Positive end-expiratory pressure prevents the loss of respiratory compliance during low tidal volume ventilation in acute lung injury patients

Study objective: 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. Setting: General ICU in a university hospital. Participants: Eight ALI patients with a lung injury score greater than 2.5. Interventions: 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. Measurements: 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. Results: 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. Conclusions: 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.