active exhalation. At ambient end-exipratory pressure, an inflatable diaphragm (mushroom) design commonly used to valve exhalation presented resistance comparable to that of an endotracheal tube with an internal diameter of 5 mm. The valve''s energy dissipation increased further as PEEP was applied. By comparison, the servo-actuated scissor valve presented less resistance during the passive deflation experiment but impeded the early phase of active exhalation. Spring-loaded PEEP attachments were prohibitively resistive in comparison with alternative methods using an underwater tube, a water column, a weighted spirometer, or an inflatible diaphragm to raise end-expiratory pressure. The exhalation valves and PEEP attachments currently available for clincal use present significant impedance to air flow. Such reistance within the exhalation pathway may be clinically important for patients supported by mechanical ventilation during the hyperpneic or weaning phases of their illness.