Influence of blood viscosity on blood flow in the forebrain but not hindbrain after carotid occlusion in rats

That cerebral blood flow remains unchanged at an increased blood viscosity, as long as the vascular supply is not compromised, was tested. To induce a reduced blood supply of some parts of the brain and to keep the supply unc hanged in others both carotid arteries were occluded in anesthetized, venti lated rats. By this procedure, blood supply to the rostral brain, but not t o the brainstem and cerebellum, was compromised. Blood viscosity was increa sed by intravenous infusion of 20% polyvinylpyrrolidone thigh viscosity gro up) or decreased by infusion of 5% albumin (low viscosity group). Cerebral blood flow was measured by the [C-14]iodoantipyrine method in 50 complete c oronal sections of the rostral brain and 22 complete coronal sections of th e brainstem and cerebellum in each rat. In the high viscosity group, mean c erebral blood flow of the rostral brain was significantly lower (46 +/- 7 m L/100 g(-1) . min(-1)) than in the low viscosity group (82 +/- 18 mL/100 g( -1) . min(-1)). No differences could be observed in brainstem and cerebellu m between both groups (162 +/- 29 mL/100 g(-1) . min(-1) vs. 156 +/- 18 mL/ 100 g(-1) . min(-1)). Local analysis of cerebral blood flow in different br ain structures of the coronal sections showed the same identical results; i .e., in 29 of the 31 brain structures analyzed in rostral brain, local cere bral blood flow was lower in the high viscosity group, whereas no differenc es could be observed in the 11 brain structures analyzed in the brainstem a nd cerebellum. It is concluded that under normal conditions cerebral blood flow can be maintained at an increased blood viscosity by a compensatory va sodilation. When the capacity for vasodilation is exhausted by occlusion of supplying arteries, an increased blood viscosity results in a decrease of cerebral blood flow.