Optimum degree of hemodilution for brain protection in a canine model of focal cerebral ischemia

The ability of hemodilution to lower blood viscosity and increase cerebral blood flow has been proven experimentally; however, the optimum hematocrit for maximum oxygen delivery to ischemic brain tissue is not known, and a study was designed to determine this. Fifty dogs were selected for inclusion in the study using criteria based on changes in somatosensory evoked potentials at the time of arterial occlusion, which were found in a previous study to predict the development of a moderate infarction of relatively constant size. Infarctions were induced by permanent occlusion of the left middle cerebral artery and the azygous anterior cerebral artery. The animals selected for inclusion were divided into five groups of 10 dogs each: 1) a control group; 2) a group with 25% hematocrit; 3) a group with 30% hematocrit; 4) a group with 35% hematocrit; and 5) a group with 40% hematocrit. Isovolemic hemodilution was accomplished 1 hour after occlusion of vessels using dextran infusion and blood withdrawal. The animals were sacrificed after 6 days and infarction volume was determined from fluorescein-stained sections. Statistical analysis was performed using Student's t-test and one-way analysis of variance. Mean infarction volume for each group, expressed as a percentage of total hemispheric volume +/- 1 standard error of the mean, was 28.3% +/- 2.8% for the control group, 33.6% +/- 3.4% for the 25% hematocrit group, 17.1% +/- 2.2% for the 30% hematocrit group, 29.2% +/- 4.3% for the 35% hematocrit group, and 29.9% +/- 2.1% for the 40% hematocrit group. The 30% hematocrit group showed the smallest average infarction size and this size differed significantly (p = 0.02) from the average infarction size in the control animals. These results show that, in this model of focal ischemia, a hematocrit of approximately 30% is optimum for protecting the brain.