Blood rheology in vitro and in vivo

Blood rheology tests are traditionally used for detection of organic disease and for monitoring disease activity. More recently they have been used for prediction of blood flow in vivo, not only in overt hyperviscosity syndromes but also in the covert hyperviscosity of low-flow states. The traditional ESR test result increases with red cell aggregation induced by increases in large, asymmetrical plasma globulins. However, small increases in haematocrit and large increases in plasma viscosity each decrease the ESR, reducing both its diagnostic utility and its ability to predict blood flow in vivo. The ESR should be corrected to a standard haematocrit, or else replaced by the ZSR or plasma viscosity, which are more rapid, simple, sensitive and independent of haematocrit. For prediction of blood flow in vivo, these tests can be supplemented by measurement of whole-blood viscosity, which can be performed simply and cheaply in capillary viscometers at high shear rates. Whole-blood viscosity is determined by plasma viscosity, haematocrit and red cell deformability at high shear rates. Its measurement is useful in overt hyperviscosity syndromes, particularly in estimating the effect of red cell transfusion in anaemic patients with plasma hyperviscosity, hyperleukocytic leukaemias or sickling disorders. Blood viscosity should be related to the haematocrit or haemoglobin concentration in order to estimate oxygen delivery to tissues. Changes in blood viscosity can be compensated readily in the normal circulation but not in the compromised, low-flow circulation. In these circumstances, systemic increases in plasma viscosity, haematocrit, whole-blood viscosity, red cell aggregation and in the numbers of circulating rigid red or white blood cells can perpetuate low-flow states and ischaemia. Red cell deformability in narrow vessels is best measured by micropore filtration systems, in which the effect of white cells has been eliminated. Red cell deformability is reduced by change in shape, decrease in the ratio of surface area to volume, decreased membrane flexibility and increased internal viscosity (MCHC and inclusions). White cells have negligible effects on bulk-blood viscosity but have important effects on blood flow in narrow vessels, due to their high internal viscosity and their adhesiveness when activated. White cell filterability is lowest for monocytes and for activated granulocytes and these adhesive and rigid cells may have important effects on microcirculatory blood flow in low-flow states.