The effects of mannitol on blood viscosity

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Abstract
To determine the effect of mannitol on blood viscosity, serial measurement were carried out on venous blood in patients undergoing craniotomies for intracranial aneurysms. Blood samples were drawn immediately prior to, and 30 min, 2 and 4 h after administration of mannitol. Complete blood counts, serum osmolarities and erythrocyte microsieving studies were performed on each sample. Whole-blood viscosity decreased at 30 min and 2 h, but not at 4 h after mannitol administration. This decrease appeared at high shear rates only, where erythrocyte deformability is critical in determining viscosity. This effect was independent of the hematocrit. Removal of mannitol from the suspension returned red cell deformability to preadministration values, indicating that the increased erythrocyte deformability required the presence of mannitol and the relative hyperosmolarity induced by this agent. The reduced erythrocyte rigidity and subsequent decreased whole-blood viscosity should enhance tissue perfusion in the microcirculation.