Regional ventilation, on the other hand, is markedly altered by the effects ofgravity. Milic-Emili and colleagues  first demonstrated a vertical gradient in alveolar size in 1966, and a vertical gradient in ventilation was demonstrated by Engel and colleagues in 1974 . In the upright, supine, and right and left lateral decubitus positions there is a gravitational gradient in end-expiratory lung volume with dependent lung regions being less well-expanded than non-dependent regions . These heterogeneities occur in conjunction with regional differences in pleural pressure (Ppl, and accordingly, with regional differences in transpulmonary pressure), and from differences in lung compliance that result from differences in alveolar volume.
The situation is considerably different in the prone position because the lung fits better inside the thorax such that there is less lung deformation in this posture.
This difference results in a much more uniformly distributed Ppl, such that the end-expiratory lung volume is much more uniformly distributed from non-dependent to dependent regions (Fig. 2). This, in turn, allows most of the lung to be at approximately the same place on the pressure-volume (PV) curve [13, 14]. Accordingly, regional ventilation during tidal breathing is much more uniform. In addition, in the supine position, the dependent portions of the lung are exposed to a positive Ppl in the setting of lung injury (Fig. 2). On turning prone, this positive pressure is markedly reduced, i.e., the compressive forces on the dependent region are diminished .
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