The gas/tissue ratio, an index of alveolar inflation in normal lung (n=14), supine (□) and prone (○) position, and lung of acute respiratory distress syndrome patients (n=20), supine (▪) and prone (•) position. The gas/tissue ratio, an index of alveolar inflation in normal lung (n=14), supine (□) and prone (○) position, and lung of acute respiratory distress syndrome patients (n=20), supine (▪) and prone (•) position. A height of 0% refers to the ventral surface in supine position and to the dorsal surface in prone position. Reproduced with permission from Gattinoni et al. 7. P. Pelosi et al. Eur Respir J 2002;20:1017-1028 ©2002 by European Respiratory Society