STUDY OF THE HUMAN BLOOD SHEAR-STRESS IN PATIENTS AND CONTROLS

It is now well recognized that blood is a yield stress fluid, that is to say that it will not flow below a critical threshold level of stres s. This is reflected in vivo with a cessation of flow threshold, of gr eat importance in circulatory physiology and pathophysiology. Values f or yield stress obtained by different techniques are scarse. Rheometry at low shear rates is one method of obtaining values. After mitigatin g the slip effects on the walls of the rheometer, it has been possible to get an approximate value of the yield stress in human blood sample s in controls and in ill subjects with different hematocrits. A Low Sh ear 40 rheometer with a controlled rotation speed was used. Home made geometries were used and the shear rate was varied in the range 10(-3) s(-1) - 100 s(-1). Fresh blood samples from heal thy donors and patie nts were anticoagulated with EDTA. Suspensions of red cells in plasma were prepared by removing plasma. The measurements were made at 25 deg rees C within four hours following blood sampling. Particular attentio n was paid to data acquisition at low shear rates (10(-3) s(-1), 3.10( -3) s(-1), 10(-2) s(-1), 3.10(-2) s(-1)) with a computer linked to the rheometer. Shear stress approched a constant value at low sheer rates for geometries with rough surfaces. Shear stress measured at 10(-3) s (-1) was taken as an approximation of yield stress. Values were 5 to 6 mPa for hematocrit 60 % and 20 to 25 mPa for hematocrit in the 80 to 90 % range. For pathological blood samples, accurate measurements were made at shear rate as low as 0.01 s(-1). Migrational effects were pre sent at 10(-1) s(-1) et 3.10(-3) s(-1) but they could be greatly mitig ated by the use of the 170 mu m surface roughness.