Abstract
Cerebral blood flow (CBF) rises as hematocrit (Hct) falls. We previously attributed this rise in CBF to two independent factors of equal importance, decreased arterial O2 content and decreased blood viscosity. We hypothesized that decreased arterial O2 content would dilate cerebral arterioles and that the magnitude of the vasodilation would depend on the magnitude of the passive fall in vascular resistance attributable to decreased viscosity. The present study was designed to test the hypothesis that anemia is accompanied by cerebral vasodilation. Using a closed cranial window, we measured the diameters of 42 pial arterioles (35-305 μm) in 7 cats as serial isovolemic hemodilution lowered Hct by 44% from 31 ± 4 to 17 ± 3%. Hemodilution increased CBF (microsphere technique) but did not change mean arterial blood pressure or arterial blood gases. Anticipated vasodilation did not occur; instead, pial arterioles constricted as Hct fell. Maximum vasoconstriction was observed when Hct reached 65-70% of the initial value. Vasoconstriction lessened as Hct was lowered further, but arteriolar diameters at the lowest Hcts remained less than base-line levels. Constriction was greater in small (< 100 μm) than in large (≥ 100 μm) arterioles. The initial constriction of pial arterioles may represent myogenic vasoconstriction in response to flow-induced vasodilation of more proximal portions of the cerebrovascular bed and/or to washout of an endogenous vasodilator. Arteriolar relaxation with more profound hemodilution may reflect superimposed metabolic vasodilation.
Original language | English (US) |
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Pages (from-to) | 26/3 |
Journal | American Journal of Physiology - Heart and Circulatory Physiology |
Volume | 257 |
Issue number | 3 |
State | Published - 1989 |
ASJC Scopus subject areas
- Physiology
- Cardiology and Cardiovascular Medicine
- Physiology (medical)