Whole blood viscosity and cerebral blood flow.

BLOOD has anomalous rheological properties, particularly in certain disease states. It may therefore be necessary to consider these properties when attempting to assess and perhaps to improve the state of a patient's cerebral circulation. So what evidence is there that cerebral blood flow (CBF) is affected by viscosity? Before attempting to answer this question, one should be aware of what influences the viscosity of blood. Viscosity is defined as the friction between adjacent layers of a fluid as they move relative to one another. It is impossible to give a single viscosity value for an individual blood sample because viscosity will vary with this relative movement, i.e. with the rate at which it is sheared. The shear rate depends on the diameter of the vessel and the rate of flow, both of which change at different phases of pulsatile flow. Slowly moving blood may have viscosities ten to twenty times higher than the same blood flowing quickly. Another problem is that we have only an approximate idea of what shear rates are relevant in different parts of the circulation. An acceptable compromise is to provide three viscosities for a sample, when measured at low, medium and high shear rates. Despite the technical problems of measuring blood viscosity and doubts about the relevance of the in vitro values to the in vivo conditions, there are three factors, in addition to shear rate, that are generally accepted to influence it. Firstly, increasing the concentration of either red or white cells raises viscosity substantially. There is a logarithmic relationship between viscosity and hematocrit (Hct) — blood with Hct of 0.50 may have twice the viscosity of that with 0.40. Secondly, raising plasma fibrinogen levels or an abnormal immunoglobulin influence whole blood viscosity both by increasing the plasma viscosity component and by increasing the reversible, red cell — red cell aggregation that occurs at low shear rates. These aggregates are largely responsible for the steep increase in viscosity as flow slows down. Thirdly, the rigidity of the red cells influences how easily they negotiate the microcirculation. Red cells are known to be more rigid in certain haematological disorders, notably in sickle-cell disease, but normal cells can become less flexible under conditions of anoxia and low pH. There are very considerable methodological problems involved in measuring erythrocyte deformability in vitro and perhaps further comment should be reserved until workers in the field can reach agreement on a reliable quantitative method.