Changes in cerebral blood flow relating to haematocrit and viscosity.

Evidence suggests that haematocrit and viscosity are important factors in the control of cerebral blood flow (CBF). Conditions in which the haematocrit is raised, such as polycythaemia rubra vera and relative polycythaemia are associated with a low CBF. Following reduction in haematocrit, there is a significant rise in CBF. It is likely that two factors are responsible for this. Firstly there is a fall in oxygen carrying capacity with venesection. Secondly there is a marked fall in whole blood viscosity. Both these changes tend to result in a rise in CBF. Further studies in patients with paraproteinaemias suggest that oxygen carriage and blood viscosity are independent variables in the control of CBF.