Comparison of cerebral blood flow after venesection of bronchitic secondary polycythaemic and primary polycythaemic patients.

Cerebral blood flow was measured before and after lowering of haematocrit in four patients with primary polycythaemia and in nine with polycythaemia secondary to chronic obstructive airways disease. Cerebral blood flow values in each group were abnormally low to a similar degree at the start of the study and the degree of rise in cerebral blood flow per unit fall in haematocrit after venesection also was similar in each. Oxygen delivery fell despite increased cerebral blood flow and symptomatic benefit was infrequent. In male secondary polycythaemic patients rise in cerebral blood flow was not associated with any improvement in hypothalamo-pituitary-testicular function which we had previously noted to be suppressed in such hypoxic subjects. Our findings suggest viscosity changes rather than alteration in blood oxygen carriage to be responsible for cerebral blood flow improvement. It is concluded that therapeutic venesection in such patients should be applied with caution.