Best compliance during a decremental, but not incremental, positive end-expiratory pressure trial is related to open-lung positive end-expiratory pressure - A mathematical model of acute respiratory distress syndrome lungs

A mathematical model of the acute respiratory distress syndrome (ARDS) lung, incorporating simulated gravitational superimposed pressure and alveolar opening and closing pressures, was used to study the mean tidal pressure-volume (PV) slope ("effective compliance") during incremental and decremental positive end-expiratory pressure (PEEP) trials with constant tidal volume (V-T) "ventilation." During incremental PEEP, the PEEP giving maximum mean tidal PV slope did not coincide with "open lung PEEP" (minimum PEEP preventing end expiratory collapse of 97.5% of alveoli inflated at end-inspiration), and it varied greatly with varying V-T and "lung mechanics." Incremental PEEP with a low V-T tests recruitment by the peak pressure, not prevention of collapse by PEEP. During decremental PEEP with a low V-T, maximum mean tidal PV slope occurred with PEEP 2-3.5 cm H2O below open-lung PEEP, unless closing pressure was high. High V-T, high "specific compliance," and high opening pressures caused slightly greater underestimation of open-lung PEEP. Maximum mean tidal PV slope was always higher (e.g., 93.7 versus 16.69 ml/cm H2O), and the variation in PV slope with PEEP was greater, during detremental PEEP. The maximum PV slope during a decremental PEEP trial with a tow V-T may be a useful method to determine open-lung PEEP in ARDS, and should be studied clinically.