Blood yield stress in systemic sclerosis.

Blood is a weak percolating physical gel at low shear rates, in which clusters of aggregates can be reversibly disaggregated or formed again. This phenomenon is of potential importance in the microvascular pathophysiology of ischemic and vasospastic disorders such as systemic sclerosis. The aim of this work was to determine blood yield stress using low-shear-rate rheometry with a homemade roughened Couette device in 10 patients with systemic sclerosis compared with 10 healthy controls. Biochemical plasmatic parameters were assessed independently. Results showed a significantly increased stress (+56%, P < 0.05 at 60% hematocrit) for scleroderma patients. The best biochemical predictor for yield stress was the ratio of albumin to globulins; 69% of its variance was explained by plasmatic factors (albumin, fibrinogen, and globulins) in scleroderma patients and 23.4% in healthy controls. Additional microscopic observations showed different microstructures. These results support the hypothesis of an abnormal red blood cell organization process in scleroderma patients that could be partly responsible for the severity of ischemic complications of the disease.

Systemic sclerosis: blood rheometry and laser Doppler imaging of digital cutaneous microcirculation during local cold exposure.

A combined study of microcirculation hemodynamics in vivo and blood rheometry has been carried out in patients with scleroderma compared to primary Raynaud's phenomenon (PRP) and healthy controls (HC). Laser Doppler perfusion imaging was used to assess superficial skin blood flow in the fingers before, during and after local cold exposure. Blood viscosity was measured at 19 and 37 degrees C. Dynamic and transient flows were also investigated. The scleroderma group showed a significantly lower level of perfusion before the cold test, that was further decreased during and after cold exposure, the difference with PRP being maximal during the rewarming period. It also showed a significantly increased viscosity (p < 0.05) at low shear rates. Hemorheological behavior of the PRP group was similar to the HC group. A significant negative correlation between the LDI perfusion and the apparent blood viscosity at low shear rates was found for the whole population. These findings suggest that rheological factors may be involved in the abnormal cold reactivity of patients with scleroderma.

Threshold of shear stress in human blood for healthy and sick subjects. / Etude du seuil de contrainte du sang humain pour des sujets sains et pathologiques.

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 stress. This is reflected in vivo with a cessation of flow threshold, of great importance in circulatory physiology and pathophysiology. Values for yield stress obtained by different techniques are scarce. Rheometry at low shear rates is one method of obtaining values. After mitigating 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 samples in controls and in ill subjects with different hematocrits. A Low Shear 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 healthy donors and patients were anticoagulated with EDTA. Suspensions of red cells in plasma were prepared by removing plasma. The measurements were made at 25 degrees C within four hours following blood sampling. Particular attention 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 approached a constant value at low shear 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 present at 10(-1) s-1 et 3.10(-3) s-1 but they could be greatly mitigated by the use of the 170 microns surface roughness.

Blood low shear rate rheometry: influence of fibrinogen level and hematocrit on slip and migrational effects.

Red blood cell (RBC) aggregation is of prime importance in vivo and in vitro for low flow rates. It may be estimated by rheometrical measurements at low shear rates, but these are perturbed by slip and migrational effects which have already been highlighted in the past. These effects lead to a torque decay with time so that the true value of the stress at low shear rates may be greatly underestimated. Elevated aggregation being associated with different diseases, pathological blood samples show more pronounced perturbing effects and a strong time dependency in low shear rate rheometry. To test the dependence of slip and migrational effects on RBC aggregation, and particularly to determine the way in which they depend upon fibrinogen concentration ([Fb]), a home-made measuring system with roughened internal and external walls (170 microns roughness) was used to study low shear rate rheometry for RBC suspensions in PBS buffer containing albumin (at 50 g/l) and fibrinogen at various concentrations. The influences of hematocrit, shear rate, and fibrinogen concentration were investigated. Particular attention was paid to data acquisition at low shear rates (10(-3) s-1 to 3 x 10(-2) s-1). The combined influence of hematocrit and fibrinogen was investigated by adjusting hematocrit to 44 or 57% and fibrinogen concentration ([Fb]) to 3.0-4.5-6.5 g/l. Microscopic observations of the blood samples at rest were performed. They showed that different structures were formed according to fibrinogen concentration. The rheometrical measurements indicated that torque decay with shearing duration was strongly dependent on fibrinogen concentration and on shear rate at fixed hematocrit. Migrational and slip effects were more pronounced as shear rate decreased, fibrinogen concentration was raised, and hematocrit was lowered. The results have been explained on the basis of the expected microstructure of flowing blood in relation to the microscopic observations at rest.