Velocity and turbulence measurements past mitral valve prostheses in a model left ventricle

Differences in Pressure Recovery Between Balloon Expandable and Self-expandable Transcatheter Aortic Valves

Pressure recovery downstream of the aortic valve constitutes an important factor affecting the calculation of pressure gradient (PG) across the valve and therefore the accuracy of the calculated aortic valve area. Some clinical studies hypothesized that stent and valve cusps design contribute to flow acceleration and Doppler-measured valve gradients across the balloon-expandable transcatheter aortic valve. This study aims at elucidating the physical mechanisms behind pressure recovery variations between Edwards SAPIEN 3 and Medtronic Evolut TAVs through the measurements of sensitive and precise axial pressure profiles. A 23 mm Edwards SAPIEN3 and a 26 mm Medtronic Evolut were deployed in a pulse duplicator. A Millar catheter was used to record 50 cycles of pressure data along the centerline of the valve chamber upstream and downstream of the valve. The peak PG obtained with SAPIEN at vena contracta (VC) is 18.83 ± 0.75 mmHg and after recovery, 9.56 ± 0.78 mmHg. For Evolut at VC, peak PG is 18.25 ± 0.63 mmHg and after recovery, 10.3 ± 0.57 mmHg. The differences in peak PG at VC and at the recovery were statistically significant (p < 0.001). With SAPIEN 3 at VC, the mean PG obtained is 10.11 ± 0.63 mmHg and after recovery 7.06 ± 0.46 mmHg. For Evolut, mean PG at VC is 10.45 ± 0.67 mmHg and after recovery 7.99 ± 0.61 mmHg. The differences between the mean PG between the two valves was not statistically significant at VC (p = 0.71) but significant post-recovery (p < 0.00001). While gradients at the VC are higher with the SAPIEN 3, the net gradient after pressure recovery is significantly lower compared to Evolut TAV. Efficiency of pressure recovery significantly depends on valve type due to stent interference with the recovering blood flow.

In vitro hemodynamic assessment of a novel polymeric transcatheter aortic valve

Transcatheter aortic valve replacement (TAVR) is a life-saving alternative to surgical intervention. However, the identification of features associated with poor outcomes, including residual paravalvular leakage (PVL), leaflet calcification, and subclinical leaflet thrombosis, are cause to be concerned about valve durablilty (Mylotte and Piazza, 2015a, 2015b; Dasi et al., 2017; Makkar et al., 2015; Kheradvar et al., 2015a). The aim of this study is to optimize the potential of a hyaluronan (HA) enhanced polymeric transcatheter aortic valve (HA-TAV) that has promised to reduce blood damage causing-turbulent flow while maintaining durability. HA-enhanced linear low-density polyethylene (LLDPE) leaflets were sutured to novel cobalt chromium stents, size 26 mm balloon expandable stents. Hemodynamic performance was assessed in a left heart simulator under physiological pressure and flow conditions and compared to a 26 mm Medtronic Evolut and 26 mm Edwards SAPIEN 3. High-speed imaging and particle image velocimetry (PIV) were performed. The HA-TAV demonstrated an effective orifice area (EOA) within one standard deviation of the leading valve, SAPIEN 3.The regurgitant fraction (RF) of the HA-TAV (11.23 ± 0.55%) is decreased in comparison the Evolut (15.74 ± 0.73%) and slightly higher than the SAPIEN 3 (10.92 ± 0.11%), which is considered trace regurgitation according to valve standards. A decreased number of higher principal Reynolds shear stresses were shown for the HA-TAV at each cardiac phase. The HA-TAV is directly comparable and in some cases superior to the leading commercially available prosthetic heart valves in in-vitro hemodynamic testing.

Sinus Hemodynamics in Representative Stenotic Native Bicuspid and Tricuspid Aortic Valves: An In-Vitro Study

(1) The study’s objective is to assess sinus hemodynamics differences between stenotic native bicuspid aortic valve (BAV) and native tricuspid aortic valve (TrAV) sinuses in order to assess sinus flow shear and vorticity dynamics in these common pathological states of the aortic valve. (2) Representative patient-specific aortic roots with BAV and TrAV were selected, segmented, and 3D printed. The flow dynamics within the sinus were assessed in-vitro using particle image velocimetry in a left heart simulator at physiological pressure and flow conditions. Hemodynamic data calculations, vortex tracking, shear stress probability density functions and sinus washout calculations based on Lagrangian particle tracking were performed. (3) (a) At peak systole, velocity and vorticity in BAV reach 0.67 ± 0.02 m/s and 374 ± 5 s−1 versus 0.49 ± 0.03 m/s and 293 ± 3 s−1 in TrAV; (b) Aortic sinus vortex is slower to form but conserved in BAV sinus; (c) BAV shear stresses exceed those of TrAV (1.05 Pa versus 0.8 Pa); (d) Complete TrAV washout was achieved after 1.5 cycles while it was not for BAV. 4) In conclusion, sinus hemodynamics dependence on the different native aortic valve types and sinus morphologies was clearly highlighted in this study.

Stented valve dynamic behavior induced by polyester fiber leaflet material in transcatheter aortic valve devices

OBJECTIVE

This study aims at assessing the global dynamic behavior, elastic deformability, closing energy and turbulence of rigid versus deformable stented (RS vs DS) valve systems with deformable and rigid textile materials (DT vs RT) through studying the stent-valve interaction compared to a bioprosthetic material in transcatheter aortic valves (TAV).

METHODS

Three 19 mm stented textile TAV designs (RS-DT, RS-RT and DS-RT) with different stent and leaflet properties were tested and compared with a control bioprosthetic TAV (RS-DB) in a left heart simulator flow loop under physiological pressure and flow. Particle Image Velocimetry and high speed imaging were performed. Pressure gradients (PG), leakage fractions (LF), Pinwheeling indices (PI), closing energy (E) and Reynolds shear stresses (RSS) were calculated.

RESULTS

(a) PGs and LFs were 11.86 ± 0.51 mmHg, 11.70 ± 0.34%; 8.84 ± 0.40 mmHg, 29.80 ± 0.76%; 11.59 ± 0.12 mmHg, 14.23 ± 1.64%; and 7.05 ± 0.09 mmHg, 12.08 ± 0.45% % for RS-DB, RS-DT, RS-RT and DS-RT respectively. (b) PIs were 15.79 ± 2.34%, 4.36 ± 0.84%, 2.47 ± 0.51% and 2.03 ± 0.33% for RS-DB, RS-DT, RS-RT and DS-RT respectively. (c) E is lowest for DS-RT (0.0010 ± 0.0002 J) followed by RS-RT (0.0017 ± 0.0002 J), RS-DB (0.0023 ± 0.0004 J) and highest with RS-DT (0.0036 ± 0.0007 J). (d) At peak systole lowest RSS was obtained with RS-DT (87.82 ± 0.58 Pa) and highest with DS-RT (122.98 ± 1.87 Pa).

CONCLUSION

PGs, LFs, PIs and E were improved with DS-RT compared to other textile TAVs and RS-DB. Despite achieving more RSS than the rest of TAVs, DS-RT still falls within the same range of RSS produced by the other 2 valves and control exceeding the threshold for platelet activation.

An in vitro evaluation of turbulence after transcatheter aortic valve implantation

BACKGROUND

This study aimed at assessment of post-transcatheter aortic valve (TAV) replacement hemodynamics and turbulence when a same-size SAPIEN 3 (Edwards Lifesciences Corp, Irvine, Calif) and Medtronic Evolut (Minneapolis, Minn) were implanted in a rigid aortic root with physiological dimensions and in a representative root with calcific leaflets obtained from patient computed tomography scans.

METHODS

TAV hemodynamics were studied by placing a SAPIEN 3 26-mm and an Evolut 26-mm in rigid aortic roots and representative root with calcific leaflets under physiological conditions. Hemodynamics were assessed using high-fidelity particle image velocimetry and high-speed imaging. Transvalvular pressure gradients (PGs), pinwheeling indices, and Reynolds shear stress (RSS) were calculated.

RESULTS

(1) PGs obtained with the Evolut and the SAPIEN 3 were comparable among the different models (10.5 ± 0.15 mm Hg vs 7.76 ± 0.083 mm Hg in the rigid model along with 13.9 ± 0.19 mm Hg vs 5.0 ± 0.09 mm Hg in representative root with calcific leaflets obtained from patient computed tomography scans respectively); (2) more pinwheeling was found in the SAPIEN 3 than the Evolut (0.231 ± 0.057 vs 0.201 ± 0.05 in the representative root with calcific leaflets and 0.366 ± 0.067 vs 0.122 ± 0.045 in the rigid model); (3) higher rates of RSS were found in the Evolut (161.27 ± 3.45 vs 122.84 ± 1.76 Pa in representative root with calcific leaflets and 337.22 ± 7.05 vs 157.91 ± 1.80 Pa in rigid models). More lateral fluctuations were found in representative root with calcific leaflets.

CONCLUSIONS

(1) Comparable PGs were found among the TAVs in different models; (2) pinwheeling indices were found to be different between both TAVs; (3) turbulence patterns among both TAVs translated according to RSS were different. Rigid aortic models yield more conservative estimates of turbulence; (4) both TAVs exhibit peak maximal RSS that exceeds platelet activation 100 Pa threshold limit.

Ventricular flow Dynamic past Bileaflet Prosthetic Heart Valves

To characterise hydrodynamic properties of prosthetic heart valves in mitral position, ultrasonic velocity measurements were performed using a cardiovascular simulator. A Duromedics and a Saint-Jude Medical bileaflet heart valve were tested. The Saint-Jude valve was oriented first in an anatomical position, i.e. the tilt axis parallel to the septal wall, and then in an anti-anatomical position. In the anti-anatomical position, from mid diastole to mid systole, two contrarotative vortices are generated in the ventricle by the interaction between the flow directed by the leaflets downstream from the lateral orifices and the ventricular wall motions. In the anatomical position, the mitral flow penetrates the ventricle principally through the lateral orifice proximal to the septal wall, due to the vortex in the atrial chamber. The mitral inflow then circulates along the septal wall to the apex, and produces a large ventricular vortex during systole. In the anatomical position, the Saint-Jude thus provides a better ventricular washout during this phase. The mitral inflow through the Duromedics in the anti-anatomical position produces two contrarotative vortices in the ventricle, but in the opposite sense than downstream the Saint-Jude valve; the flows that penetrate through the lateral orifices are directed to the ventricular walls and then recirculate to the centre of the ventricle, providing a very fluctuating flow near the apex. Thus, a slight difference in valve design produces a significant difference in the ventricular flow fields.

High-speed visualization of disturbed pathlines in axial flow ventricular assist device under pulsatile conditions

OBJECTIVE

To investigate potentially prothrombotic flow patterns within an axial flow ventricular assist device under clinically relevant pulsatile hemodynamic conditions.

METHODS

A transparent replica of the HeartMate-II left ventricular assist device (Thoratec, Pleasanton, Calif) was visualized using a high speed camera at both low and high frame rates (125 and 3000 fps). Three steady-state conditions were studied: nominal (4.5 lpm), low flow (3.0 lpm), and high flow (6.0 lpm). Time-varying conditions were introduced with an external pulsatile pump that modulated the flow rate by approximately ± 50% of the mean, corresponding to a pulsatility index of 1.0.

RESULTS

At nominal and high flow rates, the path lines within the upstream region were generally stable, well attached, and streamlined. As the flow rate was reduced below 3.8 lpm, a rapid transition to a chaotic velocity field occurred, exhibiting a large toroidal vortex adjacent to the upstream bearing. The pathlines in the downstream stator section were consistently chaotic for all hemodynamic conditions investigated. It was common to observe tracer particles trapped within recirculation bubbles and drawn retrograde, causing repeated contact with the bearing surfaces. The addition of pulsatility caused the flow field to become periodically chaotic during the diastolic portion of the cardiac cycle depending on the instantaneous flow rate and acceleration.

CONCLUSIONS

The contribution of pulsatility by the native heart may induce a periodic disturbance to an otherwise stable flow field within an axial flow ventricular assist device, particularly during the diastolic and decelerating portion of the cardiac cycle. Potentially prothrombotic flow features were found to occur periodically in the region of the upstream bearing.

Polymeric trileaflet prosthetic heart valves: evolution and path to clinical reality

Present prosthetic heart valves, while hemodynamically effective, remain limited by progressive structural deterioration of tissue valves or the burden of chronic anticoagulation for mechanical valves. An idealized valve prosthesis would eliminate these limitations. Polymeric heart valves (PHVs), fabricated from advanced polymeric materials, offer the potential of durability and hemocompatibility. Unfortunately, the clinical realization of PHVs to date has been hampered by findings of in vivo calcification, degradation and thrombosis. Here, the authors review the evolution of PHVs, evaluate the state of the art of this technology and propose a pathway towards clinical reality. In particular, the authors discuss the development of a novel aortic PHV that may be deployed via transcatheter implantation, as well as its optimization via device thrombogenicity emulation.

The Effect of Vortex Formation on Left Ventricular Filling and Mitral Valve Efficiency

A new mechanism for quantifying the filling energetics in the left ventricle (LV) and past mechanical heart valves (MHV) is identified and presented. This mechanism is attributed to vortex formation dynamics past MHV leaflets. Recent studies support the conjecture that the natural healthy left ventricle (LV) performs in an optimum, energy-preserving manner by redirecting the flow with high efficiency. Yet to date, no quantitative proof has been presented. The present work provides quantitative results and validation of a theory based on the dynamics of vortex ring formation, which is governed by a critical formation number (FN) that corresponds to the dimensionless time at which the vortex ring has reached its maximum circulation content, in support of this hypothesis. Herein, several parameters (vortex ring circulation, vortex ring energy, critical FN, hydrodynamic efficiencies, vortex ring propagation speed) have been quantified and presented as a means of bridging the physics of vortex formation in the LV. In fact, the diastolic hydrodynamic efficiencies were found to be 60, 41, and 29%, respectively, for the porcine, anti-anatomical, and anatomical valve configurations. This assessment provides quantitative proof of vortex formation, which is dependent of valve design and orientation, being an important flow characteristic and associated to LV energetics. Time resolved digital particle image velocimetry with kilohertz sampling rate was used to study the ejection of fluid into the LV and resolve the spatiotemporal evolution of the flow. The clinical significance of this study is quantifying vortex formation and the critical FN that can potentially serve as a parameter to quantify the LV filling process and the performance of heart valves.

Flow in a mechanical bileaflet heart valve at laminar and near-peak systole flow rates: CFD simulations and experiments

Time-accurate, fully 3D numerical simulations and particle image velocity laboratory experiments are carried out for flow through a fully open bileaflet mechanical heart valve under steady (nonpulsatile) inflow conditions. Flows at two different Reynolds numbers, one in the laminar regime and the other turbulent (near-peak systole flow rate), are investigated. A direct numerical simulation is carried out for the laminar flow case while the turbulent flow is investigated with two different unsteady statistical turbulence modeling approaches, unsteady Reynolds-averaged Navier-Stokes (URANS) and detached-eddy simulation (DES) approach. For both the laminar and turbulent cases the computed mean velocity profiles are in good overall agreement with the measurements. For the turbulent simulations, however, the comparisons with the measurements demonstrate clearly the superiority of the DES approach and underscore its potential as a powerful modeling tool of cardiovascular flows at physiological conditions. The study reveals numerous previously unknown features of the flow.

PIV measurements of flow in a centrifugal blood pump: steady flow

Magnetically suspended left ventricular assist devices have only one moving part, the impeller. The impeller has absolutely no contact with any of the fixed parts, thus greatly reducing the regions of stagnant or high shear stress that surround a mechanical or fluid bearing. Measurements of the mean flow patterns as well as viscous and turbulent stresses were made in a shaft-driven prototype of a magnetically suspended centrifugal blood pump at several constant flow rates (3-9 L/min) using particle image velocimetry (PIV). The chosen range of flow rates is representative of the range over which the pump may operate while implanted. Measurements on a three-dimensional measurement grid within several regions of the pump, including the inlet, blade passage, exit volute, and diffuser are reported. The measurements are used to identify regions of potential blood damage due to high shear stress and/or stagnation of the blood, both of which have been associated with blood damage within artificial heart valves and diaphragm-type pumps. Levels of turbulence intensity and Reynolds stresses that are comparable to those in artificial heart valves are reported. At the design flow rate (6 L/min), the flow is generally well behaved (no recirculation or stagnant flow) and stress levels are below levels that would be expected to contribute to hemolysis or thrombosis. The flow at both high (9 L/min) and low (3 L/min) flow rates introduces anomalies into the flow, such as recirculation, stagnation, and high stress regions. Levels of viscous and Reynolds shear stresses everywhere within the pump are below reported threshold values for damage to red cells over the entire range of flow rates investigated; however, at both high and low flow rate conditions, the flow field may promote activation of the clotting cascade due to regions of elevated shear stress adjacent to separated or stagnant flow.

Time-resolved DPIV analysis of vortex dynamics in a left ventricular model through bileaflet mechanical and porcine heart valve prostheses

The performance of the heart after a mitral valve replacement operation greatly depends on the flow character downstream of the valve. The design and implanting orientation of valves may considerably affect the flow development. A study of the hemodynamics of two orientations, anatomical and anti-anatomical, of the St. Jude Medical (SJM) bileaflet valve are presented and compared with those of the SJM Biocor porcine valve, which served also to represent the natural valve. We document the velocity field in a flexible, transparent (LV) using time-resolved digital particle image velocimetry (TRDPIV). Vortex formation and vortex interaction are two important physical phenomena that dominate the filling and emptying of the ventricle. For the three configurations, the following effects were examined: mitral valve inlet jet asymmetry, survival of vortical structures upstream of the aortic valve, vortex-induced velocities and redirection of theflow in abidance of the Biot-Savart law, domain segmentation, resonant times of vortical structures, and regions of stagnantflow. The presence of three distinct flow patterns, for the three configurations, was identified by the location of vortical structures and level of coherence corresponding to a significant variation in the turbulence level distribution inside the LV. The adverse effect of these observations could potentially compromise the efficiency of the LV and result in flow patterns that deviate from those in the natural heart.

An approach to the simulation of fluid-structure interaction in the aortic valve

A pair of finite element models has been employed to study the interaction of blood flow with the operation of the aortic valve. A three-dimensional model of the left ventricle with applied wall displacements has been used to generate data for the spatially and time-varying blood velocity profile across the aortic aperture. These data have been used as the inlet loading conditions in a three-dimensional model of the aortic valve and its surrounding structures. Both models involve fluid-structure interaction and simulate the cardiac cycle as a dynamic event. Confidence in the models was obtained by comparison with data obtained in a pulse duplicator. The results show a circulatory flow being generated in the ventricle which produces a substantially axial flow through the aortic aperture. The aortic valve behaves in an essentially symmetric way under the action of this flow, so that the pressure difference across the leaflets is approximately uniform. This work supports the use of spatially uniform but temporally variable pressure distributions across the leaflets in dry or structural models of aortic valves. The study is a major advance through its use of truly three-dimensional geometry, spatially non-uniform loading conditions for the valve leaflets and the successful modelling of progressive contact of the leaflets in a fluid environment.

Unsteady effects on the flow across tilting disk valves

The present study simulates numerically the flow across two-dimensional tilting disk models of mechanical heart valves. The time-dependent Navier-Stokes equations are solved to assess the importance of unsteady effects in the fully open position of the valve. Flow cases with steady or physiological inflow conditions and with fixed or moving valves are solved. The simulations lead into mixed conclusions. It is obvious that steady inflow cases that account for vortex shedding only cannot model realistic physiological cases. In cases with imposed physiological inflow, the details of the flow field for fixed and moving valves might differ in the fully open position as well, although the gross features are quite similar. The fixed valve case consistently results in safe estimations of several critical quantities such as the axial force, the maximal shear stress on the valve, or the transvalvular pressure drop. Thus, fixed valve simulations can provide useful information for the design of prosthetic heart valves, as long as the properties in the fully open position only are sought.

Effects of tilting disk heart valve gap width on regurgitant flow through an artificial heart mitral valve

While many investigators have measured the turbulent stresses associated with forward flow through tilting disk heart valves, only recently has attention been given to the regurgitant jets formed as fluid is squeezed through the gap between the occluder and housing of a closed valve. The objective of this investigation was to determine the effect of gap width on the turbulent stresses of the regurgitant jets through a Björk-Shiley monostrut tilting disk heart valve seated in the mitral position of a Penn State artificial heart. A 2 component laser-Doppler velocimetry system with a temporal resolution of 1 ms was used to measure the instantaneous velocities in the regurgitant jets in the major and minor orifices around the mitral valve. The gap width was controlled through temperature variation by taking advantage of the large difference between the thermal expansion coefficients of the Delrin occluder and the Stellite housing of Björk-Shiley monostrut valves. The turbulent shear stress and mean (ensemble averaged) velocity were incorporated into a model of red blood cell damage to assess the potential for hemolytic damage at each gap width investigated. The results revealed that the minor orifice tends to form stronger jets during regurgitant flow than the major orifice, indicating that the gap width is not uniform around the circumference of the valve. Based on the results of a red blood cell damage model, the hemolytic potential of the mitral valve decreases as the gap width increases. This investigation also established that the hemolytic potential of the regurgitant phase of valve operation is comparable to, if not greater than, the hemolytic potential of forward flow, consistent with experimental data on hemolysis.

Nonsymmetric leaflet motion of St. Jude Medical mitral valves simulated with a computer-controlled hydraulic mock circulator

Since the introduction of high-performance prosthetic heart valves, particularly bileaflet valves and monoleaflet valves with larger opening angles, we have observed in vivo complex leaflet motion which has not been experimentally confirmed yet. We developed a computer-controlled hydraulic mock circulator to study the motion of in vivo leaflets. A high speed CCD camera recorded the valve movement. In the mitral position, a standard St. Jude Medical valve was tilted to be horizontal or vertical. The test valve was driven with single- or double-peaked flow. The flow rate was set to 5.0 L/min at 70 bpm with a systole/diastole ratio of 0.3. We found the following results: independent of valve orientation, the valve showed a nonsymmetric leaflet motion; the valve showed unpredictable leaflet position during decreasing flow or absence of flow; and the disc closed temporarily at the lower inflow rate between the 2 flow peaks.

The effect of varying degrees of stenosis on the characteristics of turbulent pulsatile flow through heart valves

Many problems and complications associated with heart valves are related to the dynamic behavior of the valve and the resultant unsteady flow patterns. An accurate depiction of the spatial and temporal velocity and rms distributions imparts better understanding of flow related valve complications, and may be used as a guideline in valve design. While the generalized correlation between increased turbulence level and the severity of the stenosis is well established, few studies addressed the issue of the intermittent nature of turbulence and its timing in the cardiac cycle, and almost none assessed the effect of a progressive stenosis on the flow characteristics through heart valves. In this experimental work we simulated the type of flow which is present in normal and stenosed valves and conducted a comprehensive investigation of valve hemodynamics, valvular turbulence and morphology under varying degrees of stenosis. The characteristics of valves and stenoses were simulated closely, to achieve the flow conditions that initiate turbulent flow conditions. Laser Doppler anemometry (LDA) measurements were carried out in a pulse duplicator system distal to trileaflet polyurethane prosthetic heart valves, installed at mitral and aortic positions. The effect of the degree of the stenosis was comparatively studied through the structure of the turbulent jets emerging from normal and stenotic heart valves. Maximum turbulence level was achieved during the decelerating phase and correlated to the severity of the stenosis, followed by relaminarization of the flow during the acceleration phase. The intermittent nature of the turbulence emphasized the importance of realizing the timing of the turbulence production and its spatial location for optimizing current valve designs. The plug flow through the normal aortic valve prosthesis was replaced by jet like behavior for a 65% stenosis, with the jet becoming narrower and stronger for a 90% stenosis. The morphology of the velocity and turbulence waveforms was found to be governed by the stenosis geometry and the valve position (aortic, mitral).

Monitoring combined antithrombotic treatments in patients with prosthetic heart valves using transcranial Doppler and coagulation markers

BACKGROUND AND PURPOSE

The combined use of coumarin and low-dose aspirin appears to reduce the risk of systemic embolism at a low risk of bleeding. The remaining incidence of embolism of approximately 2%/y is still high. Methods for real-time detection of embolic events have not been used thus far to monitor the efficacy of therapeutic strategies. They might permit individually tailored, effective treatments.

METHODS

The frequency of embolic signals in both middle cerebral arteries was monitored using a two-channel 2-MHz transcranial Doppler system. We examined five patients with mechanical prosthetic heart valves suffering from recurrent cerebral ischemic symptoms despite adequate anticoagulant therapy (international normalized ratio, 3.0 to 4.3). Measurements were performed on coumarin alone (four baseline values) and subsequent to the addition of intravenous (500 mg bolus) and oral (100 mg/d for 10 days) aspirin or intravenous (5000 IU bolus) heparin. The prothrombotic markers thrombin-antithrombin III complex, fibrinopeptide A, D-dimer, and platelet beta-thromboglobulin were measured simultaneously.

RESULTS

None of the combined drug regimens led to a significant reduction of the emboli count. The values of thrombin-antithrombin III complex, fibrinopeptide A, and D-dimer were already within normal limits with coumarin alone. The beta-thromboglobulin levels, however, were increased, and additional aspirin or heparin did not reduce them. There was no correlation between the emboli count and the prothrombotic markers or between the prothrombotic markers and the different drug regimens.

CONCLUSIONS

The rate of cerebral emboli measured with transcranial Doppler in the group of high-risk patients studied was not influenced by additional antiplatelet therapy. The emboli are likely to be composed at least in part of platelets, and their generation seems not dependent on thrombin or cyclooxygenase. There is an apparent discrepancy between the unchanged rate of emboli during Doppler monitoring found in this and other studies and the partial efficacy of combined treatment with coumarin and aspirin in clinical long-term studies. This may be explained by differences in the composition or size of the emboli.

A relationship between Reynolds stresses and viscous dissipation: implications to red cell damage

Viscous shearing is examined as a mechanism by which turbulent flows can cause cellular damage. The use of Reynolds stress as an indicator of hemolysis is considered, and an alternative measure based on viscous dissipation is proposed. It is shown that under simple flow conditions the Reynolds stresses can be related to viscous dissipation. Data from the literature show that the instantaneous viscous shear stress at which hemolysis occurs is similar to the shear stress thresholds obtained from laminar flow studies. Also, the Kolmogorov length scales for most of the turbulent hemolysis studies are similar to the size of a red blood cell. These observations indicate that, for the jet and couette experiments examined, viscous shearing is an important mechanism in the destruction of erythrocytes by turbulence. However, pressure fluctuations may also contribute to damage for these cells and for cells of similar or larger size.

A correlation method for determining the number of sampling cycles required for pulsating flow analysis using a LDA

A correlation method has been developed for pulsatile flow analysis, which allows justification of the minimum number of cycles for ensemble averaging, to satisfy ergodicity. It can also be used to identify cycles with excessive fluctuations caused by artefact or incidental errors and, hence, enhance the accuracy of analysis.

Effects of local geometry and fluid dynamics on regional platelet deposition on artificial surfaces

An important aspect of blood-material interactions is the activation, adhesion, and subsequent aggregation of blood platelets on the artificial surface, all of which are directly affected by local fluid dynamics. The objective of this work was to directly correlate changing local fluid dynamic conditions produced by various vessel geometries, including stenosis, aneurysm, and separate contraction and expansion geometries, with quantitative in vitro measurements of regional platelet deposition. We directly measured platelet deposition as a function of axial position along four Lexan flow chambers with axisymmetric models of these geometries using 111In-labeled platelets. Platelet deposition was maximum in observed areas of flow recirculation and reattachment and minimum in locations of high shear and separation. For the stenosis geometry, two distinct regions of increased platelet deposition were apparent, one proximal to and one distal to the stenosis throat. An approximately linear increase in platelet densities was produced in the aneurysm region, increasing in the direction of flow. Through a comparison of platelet deposition with local fluid streamline orientation, we have shown that platelet deposition is increased in certain areas due to the enhanced convective transport of platelets and blood cells to the vessel wall along locally curved streamlines with velocity components perpendicular to the vessel wall.