Frequency distribution of maximal superimposed pressure and computed tomography (CT)–derived positive end-expiratory pressure (PEEP). (A) Presents the distribution of maximal superimposed pressure. Mild acute respiratory distress syndrome (ARDS) is shown with white bars, moderate ARDS with gray bars, and severe ARDS with black bars. Maximal superimposed pressure was computed dividing each lung in 10 sections along the apex-basis axis thus obtaining 20 different values of superimposed pressures and choosing the greatest one. (B) Presents the corresponding CT-derived PEEP levels (available in 50 patients). CT-derived PEEP was computed multiplying the maximal superimposed pressure times the ratio between chest wall and lung elastance. Mild ARDS is shown with white bars, moderate ARDS with gray bars, and severe ARDS with black bars.