Oxygenation index, an indicator of optimal distending pressure during high-frequency oscillatory ventilation

Objective: To test the hypothesis that, during high-frequency oscillatory ventilation (HFOV) of pigs with acute lung injury, the oxygenation index (OI = Paw*FIO2* 100/PaO2) is minimal at the lowest continuous distending pressure (Paw), where the physiological shunt fraction is below 0.1 (Paw(optimal)). Design and setting: Prospective, observational study in a university research laboratory. Subjects: Eight Yorkshire pigs weighing 12.0 +/- 0.5 kg, with lung injury induced by lung lavage. Interventions: After initiation of HFOV, the pigs were subjected to a stepwise increase of Paw to obtain under-inflation, optimal inflation and over-distension of the lungs (inflation) in series, followed by a similar decrease of Paw (deflation). Measurements and results: At each Paw level, the of and physiological shunt fraction were determined. The OI reached a minimum of 6.2 +/- 1.4 at Paw 30 +/- 4 cmH(2)O during inflation and a minimum of 2.4 +/- 0.3 at Paw 13 +/- 2 cmH(2)O during deflation. Paw(optimal) was 32 +/- 6 cmH(2)O on the inflation limb and 14 +/- 2 cmH(2)O on the deflation limb. The difference between the Paw at minimal OI and Paw(optimal) was -1.9 +/- 4.2 cmH(2)O (NS) during inflation and -1.5 +/- 1.6 cmH(2)O (p<0.05) during deflation. In 15 out of the 16 comparisons, the difference in Paw was within one step (+/- 3 cmH(2)O). Conclusion: The minimal OI is indicative for the Paw where oxygenation is optimal during HFOV in surfactant-depleted pigs.