High frequency oscillations via the pleural surface: An alternative mode of ventilation?

We applied high-frequency oscillatory ventilation (HFOV) of low amplitude to the pleural surface of the isolated rat lung (IPL) perfused at 10 ml.min^-1 with Krebs bicarbonate containing 4.5% albumin (hematocrit 34%). Lung volume was held constant by a continuous positive airways pressure (CPAP) of 5 cmH₂O. Varying CPAP from 2 to 15 cmH₂O did not affect O₂ uptake. Tidal volume (VT) was estimated with an impedance pneumograph, and it bore a direct linear relationship to the amplitude of both the loudspeaker input signal and the pressure change in the chamber up to 30 Hz; VT was inversely proportional to the frequency (f). However, at a constant loudspeaker input of 10 V, minute expired ventilation V̇E) remained constant (mean 104 ml.min^-1) as f increased from 5 to 30 Hz. Hemoglobin saturation increased by more than 80% during HFOV of 5-40 Hz and amplitude of 10 V, the maximum O₂ uptake being 14.6 ml O₂ per 100 ml perfusate. Whereas dead space was approximately 335 μl, a VT of less than 40 μl could effect normal O₂ uptake, suggesting that bulk flow is playing only a minor role in gas exchange. HFOV for 60 min (CPAP 5 cmH₂O) did not affect the amount of alveolar surfactant compared with conventional ventilation at the same mean airway pressure. We conclude that normal O₂ uptake can be maintained by applying HFOV to the pleural surface of the IPL held at constant volume.