Hemodilution, cerebral O-2 delivery, and cerebral blood flow: a study using hyperbaric oxygenation

Hemodilution reduces blood viscosity and O-2 content (Ca-O2) and increases cerebral blood flow (CBF). Viscosity and Ca-O2, may contribute to increasin g CBF after hemodilution. However, because hematocrit is the major contribu tor to blood viscosity and Ca-O2, it has been difficult to assess their rel ative importance. By varying blood viscosity without changing Ca-O2, prior investigation in hemodiluted animals has suggested that both factors play r oughly equal roles. To further investigate the relationship of hemodilution , blood viscosity, Ca-O2, and CBF, we took the opposite approach in hemodil uted animals, i.e., we varied Ca-O2 without changing blood viscosity. Hyper baric O-2 was used to restore Ca-O2 to normal after hemodilution. Pentobarb ital sodium-anesthetized rats underwent isovolumic hemodilution with 6% het astarch, and forebrain CBF was measured with [H-3]nicotine. One group of an imals did not undergo hemodilution and served as controls (Con). In the thr ee experimental groups, hematocrit was reduced from 44% to 17-19%. Con and hemodiluted (H-Dil) groups were ventilated with 40% O-2 at 101 kPa(1 atmosp here absolute), which resulted in Ca-O2 values of 19.7 +/- 1.3 and 8.1 +/- 0.7 (SD) mi O-2/dl, respectively. A second group of hemodiluted animals (H- Bar) was ventilated with 100% O-2 at 506 kPa (5 atmospheres absolute) in a hyperbaric chamber, which restored Ca-O2 to an estimated 18.5 +/- 0.5 mi O- 2/dl by increasing dissolved O-2. A fourth group of hemodiluted animals (H- Con) served as hyperbaric controls and were ventilated with 10% O-2 at 506 kPa, resulting in Ca-O2 of 9.1 +/- 0.6 ml O-2/dl. CBF was 79 +/- 19 ml . 10 0 g(-1) . min(-1) in the Con group and significantly increased to 123 +/- 9 ml . 100 g(-1) . min(-1) in the H-Dil group. When Ca-O2 was restored to ba seline with dissolved O-2 in the H-Bar group, CBF decreased to 104 +/- 20 m l . 100 g(-1) . min(-1). When normoxia was maintained during hyperbaric exp osure in the H-Con group, CBF was 125 +/- 18 ml . 100 g(-1) . min(-1), a va lue indistinguishable from that in normobaric H-Dil animals. Our data demon strate that the reduction in Ca-O2 after hemodilution is responsible for 40 -60% of the increase in CBF.