The Science Journal of the American Association for Respiratory Care
Current guidelines for conventional mechanical ventilation in adults recommend setting the PEEP at 2 cm H2O above the lower inflection point (LIP) of the inspiratory P-V curve. In HFOV currently no equivalent recommendation for the setting of the mean airway pressure (PAW) exists. We hypothesized that the PAW resulting in the best oxygenation (optimal PAW) can be related to and thus predicted from the static P-V curve of the respiratory system in a large animal model of ARDS.
Methods: In seven sheep (28±5 kg) severe lung injury was induced by repeated saline lung lavages until P/F-ratio decreased to 85±27 at PEEP 5 cm H2O. The LIP (20±0.7 cm H2O) on the inspiratory limb and the point of maximal curvature (PMC, 26±0.9 cm H2O) on the expiratory limb of the static P-V curve were determined by a blinded investigator. The sheep were then subjected to four one-hour cycles of HFOV (f 8 Hz, FiO2 1.0) at different levels of PAW (LIP+2, LIP+6, LIP+10, LIP+14 cm H2O), applied in random order. Each cycle was preceded by a standardized de-recruitment (30 s ventilator disconnection) followed by a sustained insufflation (recruitment manoeuvre) with a PAW of 50 cm H2O for 60 s.
Results: HFOV with a PAW of 6 cm H2O above LIP resulted in a highly significant improvement of oxygenation (333±176 torr, p<0.01 vs. Injury) and venous admixture (0.26±0.12, p<0.05 vs. Injury). LIP+6 significantly correlated with the PMC (r=0.77, p<0.05). Whereas no further improvement in oxygenation or venous admixture could be seen with higher PAW. Cardiac output and oxygen delivery were significantly impaired at PAW settings higher than LIP+6. Conclusion: The optimal PAW (least pressure with highest PaO2) in HFOV can be predicted from the static P-V curve. It correlates with the PMC on the expiratory limb of the P-V curve.
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