Effect of venesection on calf blood flow in polycythaemia.

Calf blood flow at rest and during postocclusive reactive hyperaemia was measured using an electrocardiogram-triggered plethysmograph in 14 patients with polycythaemia (nine with primary disease and five with polycythaemia secondary to cyanotic heart disease) before and after a course of venesection. The mean packed cell volume was reduced from 0.57 to 0.47, and whole-blood viscosity fell by 50% at low shear rates. Venesection did not affect rest flow, but peak flow was increased by 18%. The increase in peak flow failed to compensate for the reduced haemoglobin content of the blood, calculated haemoglobin delivery being reduced by 23% at rest and 10% during reactive hyperaemia. These results indicate that while venesection improves blood viscosity, this does not necessarily lead to improved delivery of oxygen to the tissues.