Physiology of prone positioning in acute respiratory distress syndrome

Shown in this figure are axial (left) and sagittal views (right) of the thoracic cage representing the changes that occur as a consequence of prone positioning compared with supine positioning. Distending pressure of lung is determined by the transpulmonary pressure (PTP). When an individual is supine, the ventral PTP (+++) significantly exceeds the dorsal PTP (–––) resulting in greater expansion of the ventral alveoli than the dorsal alveoli; this effect is exaggerated in acute respiratory distress syndrome (ARDS) such that ventral alveoli become overdistended and dorsal alveoli become atelectatic (dark purple). Prone positioning reduces the difference between the dorsal and ventral PTP, making ventilation more homogeneous, leading to a decrease in ventral alveolar overinflation and dorsal alveolar collapse and recruitment of alveoli that had collapsed during the supine ventilation. In ARDS, there is substantial ventilation-perfusion mismatch in the supine position, since blood flow and alveolar collapse are both greatest in the dependent portions of the lung. When prone, ventilation/perfusion matching improves since the previously dependent lung continues to receive the majority of the blood flow as alveoli reopen, while the newly dependent lung continues to receive the minority of the blood flow as alveoli begin to collapse. NOTE: The terms dorsal and ventral are anatomy based, rather than gravity based.