Effects of haematocrit reduction on the haemoglobin-oxygen saturation curve in polycythaemia secondary to hypoxic lung disease.

20 patients were studied with chronic airflow obstruction and arterial hypoxaemia, 10 with and the other 10 without secondary polycythaemia. There were no differences in the value of the P50 for the two patient groups (Polycythaemics 3.31 +/- 0.17 kPa; non-polycythaemics 3.47 +/- 0.25 kPa). Isovolaemic haemodilution was performed by erythrapheresis on a cell separator in the 10 polycythaemic patients. Following removal of a mean 558 ml of packed red cells, there was a fall in the mean PCV from 0.57 +/- 0.02 to 0.46 +/- 0.04. There was no change in the value of the P50 when estimated 24 hrs after treatment. Reduction of an elevated haematocrit in hypoxic polycythaemic patients had no effect on the haemoglobin-oxygen saturation curve.