Macrorheology and microrheology of blood in cerebrovascular insufficiency

The rheology of blood is increasingly appreciated as a determining factor of cerebral perfusion. A survey of the present state of the rheological theory is presented in which the significance of rheological factors in normal and disturbed flow and thence their pathogenetic role in states of hypoperfusion is outlined. The viscosity concept in the classical sense of continuum mechanics is not applicable to the analysis of blood flow in the microcirculation. It is useful, nevertheless, to assess the microrheological determinants (plasma viscosity, hematocrit value, tendency to red cell aggregation and 'red cell deformability'), which are known to determine the fluidity of blood in both the micro- and macro-vessels--as well as in the rotational viscometers. While hemorheological factors are of small relevance for the normal perfusion, they can dominate or limit the perfusion in cases of decompensated arterial obliteration. Under these conditions, therefore, beneficial effects can be expected from procedures which improve the fluidity of blood.