Respective effects of end-expiratory and end-inspiratory pressures on alveolar recruitment in acute lung injury

Objective. A low tidal volume can induce alveolar derecruitment in patients with acute lung injury. This study was undertaken to evaluate whether this resulted mainly from the decrease in tidal volume per se or from the reduction in end-inspiratory plateau pressure and whether there is any benefit in raising the level of positive end-expiratory pressure (PEEP) while plateau pressure is kept constant. Design: Prospective crossover study. Setting: Medical intensive care unit of a university teaching hospital. Patients: Fifteen adult patients ventilated for acute lung injury (PaO2/FIO2, 158 +/- 34 mm Hg; lung injury score, 2.7 +/- 0.6). Interventions: Three combinations were tested: PEEP at the lower inflection point with 6 mL/kg tidal volume, PEEP at the lower inflection point with 10 mL/kg tidal volume, and high PEEP with tidal volume at 6 mL/kg, keeping the plateau pressure similar to the preceding condition. Measurements and Main Results: Pressure-volume curves at zero PEEP and at set PEEP were recorded, and recruitment was calculated as the volume difference between both curves for pressures ranging from 15 to 30 cm H2O. Arterial blood gases were measured for all patients. For a similar PEEP at the lower inflection point (10 +/- 3 cm H2O), tidal volume reduction (10 to 6 mL/kg) led to a significant derecruitment. A low tidal volume (6 mL/kg) with high PEEP (14 +/- 3 cm H2O), however, induced a significantly greater recruitment and a higher PaO2 than the two other strategies. Conclusion. At a given plateau pressure (i.e., similar end-inspiratory distension), lowering tidal volume and increasing PEEP increase recruitment and PaO2.