Hemodilution accelerates the passage of plasma (not red cells) through cerebral microvessels in rats

Background and Purpose: Hemodilution lowers the total circulatory red cell mass and blood viscosity and thereby may alter the time of passage of red cells and plasma through cerebral microvessels. This study was designed to clarify this question. Methods: Adult Wistar-Kyoto rats, aged approximately 32 weeks, were divided into hemodilution and control groups. Local cerebral blood flow and microvascular red cells and plasma volumes in 14 brain structures were measured with the use of [¹⁴C]iodoantipyrine. ⁵⁵Fe- labeled red cells, and [¹⁴C]inulin, respectively. Results: In the control group, the hematocrit in cerebral microvessels ranged from 0.29 to 0.45 with a mean of 0.36, which was 71% of the systemic hematocrit (0.51). The mean transit times of blood, red cells, and plasma through microvessels were 0.62 to 1.77 seconds (mean, 0.92 second), 0.44 to 1.15 seconds (mean, 0.65 second), and 0.78 to 2.5 seconds (mean, 1.25 seconds), respectively. In the hemodilution group, the mean hematocrit in microvessels was 0.28, which was 89% of the systemic hematocrit (0.32). Local cerebral blood flow was approximately 59% higher (P<.01) than that of the control animals. The rate of oxygen delivered to the brain was slightly increased (9%) after hemodilution. Blood volume in cerebral microvessels was similar to that of the control group. Mean transit time of blood was 0.62 second (68% of the control), transit time of red cells was 0.53 second (85% of the control), and transit time of plasma was 0.67 second (54% of the control). Conclusions: These findings indicate that isovolemic hemodilution accelerates the plasma (not red cell) flow velocity in cerebral microvessels.