Cerebral blood-flow and viscosity in relative polycythaemia.

Cerebral blood flow (CBF), red-cell mass, and plasma volume were measured in 39 patients with venous haematocrit values in the range 0.47--0.58. Patients with true polycythaemia were thus excluded. The 39 patients studied fell into two groups--those with a measured red-cell mass in the high-normal range and those with normal red-cell-mass values but reduced plasma-volume values (the relative-polycythaemia, low-plasma-volume group). The mean CBF was low in both the high-normal red-cell-mass (HNRCM) group (45.8 ml/100 g/min) and low-plasma-volume (LPV) group (48.8 ml/100 g/min). The haematocrit in the HNRCM group was 0.504 and in the LPV group 0.513. In a control group of subjects with a mean haematocrit of 0.421 CBF was 68.6 ml/100 g/min. Venesection was associated with a significant rise in CBF in both groups of patients--to 59.7 ml/100 g/min in the HNRCM group and 65.0 ml/100 g/min in the LPV group. Whole-blood viscosity fell significantly in both groups. Both groups demonstrated significant inverse relationships between CBF and haematocrit and between CBF and blood viscosity.