Clinical Hemorheology and Microcirculation

Pulsatile flow, wall distensibility, non‐Newtonian flow characteristics of blood in flow separation regions, and high/low blood pressure were studied in elastic silicon rubber models having a compliance similar to human vessels and the same surface structure as the biological intima models of (1) a healthy carotid artery model, (2) a 90% stenosis in the ICA, and (3) 80% stenosis in both the internal and external carotid arteries. Flow was visualized for steady flow and pulsatile studies to localize flow separation regions and reattachment points. Local velocity was measured with a 1‐, 2‐, or 3‐D laser‐Doppler‐anemometer (LDA). Flow in the unstenosed model was Re $=$ 250. In the stenosed models, the Re number decreased to Re $=$ 180 and 213 under the same experimental conditions. High velocity fluctuations with vortices were found in the stenosed models. The jet flow in the stenosis increased up to 4 m/s. With an increasing bifurcation angle, the separation regions in the ECA and ICA increased. Increased flow (Re $=$ 350) led to an increase in flow separation and high velocity shear gradients. The highest shear stresses were nearly 20 times higher than normal. The 90% stenosis created high velocity shear gradients and velocity fluctuations. Downstream of the stenoses, eddies were found over the whole cross‐section. In the healthy model a slight flow separation region was observed in the ICA at the branching cross‐section whereas in the stenosed models, the flow separation regions extended far into the ICA. We conclude that a detailed understanding of flow is necessary before vascular surgery is performed especially before artificial grafts or patches are implanted.

RheothRx®injection is an intravenous formulation of Poloxamer 188, a non‐ionic block copolymer surfactant which is actually used in clinical studies, e.g., during thrombolysis in acute myocardial infarction. We have analysed the influence of RheothRx®on plasma and whole blood viscosity of healthy individuals in vitro. For that purpose plasma was incubated with 0, 0.75, 3.75 and 18.75 mg/ml RheothRx®and whole blood with a constant hematocrit of 41.4% with 0, 0.4, 2 and 10 mg/ml RheothRx®at 37 ${}^\circ$ C. Viscosity was measured with a Couette viscometer at shear rates of 94, 11, 0.9 and 0.1 s $^{-1}$ as well as with a newly developed oscillating viscometer. In contrast to other studies, we found no favourable effect of RheothRx®on plasma and whole blood viscosity. At the highest RheothRx®concentration an increase in high and low shear viscosity was observed. Erythrocyte morphology remained unchanged. These data, although on normal blood in vitro, suggest that the positive effects of RheothRx®in vivo may not be caused by improved flow properties of blood, but could rely on other mechanisms.

In a group of 36 patients with acute ischemic stroke, a study of the influence of arterial hypertension on the red cells and fibrinogen interaction was carried out by the aspect of fibrinogen molecules’ contribution to this phenomenon, respecting the role of other plasma biochemical factors. Patients were divided into two clinical groups: with and without arterial hypertension. In the blood samples with stable haematocrit ratio the following rheological properties were estimated: plasma viscosity, yield shear stress (YSS), fibrinogen level and parameter $R$ (the difference between two measurements of fibrinogen concentration). Also measured were the levels of albumin, IgG, IgA, IgM, cholesterol, HDL and LDL. The value of parameter $R$ in patients with arterial hypertension was lower than in the group without accompanying disease (statistical significance was indicated for 80 and 60% of plasma dilution). The first group of patients was also characterized by a significant increase of IgA level (in comparison with the control group), whereas the red cells and fibrinogen interaction measured as a value of YSS was similar in both analysed clinical groups. This study indicated a specific character of some hemorheological changes in relation to a certain vascular pathology.

Furosemide is a diuretic agent which is often given in high concentrations intravenously. Since erythrocytes have a furosemide‐sensitive transport system and oral application of furosemide was found to induce an echinocytic shape transformation in horses, we have analysed the influence of furosemide on erythrocyte shape, volume and blood viscosity in vitro. Increasing plasma furosemide concentrations of 0, 1, 10, 100 and 1000 $\mu$ g/ml did not affect the erythrocyte volume or shape and left blood viscosity unaffected. We conclude that furosemide has no influence on blood rheology in humans.

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 ${}^\circ$ 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.

A newly designed type of oscillating viscometer was tested for blood viscosity measurements. The viscometer consists of a probe (either a tube or a rod) oscillating at a resonance frequency with amplitudes in the micro‐ and nanometer range. The torsional oscillations are dampened by fluids flowing through the tube or surrounding the rod. The degree of damping depends on the viscosity of the fluid, which allows to measure viscosity. Data obtained with these instruments were compared with those obtained with a conventional Couette viscometer. An increase of erythrocyte aggregation by the addition of dextran 70 in vitro led to the expected increase of viscosity in the Couette viscometer; in the oscillating tube viscometer, however, it remained unchanged, which may be explained by a decreased erythrocyte concentration near the tube wall due to increased aggregation and flow of erythrocytes in the tube center. In ex vivo experiments on blood flowing without anticoagulant directly through the tube viscometer an inverse correlation between viscosity and fibrinogen concentration was found. This is in contrast to actual knowledge and may indicate that high fibrinogen levels have a beneficial rheological effect at the tube or vessel wall. Our data suggest that the new oscillating tube viscometer is an interesting tool, which may contribute to a more comprehensive understanding of blood flow.

We hypothesized that due to monthly bloodloss, the mechanical properties of blood of premenopausal women are superior to men, and place them at less risk for cardiovascular diseases than men in any age group. Rheological properties of blood of premenopausal women and age‐matched men were compared. It was found that male blood possesses an increased viscosity, RBC aggregability and RBC rigidity. Additionaly, male RBCs were found to have higher mechanical fragility. Since women in reproductive age have almost half as many old RBCs and almost twice as many young RBCs as men, we investigated the effect of in vivo aging of RBCs on their mechanical properties. Old RBCs were shown to have an increased mechanical fragility and aggregability, and decreased deformability as compared to young RBCs. Decreased deformability and increased aggregability of RBCs cause an increase in blood viscosity and are known as risk factors of cardiovascular diseases. Since men possess a higher number of old RBCs with suboptimum mechanical properties than premenopousal women, who due to monthly bloodloss have a higher number of young cells and a lower number of old RBCs than their male counterparts, our results suggest that an elevated hemorheological risk for males is associated with the age distribution of RBCs. This, in addition to significantly higher hematocrit, may be the reason for the increased risk of morbidity and mortality from cardiovascular diseases of men as compared to women of reproductive age.

Using the method of inducing spherocytic anemia in the rabbit with antibody serum, we have developed an animal model in which red blood cells (RBCs) can nearly grow synchronously. With this model, we determined that the surface charge density on the RBC membrane decreased with cell aging. The change was not linear, being much more profound in the latter half of RBC life span. There was a positive correlation between the mean RBC density and its “age” ( $r=0.847$ , $p<0.01$ ). However, the density distribution of the RBCs at the same “age” showed a broad range, and the density values for RBC groups with different ages showed considerable overlap. This indicates that the density gradient technique can be used to separate RBC population into fractions with different mean ages, but has a low resolving power for obtaining individual RBCs of a given “age”.