Management of relative polycythaemia: studies of cerebral blood flow and viscosity.

Cerebral blood flow has been shown to be reduced in a group of patients with high packed cell volume due to a reduced plasma volume (relative polycythaemia). 500 ml of intravenous Dextran 40 resulted in a significant fall in PCV and whole blood viscosity with a concomitant rise in cerebral blood flow (CBF). This effect, however, lasted less than 24 h. Oral fludrocortisone therapy (0.2 mg daily) resulted in no definite change in either PCV, viscosity or CBF. Venesection brought about a significant fall in PCV and whole blood viscosity with a significant rise in CBF. Of the methods used, venesection was the only practical long-term method of lowering PCV in this group of patients.