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

Objective: To examine the influence of positive end-expiratory pressure (PE EP) 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 supin e 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 (simila r to 1.7 cm(3)) lung regions. Pulmonary blood flow heterogeneity was evalua ted using the coefficient of variation of blood flow of the lung regions. The number of regions analyzed were 1290 +/- 154 (SD) per animal. PEEP incr eased 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 (gravitationa l) flow gradients were present only in the supine position, and increased w ith PEEP (-7.2%/cm vs. -10.4%/cm, p<.001). Conclusions: PEEP redistributes pulmonary perfusion to dependent lung regio ns in supine, but not in prone, anesthetized and mechanically-ventilated sh eep.