Reactivity of the cerebrovascular bed to carbon dioxide in patients with primary high haematocrit before and after venesection

Paired cerebral blood flow measurements at 2 levels of arterial carbon dioxide tension (PaCO2) were made in 10 subjects with an elevated haematocrit and blood viscosity. A value of 32 was obtained for the percentage change in flow per kPa change in PaCO2 (CO2 reactivity). The haematocrit was then lowered by venesection from 0.536 to 0.458 and the measurements were repeated. The post-venesection CO2 reactivity was 29, which was not significantly different from the pre-venesection value. Thus CO2 reactivity is not comprised in patients with low cerebral blood flow secondary to an elevated haematocrit.