Mean airway pressure as an index of mean alveolar pressure - Effect of expiratory flow limitation

Mean airway pressure ((P) over bar ao) has been advocated as a useful index for monitoring hemodynamic performance and risk for barotrauma during mechanical ventilation. This is based on the assumption that (P) over bar ao closely reflects mean alveolar pressure ((P) over bar alv). In the present study we have compared (P) over bar ao with (P) over bar alv in 12 sedated, paralyzed, mechanically ventilated patients. External PEEP ranged from 0.3 to 8.9 cm H2O. (P) over bar alv was estimated by measuring (P) over bar ao after rapid flow interruptions made at different points in time of the breathing cycle, using a modification of the method of Fuhrman and coworkers (4). All subjects exhibited intrinsic PEEP (PEEPi), which ranged from 0.5 to 9.4 cm H2O. A significant negative correlation (p < 0.001) was found between (P) over bar ao/(P) over bar alv and PEEPi. On average, at PEEPi of 10 cm H2O, (P) over bar ao underestimated (P) over bar alv by about 50%. We conclude that (P) over bar ao cannot be taken as an index of (P) over bar alv in patients who exhibit dynamic hyperinflation and PEEPi caused by expiratory flow limitation.