Traditionally, blood rheology tests have been used in diagnosis and monitoring of
infection,
rheumatic diseases and
malignancy, and are still of clinical value in these conditions. In the last twenty years, clinical and epidemiological studies have shown that the haematological determinants of blood flow resistance (haematocrit,
fibrinogen, white cell count and altered red and white cell rigidity) are also associated with nutritional, metabolic, endocrine and vascular disorders. Decreased red cell deformability may contribute to reduced red cell survival and anaemia in
burns,
malaria,
liver disease and
kidney failure. In
trauma and inflammatory disease, overt hyperviscosity is usually prevented by vasodilatation and reduction in the haematocrit. However, low-flow states may arise systemically from haemoconcentration (contracted plasma volume, Chapter 3) in severe
burns, inappropriate red cell transfusion, or
dehydration due to illness; systemically in circulatory
shock; and locally in
venous thrombosis or arterial disease. In such circumstances, the intrinsic flow resistance of blood may perpetuate flow disturbance, ischaemia and
thrombosis. Conversely, optimal levels of haematocrit,
fibrinogen and white cell count may be lower than normal in low-flow states. Haemodilution by
colloid infusion is beneficial in
burns,
shock, major surgery, prevention of postoperative
venous thrombosis, chronic stable claudication and possibly in
acute stroke and
retinal vein thrombosis.
Plasma exchange may be beneficial in severe Raynaud's phenomenon. Defibrination with
ancrod is effective in prevention and treatment of
venous thrombosis but its role in arterial disease is unproven. The benefits of
streptokinase therapy in venous thrombo-
embolism and acute
myocardial infarction may be partly rheological, due to
fibrinogen depletion. Drugs with rheological effects may be beneficial in
intermittent claudication.