Effect of I/E ratio on mean alveolar pressure during high-frequency oscillatory ventilation

This study investigated factors contributing to differences between mean al veolar pressure <(PA)over bar> and mean pressure at the airway opening (Pao ) during high-frequency oscillatory ventilation (HFOV). The effect of the i nspiratory-to-expiratory time (I/E) ratio and amplitude of oscillation on t he magnitude of <(PA)over bar> - (Pao) over bar (<(Pdiff)over bar>) was exa mined by using the alveolar capsule technique in normal rabbit lungs (n = 4 ) and an in vitro lung model. The effect of ventilator frequency a:nd endot racheal tube (ETT) diameter on <(Pdiff)over bar> was further examined in th e in vitro lung model at an I/E ratio of 1:2. In both lung models, PA fell below Pao during HFOV when inspiratory time was shorter than expiratory tim e. Under these conditions, differences between inspiratory and expiratory f lows, combined with the nonlinear relationship between resistive pressure d rop and flow in the ETT, are the principal determinants of <(Pdiff)over bar >. In our experiments, the magnitude of <(Pdiff)over bar> at each combinati on of I/E, frequency, lung compliance, and ETT resistance could be predicte d from the difference between the mean squared inspiratory and expiratory v elocities in the ETT. These observations provide an explanation for the mea sured differences in mean pressure between the airway opening and the alveo li during HFOV and will assist in the development of optimal strategies for the clinical application of this technique.