Under physiological conditions, moderate changes of either blood viscosity or oxygen content are not followed by a change in oxygen delivery indicating a vascular compensation ( 10, 17, 22).
The blood components relevant for the oxygen delivery to the brain are viscosity and oxygen content. Compensatory mechanisms were exhausted at a Hct of 19% and lower as indicated by the reduction of CBF and cerebral oxygen delivery.Īdequate oxygen delivery to the brain is important for its function because of its low capacity for anaerobic metabolism. We concluded that vasodilating mechanisms compensated for B vis increases thereby maintaining constant cerebral oxygen delivery.
The resulting decrease in cerebral oxygen delivery in the BR/PVP group indicates a global loss of vascular compensation. At the lowest Hct of 15%, LCBF of 15 brain structures and mean CBF were reduced in BR/PVP. At an Hct of 19%, local cerebral blood flow (LCBF) in some brain structures was lower in BR/PVP rats than in BR rats. Despite different B vis, CBF did not differ between BR and BR/PVP rats at Hct values of 30 and 22%, indicating a complete vascular compensation of the increased B vis at decreased Ca O 2. B vis was reduced in the BR group and was doubled in the BR/PVP. The second group received an additional high viscosity solution of polyvinylpyrrolidone (BR/PVP). One group received blood replacement (BR) only and served as the control. Ca O 2 was decreased by hemodilution to hematocrits (Hct) of 30, 22, 19, and 15% in two groups. We addressed the question to which extent cerebral blood flow (CBF) is maintained when, in addition to a high blood viscosity (B vis) arterial oxygen content (Ca O 2) is gradually decreased.
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