Alveolar pressures and lung volumes during high-frequency oscillatory ventilation in dogs

A constant-volume plethysmograph was used to measure functional residual capacity (FRC) in anesthetized dogs before, during, and after high-frequency oscillatory ventilation (HFO). The phrenic nerves were electrically stimulated to provide the necessary respiratory effort. In addition to FRC, mean alveolar pressure (PA) was obtained from the pressure axis of the static pressure/volume (P/V) curve at FRC. In eight dogs, the first min of HFO at 22 ml, 20 Hz, and a 4 L/min bias flow of air increased FRC by an average of 63.6% above the apneic value, where it remained for the subsequent 15 min. PA values were significantly higher than mean airway pressures (Paw) measured in the endotracheal tube, indicating higher pressure in the terminal lung units. In five other dogs, FRC, Paw, and mean pleural pressure increased as HFO was increased from 15 to 30 Hz with 1 and 4 L/min bias flows of air. We speculate that these increases may account for both the impairment of cardiovascular function and improvements in oxygen tensions often seen with HFO.