THE physiologic rationale for the prone position in mechanically ventilated subjects was articulated by A.C. Bryan almost 50 yr ago. Simply stated, this rationale is as follows: in the supine position, the hydrostatic pressure exerted by abdominal contents curves the dependent portion of the diaphragm cranially, resulting in loss of lung gas volume in dependent caudal regions. In the spontaneously breathing subject, this loss is partially countered by the local mechanical advantage conferred by the smaller radius of curvature of the cranially displaced dependent diaphragm. Because of Laplace’s law, this portion of the diaphragm will generate a higher transpulmonary pressure and hence attract ventilation primarily to the caudal dependent lung. In the passively ventilated subject, this mechanical advantage is absent and ventilation distributes predominantly to nondependent regions, where chest wall elastance and intraabdominal pressure are lower....

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