Effect of continuous positive airway pressure on lung mechanics during high-frequency jet ventilation

Six mongrel dogs were studied in a body plethysmograph to ascertain the effects of continuous positive airway pressure (CPAP) during high-frequency jet ventilation (HFJV), using an open system allowing gas entrainment. Increases in CPAP significantly reduced tidal volume. Increases in HFJV and inspiratory fraction caused progressively larger increases in functional residual capacity (FRC). Higher levels of CPAP dramatically reduced HFJV's effect on FRC, but lower levels of CPAP augmented this effect. At constant CPAP, tidal volume correlated well with the difference between peak airway pressure and CPAP, while the FRC change was correlated with the difference between end-expiratory pressure and CPAP. The relationship between end-expiratory airway pressure and total change in FRC was predictable from lung compliance at all levels of CPAP.