Regional ventilation-perfusion distribution is more uniform in the prone position

The arterial blood PO2 is increased in the prone position in animals and humans because of an improvement in ventilation ((V) over dot A) and perfusion ((Q) over dot) matching. However, the mechanism of improved (V) over dot A/(Q) over dot is unknown. This experiment measured regional (V) over dot A/(Q) over dot heterogeneity and the correlation between (V) over dot A and (Q) over dot in supine and prone positions in pigs. Eight ketamine diazepam-anesthetized, mechanically ventilated pigs were studied in supine and prone positions in random order. Regional (V) over dot A and (Q) over dot were measured using fluorescent-labeled aerosols and radioactive-labeled microspheres, respectively. The lungs were dried at total lung capacity and cubed into 603-967 small (similar to 1.7-cm(3)) pieces. In the prone position the homogeneity of the ventilation distribution increased (P = 0.030) and the correlation between (V) over dot A and (Q) over dot increased (correlation coefficient = 0.72 +/- 0.08 and 0.82 +/- 0.06 in supine and prone positions, respectively, P = 0.03). The homogeneity of the (V) over dot A/(Q) over dot distribution increased in the prone position (P = 0.028). We conclude that the improvement in (V) over dot A/(Q) over dot matching in the prone position is secondary to increased homogeneity of the (V) over dot A distribution and increased correlation of regional (V) over dot A and (Q) over dot.