Free oscillation rheometry detects changes in clot properties in pregnancy and thrombocytopenia.

Improved methods are needed to identify patients at risk for thrombotic or bleeding events. Free oscillation rheometry (FOR) is a technique that offers information on coagulation, based on contributions of all blood components, by measurement of clotting time and changes in clot elasticity. This is the first study that evaluates FOR parameters in subjects likely to represent hypercoagulability (pregnant women) and hypocoagulability (thrombocytopenic patients). Clotting time and blood clot elasticity were measured by FOR in blood samples obtained from women in different pregnancy trimesters (n = 58), in thrombocytopenic patients before and after a platelet transfusion (n = 20) and in healthy blood donors (n = 60). The clotting time was shorter and the clot elasticity higher in pregnant women compared to the non-pregnant female blood donors. The elasticity was higher in late pregnancy compared to early pregnancy. Compared to the blood donors, the thrombocytopenic patients had lower elasticity, which was increased by a platelet transfusion, but there was no difference in clotting time. The results suggest that FOR can provide new information on the haemostatic status of patients at risk of thrombotic or bleeding events as well as information on the haemostatic effect of a platelet transfusion.

Hemodynamic diagnostics of epicardial coronary stenoses: in-vitro experimental and computational study.

BACKGROUND: The severity of epicardial coronary stenosis can be assessed by invasive measurements of trans-stenotic pressure drop and flow. A pressure or flow sensor-tipped guidewire inserted across the coronary stenosis causes an overestimation in true trans-stenotic pressure drop and reduction in coronary flow. This may mask the true severity of coronary stenosis. In order to unmask the true severity of epicardial stenosis, we evaluate a diagnostic parameter, which is obtained from fundamental fluid dynamics principles. This experimental and numerical study focuses on the characterization of the diagnostic parameter, pressure drop coefficient, and also evaluates the pressure recovery downstream of stenoses. METHODS: Three models of coronary stenosis namely, moderate, intermediate and severe stenosis, were manufactured and tested in the in-vitro set-up simulating the epicardial coronary network. The trans-stenotic pressure drop and flow distal to stenosis models were measured by non-invasive method, using external pressure and flow sensors, and by invasive method, following guidewire insertion across the stenosis. The viscous and momentum-change components of the pressure drop for various flow rates were evaluated from quadratic relation between pressure drop and flow. Finally, the pressure drop coefficient (CDPe) was calculated as the ratio of pressure drop and distal dynamic pressure. The pressure recovery factor (eta) was calculated as the ratio of pressure recovery coefficient and the area blockage. RESULTS: The mean pressure drop-flow characteristics before and during guidewire insertion indicated that increasing stenosis causes a shift in dominance from viscous pressure to momentum forces. However, for intermediate (approximately 80%) area stenosis, which is between moderate (approximately 65%) and severe (approximately 90%) area stenoses, both losses were similar in magnitude. Therefore, guidewire insertion plays a critical role in evaluating the hemodynamic severity of coronary stenosis. More importantly, mean CDPe increased (17 +/- 3.3 to 287 +/- 52, n = 3, p < 0.01) and mean eta decreased (0.54 +/- 0.04 to 0.37 +/- 0.05, p < 0.01) from moderate to severe stenosis during guidewire insertion. CONCLUSION: The wide range of CDPe is not affected that much by the presence of guidewire. CDPe can be used in clinical practice to evaluate the true severity of coronary stenosis due to its significant difference between values measured at moderate and severe stenoses.

Threshold response of initiation of blood coagulation by tissue factor in patterned microfluidic capillaries is controlled by shear rate.

OBJECTIVE: Blood flow is considered one of the important parameters that contribute to venous thrombosis. We quantitatively test the relationship between initiation of coagulation and shear rate and suggest a biophysical mechanism to understand this relationship. METHODS AND RESULTS: Flowing human blood and plasma were exposed to cylindrical surfaces patterned with patches of tissue factor (TF) by using microfluidics. Initiation of coagulation of normal pooled plasma depended on shear rate, not volumetric flow rate or flow velocity, and coagulation initiated only at shear rates below a critical value. Initiation of coagulation of platelet-rich plasma and whole blood showed similar behavior. At constant shear rate, coagulation of plasma also showed a threshold response to the size of a patch of TF, consistent with our previous work in the absence of flow. CONCLUSIONS: Initiation of coagulation of flowing blood displays a threshold response to shear rate and to the size of a surface patch of TF. Combined with the results of others, these results set the range of shear rates that limit initiation of coagulation by small surface areas of TF and by shear activation of platelets. This range fits the relatively narrow range of physiological shear rates described by Murray's law.

[Rheology, hemolysis and peripheral blood indices in macaccus rhesus monkeys in health and posthemorrhagic period].

Twenty monkeys (macaccus rhesus) were examined by the following parameters: deformability of erythrocytes and mononuclear leukocytes, hemolytic activity of the serum, free hemoglobin of the serum and viscosity of plasma. The data obtained are accepted as standard because literature data on hemorrheology of monkeys are absent. Compared to relevant indices in humans, monkeys have increased hematocrit, serum hemolytic activity, erythrocytic rigidity index. Seven monkeys were used as the model of "donor" (6-8% blood loss). The above indices were measured before exfusion and on days 2, 7, 14 and 21 after exfusion. Maximal changes in rheology and hemolysis were observed on day 2 and 7 after blood loss which correlated with deviations of the red sprout of hemogeny.

Modelling thrombosis using dissipative particle dynamics method.

AIM: Arterial occlusion is a leading cause of cardiovascular disease. The main mechanism causing vessel occlusion is thrombus formation, which may be initiated by the activation of platelets. The focus of this study is on the mechanical aspects of platelet-mediated thrombosis which includes the motion, collision, adhesion and aggregation of activated platelets in the blood. A review of the existing continuum-based models is given. A mechanical model of platelet accumulation onto the vessel wall is developed using the dissipative particle dynamics (DPD) method in which the blood (i.e. colloidal-composed medium) is treated as a group of mesoscale particles interacting through conservative, dissipative, attractive and random forces. METHODS: Colloidal fluid components (plasma and platelets) are discretized by mesoscopic (micrometre-size) particles that move according to Newton's law. The size of each mesoscopic particle is small enough to allow tracking of each constituent of the colloidal fluid, but significantly larger than the size of atoms such that, in contrast to the molecular dynamics approach, detailed atomic level analysis is not required. RESULTS: To test this model, we simulated the deposition of platelets onto the wall of an expanded tube and compared our computed results with the experimental data of Karino et al. (Miscrovasc. Res. 17, 238-269, 1977). By matching our simulations to the experimental results, the platelet aggregation/adhesion binding force (characterized by an effective spring constant) was determined and found to be within a physiologically reasonable range. CONCLUSION: Our results suggest that the DPD method offers a promising new approach to the modelling of platelet-mediated thrombosis. The DPD model includes interaction forces between platelets both when they are in the resting state (non-activated) and when they are activated, and therefore it can be extended to the analysis of kinetics of binding and other phenomena relevant to thrombosis.

A detailed fluid mechanics study of tilting disk mechanical heart valve closure and the implications to blood damage.

Hemolysis and thrombosis are among the most detrimental effects associated with mechanical heart valves. The strength and structure of the flows generated by the closure of mechanical heart valves can be correlated with the extent of blood damage. In this in vitro study, a tilting disk mechanical heart valve has been modified to measure the flow created within the valve housing during the closing phase. This is the first study to focus on the region just upstream of the mitral valve occluder during this part of the cardiac cycle, where cavitation is known to occur and blood damage is most severe. Closure of the tilting disk valve was studied in a "single shot" chamber driven by a pneumatic pump. Laser Doppler velocimetry was used to measure all three velocity components over a 30 ms period encompassing the initial valve impact and rebound. An acrylic window placed in the housing enabled us to make flow measurements as close as 200 microm away from the closed occluder. Velocity profiles reveal the development of an atrial vortex on the major orifice side of the valve shed off the tip of the leaflet. The vortex strength makes this region susceptible to cavitation. Mean and maximum axial velocities as high as 7 ms and 20 ms were recorded, respectively. At closure, peak wall shear rates of 80,000 s(-1) were calculated close to the valve tip. The region of the flow examined here has been identified as a likely location of hemolysis and thrombosis in tilting disk valves. The results of this first comprehensive study measuring the flow within the housing of a tilting disk valve may be helpful in minimizing the extent of blood damage through the combined efforts of experimental and computational fluid dynamics to improve mechanical heart valve designs.

Relationship of 24-hour blood pressure rhythm with endothelial function and blood rheology.

The purpose of the study was to investigate relationships between sex, blood pressure level, duration of arterial hypertension (AH), 24-hour blood pressure (BP) rhythm, endothelial function (EF) and blood rheological parameters. 23 (mean age 50+/-8.73) outpatients with AH were included in the study. All subjects underwent off-therapy 24-hour ambulatory BP monitoring, investigation of blood rheological parameters and high resolution vascular dopplerography Dipper patients showed lower rate of platelet aggregation and platelet adhesion, than it was in non-dippers (86.42+/-8.20 vs. 99.36+/-5.93; P=0.0018 and 24.2+/-11.54 vs. 42.16+/-13.71; P=0.01). Viscosity was higher in patients with endothelial dysfunction, than it was in patients without it (0.064+/-0.01 vs. 0.051+/-0.0035; P=0.004). Compared with dippers, non-dippers showed an impaired EF (11.4+/-2% vs. 3.5+/-2.6%; p= 0.0006). The present data suggest the presence of disturbed endothelium-dependent vasodilatation and alterations in rheological indices in AH. According to the results obtained, the main factor, which leads to endothelial dysfunction and increase in platelet aggregative and adhesive activity, is the shortage in the lowering BP during the night. Consequently, hypertensive patients with non-dipper circadian blood pressure profile have to be assessed as a high risk group for development of future cardiovascular and cerebrovascular complications.

Design of an ex vivo culture system to investigate the effects of shear stress on cardiovascular tissue.

Mechanical forces are known to affect the biomechanical properties of native and engineered cardiovascular tissue. In particular, shear stress that results from the relative motion of heart valve leaflets with respect to the blood flow is one important component of their mechanical environment in vivo. Although different types of bioreactors have been designed to subject cells to shear stress, devices to expose biological tissue are few. In an effort to address this issue, the aim of this study was to design an ex vivo tissue culture system to characterize the biological response of heart valve leaflets subjected to a well-defined steady or time-varying shear stress environment. The novel apparatus was designed based on a cone-and-plate viscometer. The device characteristics were defined to limit the secondary flow effects inherent to this particular geometry. The determination of the operating conditions producing the desired shear stress profile was streamlined using a computational fluid dynamic (CFD) model validated with laser Doppler velocimetry. The novel ex vivo tissue culture system was validated in terms of its capability to reproduce a desired cone rotation and to maintain sterile conditions. The CFD results demonstrated that a cone angle of 0.5 deg, a cone radius of 40 mm, and a gap of 0.2 mm between the cone apex and the plate could limit radial secondary flow effects. The novel cone-and-plate permits to expose nine tissue specimens to an identical shear stress waveform. The whole setup is capable of accommodating four cone-and-plate systems, thus concomitantly subjecting 36 tissue samples to desired shear stress condition. The innovative design enables the tissue specimens to be flush mounted in the plate in order to limit flow perturbations caused by the tissue thickness. The device is capable of producing shear stress rates of up to 650 dyn cm(-2) s(-1) (i.e., maximum shear stress rate experienced by the ventricular surface of an aortic valve leaflet) and was shown to maintain tissue under sterile conditions for 120 h. The novel ex vivo tissue culture system constitutes a valuable tool toward elucidating heart valve mechanobiology. Ultimately, this knowledge will permit the production of functional tissue engineered heart valves, and a better understanding of heart valve biology and disease progression.

Microcirculatory dysfunction in cardiac syndrome X: role of abnormal blood rheology.

OBJECTIVE: Cardiac syndrome X (CSX) is of clinical interest, yet the underlying pathophysiological mechanisms have not been fully elucidated. It is well known that elevated blood viscosity and red blood cell (RBC) aggregation can adversely affect microcirculatory blood flow. The present study was designed to explore whether CSX is associated with abnormalities of blood rheology. METHODS: Blood samples were obtained from 152 adult angina patients undergoing diagnostic coronary angiography; geometric and flow-velocity data were obtained. Rheologic measurements were performed in a blinded manner; 21 subjects were later identified with CSX. Hemorheologic and clinical laboratory data were compared to 21 age- and gender-matched healthy controls. RESULTS: CSX patients had markedly abnormal blood rheology: (1) higher RBC aggregation and aggregability as judged by erythrocyte sedimentation rate and Myrenne indices at stasis and low shear (p < 0.001) and (2) elevated hematocrit-corrected blood viscosity, plasma viscosity (p < 0.001), and yield stress (p < 0.01). White blood cell counts and high-sensitivity C-reactive protein levels were significantly elevated in CSX; coronary-flow velocities were below normal. CONCLUSIONS: Abnormal hemorheologic parameters exist in subjects with CSX and may contribute to the pathophysiology of the disease, presumably via adversely affecting blood flow in the coronary microcirculation. Therapeutic measures aimed at normalizing blood rheology and hence microcirculatory flow should be explored.

Preliminary study of hemodynamic distribution in patient-specific stenotic carotid bifurcation by image-based computational fluid dynamics.

BACKGROUND: Regions prone to atherosclerosis, such as bends and bifurcations, tend to exhibit a certain degree of non-planarity or curvature, and these geometric features are known to strongly influence local flow patterns. Recently, computational fluid dynamics (CFD) has been used as a means of enhancing understanding of the mechanisms involved in atherosclerotic plaque formation and development. PURPOSE: To analyze flow patterns and hemodynamic distribution in stenotic carotid bifurcation in vivo by combining CFD with magnetic resonance angiography (MRA). MATERIAL AND METHODS: Twenty-one patients with carotid atherosclerosis proved by digital subtraction angiography (DSA) and/or Doppler ultrasound underwent contrast-enhanced MR angiography of the carotid bifurcation by a 3.0T MR scanner. Hemodynamic variables and flow patterns of the carotid bifurcation were calculated and visualized by combining vascular imaging postprocessing with CFD. RESULTS: In mild stenotic cases, there was much more streamlined flow in the bulbs, with reduced or disappeared areas of weakly turbulent flow. Also, the corresponding areas of low wall shear stress (WSS) were reduced or even disappeared. As the extent of stenosis increased, stronger blood jets formed at the portion of narrowing, and more prominent eddy flows and slow back flows were noted in the lee of the stenosis. Regions of elevated WSS were predicted at the portion of stenosis and in the path of the downstream jet. Areas of low WSS were predicted on the leeward side of the stenosis, corresponding with the location of slowly turbulent flows. CONCLUSION: CFD combined with MRA can simulate flow patterns and calculate hemodynamic variables in stenotic carotid bifurcations as well as normal ones. It provides a new method to investigate the relationship of vascular geometry and flow condition with atherosclerotic pathological changes.

Haemorheological parameters in young patients with acute myocardial infarction.

The role played by hemorheological alterations on acute myocardial infarction (AMI) in young patients remains a question of debate. We have carried out a case-control study of 84 AMI patients aged <45 years and 135 sex and age matched controls, in which blood viscosity (BV), plasma viscosity (PV), erythrocyte aggregation (EA) performed with the Myrenne (EA0, EA1) and the Sefam aggregometer (Ta, AI10, gammaD), erythrocyte deformability (ED) along with fibrinogen (Fbg), C-reactive protein (CRP) and plasmatic lipids i.e. total cholesterol (T-Chol) and triglycerides (TG) were determined. AMI patients showed higher, Fbg, TG, EA0, EA1, IA10, gammaD and lower Ta than controls (p=0.029, p<0.001, p=0.013, p=0.003, p=0.010, p=0.025) respectively. No differences in the other rheological parameters were observed. No differences in any rheological parameter were observed regarding the AMI type, number and score of stenosed vessels and the time elapsed since the thrombotic event. After multivariate adjustment, Fbg>380 ml/dl and TG>185 ml/dl were independently associated with a higher risk of erythrocyte hyperaggregability (OR: 5.5 CI 95% 1.04-29.27 and OR: 7.3 CI 95% 2.66-20.03) respectively. EA>8.85 was associated with a increased AMI risk (OR: 5.3 CI 95% 1.98-14.5). These results reinforces the view that in young AMI patients increased Fbg and TG may promote the development of ischaemic events not only through its known mechanism but also by altering rheological blood behaviour, mainly increasing EA.

Hemorheological abnormalities in stable angina and acute coronary syndromes.

The pathophysiological abnormalities of stable angina (SA) and acute coronary syndromes (ACS) may, in part, be promoted by fluid forces associated with local blood flow and hence by the rheological properties of blood. This study evaluated several hemorheological parameters in 16 healthy controls and in 16 SA, 18 unstable angina (UA) and 19 acute myocardial infarct (AMI) patients; all patients underwent diagnostic angiography following blood sampling. Rheological measurements included whole blood viscosity, plasma viscosity and RBC aggregation via erythrocyte sedimentation rate (ESR) and Myrenne aggregometer indices. Compared to controls, RBC aggregation was significantly elevated in all patient groups (p<0.001), with the rank being AMI>UA>SA. RBC aggregability as tested in 70 kDa dextran exceeded control in all patients. Blood viscosity values calculated at 40% Hct, plasma viscosity and yield shear stress values followed the same pattern (AMI>UA>SA>control); increases of inflammatory markers (i.e., WBC count, hs-CRP) were elevated in all patient groups in the order AMI>UA>SA. Our study thus indicates an association between hemorheological abnormalities and the severity of coronary artery disease, and suggests the merit of evaluating whether therapeutic interventions that normalize blood rheology may reduce the incidence and/or progression of coronary artery disease.

Hemorheological changes and characteristic parameters derived from whole blood viscometry in chronic heroin addicts.

A group of 15 chronic opioid addicts (DA) with mean age 26.5+/-7.3 years was studied by means of a rotational Contraves Low Shear 30 viscometer and the results have been compared with a control group of 19 healthy subjects. It was found that the mean whole blood viscosity values of the investigated group of heroin abusers (n=15) were elevated compared to that of healthy persons (n=19) over the whole shear rate range and fell by more than ten orders of magnitude (Savov et al., 2006). The present investigation uses the coefficients of the models of Ostwald-de-Walle (power law) and Herschel-Bulkley law, which describe whole blood flow curves (tau-gamma) within the shear rates range from 10(-2) to 10(2) s(-1) and itself incorporate whole blood viscosity data in the entire shear rate range. A significant difference in the mean yield shear stress tau(0) values of the drug abusers' group compared to the controls was found. A strong positive linear correlation was determined between the parameters of RBC aggregation in the group of heroin addicts confirming our previous results (Ivanov and Antonova, 2005; Savov, Zvetkova et al., 2007; Savov, Antonova et al., 2007) for intensive RBC and platelet aggregation and morphological changes in DA group.

Age dependent changes of arterial wall viscoelasticity.

Viscoelastic characteristics (VEC) of old rat aorta (Wistar, 10 months) were obtained by sinusoidal excitation of intraluminal pressure (p) in cylindrical arterial preparations. The pressure excitation frequency (f(exc)) was swept in the range 3-30 Hz up and down at several mean-pressure levels while response volume oscillations were recorded and resonance curves were plotted. Natural frequency (f(0)), dynamic modulus of elasticity (E') and coefficient of viscosity (beta) were estimated from resonance curves and the dependences of VEC on p were drawn. The results showed that f(0) decreased linearly with p whereas our previous data for young rat aorta (Wistar, 4 months) showed independence of f(0) on p. E' increased nonlinearly with p with the values being higher in comparison to young rat aorta. This means stiffening of rat aorta with age in accordance with the known literature data. beta-values increased linearly with p being higher in comparison to young rat aorta, demonstrative of raised intrinsic friction in the wall. VEC values were higher at decreasing f(exc) suggesting that the direction of excitation sweeping also determines the arterial wall biomechanical behaviour. It could be concluded that blood vessels VEC worsen with age, which endangers the arterial wall integrity, especially at higher intraluminal pressure.

Time dependent variation of human blood conductivity as a method for an estimation of RBC aggregation.

Time variation of whole human blood conductivity and shear stresses were investigated at rectangular and trapezium-shaped Couette viscometric flow under electric field of 2 kHz. The kinetics of conductivity signals were recorded both under transient flow and after the complete stoppage of shearing at shear rates from 0.94 to 94.5 s(-1) and temperatures T=25 degrees C and 37 degrees C. Contraves Low Shear 30 rotational viscometer as a base unit and a concurrent measuring system, including a device, developed by the conductometric method with a software for measurement of conductivity of biological fluids (data acquisition system), previously described (IFMBE Proceedings Series, Vol. 11, 2005, pp. 4247-4252; Clin. Hemorheol. Microcirc. 35(1/2) (2006), 19-29), were used for the experiments. The obtained experimental relationships show that the human blood conductivity is time, shear rate, hematocrit and temperature dependent under transient flow. It is also dependent on the regime of the applied shear rates. Non-linear curve approximation of the growth and relaxation curves is done. The results show that valuable information could be received about the kinetics of "rouleaux formation" and the time dependences of the blood conductivity follow the structural transformations of RBC aggregates during the aggregation-disaggregation processes. The results suggest that this technique may be used to clarify the mechanism of dynamics of RBC aggregates.

The coronary slow flow phenomenon: a new cardiac "Y" syndrome?

The coronary slow flow phenomenon (CSFP) is an angiographic finding that is characterised by delayed progression of the contrast medium during coronary angiography. The mechanism of this phenomenon remains unknown. In the present paper, we revise the current evidence regarding this phenomenon and discuss recent findings from our group reporting increased resting resistances in patients with the CSFP. We report that these patients had preserved blood flow responses to the intracoronary infusion of the vasodilator papaverine, demonstrating that the CSFP is not necessarily associated with an abnormal coronary flow reserve. Based on these findings and on the review of the current literature, we concur with the concept proposed by Beltrame et al. that the CSFP should be considered a separate clinical entity. Further studies are necessary to describe the clinical characteristics, including the prognosis, of these patients and to identify potential treatments.

Studies on the aggregation behaviour of pegylated human red blood cells with the Zeta sedimentation technique.

Covalent binding of poly(ethylene glycol), abbreviated as PEG, to red blood cells (RBC) surface leads to masking of the RBC blood group determinants and the PEG layer on the cell surface sterically hinders RBC-RBC and RBC-plasma protein interactions. We cross-linked linear mPEG-SPA of various molecular mass (2000, 5000, 20000) to washed human RBC under varying incubation ratios polymer to RBC. The electrophoretic mobility (EM) of the modified RBC decreases with increasing of chain length and concentration of PEG up to 50%. It may reflect the alteration in the surface layer thickness and friction. The aggregation behaviour of the pegylated RBC was studied with the Zeta sedimentation technique modifying the cell-cell interactions pressing them toward each other under centrifugal forces of various magnitudes. As a rule at low centrifugation forces the increase in chain length and concentration of PEG linked to RBC surface reduces the dextran-induced aggregation probably via elevation of the steric repulsion, which counteracts the depletion force generated by the free polymer. This effect was reversed to some extent by elevation of free dextran concentration and centrifugation forces. If cell-cell polymer bridging starts playing a role under these conditions requires further experimental and theoretical investigations.

On the effect of microstructural changes of blood on energy dissipation in Couette flow.

Red blood cell aggregation affects the flow of blood at low shear rates; not only the behaviour of the fluid deviates from its Newtonian characteristics, but, depending on the shearing history of the flow, the non-Newtonian characteristics may be influenced. It is not clear how the time and flow-dependent characteristics of the microstructural network developed in blood affect its mechanical properties. The present study aims to improve understanding of the effect of dynamic flow conditions on microstructural characteristics and consequently on the mechanical properties of the fluid. Viscosity measurements on blood samples from healthy volunteers (H=0.45) were taken with a double-walled Couette rheometric cell, under unsteady and quasi-unsteady flow conditions. The aggregation extent index A(alpha), and the microstructural integrity index A(I) were assessed with an optical shearing system and image analysis. Results showed that energy losses in Couette geometries may depend on the structural integrity of the developed RBC network.