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 (Pawoptimal). 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 OI and physiological shunt fraction were determined. The OI reached a minimum of 6.2±1.4 at Paw 30±4 cmH2O during inflation and a minimum of 2.4±0.3 at Paw 13±2 cmH2O during deflation. Pawoptimal was 32±6 cmH2O on the inflation limb and 14±2 cmH2O on the deflation limb. The difference between the Paw at minimal OI and Pawoptimal was -1.9±4.2 cmH2O (NS) during inflation and -1.5±1.6 cmH2O (p<0.05) during deflation. In 15 out of the 16 comparisons, the difference in Paw was within one step (±3 cmH2O). Conclusion: The minimal OI is indicative for the Paw where oxygenation is optimal during HFOV in surfactant-depleted pigs.

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