Hemodilution, cerebral O2delivery, and cerebral blood flow: a study using hyperbaric oxygenation

Abstract

Hemodilution reduces blood viscosity and O2content ([Formula: see text]) and increases cerebral blood flow (CBF). Viscosity and [Formula: see text] may contribute to increasing CBF after hemodilution. However, because hematocrit is the major contributor to blood viscosity and[Formula: see text], it has been difficult to assess their relative importance. By varying blood viscosity without changing[Formula: see text], prior investigation in hemodiluted animals has suggested that both factors play roughly equal roles. To further investigate the relationship of hemodilution, blood viscosity,[Formula: see text], and CBF, we took the opposite approach in hemodiluted animals, i.e., we varied[Formula: see text] without changing blood viscosity. Hyperbaric O2 was used to restore[Formula: see text] 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 (HDil) groups were ventilated with 40% O2 at 101 kPa (1 atmosphere absolute), which resulted in[Formula: see text] values of 19.7 ± 1.3 and 8.1 ± 0.7 (SD) ml O2/dl, respectively. A second group of hemodiluted animals (HBar) was ventilated with 100% O2 at 506 kPa (5 atmospheres absolute) in a hyperbaric chamber, which restored[Formula: see text] to an estimated 18.5 ± 0.5 ml O2/dl by increasing dissolved O2. A fourth group of hemodiluted animals (HCon) served as hyperbaric controls and were ventilated with 10% O2 at 506 kPa, resulting in[Formula: see text] of 9.1 ± 0.6 ml O2/dl. CBF was 79 ± 19 ml ⋅ 100 g−1 ⋅ min−1in the Con group and significantly increased to 123 ± 9 ml ⋅ 100 g−1 ⋅ min−1in the HDil group. When[Formula: see text] was restored to baseline with dissolved O2 in the HBar group, CBF decreased to 104 ± 20 ml ⋅ 100 g−1 ⋅ min−1. When normoxia was maintained during hyperbaric exposure in the HCon group, CBF was 125 ± 18 ml ⋅ 100 g−1 ⋅ min−1, a value indistinguishable from that in normobaric HDil animals. Our data demonstrate that the reduction in [Formula: see text] after hemodilution is responsible for 40–60% of the increase in CBF.