Recent studies using microspheres (both fluorescent and radiolabelled) in dogs, pigs and goats have demonstrated considerable heterogeneity of pulmonary perfusion within isogravitational planes. To test whether a gravitational gradient would be more apparent in an animal with large vertical lung height, we measured perfusion heterogeneity in horses (vertical lung ht=~55 cm) at rest and at three levels of exercise (30%, 60% and 90% of VO2max). Four thoroughbred horses (Wt=422-500kg) were studied awake in the standing position, with fluorescent microspheres. Between 2303 and 2656 (1.13 cm3 in volume) pieces were obtained from each horse with spatial coordinates and cardiac output (Q) determined for each piece. The coefficient of variation (CV) of Q varied between 23% and 60% among the horses. Considerable heterogeneity is seen in each isogravitational plane, comparable in degree to that previously noted in smaller animals. CV of Q did not change with increasing exercise levels. Greater than 70% of pulmonary blood flow variation across rest to high exercise states is determined by a fixed, spatial pattern. 30% of the variation resulted from exercise and experiemental variation. The majority of flow redistribution was due to flow increasing to the dorsal regions of the lung. We conclude that in resting unanesthetized horses, there is no vertical dependence of Q and that there is considerable heterogeneity of Q within isogravitational planes. With exercise, blood flow increases primarily in the dorsal aspects of the lung.
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