Ventilation and pulmonary mechanics during high-frequency oscillation of immature lungs at birth

Ventilation was initiated and maintained by high-frequency oscillation (HFO) in nine immature lambs not expected to survive for more than a few minutes out of the uterus. They were delivered to HFO (18 to 20 Hz) and ventilated only with air at the setting that yielded optimal blood gases and pH. Once removed from HFO, cardiorespiratory arrest occurred within minutes despite the return of previously inhibited natural inspiratory efforts. At autopsy, the lungs were liquid-filled (i.e., fetal). Thus, HFO had maintained viability without promoting either lung stability or fetal pulmonary fluid (FPF) absorption. Additional experiments on excised immature lungs showed that aeration is determined by distending pressure, which is regulated by the flow through the oscillator circuit and monitored most accurately as transpulmonary pressure (TPP). Once air-entry has begun, alveolar recruitment and negative compliance establish the limits of tolerable lung expansion via TPP. Within these limits, the immature lung can be ventilated effectively with air. However, since these lungs do not produce intrapulmonary foam, lung stability and FPF absorption are not effected. Complications of HFO (including pneumothorax, decreased arterial pressure and heart rate, and metabolic acidemia) can be related to overdistention of the lungs.