Oxygen delivery at high blood viscosity and decreased arterial oxygen content to brains of conscious rats

We addressed the question to which extent cerebral blood flow (CBF) is main tained when, in addition to a high blood viscosity (B-vis) arterial oxygen content (Ca-O2) is gradually decreased. Ca-O2 was decreased by hemodilution to hematocrits (Hct) of 30, 22, 19, and 15% in two groups. One group recei ved blood replacement (BR) only and served as the control. The second group received an additional high viscosity solution of polyvinylpyrrolidone (BR /PVP). B-vis was reduced in the BR group and was doubled in the BR/PVP. Des pite 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 in creased B-vis at decreased Ca-O2. At an Hct of 19%, local cerebral blood fl ow (LCBF) in some brain structures was lower in BR/PVP rats than in BR rats . At the lowest Hct of 15%, LCBF of 15 brain structures and mean CBF were r educed in BR/PVP. The resulting decrease in cerebral oxygen delivery in the BR/PVP group indicates a global loss of vascular compensation. We conclude d that vasodilating mechanisms compensated for Bvis increases thereby maint aining constant cerebral oxygen delivery. Compensatory mechanisms were exha usted at a Hct of 19% and lower as indicated by the reduction of CBF and ce rebral oxygen delivery.