Rheology of the absolute polycythaemias

The absolute polycythaemias include all patients who have a raised red cell mass. They may be divided by clinical and laboratory investigation into: primary proliferative polycythaemia (polycythaemia rubra vera), secondary polycythaemias, and idiopathic erythrocytosis. In vitro, the PCV is the single most important determinant of whole blood viscosity with the most marked effects at low shear rates. Yield stress is also dominantly influenced by PCV. Thus in the absolute polycythaemias, from whatever cause, whole blood viscosity and yield stress are high. However, flow conditions in vivo are much more complex than those usually used in in-vitro measurements. Under normal physiological conditions, whole blood viscosity probably does not determine flow in vivo. The observed in-vitro changes are only of relevance in vivo when local or general abnormalities of flow occur. In primary proliferative polycythaemia (PPP) and idiopathic erythrocytosis, presentation with ischaemia or thrombosis (either arterial or venous) is common, and the cerebral circulation is at particular risk. The incidence of vascular occlusion is positively correlated with the PCV. There is an inverse correlation between PCV and cerebral blood flow (CBF), with untreated patients having low CBF values. This is probably a 'physiological' change related to the increased oxygen carrying capacity at high PCV values. However, in many patients, cerebral oxygen carriage actually increases on reduction of the PCV to normal. This explains the improvement in cerebral ischaemic symptoms and mental performance which may be observed following treatment. Platelet contact and adhesion to the vessel wall are increased at high PCV values and, combined with the lower blood flow in both the arteries and veins, tend to favour thrombus formation. Ischaemia causes vessel dilatation but when this is maximal the high whole-blood viscosity demonstrated in vitro is a major determinant of flow. Hence in man and experimental animals, as demonstrated in the brain, the area of ischaemia or non-perfusion following arterial occlusion is greatest at high PCV values. Patients with polycythaemia due to hypoxic lung disease have a poor prognosis. The hypoxic pulmonary vasoconstriction combined with the high blood viscosity causes an increase in pulmonary vessel resistance. PCV reduction of 0.50-0.52 leads to an improvement in work performance and mental alertness, and reduction in pulmonary vascular resistance.(ABSTRACT TRUNCATED AT 400 WORDS)