Optimal Mean Airway Pressure during High-frequency Oscillation

Abstract

Background A number of groups have recommended setting positive end-expiratory pressure during conventional mechanical ventilation in adults at 2 cm H2O above the lower corner pressure (P(CL)) of the inspiratory pressure-volume (P-V) curve of the respiratory system. No equivalent recommendations for the setting of the mean airway pressure (Paw) during high-frequency oscillation (HFO) exist. The authors questioned if the Paw resulting in the best oxygenation without hemodynamic compromise during HFO is related to the static P-V curve in a large animal model of acute respiratory distress syndrome. Methods Saline lung lavage was performed in seven sheep (28+/-5 kg, mean +/- SD) until the arterial oxygen partial pressure/fraction of inspired oxygen ratio decreased to 85+/-27 mmHg at a positive end-expiratory pressure of 5 cm H2O (initial injury). The PCL (20+/-1 cm H2O) on the inflation limb and the point of maximum curvature change (PMC; 26+/-1 cm H2O) on the deflation limb of the static P-V curve were determined. The sheep were subjected to four 1-h cycles of HFO at different levels of Paw (P(CL) + 2, + 6, + 10, + 14 cm H2O), applied in random order. Each cycle was preceded by a recruitment maneuver at a sustained Paw of 50 cm H2O for 60 s. Results High-frequency oscillation with a Paw of 6 cm H2O above P(CL) (P(CL) + 6) resulted in a significant improvement in oxygenation (P < 0.01 vs. initial injury). No further improvement in oxygenation was observed with higher Paw, but cardiac output decreased, pulmonary vascular resistance increased, and oxygen delivery decreased at Paw greater than P(CL) + 6. The PMC on the deflation limb of the P-V curve was equal to the P(CL) + 6 (r = 0.77, P < 0.05). Conclusion In this model of acute respiratory distress syndrome, optimal Paw during HFO is equal to P(CL) + 6, which correlates with the PMC.