Effect of different seated positions on lung volume and oxygenation in acute respiratory distress syndrome

Lung volume available for ventilation is markedly decreased during acute respiratory distress syndrome. Body positioning may contribute to increase lung volume and partial verticalization is simple to perform. This study evaluated whether verticalization had parallel effects on oxygenation and end expiratory lung volume (EELV). Prospective multicenter study in 40 mechanically ventilated patients with ALI/ARDS in five university hospital MICUs. We evaluated four 45-min successive trunk position epochs (supine slightly elevated at 15A degrees; semi recumbent with trunk elevated at 45A degrees; seated with trunk elevated at 60A degrees and legs down at 45A degrees; back to supine). Arterial blood gases, EELV measured using the nitrogen washin/washout, and static compliance were measured. Responders were defined by a PaO2/FiO(2) increase > 20 % between supine and seated position. Results are median [25th-75th percentiles]. With median PEEP = 10 cmH(2)O, verticalization increased lung volume but only responders (13 patients, 32 %) had a significant increase in EELV/PBW (predicted body weight) compared to baseline. This increase persisted at least partially when patients were positioned back to supine. Responders had a lower EELV/PBW supine [14 mL/kg (13-15) vs. 18 mL/kg (15-27) (p = 0.005)] and a lower compliance [30 mL/cmH(2)O (22-38) vs. 42 (30-46) (p = 0.01)] than non-responders. Strain decreased with verticalization for responders. EELV/PBW increase and PaO2/FiO(2) increase were not correlated. Verticalization is easily achieved and improves oxygenation in approximately 32 % of the patients together with an increase in EELV. Nonetheless, effect of verticalization on EELV/PBW is not predictable by PaO2/FiO(2) increase, its monitoring may be helpful for strain optimization.