Influence of positioning on ventilation-perfusion relationships in severe adult respiratory distress syndrome.

In 12 patients with severe adult respiratory distress syndrome (ARDS), pulmonary gas exchange and hemodynamics were evaluated before, during, and after a 2-h period of pressure-controlled mechanical ventilation with the patient in the prone position. Ventilation-perfusion relationships (VA/Q) were assessed by a multiple inert gas elimination technique. Pressure-controlled mechanical ventilation in the prone position resulted in an overall increase (p < or = 0.05) of arterial oxygenation after 120 min (98.4 +/- 50.3 to 146.2 +/- 94.9 mm Hg). Whereas eight patients revealed an improvement of PaO2 of more than 10 mm Hg after 30 min in the prone position (responders), four patients reacted to positional changes with a deterioration of arterial oxygenation (nonresponders). Data about the continuous distribution of ventilation-perfusion ratios revealed that in the responder group positioning caused a decrease of shunt perfusion of 11 +/- 5% and a concomitant increase of normal VA/Q by 12 +/- 4% after 30 min. There was no change demonstrable within low VA/Q areas. Returning the patient to the supine position reversed the improvement in gas exchange. The nonresponder group did not show any significant alteration in the distribution of VA/Q during the study. We concluded that improvement of oxygenation during pressure-controlled mechanical ventilation in the prone position is due to a shift of blood flow away from shunt regions, thus increasing areas with normal VA/Q. This redistribution of blood flow is most likely caused by a recruitment of previously ateletatic but nondiseased areas induced by altered gravitational forces.