The effects of isovolumic hemodilution on ocular blood flow.

Techniques by which retinal blood flow may be increased safely are potentially important in the treatment of retinal vascular disease. It was hypothesized that hemodilution, which increases cerebral blood flow, would also increase retinal blood flow. To investigate the physiological effects of hemodilution in the eye, ocular blood flow was measured in 14 cats using the radioactively labeled microsphere method. After the animals were anesthetized with halothane and oxygen, intraocular and systemic arterial pressure were recorded; blood flows were measured before and after isovolumic hemodilution to a hematocrit of 20-22% using 6% hydroxyethyl starch (a synthetic plasma expander with a molecular weight of 450 in 0.9% saline). In hemodiluted cats, retinal blood flow increased 71% from its baseline value (36.7 +/- 6.4 ml 100 g-1 min-1 to 62.9 +/- 6.4 ml 100 g-1 min-1, mean +/- S.E.M., P < 0.0001). Calculated retinal O2 delivery remained approximately constant, as the increased blood flow countered a significant decrease in arterial O2 content. Choroidal blood flow decreased (1297 +/- 140 ml 100 g-1 min-1 to 1051 +/- 144 ml 100 g-1 min-1) but the change was not statistically significant. Blood flows in the iris and sclera were not significantly altered. Hemodilution increased retinal blood flow without causing a redistribution in ocular blood flow.