Impact of Supine Versus Semirecumbent Body Posture on the Distribution of Ventilation in Acute Respiratory Distress Syndrome

In some patients with acute respiratory distress syndrome (ARDS), a paradoxical improvement in respiratory system compliance (C_RS) has been observed when assuming a supine (head of bed [HOB] 0°) compared with semirecumbent (HOB 35–40°) posture. We sought to test the hypothesis that mechanically ventilated patients with ARDS would have improved C_RS, due to changes in ventilation distribution, when moving from the semirecumbent to supine position. We conducted a prospective, observational ICU study including 14 mechanically ventilated patients with ARDS. For each patient, ventilation distribution (assessed by electrical impedance tomography) and pulmonary mechanics were compared in supine versus semirecumbent postures. Compared with semirecumbent, in the supine posture C_RS increased (33 ± 21 vs. 26 ± 14 mL/cm H₂O, p = 0.005), driving pressure was reduced (14 ± 6 vs. 17 ± 7 cm H₂O, p < 0.001), and dorsal fraction of ventilation was decreased (48.5 ± 14.1% vs. 54.5 ± 12.0%, p = 0.003). Posture change from semirecumbent to supine resulted in a favorable physiologic response in terms of improved C_RS and reduced driving pressure—with a corresponding increase in ventral ventilation, possibly related to reduced ventral overdistension.