Airway pressure gradient during high-frequency ventilation.

Six mongrel dogs, each serving as its own control, underwent ventilatory trials on high-frequency positive-pressure ventilation (HFPPV) and high-frequency oscillation (HFO), before and after oleic acid-induced lung injury. Central and distal airway pressures were compared to simultaneous measurements of oxygen transport and shunt fraction. Airway pressure differences were more pronounced with HFO. Mean proximal pressure with HFO was 4.91 +/- 1.9 mm Hg, rising to 11.6 +/- 3.6 mm Hg distally. HFPPV produced somewhat higher proximal and distal pressures of 9.5 +/- 5.2 and 14.2 +/- 6.9 mm Hg, respectively. After induction of lung injury with a subsequent decrease in compliance, central (proximal) airway pressures remained relatively unchanged, while distal pressures showed a further rise, thus increasing the pressure gradient to 9.2 +/- 3.8 mm Hg for HFO and 6.6 +/- 4.6 mm Hg for HFPPV. Shunt fraction increased with decreased oxygen transport after lung injury in both models; however, the increase was less pronounced with HFO, which maintained a greater pressure gradient. The distal increase in airway pressures may be important in maintaining critical closing volumes in the diseased lung. By creating a pressure gradient, high-frequency ventilation by either jet or oscillator may improve functional residual capacity and oxygenation.