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 embolic 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 ischemic animals, there was a significant fall in cerebral blood flow in the ischemic region in all groups after arterial occlusion. Hemodilution resulted in a progressive increase in cerebral blood flow in both ischemic and nonischemic regions. With hypoxic hypoxia, however, cerebral blood flow in the ischemic 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 ischemic brain. In contrast, hemodilution resulting in a comparable degree of hypoxemia is capable of significantly increasing cerebral blood flow in ischemic brain. Therefore, the mechanism of blood flow augmentation by hemodilution in ischemic brain is probably related to a direct hemorheologic effect rather than to the resulting hypoxemia. (Stroke 1992;23:1487-1493 KEY WORDS • cerebral blood flow • cerebral ischemia • hemodilution • rabbits

Dynamic Cerebral Autoregulation in Homozygous Sickle Cell Disease

“…An inverse relationship between CBF and hematocrit level has been reported under physiological conditions both in animal models, [32][33][34] and in humans. 35,36 In the present study, no relationship was found between either CBFV or dynamic cerebral autoregulatory capacity and Hb, rendering a role for anemia as an independent factor in the impairment of dynamic cerebral autoregulation unlikely.…”

Focal Cerebral Ischemia in Rats: Effect of Hemodilution with α-α Cross-Linked Hemoglobin on Brain Injury and Edema

“…5 Other investigators have reported findings that support viscosity as the predominant mechanism of hemodilution-induced increases in CBF during ischemia. 37 " 40 A potential problem of our methodology was the hemodynamic effects of DCLHb. Although the mechanism is not clear, hemoglobin substitutes are known to increase blood pressure.…”

AltitudeOmics: The Basic Biology of Human Acclimatization to High Altitude. Addendum

“…Below this threshold, the degree of vasodilatation increases exponentially and outweighs the degree of hypocapnic vasoconstriction A. W. Subudhi and others (Mardimae et al 2012;Willie et al 2012); presumably, to provide greater blood flow in a time of need. While the correlation between changes in gCBF and C aO 2 was not significant, the change in C aO 2 from SL to ALT1 was similar among all subjects and may not have afforded an appropriate range of values to detect the relationship that has previously been shown with progressive haemodilution (Korosue & Heros, 1992). Qualitatively, the ß70% increase in gCBF was within the expected range during acute hypocapnic hypoxia (Severinghaus, , 2001Jensen et al 1990;Brugniaux et al 2007) and proportional to the ß60% reduction in P aO 2 that was responsible for the reduction in C aO 2 .…”