Pulmonary gas exchange improves in the prone position with abdominal distension

Arterial blood oxygenation in patients with adult respiratory distress syndrome is often improved in the prone position. Critically ill patients often have abdominal distension and whether similar improvements in gas exchange occur with the prone position is not known. We therefore studied the effect of posture on gas exchange in eight ketamine-anesthetized pigs with abdominal distension. A rubber balloon, placed in the abdominal cavity, was filled with water to increase intra-abdominal pressure. The animals were mechanically ventilated with FIO2 = 0.4, and PaCO2 was kept constant. Gas exchange was measured in the supine and prone positions, with and without abdominal distension, in random order, using the multiple inert gas elimination technique (MIGET). When the abdomen was normal, the prone position increased PaO2 by 16 +/- 21 mm Hg (p < 0.05), accompanied by a small, but statistically insignificant, decrease in AaPO2 (p = 0.08) and no change in ventilation/perfusion (V A/Q) heterogeneity measured by MIGET. In the presence of abdominal distension, the prone position increased Pa O2 by 26 +/- 18 mm Hg (p < 0.01) and decreased AaPO2 (p < 0.05) and V A/Q heterogeneity as measured by the log standard deviation of the perfusion distribution (p < 0.01) and the arterial-alveolar difference area (p < 0.05). In addition, intragastric pressure was lower in the prone position (p < 0.01). We conclude that in anesthetized, mechanically ventilated pigs, the prone position improves pulmonary gas exchange to a greater degree in the presence of abdominal distension than when the abdomen is normal.