Positive end-expiratory pressure redistributes perfusion to dependent lung regions in supine but not in prone lambs

Objective: To examine the influence of positive end-expiratory pressure (PEEP) and posture on the distribution of pulmonary blood flow. Design: Experimental study, Setting: University animal laboratory. Subjects: Seven anesthetized and mechanically ventilated lambs. Interventions: Four conditions were studied in random order: prone or supine position, with or without 5 cm H2O PEEP. Measurements and Main Results: The distribution of pulmonary blood flow was assessed using fluorescent labeled microspheres (15 mu m) in small (similar to 1.7 cm(3)) 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). Conclusions: PEEP redistributes pulmonary perfusion to dependent lung regions in supine, but not in prone, anesthetized and mechanically-ventilated sheep.