Plasma Volume and Splenomegaly in Polycythaemia Vera

Summary Blood volume studies in 23 patients with polycythaemia vera showed the mean plasma volume to be normal when compared with control subjects, and variation in plasma volume for the group as a whole was no greater than in normal subjects. However, the polycythaemic subjects proved to be heterogeneous with respect to plasma volume: in one subgroup, showing splenomegaly with demonstrable intrasplenic red cell pooling, there was a moderate increase in mean plasma volume. In addition, the effect of intrasplenic red cell pooling on the whole body haematocrit/venous haematocrit ratio was found to differ in this condition from the effect in non‐polycythaemic subjects: the splenic pool failed to concentrate red cells significantly above the levels present in the extrasplenic circulation, and did not result in a rise in the Hb/Hv ratio.