Transport of oxygen to the brain in patients with elevated haematocrit values before and after venesection.

Cerebral blood flow and oxygen transport to the brain have been studied in 20 patients with primary polycythaemia, before and after venesection. Cerebral blood flow rose by an average of 30 per cent (P less than 0.001) when the mean haematocrit was reduced from 0.537 to 0.444. Whole blood viscosity fell predictably and the correlation coefficient between haematocrit and blood viscosity at a shear rate of 91 s-1 was 0.926 (P less than 0.001). Mean arterial blood pressure, arterial PCO2 and pH were not influenced by venesection. A small but significant overall rise in arterial PO2 was observed but the arterial oxygen content fell with the reduction in haemoglobin concentration. The product of cerebral blood flow and arterial oxygen content rose by an average of 8 per cent (P less than 0.001) and this was considered physiologically significant because the position of the oxygen dissociation curve did not change. The increase in oxygen transport to the brain may serve to protect against ischaemic insult and other mechanisms whereby elevated haemotocrit might predispose to cerebral infarction are briefly discussed.