[Blood hyperviscosity syndromes. Classification and physiopathological understanding. Therapeutic deductions]

Blood has a number of rheological properties which partially determine flow, especially at capillary level, and its capacity to deliver oxygen. It is non-Newtonian, pseudoplastic, thixotropic and viscoelastic. Viscosity can be studied with different types of viscosimeters (coaxial cylinder or capillary viscosimeters). It can be defined by the ratio of stress of deformation to rate of deformation. Viscosity depends on macrorheological parameters: hematocrit, serum proteins, especially fibrinogen and globulins, and also on microrheological parameters: degree of aggregation and red blood cell deformability. Viscosity rises when the temperature falls and decreases with the radius of the tube through which the blood flows (Fahraeus-Linqvist effects). Blood viscosity is studied clinically at different temperatures, and, above all, at different rates of deformation by carefully recording the hematocrit. Plasma viscosity, fibrinogen, albumia and immunoglobulin levels, the viscosity of blood cell suspensions in normal saline must also be taken into consideration. Special investigations (rheoscopy, filtrability) provide information about red cell aggregation and deformability. Hyperviscosity syndromes are observed with: --raised hematocrit (polycythemia and pseudopolycythemia), --conditions with raised serum proteins or changes in their composition (especially hyperfibrinogenemia, raised immunoglobulins, low albumin levels); inflammatory syndromes, dysglobulinemias (Fahey's syndrome of plasma hyperviscosity), --low temperature (hypothermia), --increased red cell aggregability (shock, fat embolism), --reduced red cell deformability due to various congenital and acquired conditions (sickle cell anemia, renal failure, hyperlipoproteinemia, thrombosis, diabetes). Conversely, hypoviscosity may occur with a low hematocrit, hypoproteinemia, hypofibrinogenemia, and hyperthermia. Increased viscosity results in a slowing of blood flow, stagnation of its constituents and in ischemia. Therapeutic interventions may be considered on the different components of the hyperviscosity syndrome: hemodilation, plasmapheresis, dispersion of aggregants, agents acting on red cell deformability.