Fig. 2
From: Chest wall loading in the ICU: pushes, weights, and positions
Effect of upright positioning on end-tidal alveolar distension under passive conditions. The weight of the abdomen is off-loaded with upright positioning, resulting in diaphragmatic descent (dashed arrows), increased transpulmonary pressure (PL), and increased lung volumes. In the ‘healthy’ lung (A), chest wall compliance (CCW) improves, and lung compliance is minimally affected, resulting in improved compliance of the respiratory system (CRS). In early ARDS (B), increased PL associated with more upright positioning leads to recruitment of lung units that were previously atelectatic (compressed lung units at the bases) or fluid filled (ovals with light shading). While some end-tidal overdistension may occur in non-dependent regions (ovals with thick outline), recruitment exceeds overdistension, resulting in increased CL and CRS. In late-stage, unresolving ARDS (C), there is extensive loss of aeratable lung units as edema and atelectasis are replaced by fibrosis and consolidation (ovals with dark shading). Increased PL in more upright positioning then results in minimal recruitment and widespread overdistension; the improved CCW associated with upright positioning is offset by a relatively greater decline in CL, leading to a paradoxical decrease in CRS