We tested the hypothesis that during high-frequency oscillatory ventilation, the oscillatory pressure ratio (OPR) is minimal at the optimal mean airway pressure (Paw). OPR is defined as the ratio of pressure swings at the distal end and the proximal opening of the endotracheal tube. Optimal Paw was assumed to be the lowest Paw at which the physiological shunt fraction was below 0.1. Acute lung injury was produced by saline lung lavage of pigs who were then subjected to a stepwise increase of Paw to impose underinflation, optimal inflation, and overdistention (inflation phase), followed by a stepwise decrease of Paw (deflation phase). OPR reached a minimum of 0.10 +/- 0.01 at Paw = 31 +/- 4 cm H(2)O during the inflation phase and a minimum of 0.04 +/- 0.01 at Paw = 18 +/- 1 cm H(2)O during the deflation phase. Optimal Paw was 31 +/- 4 cm H(2)O on the inflation limb and 14 +/- 2 cm H(2)O on the deflation limb. Paw at the minimal OPR was not significantly different from the optimal Paw during the inflation phase, and slightly but significantly higher (4.1 +/- 1.6 cm H(2)O) during the deflation phase. In conclusion, a consistent relationship was found between OPR and Paw, with a minimum in all animals. The minimal OPR coincides fairly well with the Paw where oxygenation is optimal.
Copyright 2002 Wiley-Liss, Inc.