REGIONAL DISTRIBUTION OF BLOOD-FLOW IN THE BRAIN OF HORSES AT REST AND DURING EXERCISE

Objective-To examine regional distribution of blood flow in the brain of horses at rest and during exercise. Animals-9 clinically normal hor ses. Procedure-Regional brain blood flow was measured using radionucli de-labeled 15-mu m-diameter microspheres injected into the left ventri cle, while reference blood samples were obtained from the aorta. Resul ts-At rest, cerebral cortex and caudate nuclei received significantly higher blood flow, compared with cerebral white matter. A similar perf usion heterogeneity existed in the cerebellum. In the brain stem, a gr adual tapering of blood flow from thalamus-hypothalamus towards medull a was observed in standing horses. Progressive significant increases i n heart rate and in aortic and right atrial pressures occurred during exercise at 8 and 13 m/s, and horses developed significant arterial hy poxemia and hypercapnia. Cerebral and cerebellar gray- to white-matter perfusion heterogeneity was maintained during exercise, indicating di fferential metabolic O-2 needs. Despite arterial hypoxemia, hypercapni a, and hypertension, exercise did not result in significant changes in blood flow to the cerebral cortex and caudate nuclei whereas, in cere bral white matter, a significant decrease in blood flow was observed. In all cerebral tissues,vascular resistance increased during exercise, indicating autoregulation of cerebral blood flow. In the cerebellar c ortex, blood flow increased significantly with strenuous exercise as v asodilation occurred. Vascular resistance in cerebellar white matter i ncreased during exercise at 13 m/s. Blood flow in the medulla, pens, m idbrain, and thalamus-hypothalamus was not significantly altered durin g exercise from that at rest. Conclusion-Despite arterial hypoxemia, h ypercapnia, and hypertension, autoregulation of cerebral and cerebella r blood flow is maintained in horses during exercise.