Mechanism of cerebral blood flow augmentation by hemodilution in rabbits

Abstract

Background and Purpose: Hemodilution is known to increase cerebral blood flow, but it is not known whether the increase in flow is a direct result of a decrease in viscosity or whether it may be due to compensatory vasodilatation in response to the decrease in oxygen carrying capacity that results from hemodilution. This study is designed to investigate this question. Methods: Changes in regional cerebral blood flow were studied in normal and ischemic brains of 15 and 18 rabbits, respectively. In one group of rabbits graded hemodilution was used to reduce arterial oxygen content progressively in stages; in the second group the arterial oxygen content was reduced in similar stages by progressively larger reductions in the concentration of inspired oxygen (hypoxic hypoxia). In the ischemic animals focal ischemia was produced by embolie occlusion of the right middle cerebral artery. Results: In the normal rabbits, hypoxic hypoxia and hemodilution resulted in similar progressive increases in cerebral blood flow as arterial oxygen content fell. In the ischemie animals, there was a significant fall in cerebral blood flow in the ischemie region in all groups after arterial occlusion. Hemodilution resulted in a progressive increase in cerebral blood flow in both ischemie and nonischemic regions. With hypoxic hypoxia, however, cerebral blood flow in the ischemie region showed no increase or a slight decrease. Conclusions: Even though hypoxic hypoxia results in a marked increase in cerebral blood flow in normal brain, it does not significantly change cerebral blood flow in ischemie brain. In contrast, hemodilution resulting in a comparable degree of hypoxemia is capable of significantly increasing cerebral blood flow in ischemie brain. Therefore, the mechanism of blood flow augmentation by hemodilution in ischemie brain is probably related to a direct hemorheologic effect rather than to the resulting hypoxemia. © 1992 American Heart Association, Inc.