Positive end-expiratory pressure prevents the loss of respiratory compliance during low tidal volume ventilation in acute lung injury patients. 
To study the effect of positive end-expiratory pressure (PEEP) on the decay of respiratory system compliance (Cpl,rs) due to low tidal volume (VT) ventilation in acute lung injury (ALI) patients.  General ICU in a university hospital.  Eight ALI patients with a lung injury score greater than 2.5.  Pressure-controlled ventilation (PCV) and volume-controlled ventilation (VCV), with an average VT of 8.5 +/- 0.4 mL/kg, were applied at three levels of PEEP (5, 10, and 15 cm H2O).  Before each PCV and VCV period, lung volume history was standardized by manual hyperinflation maneuvers.  We measured Cpl,rs at time 0 (start), 10, 20, and 30 (end) min from the beginning of each PCV and VCV period.  Gas exchange and hemodynamic data were collected at end.  At PEEP 5 and 10 cm H2O, we observed a progressive Cpl,rs decay with both PCV and VCV modes.  At PEEP 5 cm H2O, we detected a higher Cpl,rs decrease during PCV, due to a higher Cpl,rs at start, compared with VCV.  At PEEP 15 cm H2O, Cpl,rs did not decrease significantly.  Cpl,rs values measured at end as well as oxygenation and hemodynamic data did not differ between PCV and VCV.  At PEEP 15 cm H2O, PCV provided lower PaCO2 than VCV.  A PEEP of at least 15 cm H2O was needed to prevent Cpl,rs decay.  The progressive Cpl,rs loss we observed at lower PEEP probably reflects alveolar instability.