Blood Volume, Hematocrit and Pressure Relationships in the Isolated Perfused Dog Brain

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Abstract
The intracranial blood volume and hematocrit of the isolated perfused dog brain preparation were determined with Cr 51 -labeIed erythrocytes. The intracranial blood volume was determined to be 8.6 ml/100 gm brain (SE±1.2 ml). Over the range of perfusing hematocrits studied, the relationship of perfusing hematocrit (Hct PR ) to intracranial hematocrit (Hct IC ) was found to be a linear function (Hct IC = 5.44 + 0.702 Hct PR ). At a peripheral hematocrit of 20% the Hct IC /Hct PR ≈ 1.0, whereas at hematocrits of 50 to 55% this ratio dropped to 0.87. With increasing perfusing hematocrits the carotid artery pressure rose markedly and there was flattening of the EEG. Since the isolated dog brain is a closed system void of extracranial anastomoses, high viscosity blood cannot circumvent the cerebral circulation but must follow the high resistance networks of the deep cerebral vasculature until sludging of the erythrocytes becomes sufficient to cause anoxia and loss of autoregulation. In the intact erythrocythemic animal, however, increases in arteriolar pressure can be reflected downstream to the large cerebral and meningeal vessels with extracranial shunting of high hematocrit (high viscosity) blood from the brain.