ObjectiveTo examine the influence of positive end-expiratory pressure (PEEP) and posture on the distribution of pulmonary blood flow.DesignExperimental study.SettingUniversity animal laboratory.SubjectsSeven anesthetized and mechanically-ventilated lambs.InterventionsFour conditions were studied in random order: prone or supine position, with or without 5 cm H2O PEEP.Measurements and Main ResultsThe distribution of pulmonary blood flow was assessed using fluorescent-labeled microspheres (15 [micro sign]m) in small ([similar]1.7 cm3) lung regions. Pulmonary blood flow heterogeneity was evaluated using the coefficient of variation of blood flow of the lung regions.The number of regions analyzed were 1290 +/-154 (SD) per animal. PEEP increased pulmonary blood flow heterogeneity in the supine position (47.0 +/- 7.7% to 54.1 +/- 7.3%, p<.01, paired Student's t-test), but not in the prone position (40.4 +/- 8.1% to 39.6 +/- 11.5). Dorsal to ventral (gravitational) flow gradients were present only in the supine position, and increased with PEEP (-7.2%/cm vs. -10.4%/cm, p<.001).ConclusionsPEEP redistributes pulmonary perfusion to dependent lung regions in supine, but not in prone, anesthetized and mechanically-ventilated sheep. (Crit Care Med 1999; 27:37-45)