Prone position improves gas exchange--but how?

* Mechanically ventilated patients with severe acute lung insufficiencies dramatically improve their gas exchange when treated in the prone position. * ventilation heterogeneity is greater in the supine then in the prone position during CMV. * the dominant dorsal Q while supine is not turned into a dominant ventral Q in the prone position. * in the presence of an abdominal distension, the prone position more clearly improves gas exchange than at normal abdominal pressures. * CPAP enhances the dominant dorsal lung perfusion while supine. In the prone position lung perfusion is more uniform. * V/Q matching is improved in the prone position during CMV.