High frequency jet ventilation in acute respiratory failure: which ventilator settings?

Seven hypoxaemic patients with acute respiratory failure were ventilated with HFJV (Ventilator VS 600). Arterial oxygenation was improved in each patient by the increases induced in mean airway pressure (PAW) (to 20 cm H2O) using three different ventilatory settings applied in a random order: technique A: I:E ratio 0.43, driving pressure 2.9 bar, no PEEP; technique B: I:E ratio 1.0, driving pressure 1.9 bar, no PEEP; technique C: I:E ratio 0.43, driving pressure 1.8 bar, PEEP 11 cm H2O. Respiratory frequency was maintained at 250 b.p.m. throughout the study. There were no significant differences in PaO2 (FlO2 = 1) or Qs/Qt between the three techniques. In contrast, carbon dioxide elimination was markedly affected by the method used to increase PAW:PaCO2 was significantly higher during technique C (8.5 +/- 3.6 kPa) and technique B (6.6 +/- 2.1 kPa) than during technique A (4.8 +/- 0.9 kPa). Significant increases in cardiac index, heartrate, mean pulmonary arterial pressure and a decrease in the arterio-venous oxygen content difference were observed when PaCO2 increased. We conclude that, to obtain the PAW necessary to improve pulmonary oxygen exchange, more effective carbon dioxide elimination is achieved by increasing the driving pressure, rather than by increasing the I:E ratio, or using a PEEP valve.