Bench study on active exhalation valve performance.

BACKGROUND Ventilator exhalation-valve performance during the expiratory phase has been studied in depth. An active exhalation valve uses servo-control technology that allows gas to be released from the exhalation valve during the inspiratory phase if the patient makes an expiratory effort. We conducted a bench study of active exhalation valve response to expiratory effort during the inspiratory phase. METHODS We studied 4 ventilators that have active exhalation valves (Maquet Servo-i, Newport e500, Puritan Bennett 840, and Evita XL) and one that does not (Puritan Bennett 7200ae). With an active test lung we simulated various magnitudes of expiratory effort during the middle of the inspiratory phase. We measured the exhalation resistance and pressure over-shoot during the expiratory effort, and we measured the pressure under-shoot after the expiratory effort. The exhalation resistance of the 7200ae could not be determined because this ventilator did not allow any gas-release through the exhalation valve during the expiratory effort. RESULTS The exhalation resistance of the Evita XL (6.6 +/- 1.8 cm H(2)O/L/s) was higher than that of the Servo-i (3.0 +/- 1.3 cm H(2)O/L/s), e500 (2.6 +/- 0.8 cm H(2)O/L/s), and 840 (3.5 +/- 0.8 cm H(2)O/L/s) (all P < .001). The magnitude of pressure over-shoot during the expiratory efforts was not significantly different among the 4 ventilators with active exhalation valves. Pressure over-shoot was significantly higher with the 7200ae than with any of other ventilators (all P < .001). CONCLUSIONS There was a significant difference in exhalation resistance between the Evita XL and the other 3 ventilators with active exhalation valves. All 4 ventilators with active exhalation valves had lower exhalation resistance than the 7200ae.