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The arterial blood is increased in the prone position in animals and humans because of an improvement in ventilation (V˙a) and perfusion (Q˙) matching. However, the mechanism of improvedV˙a/Q˙ is unknown. This experiment measured regionalV˙a/Q˙ heterogeneity and the correlation between V˙a andQ˙ in supine and prone positions in pigs. Eight ketamine-diazepam-anesthetized, mechanically ventilated pigs were studied in supine and prone positions in random order. RegionalV˙a and Q˙ 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 betweenV˙a and Q˙ increased (correlation coefficient = 0.72 ± 0.08 and 0.82 ± 0.06 in supine and prone positions …