Impact of changes in operating pressure during high-frequency jet ventilation.

Fifteen critically ill patients with acute respiratory failure were ventilated with high-frequency jet ventilation (HFJV; frequency 100 breaths/min; I/E ratio 0.43; 1.8-mm internal diameter injector cannula). The patients were divided into two groups according to their initial PaO2 during intermittent positive pressure ventilation at FIO21: in eight patients (group I) the mean PaO2 was 141 +/- 34 mm Hg, and in seven patients (group II) mean PaO2 was 376 +/- 75 mm Hg. During HFJV three different operating pressures were used at random: 1.3, 1.8, and 2.3 bars. Increase in operating pressures significantly decreased PaCO2 and significantly increased mean lung volume above apneic FRC, mean airway pressure, and mean alveolar pressure in both groups. A close relationship was found between variations in mean airway pressure and mean alveolar pressure (r = 0.99, P less than 0.001). Significant increases in PaO2 with increasing operating pressures were observed only in group I. In group II, PaCO2 was significantly lower than in group I for a given operating pressure. We conclude that operating pressure is a main determinant of arterial oxygenation during HFJV because of the concomitant increases in intrathoracic pressures and lung volume. Operating pressure also influences carbon dioxide clearance, PaCO2 varies inversely with operating pressure.