Effects of secondary polycythemia on cerebral blood flow in chronic obstructive pulmonary disease.

Cerebral blood flow was measured before and after venesection in 6 patients with chronic obstructive pulmonary disease and secondary polycythemia. The hematocrit was decreased from a mean of 59.6 to 43.9%, and whole blood viscosity was decreased by 35%. The supratentorial blood flow increased 78.2% (p < 0.005) after venesection; subtentorial flow also increased, but not significantly. The improvement in cerebral blood flow appeared to be related to a decrease in blood viscosity, because pulmonary function and arterial blood gases did not change significantly. There was subjective improvement in all patients, and this improvement might have been due to the increased supratentorial blood flow.