131 PEAK INSPIRATORY PRESSURE (PIP) OVERSHOOTS ON SOME INFANT VENTILATORS DURING ACTIVE EXPIRATION

We examined the performance of 4 infant ventilators (Bourns BP 200, Healthdyne 105, Sechrist IV-100B, Infrasonic Infant Star) during a simulated expiration. A test lung was connected via a conventional circuit to the ventilator which was set to deliver a PIP of 20 cm H2O. A Statham pressure transducer was placed proximal to the test lung. During the inspiratory phase of the ventilator, an expiratory flow was introduced. This varied from +4 to +12 L/min. We had previously recorded such peak crying expiratory flow rates on intubated infants (birthweight 0.5 to 2.5 kg). Three ventilators of each type were tested at all 5 expiratory flow rates and each ventilator was examined twice. Increments of PIP above the set value of 20 cm H2O are tabulated below. Values represent X ± S.D. of 6 records.Because these findings were consistent over a range of tidal volumes of 10 to 30 ml, we conclude that expiration during the inspiratory cycle of some ventilators considerably increases PIP in a flow dependent manner. This PIP overshoot is remarkably different in currently used infant ventilators. It is speculated that this PIP overshoot contributes to the development of air leaks in actively expiring ventilated infants.