Effects of Prone Positioning on Lung Protection in Patients with Acute Respiratory Distress Syndrome

Rationale: Positive end-expiratory pressure (PEEP) and prone positioning may induce lung recruitment and affect alveolar dynamics in acute respiratory distress syndrome (ARDS). Whether there is interdependence between the effects of PEEP and prone positioning on these variables is unknown. Objectives: To determine the effects of high PEEP and prone positioning on lung recruitment, cyclic recruitment/derecruitment, and tidal hyperinflation and how these effects are influenced by lung recruitability. Methods: Mechanically ventilated patients (VT 6 ml/kg ideal body weight) underwent whole-lung computed tomography (CT) during breath-holding sessions at airway pressures of 5, 15, and 45 cm H2O and Cine-CTs on a fixed thoracic transverse slice at PEEP 5 and 15 cm H2O. CT images were repeated in supine and prone positioning. A recruitment maneuver at 45 cm H2O was performed before each PEEP change. Lung recruitability was defined as the difference in percentage of nonaerated tissue between 5 and 45 cm H2O. Cyclic recruitment/derecruitment and tidal hyperinflation were determined as tidal changes in percentage of nonaerated and hyperinflated tissue, respectively Measurements and Main Results: Twenty-four patients with ARDS were included. Increasing PEEP from 5 to 15 cm H2O decreased nonaerated tissue (501 +/- 201 to 322 +/- 132 grams; P < 0.001) and increased tidal-hyperinflation (0.41 +/- 0.26 to 0.57 +/- 0.30%; P = 0.004) in supine. Prone positioning further decreased nonaerated tissue (322 +/- 132 to290 +/- 141 grams; P = 0.028) and reduced tidal hyperinflation observed at PEEP 15 in supine patients (0.57 +/- 0.30 to 0.41 +/- 0.22%). Cyclic recruitment/derecruitment only decreased when high PEEP and prone positioning were applied together (4.1 +/- 1.9 to 2.9 +/- 0.9%; P=0.003), particularly in patients with high lung recruitability. Conclusions: Prone positioning enhances lung recruitment and decreases alveolar instability and hyperinflation observed at high PEEP in patients with ARDS.