Effect of haematocrit on cerebral blood-flow in man.

Cerebral blood-flow (C.B.F.) was measured in 38 patients with haematocrit values (Hct) in the range 0-47--0-53 and was found to be significantly lower than in 43 patients with Hct in a lower range (0-36--0-46). After reduction of Hct in the higher Hct group by venesection, flow increased by a mean of 50%. This improvement in flow was largely due to a reduction in viscosity. Hct around the generally accepted upper limit of normal may be an important factor in the causation of occlusive vascular disease.