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

Abstract

The arterial blood PO2 is increased in the prone position in animals and humans because of an improvement in ventilation (V̇A) and perfusion (Q̇) matching. However, the mechanism of improved V̇A/Q̇ is unknown. This experiment measured regional V̇A/Q̇ heterogeneity and the correlation between V̇A and Q̇ 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 V̇A and Q̇ 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 (~1.7-cm3) pieces. In the prone position the homogeneity of the ventilation distribution increased (P = 0.030) and the correlation between VA and 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 V̇A/Q̇ distribution increased in the prone position (P = 0.028). We conclude that the improvement in V̇A/Q̇ matching in the prone position is secondary to increased homogeneity of the V̇A distribution and increased correlation of regional V̇A and Q̇.