Immediate Effects of Positive End-Expiratory Pressure and Low and High Tidal Volume Ventilation upon Gas Exchange and Compliance in Patients with Acute Lung Injury

BACKGROUND: Protective ventilation, in general, includes low tidal volume ventilation and maintaining end-inspiratory plateau pressures less than 35 cmH2O. Recent clinical studies have determined that such an approach results in improved survival in patients with moderate to severe acute lung injury and acute respiratory distress syndrome. However, experimental evidence suggests that repeated end-expiratory collapse and reexpansion contributes to ventilator-induced lung injury. We sought to determine the immediate effects of specific tidal volume-PEEP combinations upon oxygenation and static compliance in patients with moderate to severe acute lung injury. METHODS: Fourteen patients were prospectively studied and were treated with each of 10 tidal volume-PEEP combinations, applied in random order. After 5 minutes at each tidal volume-PEEP combination, PaO2/FIO2 and static compliance were recorded. Comparisons were made between low and high tidal volume ventilation as well as across five PEEP levels. RESULTS: At both low (6 mL/kg) and high (10 mL/kg) tidal volume ventilation, PaO2/FIO2 increased with increasing PEEP, up to 20 cmH2O. Similar changes in static compliance were not evident. Static compliance was highest at PEEP of 10 and 15 cmH2O, regardless of tidal volume. With PEEP set at 5 cmH2O, static compliance was significantly lower with 6 mL/kg than with 10 mL/kg tidal volumes. Overall, static compliance was lowest for both tidal volume conditions with PEEP set at 25 cmH2O. CONCLUSION: Low tidal volume ventilation with PEEP set at 5 cmH2O results in poor oxygenation and compliance in patients with moderate to severe acute lung injury. Similarly, PEEP set at 25 cmH2O did not improve oxygenation or compliance.