EFFECT OF MEAN AIRWAY PRESSURE ON GAS-EXCHANGE DURING HIGH-FREQUENCY OSCILLATORY VENTILATION

We studied the effect of mean airway pressure (PBARaw) on gas exchange during high-frequency oscillatory ventilation in 14 adult rabbits before and after pulmonary saline lavage. Sinusoidal volume changes were delivered through a tracheostomy at 16 Hz, a tidal volume of 1 or 2 ml/kg, and inspired O2 fraction of 0.5. Arterial PO2 and PCO2 (Pa(o2), Pa(CO2)), lung volume change, and venous admixture were measured at PBARaw from 5 to 25 cmH2O after either deflation from total lung capacity or inflation from relaxation volume (Vr). The rabbits were lavaged with saline until Pa(o2) was < 70 Torr, and all measurements were repeated. Lung volume change was measured in a pressure plethysmograph. Raising PBARaw from 5 to 25 cmH2O increased lung volume by 48-50 ml above Vr in both healthy and lavaged rabbits. Before lavage, Pa(o2) was relatively insensitive to changes in PBARaw, but after lavage Pa(o2) increased with PBARaw from 42.8 +/- 7.8 to 137.3 +/- 18.3 (SE) Torr (P < 0.001). Pa(CO2) was insensitive to PBARaw change before and after lavage. At each PBARaw after lavage, lung volume was larger, venous admixture smaller, and Pa(O2) higher after deflation from total lung capacity than after inflation from Vr. This study shows that the effect of increased PBARaw on Pa(o2) is mediated through an increase in lung volume. In saline-lavaged lungs, equal distending pressures do not necessarily imply equal lung volumes and thus do not imply equal Pa(o2).