Rheological and flow properties of blood investigated by ultrasound

Ultrasonic waves of 1-15 MHz frequencies easily propagate through soft biological tissues, thus providing qualitative and quantitative information on mechanical and flow properties of blood and red blood cell (RBC) suspensions. Two types of techniques allow to investigate blood behaviors: echographic devices via amplitude detection and Doppler effect based devices via frequency detection of the ultrasonic signal. When ever B mode serves to construct images of tissue slabs from the ultrasonic backscattering coefficient and can give qualitative information on the mechanical properties of blood, A-mode allows to quantify the ultrasonic backscattering coefficient. Ultrasonic Doppler modes also provide both qualitative and quantitative information on blood flow velocity: continuous and pulsed Doppler modes provide curves of blood flow versus time when color Doppler and power Doppler imaging visualize blood flowing in human vessels. Association of echographic and Doppler modes to investigate simultaneously structure and velocity of blood is commercially available. Some examples of results given by such ultrasonic techniques that contribute to characterize, both in vitro and in vivo, structure and flow properties of blood or red blood cell (RBC) suspensions are presented.

Rheology and ultrasound scattering from aggregated red cell suspensions in shear flow

The shear flow dynamics of reversible red cell aggregates in dense suspensions were investigated by ultrasound scattering, to study the shear disruption processes of Rayleigh clusters and examine the effective mean field approximation used in microrheological models. In a first section, a rheo-acoustical model, in the Rayleigh scattering regime, is proposed to describe the shear stress dependence of the low frequency scattered power in relation to structural parameters. The fractal scattering regime characterizing the anisotropic scattering from flocs of size larger than the ultrasound wavelength is further discussed. In the second section, we report flow-dependent changes in the low-frequency scattering coefficient in a plane-plane flow geometry to analyze the shear disruption processes of hardened or deformable red cell aggregates in neutral dextran polymer solution. Rheo-acoustical experiments are examined on the basis of the rheo-acoustical model and the effective medium approximation. The ability of ultrasound scattering technique to determine the critical disaggregation shear stress and to give quantitative information on particle surface adhesive energy is analyzed. Lastly, the shear-thinning behavior of weakly aggregated hardened or deformable red cells is described.

Ultrasound scattering from concentrated suspensions of aggregated red cells in shear flow

Ultrasound scattering technique is used to investigate dynamics of reversible fractal aggregates in dense suspensions and analyze shear break-up processes of Rayleigh fractal clusters. On the basis of an homogenous fractal flocculation and the hybrid scattering model in the Rayleigh scattering regime, a first order expression of the ultrasound scattering cross-sectional area per unit of volume (backscattering coefficient) is derived for a dense distribution of correlated Rayleigh fractal clusters. From the scaling laws for shear break-up of reversible aggregates in concentrated suspensions, a rheo-acoustical study is then proposed to describe the shear stress dependence of the low frequency scattered power per unit of volume. In a second part, experimental flow dependent changes of the ultrasound backscattering coefficient in a plane-plane flow geometry were reported to analyze shear break-up processes of hardened or deformable red cell aggregates in polymer solution (neutral dextran polymer). Rheo-acoustical experiments were examined within the framework of the effective mean field approximation and the proposed rheo-acoustical model. The ability of ultrasound scattering to determine the critical disaggregation shear stress inducing a complete disaggregation and to give quantitative information on particle surface adhesive energy are finally analyzed.

Sensitivity of the ultrasonic interferometry method (Echo-Cell) to changes of red cell aggregation: application to diabetes

The sensitivity of the ultrasonic interferometry method (Echo-Cell) to changes in red blood cell (RBC) aggregation was investigated in comparison to the Regulest erythroaggregometry known as a reference method. In experiments where different concentrations of dextrans of 40 or 70 kD molecular weights were added to normal RBCs, the Echo-Cell was proved as sensitive as erythroaggregometry. A comparative study using RBC samples from normal and diabetic subjects showed that the Echo-Cell was much more sensitive when aggregation was measured in diluted than undiluted plasma. The sensitivity of Echo-Cell measurements in diluted plasma was similar to that of erythroaggregometry. Further analysis revealed that RBC aggregation was underestimated by Echo-Cell when measurements were made in undiluted plasma containing high fibrinogen levels, implying that in that case an elevated plasma viscosity might indirectly affect the sensitivity of the Echo-Cell. The low sensitivity of the Echo-Cell to detect an abnormal RBC aggregation when suspensions were prepared in undiluted plasma, is likely related to a relatively high shear stress exerted on RBC aggregates by the suspending medium. In conclusion, the sensitivity of the Echo-Cell to detect abnormal changes in RBC aggregation can be optimized by diluting the plasma.

Mechanical properties of bovine aortic endothelial cells in suspension studied by ultrasonic interferometry

OBJECTIVE

Cell adhesion phenomenon has been extensively studied in the last decade and was shown to be mediated by specialized molecules and driven by physical forces. Cohesion of the vessel wall cells is also dependent on adhesion molecules but less is known about the physical forces involved. To investigate endothelial cell/endothelial cell interaction from a mechanical point of view, we have used an ultrasonic interferometry device, named EchoCell, which has been previously designed to study red blood cell-red bood cell (RBC-RBC) interaction.

METHODS

Bovine aortic endothelial (BAE) cells were cultured, detached, then suspended in buffer and their mechanical and geometrical properties studied with the EchoCell system. The ultrasonic apparatus measures both the accumulation rate of cells in suspension on a solid plate and the acoustical impedances of the suspension and the sediment.

RESULTS

In suspension, BAE exhibited, in our experimental conditions (3x10(6) cells per ml), a spherical size evaluated by calculation at a mean radius of 7+/-2 microm. Moreover, no BAE aggregation occurred at the concentrations used. The acoustical impedance of the BAE suspensions calculated from all the samples studied, in the cell concentration range from 1.5x10(6) to 6x10(6) cells per ml, was 1.52x10(6) Rayl (kg m(-2) s(-1)). Furthermore, the acoustical impedance of the cell sediment was found to be independent on the initial cell suspension concentration and equal to 1.63x10(6) Rayl (kg m(-2) s(-1)). Estimation of the volume fraction of BAE inside the sediment allows to evaluate the ultrasonic velocity and the elastic bulk modulus of cells.

CONCLUSION

The ultrasonic interferometry method appears particularly interesting to study geometrical and mechanical (acoustical impedance, sound velocity, elastic bulk modulus) properties of BAE cells.

Rheo-acoustical study of the shear disruption of reversible aggregates. Ultrasound scattering from concentrated suspensions of red cell aggregates

Shear-induced disruption of reversible aggregates or clusters in a concentrated suspension is investigated by ultrasound backscattering in the low shear regime. Fractal aggregates are considered as non-Brownian scatterers much smaller than the wavelength with acoustic properties close to those of the surrounding liquid, so that the attenuation of the coherent field is weak and multiple scattering can be neglected. The concept of variance in local particle volume fraction is used to deduce a first-order expression of the ultrasound scattering cross section per unit volume for Rayleigh scatterers in a dense suspension. On the basis of a scaling law for the shear-induced disruption of aggregates, the shear stress dependence of the ultrasonic scattered intensity from a dense suspension of clusters is derived. In a second part, the shear breakup of hardened red blood cell aggregates is investigated in plane-plane flow geometry by ultrasound scattering. Rheo-acoustical experiments are analyzed within the framework of the self-consistent field approximation and the scaling laws currently used in microrheological models. Finally, the ability of ultrasonic, light reflectometry and viscometry methods to provide quantitative information about red blood cell aggregation and membrane adhesiveness is discussed.

Covalent binding of polyethylene glycol to the surface of red blood cells as detected and followed up by cell electrophoresis and rheological methods

Cyanuric chloride activated polyethylene glycol (PEG)-5000 was covalently coupled to murine and human red blood cells (pegylated RBC). Our purpose was to camouflage RBC receptors, which is necessary for parasite invasion, a process essential to sustain parasitemia. Cell electrophoretic mobility analysis (CEM) of pegylated RBC distinguished a new population of cells bearing characteristic CEM. Pegylation of RBC also modified their rheological properties, which were documented by evaluation of cell deformability (based on cell transit time through calibrated micropores) and cell aggregation (as measured by ultrasonic interferometry). Homologous transfusion of pegylated RBC into murine malaria-infected mice had no significant effect on the cerebral malaria death rate in Plasmodium berghei-infected mice, but it reduced the peripheral blood parasitemia by a factor 2 while in Plasmodium yoelii infected mice, the parasitemia was dramatically reduced by a factor of 4. These experiments demonstrate that transfusion of pegylated RBC may inhibit peripheral parasitemia. Cell electrophoresis appears to be a useful tool to allow in vivo detection and to investigate the fate of transfused pegylated RBC.

Gender, menstrual cycle, oral contraceptives and red blood cell deformability in healthy adult subjects

Gender, menstrual cycle and oral contraceptives may have influence on mechanical properties of Red Blood Cell (RBC) and particularly on RBC deformability. So cell transit parameters have been assessed by filtration with the Cell Transit Analyser (CTA) for a large healthy adult population (seventy-nine males and one-hundred-fifteen females). The CTA provides the distribution of cell transit times of 5000 red blood cells, the mean transit time of the population and different percentiles such as p50, p75, p90 and p95. No effect of oral contraceptives was found. Nevertheless, influence of sex and menstrual cycle were demonstrated. A significant increase of the filtration parameters measured in the female population with respect to the male population and during menstruation, preovulation and post-ovulation periods was observed. During ovulation, the CTA parameters are comparable to the same parameters found in males.

Casein peptide release and passage to the blood in humans during digestion of milk or yogurt

In adult humans, after milk or yogurt ingestion, many peptides derived from alpha s1-, beta- or kappa-caseins were detected in stomach, including the kappa-caseinoglycopeptide, an inhibitor of platelet aggregation. Smaller peptides derived from casein and lactoferrin were recovered from duodenum. Two long peptides, the kappa-caseinoglycopeptide and the N-terminal peptide of alpha s1-casein, were absorbed and detected in plasma. These results support the concept that food-born peptides could have physiological activities in man.

Measurement of platelet aggregation peptide inhibitors by ultrasonic interferometry

Several peptide inhibitors of thrombin- or collagen-induced platelet aggregation and of the interaction between glycoprotein Ib and von Willebrand factor were studied by a new method--ultrasonic interferometry (Echo Cell). Inhibition of aggregate formation in a concentration-dependent manner was observed. The sensitivity of the method was 3 to 40 times higher than that of classical turbidimetry.

[Effect of smoking on blood rheology]

The aim of this study was to compare the effects of cigarette smoking on biologic and rheologic tests, chiefly on the red blood cells (RBC) in measuring the deformability by the Cell Transit Analyser (CTA) and their aggregation by using an ultrasonic interferometry method based on A-mode echography allowed for the measurement of the accumulation rate of particles in a solid plate which is related to their sedimentation rate (Echo-Cell). Nine male smoker subjects with a high nicotine addiction measured by Fagerström questionnaire (> 8) and level of carbon monoxide (CM) in the breathed out air (> 20 ppm), have been compared with ten healthy no-smoker volunteers (CM < 3 ppm). One smoker has been eliminated of statistic evaluations because his glucose level showed a diabetes (10.5 mmol/l). A nailfold capillaroscopy performed in all subjects has eliminated the patterns of latent vasculitis or scleroderma. RBC and platelets counts, hemoglobin, ionogram, gamma GT, ASAT, ALAT, uric acid, total cholesterol and glucose levels were not significantly different between the two groups. On the other hand, in the smoker group, white blood cells count, serum triglycerides and especially fibrinogen values were higher than in the non-smoker's group. RBC sedimentation rate was normal in the two groups but was higher in smoker's group too. Without consumption of alcohol, the mean RBC volume was more important in smokers (91.9 +/- 1.2 versus 87.5 +/- 0.4, p = 0.003). Rheologic tests were more pathologic in smokers. The transit time or RBC by CTA was longer than in control group (1.6 ms +/- 0.02 versus 1.2 +/- 0.05, p = 0.0003). Echo-Cell technic showed a number and size of RBC aggregates more important with a rate of speed of accumulation higher than in the control group. These results demonstrated the toxic effects of smoking alone on blood toward a propensity for thrombotic status.

Membrane modifications of red blood cells in Alzheimer's disease

Red blood cells (RBC) from 24 Alzheimer's disease (AD) patients, 18 age- and sex-matched nondemented (ND) patients, hospitalized in the same facility for orthopedic problems, and 18 healthy volunteers aged 30-52 years were studied in order to gain insight into the nature of RBC membrane modifications in AD. Significant differences were found between RBC from AD and ND patients or young controls respectively for annexin V-binding (45.5 +/- 18.0% vs 27.1 +/- 14.7 and 2.7 +/- 1.9, p = .003), fraction of glycerol resistant cells (30.8 +/- 11.1% vs 19.6 +/- 6.4 and 10.2 +/- 3.1, p = .026), cell electrophoretic mobility in polymer (1.028 +/- 0.022 microns sec-1 V-1 cm vs 1.046 +/- 0.022 and 1.053 +/- 0.021, p = .02) and only limited significance for the filterability (1.46 +/- 0.12 msec vs 1.58 +/- 0.11 and 1.54 +/- 0.11, p = 0.1). A logistic analysis, using simultaneously several features as independent variables, suggested the combined use of annexinV- binding, glycerol resistance, and cell filterability which allowed the assignment of 95% of patients from this cohort to the right group. A prospective analysis of a larger cohort is required for the estimation of the diagnostic value of this test battery. In addition, the high level of annexin binding is characteristic of a disruption of the phospholipid asymmetry in aged or damaged cells, while the high glycerol resistance combined with low electrophoretic mobility an rigidity characterize young RBC, thus indicating an enhanced turnover of RBC in Alzheimer's disease.

Characterization of red blood cell aggregate formation using an analytical model of the ultrasonic backscattering coefficient

Ultrasound backscattering is well adapted to study the red blood cell (RBC) aggregation phenomenon and growth of RBC aggregates since the backscattered ultrasonic intensity depends on the sixth power of the mean radius of the scattering centers when considered as spherical. Thus, small variations of aggregate size induce large variations of the backscattered intensity. From measurements of the ultrasonic backscattering coefficient (ultrasonic backscattering cross section per unit volume of suspension), an analytical model describing its variation versus time, for human aggregated red blood cells in sedimentation, is proposed. Results given by the model allow to define three phases in the phenomenon: 1) a starting phase characterized by a duration ts; 2) a stationary final phase beginning at time tf; 3) a growing intermediate phase characterized by its duration tf - ts. The analytical model has been applied to describe RBC aggregation in dextran 70,000 dalton of different concentrations, and at various hematocrits. Knowledge of the durations ts, tf and the maximum slope s of the curve during the intermediate phase, determined with the model, allows a means to study RBC aggregate growth.

[Spontaneous intracardiac contrast and embolic risk]

Spontaneous contrast in the form of smokey echos in the cardiac chambers is considered to be a risk factor for thromboembolism. The aggregation of red blood cells results in larger target which diffuse a measurable in vitro ultrasonic signal. The phenomenon of erythrocytic aggregation is dependent on the red cells themselves, the plasma fibrinogen and conditions of blood flow. The other constituents of the blood only reflect a small amount of ultrasound, usually undetectable. Transoesophageal echocardiography with high frequency transducers (5 MHz) positioned in close proximity to the cardiac chambers, has become the reference method for detecting spontaneous contrast. This phenomenon is almost exclusively observed in the left atrium and left auricle and rarely in the other cardiac chambers or descending aorta. In pathological situations, spontaneous contrast is essentially implicated in two conditions: mitral valve obstacles and non-valvular atrial fibrillation. Conversely, moderate to severe mitral regurgitation is a negative predictive factor of spontaneous contrast. However, a purely qualitative appreciation of spontaneous contrast which may be influenced by the gain setting and technical specifications of the echocardiograph, and the subjectivity of the operator, is an important limitation. Therefore, the identification of quantitative markers of spontaneous contrats and new therapeutic antithrombotic protocols remain essential.

Measurement of human platelet microaggregates by a new method: ultrasonic interferometry

We have adapted the ultrasonic interferometry technique (Echo-Cell), which was initially designed to study red blood cell aggregation and agglutination, to the detection of human platelet microaggregates. The experimental parameter chosen was the slope of the signal over the first 5 minutes of sedimentation. We compared our new method with the conventional aggregometry for the measurement of aggregates after thrombin-, collagen-, and epinephrine-induced platelet activation. Under these conditions we demonstrated the particular sensibility of the present method in detecting small platelet aggregates induced in the first phase of aggregation and formed by low concentrations of agonists. Furthermore, as an illustration of this method, we showed an inhibition of the formation of thrombin-induced platelet aggregates in a concentration-dependent manner by the well known antagonist arginine-glycine-aspartic acid-serine with a median inhibitory concentration of 0.4 micromol/L, which is 30 times lower than the median inhibitory concentration found by aggregometry.

[Measurement of immunologic agglutination of erythrocytes by ultrasonic interferometry]

Ultrasound interferometry is a new methodology which has been developed in our laboratories in order to measure precisely and quickly the size of particles sedimenting in liquid on horizontal surface, upon gravity. Applied to red blood cells, this method evaluates the sedimentation of erythrocytes, their aggregation induced by proteins or aggregating compounds as well as their agglutination upon immune reactions. The quantitative assessment of red cell agglutination was applied to the study of blood groups and to the search for red cell antibodies. Preliminary results show that ultrasound interferometry is 1) quantitative, measuring the size of agglutinates; 2) sensitive; 3) specific; 4) fast; 5) able to detect irregular antibodies.

Red blood cell deformability and alcohol dependence in humans

Studies of DPH fluorescence polarization and deformability have shown that alcohol induces rigidification of the red blood cell (RBC) membrane. We investigated a possible link between RBC membrane fluidity and deformability by studying both parameters simultaneously in samples from alcohol-dependent patients (group 1, N = 19), social drinkers (group 2, N = 12) and long-term abstaining alcoholics (group 3, N = 8). The active drinkers showed disturbances of several RBC membrane parameters, including abnormal microorganization of the membrane surface, a decrease in sialic acid content, and resistance to the fluidizing effect of ethanol, that were not completely corrected in the abstinent alcoholics. The RBC transit time was significantly longer in the active drinkers than in the abstainers but not the social drinkers. There were no significant differences between the groups with regard to membrane lipid core fluidity. The main abnormality (fluidization) in RBC from the active alcoholics involved the polar surface of the membrane (probed using TMA-DPH), and correlated with the decrease in sialic acid content but not with RBC deformability.

Ultrasonic interferometry: study of particle sedimentation in liquid

An ultrasonic interferometry method was designed to study sedimentation of particles in liquid. The method, based on A mode echography, measures the amplitude of ultrasonic waves reflected (echo E1) by a fixed interface I1 called "solid plate-sediment" interface formed when particles are sedimenting on a solid plate. The amplitude of the echo depends both on mechanical properties of the three media (solid plate, sediment and suspension) on the thickness of the sediment and on the presence of a second mobile interface I2 called "sediment-suspension" interface. In the first phase of sedimentation when the second interface is very close to the first, two reflected waves interfere. Then, in the second phase of sedimentation when the sediment is thick enough, the amplitude of the echo E1 depends only on the sediment and solid plate properties. The first phase will give information on the sedimentation rate of particles (SR). We have compared SR of particles determined by this method with SR measured in a cylindrical tube of the same geometry as the ultrasonic measurement cell and with theoretical values of the sedimentation rate given by theoretical models.

Assessment of red blood cell aggregation with dextran by ultrasonic interferometry

Aggregation of human red blood cells (RBCs) induced by dextrans of various molecular weight has been studied by using a new ultrasonic interferometry method. This method, based on A-mode echography, allowed for the measurement of the accumulation rate of particles on a solid plate which is related to their sedimentation rate (i.e., to their mean size). The initial aggregation process, the mean and the maximum sedimentation rate of aggregates and the packing of the sedimented RBCs have been investigated. Effects of hematocrit, molecular weight of dextrans and inhibition by dextran 40 on the RBC aggregation induced by dextran of higher molecular weight have been determined by analysing variations of the aggregate size. Results obtained confirm the aggregation effect of dextrans of molecular weights equal or higher than 70,000 dalton and disaggregation effect of dextran 40,000 dalton on aggregation by dextrans of higher molecular weight.

Size determination of red blood cell aggregates induced by dextran using ultrasound backscattering phenomenon

Different methods are commonly used to study the red blood cell aggregation phenomenon. The major interest of the ultrasonic method presently discussed is to assess the mean size of red blood cell (RBC) aggregates by measuring ultrasonic intensity backscattered by blood. Applying Rayleigh theory of sound to blood medium, one can show that the scattered ultrasonic intensity is proportional to the 6th power of the size of the RBC aggregates. The ultrasonic method is used to evaluate the mean size of RBC aggregates induced by dextrans. RBCs are suspended at various hematocrits H, in solution of dextrans of different molecular weights M and at different weight concentrations Cw. Results are presented by using the ultrasonic backscattering coefficient chi which is a relevant quantity in a scattering experiment. For suspensions of RBCs aggregated with dextran of molecular weight 70,000 dalton (dextran 70) at concentration Cw = 40 g/l, variations of chi as a function of H are similar to those obtained for normal blood. At a fixed hematocrit, variation of chi versus Cw for dextran 70 exhibits a maximum at 40 g/l. In the case of RBCs suspended at hematocrit 20% and aggregated with dextrans of molecular weight M, 70,000 less than or equal to M less than or equal to 2,000,000, the variations of chi versus molar concentration Cm are similar to those of the microscopic aggregation index defined by Chien (1). Finally, a statistical model of the blood structure previously described (2) is applied to evaluate the mean size of the aggregates. According to this model, the mean size of aggregates is independent of hematocrit for H less than or equal to 40% and independent of the molecular weight of dextran for M greater than or equal to 150,000 dalton.

Determination of erythrocyte transit times through micropores. I--Basic operational principles

To study the transit times of each red blood cell passing through cylindrical micropores and in order to evaluate sub-population of cells with regard to their deformability, we have developed a new system called the cell transit time analyser (CTTA). By using an AC voltage (100 KHz) across a special filter, we measure the electrical conductance change produced by the cells passing through the pores under a known driving pressure. This computer based device provides the distribution of transit times tau for 2000 cells in 1 minute and as a result the mean transit time [tau]. Experiments with red cells were designed to evaluate the flow behavior of both normal cells and cells whose mechanical properties were artificially altered. Cell volume was changed by use of non-isotonic media. Cell shape and cell volume were modified by varying the pH of the suspending buffer. Results of these experiments are: 1) a skew distribution of transit times towards high tau values for both control cells and artificially altered cells is observed: 2) [tau] is minimum for isotonic conditions and increases sharply for either hypotonic or hypertonic media: 3) [tau] is minimum at physiological pH and increases for either acid or alcaline changes of pH.

Aggregation of red blood cells studied by ultrasound backscattering

The erythrocyte aggregation phenomenon is an important factor in capillary circulation. This phenomenon can be evaluated by a number of methods (microscopic observations, viscometry, light measurements) which cannot be applied simply to in vivo measurements. In contrast, ultrasound which propagates through soft tissues allows measurement of the mechanical properties of red blood cell (RBC) suspensions which depend on the aggregation phenomenon. We devised an apparatus in order to measure in vitro the ultrasonic backscattering intensity of RBC suspensions. First, with latex particles of different sizes, the ultrasonic backscattering coefficient has been measured in order to evaluate the apparatus response. Then, the ultrasonic backscattering coefficient of different aggregated erythrocyte suspensions has been measured and correlated with the erythrocyte sedimentation rate. Finally, the size of RBC aggregates of different suspensions has been evaluated.

Erythrocyte sedimentation studied by an echographic method

The erythrocytes sedimentation can be monitored by an ultrasonic echographic method which is described. It is shown that this technique can give some information on the hematocrit profile in the sedimentation tube.