Model analysis of the enhancement of tissue oxygenation by hemodilution due to increased microvascular flow velocity.

The effect of hemodilution on oxygen delivery to the tissue was investigated analytically by taking into consideration the oxygen loss that occurs along the arterial microvasculature due to diffusion into the tissue and shunting to parallel running venules. The theoretical findings were related to experimental data on microvascular oxygen distribution and the blood flow weighted by the oxygen-carrying capacity of plasma and red blood cells (Q) during hemodilution. It was found that at 30-33% hematocrit, the amount of oxygen brought to the tissues is increased by 5% when diffusion is the only mechanism of oxygen loss, and by 15% when the loss is due to arteriole-venule (A-V) shunting. These increases are relative to the conditions of normal hematocrit, and are in addition to the 10% increase due to the enhancement of Q caused by hemodilution. The analysis was extended to conditions of low oxygen tension and flow condition, characteristic of ischemia. In the case of severe ischemia the total increase in oxygen delivery at hematocrit 30-33% was 38% for tissues with diffusion losses only, and 66% when shunting losses are predominant. These results suggest that hemodilution is particularly effective in increasing oxygenation in ischemic tissue, while it has a comparatively small effect in normal conditions.