Hemodilution reduces blood viscosity and O₂ content (Ca_O2) and increases cerebral blood flow (CBF). Viscosity and Ca_O2 may contribute to increasing CBF after hemodilution. However, because hematocrit is the major contributor to blood viscosity and Ca_O2, it has been difficult to assess their relative importance. By varying blood viscosity without changing Ca_O2, prior investigation in hemodiluted animals has suggested that both factors play roughly equal roles. To further investigate the relationship of hemodilution, blood viscosity, Ca_O2, and CBF, we took the opposite approach in hemodiluted animals, i.e., we varied Ca_O2 without changing blood viscosity. Hyperbaric O₂ was used to restore Ca_O2 to normal after hemodilution. Pentobarbital sodium-anesthetized rats underwent isovolumic hemodilution with 6% hetastarch, and forebrain CBF was measured with [³H]nicotine. One group of animals did not undergo hemodilution and served as controls (Con). In the three experimental groups, hematocrit was reduced from 44% to 17-19%. Con and hemodiluted (H_Dil) groups were ventilated with 40% O₂ at 101 kPa (1 atmosphere absolute), which resulted in Ca_O2 values of 19.7 ± 1.3 and 8.1 ± 0.7 (SD) ml O₂/dl, respectively. A second group of hemodiluted animals (H_Bar) was ventilated with 100% O₂ at 506 kPa (5 atmospheres absolute) in a hyperbaric chamber, which restored Ca_O2 to an estimated 18.5 ± 0.5 ml O₂/dl by increasing dissolved O₂. A fourth group of hemodiluted animals (H_Con) served as hyperbaric controls and were ventilated with 10% O₂ at 506 kPa, resulting in Ca_O2 of 9.1 ± 0.6 ml O₂/dl. CBF was 79 ± 19 ml · 100 g¹ · min¹ in the Con group and significantly increased to 123 ± 9 ml · 100 g¹ · min¹ in the H_Dil group. When Ca_O2 was restored to baseline with dissolved O₂ in the H_Bar group, CBF decreased to 104 ± 20 ml · 100 g¹ · min¹. When normoxia was maintained during hyperbaric exposure in the H_Con group, CBF was 125 ± 18 ml · 100 g¹ · min¹, a value indistinguishable from that in normobaric H_Dil animals. Our data demonstrate that the reduction in Ca_O2 after hemodilution is responsible for 40-60% of the increase in CBF.

oxygen content; viscosity

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