A Method for Determining Optimal Mean Airway Pressure in High-Frequency Oscillatory Ventilation

"Optimal" mean airway pressure (MAP) during high-frequency oscillatory ventilation (HFOV) can be defined as the pressure that allows for maximal alveolar recruitment while minimizing alveolar overdistension. Choosing a MAP near or just below the point of maximal curvature (PMC) of the volume-pressure characteristics of the lung can serve as a guide to avoid overdistention during HFOV, while simultaneously preventing derecruitment. The purpose of this study was to assess whether optimal MAP at the PMC can be determined by using measures of PaO2 in patients with acute respiratory distress syndrome (ARDS) undergoing HFOV. We prospectively studied seven patients with ARDS who underwent HFOV after failed conventional ventilation. In addition, 11 healthy subjects were studied to validate measurements of changes in end-expiratory lung volume (a dagger EELV) using magnetometers. Using this validated method, plots of a dagger EELV and MAP were constructed during decremental changes in MAP following a recruitment maneuver in seven ventilated patients with ARDS. The PMC was defined as the point where the slope of the a dagger EELV versus MAP curve acutely changed. The MAP at the PMC was compared to that determined from plots of PaO2 versus MAP. In the healthy cohort, measurements of a dagger EELV obtained by magnetometry approximated the line of identity when compared to those obtained by spirometry. The MAP determined using either the a dagger EELV or PaO2 techniques were identical in all seven HFOV ventilated patients. Additionally, there was a significant correlation between the MAP associated changes in PaO2 and the MAP associated changes in a dagger EELV (p < 0.001). The finding that MAP at the PMC is the same whether determined by measures of a dagger EELV or PaO2 suggest that bedside measures PaO2 may provide an acceptable surrogate for measures of EELV when determining "optimal" MAP during HFOV.