Red cell mass, plasma volume and blood volume before and after venesection in relative polycythaemia.

Seven male patients with relative polycythaemia have had red cell mass, plasma volume and blood volume measurements made before and after venesection. The mean PCV prior to venesection was 0.522. Using a small volume (250 ml) serial venesection technique, in each the PCV was reduced to or below 0.45, the overall mean value being 0.431. Venesection resulted in a rise in plasma volume with no significant reduction in blood volume in six of the patients. Thus hypovolaemia does not occur in the majority of the patients. One patient did not increase his plasma volume on packed cell volume reduction. His blood volume fell by 15%. None of the patients developed any symptoms of hypovolaemia. To maintain the PCV at values less than 0.45, venesection was required on average once every 2 months. It is suggested that venesection of small volumes is an acceptable long-term method of lowering the PCV in patients with relative polycythaemia.