Effects of vertical positioning on gas exchange and lung volumes in acute respiratory distress syndrome

Objective: Supine position may contribute to the loss of aerated lung volume in patients with acute respiratory distress syndrome (ARDS). We hypothesized that verticalization increases lung volume and improves gas exchange by reducing the pressure surrounding lung bases. Design and setting: Prospective observational physiological study in a medical ICU. Subjects and intervention: In 16 patients with ARDS we measured arterial blood gases, pressure-volume curves of the respiratory system recorded from positive-end expiratory pressure (PEEP), and changes in lung volume in supine and vertical positions (trunk elevated at 45 degrees and legs down at 45 degrees). Measurements and results: Vertical positioning increased PaO2 significantly from 94 +/- 33 to 142 +/- 49 mmHg, with an increase higher than 40% in 11 responders. The volume at 20 cmH(2)O measured on the PV curve from PEEP increased using the vertical position only in responders (233 +/- 146 vs. -8 +/- 91 ml in nonresponders); this change was correlated to oxygenation change (p = 0.55). End-expiratory lung volume variation from supine to vertical and 1 h later back to supine, measured in 12 patients showed a significant increase during the 1-h upright period in responders (n =7) but not in nonresponders (n = 5; 215 +/- 220 vs. 10 +/- 22 ml), suggesting a time-dependent recruitment. Conclusions: Vertical positioning is a simple technique that may improve oxygenation and lung recruitment in ARDS patients.