HEMODILUTION ACCELERATES THE PASSAGE OF PLASMA (NOT RED-CELLS) THROUGH CEREBRAL MICROVESSELS IN RATS

Background and Purpose Hemodilution lowers the total circulatory red c ell 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, ag ed approximately 32 weeks, were divided into hemodilution and control groups. Local cerebral blood flow and microvascular red cell and plasm a volumes in 14 brain structures were measured with the use of [C-14]i odoantipyrine, Fe-55-labeled red cells, and [C-14]inulin, respectively . Results In the control group, the hematocrit in cerebral microvessel s ranged from 0.29 to 0.45 with a mean of 0.36, which was 71% of the s ystemic 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 se conds (mean, 1.25 seconds), respectively. In the hemodilution group, t he mean hematocrit in microvessels was 0.28, which was 89% of the syst emic hematocrit (0.32). Local cerebral blood flow was approximately 59 % higher (P < .01) than that of the control animals. The rate of oxyge n delivered to the brain was slightly increased (9%) after hemodilutio n. Blood volume in cerebral microvessels was similar to that of the co ntrol group. Mean transit time of blood was 0.62 second (68% of the co ntrol), 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). Con clusions These findings indicate that isovolemic hemodilution accelera tes the plasma (not red cell) flow velocity in cerebral microvessels.