Numerical Investigation of the Performance of Three Hinge Designs of Bileaflet Mechanical Heart Valves

Thromboembolic complications (TECs) of bileaflet mechanical heart valves (BMHVs) are believed to be due to the nonphysiologic mechanical stresses imposed on blood elements by the hinge flows. Relating hinge flow features to design features is, therefore, essential to ultimately design BMHVs with lower TEC rates. This study aims at simulating the pulsatile three-dimensional hinge flows of three BMHVs and estimating the TEC potential associated with each hinge design. Hinge geometries are constructed from micro-computed tomography scans of BMHVs. Simulations are conducted using a Cartesian sharp-interface immersed-boundary methodology combined with a second-order accurate fractional-step method. Leaflet motion and flow boundary conditions are extracted from fluid–structure-interaction simulations of BMHV bulk flow. The numerical results are analyzed using a particle-tracking approach coupled with existing blood damage models. The gap width and, more importantly, the shape of the recess and leaflet are found to impact the flow distribution and TEC potential. Smooth, streamlined surfaces appear to be more favorable than sharp corners or sudden shape transitions. The developed framework will enable pragmatic and cost-efficient preclinical evaluation of BMHV prototypes prior to valve manufacturing. Application to a wide range of hinges with varying design parameters will eventually help in determining the optimal hinge design.     

Characterization of Hemodynamic Forces Induced by Mechanical Heart Valves: Reynolds vs. Viscous Stresses

Bileaflet mechanical heart valves (BMHV) are widely used to replace diseased heart valves. Implantation of BMHV, however, has been linked with major complications, which are generally considered to be caused by mechanically induced damage of blood cells resulting from the non-physiological hemodynamics environment induced by BMHV, including regions of recirculating flow and elevated Reynolds (turbulence) shear stress levels. In this article, we analyze the results of 2D high-resolution velocity measurements and full 3D numerical simulation for pulsatile flow through a BMHV mounted in a model axisymmetric aorta to investigate the mechanical environment experienced by blood elements under physiologic conditions. We show that the so-called Reynolds shear stresses neither directly contribute to the mechanical load on blood cells nor is a proper measurement of the mechanical load experienced by blood cells. We also show that the overall levels of the viscous stresses, which comprise the actual flow environment experienced by cells, are apparently too low to induce damage to red blood cells, but could potentially damage platelets. The maximum instantaneous viscous shear stress observed throughout a cardiac cycle is <15 N/m². Our analysis is restricted to the flow downstream of the valve leaflets and thus does not address other areas within the BMHV where potentially hemodynamically hazardous levels of viscous stresses could still occur (such as in the hinge gaps and leakage jets).     

Simulation of the Three-Dimensional Hinge Flow Fields of a Bileaflet Mechanical Heart Valve Under Aortic Conditions

Thromboembolic complications of bileaflet mechanical heart valves (BMHV) are believed to be due to detrimental stresses imposed on blood elements by the hinge flows. Characterization of these flows is thus crucial to identify the underlying causes for complications. In this study, we conduct three-dimensional pulsatile flow simulations through the hinge of a BMHV under aortic conditions. Hinge and leaflet geometries are reconstructed from the Micro-Computed Tomography scans of a BMHV. Simulations are conducted using a Cartesian sharp-interface immersed-boundary methodology combined with a second-order accurate fractional-step method. Physiologic flow boundary conditions and leaflet motion are extracted from the Fluid–Structure Interaction simulations of the bulk of the flow through a BMHV. Calculations reveal the presence, throughout the cardiac cycle, of flow patterns known to be detrimental to blood elements. Flow fields are characterized by: (1) complex systolic flows, with rotating structures and slow reverse flow pattern, and (2) two strong diastolic leakage jets accompanied by fast reverse flow at the hinge bottom. Elevated shear stresses, up to 1920 dyn/cm² during systole and 6115 dyn/cm² during diastole, are reported. This study underscores the need to conduct three-dimensional simulations throughout the cardiac cycle to fully characterize the complexity and thromboembolic potential of the hinge flows.     

Micro Particle Image Velocimetry Measurements of Steady Diastolic Leakage Flow in the Hinge of a St. Jude Medical® Regent™ Mechanical Heart Valve

A number of clinical, in vitro and computational studies have shown the potential for thromboembolic complications in bileaflet mechanical heart valves (BMHV), primarily due to the complex and unsteady flows in the valve hinges. These studies have focused on quantitative and qualitative parameters such as velocity magnitude, turbulent shear stresses, vortex formation, and platelet activation to identify potential for blood damage. However, experimental characterization of the whole flow fields within the valve hinges has not yet been conducted. This information can be utilized to investigate instantaneous damage to blood elements and also to validate numerical studies focusing on the hinge’s complex fluid dynamics. The objective of this study was therefore to develop a high-resolution imaging system to characterize the flow fields and global velocity maps in a BMHV hinge. In this study, the steady leakage hinge flow fields representing the diastolic phase during the cardiac cycle in a 23 mm St. Jude Medical regent BMHV in the aortic position were characterized using a two-dimensional micro particle image velocimetry system. Diastolic flow was simulated by imposing a static pressure head on the aortic side. Under these conditions, a reverse flow jet from the aortic to the ventricular side was observed with velocities in the range of 1.47–3.24 m/s, whereas low flow regions were observed on the ventricular side of the hinge with viscous shear stress magnitude up to 60 N/m². High velocities and viscous shearing may be associated with platelet activation and hemolysis, while low flow zones can cause thrombosis due to increased residence time in the hinge. Overall, this study provides a high spatial resolution experimental technique to map the fluid velocity in the BMHV hinge, which can be extended to investigate micron-scale flow domains in various prosthetic devices under different hemodynamic conditions.     

Numerical investigation of the effects of channel geometry on platelet activation and blood damage

Thromboembolic complications in Bileaflet mechanical heart valves (BMHVs) are believed to be due to the combination of high shear stresses and large recirculation regions. Relating blood damage to design geometry is therefore essential to ultimately optimize the design of BMHVs. The aim of this research is to quantitatively study the effect of 3D channel geometry on shear-induced platelet activation and aggregation, and to choose an appropriate blood damage index (BDI) model for future numerical simulations. The simulations in this study use a recently developed lattice-Boltzmann with external boundary force (LBM-EBF) method [Wu, J., and C. K. Aidun. Int. J. Numer. Method Fluids 62(7):765–783, 2010; Wu, J., and C. K. Aidun. Int. J. Multiphase flow 36:202–209, 2010]. The channel geometries and flow conditions are re-constructed from recent experiments by Fallon [The Development of a Novel in vitro Flow System to Evaluate Platelet Activation and Procoagulant Potential Induced by Bileaflet Mechanical Heart Valve Leakage Jets in School of Chemical and Biomolecular Engineering. Atlanta: Georgia Institute of Technology] and Fallon et al. [Ann. Biomed. Eng. 36(1):1]. The fluid flow is computed on a fixed regular ‘lattice’ using the LBM, and each platelet is mapped onto a Lagrangian frame moving continuously throughout the fluid domain. The two-way fluid–solid interactions are determined by the EBF method by enforcing a no-slip condition on the platelet surface. The motion and orientation of the platelet are obtained from Newtonian dynamics equations. The numerical results show that sharp corners or sudden shape transitions will increase blood damage. Fallon’s experimental results were used as a basis for choosing the appropriate BDI model for use in future computational simulations of flow through BMHVs.     

Comparison of the Hinge Flow Fields of Two Bileaflet Mechanical Heart Valves under Aortic and Mitral Conditions

Background: Animal and clinical studies have shown that bileaflet mechanical heart valve designs are plagued by thromboembolic complications, with higher rates in the mitral than in the aortic position. This study evaluated the hinge flow dynamic of the 23 mm St. Jude Medical (SJM) Regent and the 23 mm CarboMedics (CM) valves under aortic conditions and compared these results with previous findings under mitral conditions. Method: Velocity and Reynolds shear stress fields were captured using two-component laser Doppler velocimetry. Results: Under aortic conditions, both the SJM and CM hinge flow fields exhibited a strong forward flow pattern during systole (maximum velocities of 2.31 and 1.75 m/s, respectively) and two main leakage jets during diastole (maximum velocities of 3.08 and 2.27 m/s, respectively). Conclusions: Aortic and mitral flow patterns within the two hinges were similar, but with a more dynamic flow during the forward flow phase under aortic conditions. Velocity magnitudes and shear stresses measured under mitral conditions were generally higher than those obtained in the aortic position, which may explain the higher rates of thromboembolism in the mitral implants when compared with the aortic implants.     

Hemodynamic Performance and Thrombogenic Properties of a Superhydrophobic Bileaflet Mechanical Heart Valve

In this study, we explore how blood-material interactions and hemodynamics are impacted by rendering a clinical quality 25 mm St. Jude Medical Bileaflet mechanical heart valve (BMHV) superhydrophobic (SH) with the aim of reducing thrombo-embolic complications associated with BMHVs. Basic cell adhesion is evaluated to assess blood-material interactions, while hemodynamic performance is analyzed with and without the SH coating. Results show that a SH coating with a receding contact angle (CA) of 160° strikingly eliminates platelet and leukocyte adhesion to the surface. Alternatively, many platelets attach to and activate on pyrolytic carbon (receding CA = 47), the base material for BMHVs. We further show that the performance index increases by 2.5% for coated valve relative to an uncoated valve, with a maximum possible improved performance of 5%. Both valves exhibit instantaneous shear stress below 10 N/m² and Reynolds Shear Stress below 100 N/m². Therefore, a SH BMHV has the potential to relax the requirement for antiplatelet and anticoagulant drug regimens typically required for patients receiving MHVs by minimizing blood-material interactions, while having a minimal impact on hemodynamics. We show for the first time that SH-coated surfaces may be a promising direction to minimize thrombotic complications in complex devices such as heart valves.     

A review of state-of-the-art numerical methods for simulating flow through mechanical heart valves

In nearly half of the heart valve replacement surgeries performed annually, surgeons prefer to implant bileaflet mechanical heart valves (BMHV) because of their durability and long life span. All current BMHV designs, however, are prone to thromboembolic complications and implant recipients need to be on a life-long anticoagulant medication regiment. Non-physiologic flow patterns and turbulence generated by the valve leaflets are believed to be the major culprit for the increased risk of thromboembolism in BMHV implant recipients. In this paper, we review recent advances in developing predictive fluid–structure interaction (FSI) algorithms that can simulate BMHV flows at physiologic conditions and at resolution sufficiently fine to start probing the links between hemodynamics and blood-cell damage. Numerical simulations have provided the first glimpse into the complex hemodynamic environment experienced by blood cells downstream of the valve leaflets and successfully resolved for the first time the experimentally observed explosive transition to a turbulent-like state at the start of the decelerating flow phase. The simulations have also resolved a number of subtle features of experimentally observed valve kinematics, such as the asymmetric opening and closing of the leaflets and the leaflet rebound during closing. The paper also discusses a future research agenda toward developing a powerful patient-specific computational framework for optimizing valve design and implantation in a virtual surgery environment.     

Effect of Hypertension on the Closing Dynamics and Lagrangian Blood Damage Index Measure of the B-Datum Regurgitant Jet in a Bileaflet Mechanical Heart Valve

We hypothesize that the formation of the closing vortex and subsequent b-datum regurgitation jet in bileaflet mechanical heart valves is governed by the magnitude of the driving mean aortic pressure (MAP), and that this sensitivity does impact the blood damage index (BDI) corresponding to platelet activation and lysis. High spatial resolution time resolved (1 kHz) as well as phase locked particle image velocimetry techniques captured the dynamic leaflet closure and regurgitation jet of a model 25 mm St. Jude Medical BMHV. Cell trajectories were estimated using Lagrangian particle tracking analysis while the leaflet kinematics was quantified by tracking the leaflet tip-position throughout closure. The non-principal as well as principal shear stress loading histories along each cell trajectory revealed BDI for platelet activation and lysis as a function of cell initial position, release time-point, and blood pressure. Results show that the leaflet closing time reduces by roughly 10 ms, in response to an increase in MAP by 40 mmHg. We report that higher MAP leads to a stronger b-datum vortex and jet formation. Platelet activation BDI lowers with a higher MAP due to reduction in exposure times despite an increase in principal shear stress experienced. Platelet lysis BDI however increases with higher MAP. Maximum BDI may occur for cells initially in the b-datum zone during the onset of leaflet closure. Our results provide a better understanding of BDI of the regurgitant b-datum jet and sheds light on the potential importance of blood damage testing under hypertensive conditions.     

Spatio-temporal Flow Analysis in Bileaflet Heart Valve Hinge Regions: Potential Analysis for Blood Element Damage

Point-wise velocity measurements have been traditionally acquired to estimate blood damage potential induced by prosthetic heart valves with emphasis on peak values of velocity magnitude and Reynolds stresses. However, the inherently Lagrangian nature of platelet activation and hemolysis makes such measurements of limited predictive value. This study provides a refined fluid mechanical analysis, including blood element paths and stress exposure times, of the hinge flows of a CarboMedics bileaflet mechanical heart valve placed under both mitral and aortic conditions and a St Jude Medical bileaflet valve placed under aortic conditions. The hinge area was partitioned into characteristic regions based on dominant flow structures and spatio-temporal averaging was performed on the measured velocities and Reynolds shear stresses to estimate the average bulk stresses acting on blood elements transiting through the hinge. A first-order estimate of viscous stress levels and exposure times were computed. Both forward and leakage flow phases were characterized in each partition by dynamic flows dependent on subtle leaflet movements and transvalvular pressure fluctuations. Blood elements trapped in recirculation regions may experience exposure times as long as the entire forward flow phase duration. Most calculated stresses were below the accepted blood damage threshold. Estimates of the stress levels indicate that the flow conditions within the boundary layers near the hinge and leaflet walls may be more detrimental to blood cells than bulk flow conditions, while recirculation regions may promote thrombus buildup.     

Velocities,shear stresses and blood damage potential of the leakage jets of the Medtronic Parallel bileaflet valve

Even nowadays, the essential problem of mechanical heart valve prostheses is the risk of thromboembolic events mainly caused by unnatural hemodynamics, e.g. just a few years ago the Medtronic Parallel (MP) showed unsatisfactory clinical results caused by thrombi. Therefore, in vitro investigations of the whole leakage jets were performed at the MP in mitral position by means of a pulse duplicator using a two channel laser Doppler anemometer. From the measured data, mean velocity profiles and the distribution of Reynolds shear stresses, as a function of the location within the jet, were calculated. From this data the potential of blood damage is evaluated computing a Blood Damage Index (BDI) of hemolysis and platelet damage. Four regurgitant free jets right above the hinges were observed during systole at the inflow side of the MP. The peak velocities at the origin of the jets were in the order of 1.6-2.1 m/s. Two jets experienced maximum turbulent shear stresses around 100 N/m2 within this area. The BDI for platelets of the MP is around ten times higher than the BDI of the St.-Jude-Medical. The study shows that besides the flow structure within the hinges of a mechanical heart valve, the whole regurgitant jet has a large blood damage potential. This potential is measurable, respectively calculable and seems to be (on account of it's support of the clinical outcome) one piece of the puzzle that explains the negative trials of the MP.     

Flow analysis of the bileaflet mechanical prosthetic heart valves using laser Doppler anemometer: Effect of the valve designs and installed orientations to the flow inside the simulated left ventricle

Ever since the first introduction of the ball-type valve by Hufnagel in 1952, which was installed in the descending aorta to correct aortic valve insufficiency, great efforts have been aimed to produce a hemodynamically and structurally superior prosthetic heart valve. Bileaflet valves, commercially initiated by the St. Jude medical (SJM) valve, perform satisfactorily, and now the majority of the mechanical-type prosthetic heart valves used clinically are of this type. The recent trend in bileaflet valve design seems to be concentrated on the hinge mechanism and leaflet design to improve performance against thromboembolic complications and hemolysis. This paper studied the effects of hinge location, leaflet configuration, valve opening angle, and valve installed orientation to the flow field inside the simulated ventricle using laser Doppler anemometry. As a model prosthetic valve, the SJM valve was selected as a reference, and newer bileaflet valves, including the ATS, the Carbomedics (CM), and the Jyros (JR) valves, were selected for comparison. The test program also utilized a flow visualization technique to map the velocity field inside the simulated ventricle to complement the information obtained using the LDA system. Comparison of the velocity profiles at corresponding flow phases revealed the effects of the differences in valve design and orientation. Based on precise examination of the data, the following general conclusions can be made: all valves (SJM, ATS, CM, and JR) show distinct circulatory flow patterns when the valve is installed in the antianatomical orientation. The small differences in hinge location and leaflet configuration can generate noticeable differences, particularly during the accelerating flow phase of the valve. The ATS and the CM valves open less during the forward flow phase, and this results in generally diverse and less distinct flow patterns and slower velocity. This is particularly noticeable for the flow through the central orifice. The SJM valve maintains a relatively higher velocity through the central orifice. The curved leaflet JR valve generates higher but divergent flow during the accelerating and peak flow phases.     

Advances in prosthetic heart valves: fluid mechanics of aortic valve designs

The in vitro hemodynamic characteristics of a variety of mechanical and tissue heart valve designs used during the past two decades were investigated in the aortic position under pulsatile flow conditions. The following valve designs were studied: Starr-Edwards ball and cage (model 1260), Bj?rk-Shiley tilting disc (convexo-concave model), Medtronic-Hall tilting disc, St. Jude Medical bileaflet, Carpentier-Edwards porcine and pericardial (models 2625, 2650 and 2900), Hancock porcine (models 250 and 410) and Ionescu-Shiley standard pericardial. The Starr-Edward ball and cage, Bj?rk-Shiley tilting disc, Carpentier-Edwards porcine (model 2625) and Ionescu-Shiley standard pericardial valves were designed prior to 1975, while the Medtronic-Hall tilting disc, St. Jude Medical bileaflet, Hancock porcine (model 250), Hancock II porcine (model 410), Carpentier-Edwards porcine (model 2650) and Carpentier-Edwards pericardial (model 2900) valves were designed after 1975. The pressure drop results indicated that the valves designed prior to 1975 had performance indices of 0.30 to 0.45, whereas the valves designed after 1975 had performance indices of 0.40 to 0.70. The regurgitant volumes were higher for the mechanical designs (5.0 to 11.0 cm3/beat) compared to the tissue bioprostheses (1.0 to 5.0 cm3/beat). Two-dimensional laser Doppler anemometry studies indicated that the valves designed after 1975 tended to create more centralized flow fields, with reduced levels of turbulent shear stresses. However, none of the current valve designs is ideal: they all create areas of stasis and/or regions of low velocity reverse flow; and regions of elevated turbulent shear stresses that are capable of causing sub-lethal and/or lethal damage to the formed elements of blood.     

Relative blood damage index of the jellyfish valve and the Bjork-Shiley tilting-disk valve

Determination of the potential for blood cell damage induced by artificial heart valves is essential in deciding the suitability of the valve for clinical use. Both the magnitude and the duration of the shear stress influence the onset and severity of the damage to the constitutents of blood. In this study, in vitro shear stress measurements of the mitral jellyfish and Bjork-Shiley tilting-disk (mono) prosthetic valves under physiological pulsatile flow conditions were conducted. The data indicate that elevated levels of shear stress occurred mainly 1D downstream from both valves. With the aid of a mathematical model and using the elevated shear stress data, the relative release of hemoglobin by damaged red blood cells and of lactate dehydrogenase by platelets was computed for both valves. For the operating conditions examined, the jellyfish valve was found to cause the least damage to blood, with a relative blood damage index of 0.27 against a value of 0.47 for the Bjork-Shiley valve.     

Validation of a numerical FSI simulation of an aortic BMHV by in vitro PIV experiments

In this paper, a validation of a recently developed fluid–structure interaction (FSI) coupling algorithm to simulate numerically the dynamics of an aortic bileaflet mechanical heart valve (BMHV) is performed. This validation is done by comparing the numerical simulation results with in vitro experiments. For the in vitro experiments, the leaflet kinematics and flow fields are obtained via the particle image velocimetry (PIV) technique. Subsequently, the same case is numerically simulated by the coupling algorithm and the resulting leaflet kinematics and flow fields are obtained. Finally, the results are compared, revealing great similarity in leaflet motion and flow fields between the numerical simulation and the experimental test. Therefore, it is concluded that the developed algorithm is able to capture very accurately all the major leaflet kinematics and dynamics and can be used to study and optimize the design of BMHVs.     

Laser Doppler Anemometry Study of Bidimensional Flows Downstream of Three 19 mm Bileaflet Valves in the Mitral Position, Under Kinematic Similarity

Three small-size (nominal size: 19 mm) bileaflet valves, CarboMedics R (CM), St Jude Standard (SJ) and Sorin Bicarbon (SB), have been tested by means of a two-component laser Doppler anemometry (LDA) system, in the mitral position, in order to assess the potential damage to blood elements entailed by the turbulent flow through them. A high regime (6 l/min cardiac output) was chosen to perform measurements for the worst case in generated turbulence. Two half-diameter profiles, at 13 and 26 mm downstream of the valve plane, have been investigated for each model. Besides velocity profiles, turbulence shear stresses (TSS) are reported, after the application of the stress analysis technique, in order to assess the maximum values of TSS (TSS_max exerted on blood particles. Results show the typical bileaflet-type velocity profile for SB and SJ, with three jets exiting the valve, whereas CM lacks the central jet, due to instabilities of its flow field. As for TSS_max, CM reaches the highest values, presumably due to leaflets fluttering. SJ's TSS_max profiles maintain similar shapes at the two downstream locations, whereas SB presents an unexpected increase in the peak value of TSS_max from 13 to 26 mm downstream of the valve plane, probably due to the curved leaflet design. The three prosthetic heart valves (PHVs) tested show many differences as for their turbulence properties, although they are similarly constructed.     

High-Resolution Fluid–Structure Interaction Simulations of Flow Through a Bi-Leaflet Mechanical Heart Valve in an Anatomic Aorta

We have performed high-resolution fluid–structure interaction simulations of physiologic pulsatile flow through a bi-leaflet mechanical heart valve (BMHV) in an anatomically realistic aorta. The results are compared with numerical simulations of the flow through an identical BMHV implanted in a straight aorta. The comparisons show that although some of the salient features of the flow remain the same, the aorta geometry can have a major effect on both the flow patterns and the motion of the valve leaflets. For the studied configuration, for instance, the BMHV leaflets in the anatomic aorta open much faster and undergo a greater rebound during closing than the same valve in the straight axisymmetric aorta. Even though the characteristic triple-jet structure does emerge downstream of the leaflets for both cases, for the anatomic case the leaflet jets spread laterally and diffuse much faster than in the straight aorta due to the aortic curvature and complex shape of the anatomic sinus. Consequently the leaflet shear layers in the anatomic case remain laminar and organized for a larger portion of the accelerating phase as compared to the shear layers in the straight aorta, which begin to undergo laminar instabilities well before peak systole is reached. For both cases, however, the flow undergoes a very similar explosive transition to the small-scale, turbulent-like state just prior to reaching peak systole. The local maximum shear stress is used as a metric to characterize the mechanical environment experienced by blood cells. Pockets of high local maximum shear are found to be significantly more widespread in the anatomic aorta than in the straight aorta throughout the cardiac cycle. Pockets of high local maximum shear were located near the leaflets and in the aortic arc region. This work clearly demonstrates the importance of the aortic geometry on the flow phenomena in a BMHV and demonstrates the potential of our computational method to carry out image-based patient-specific simulations for clinically relevant studies of heart valve hemodynamics.     

The Hemodynamics of the Berlin Pulsatile VAD and the Role of its MHV Configuration

The 3D flow in a model of the Berlin ventricular assist device (VAD) chamber with monoleaflet valves placed in S-shape conduits was simulated numerically. The blood flow dynamics were described in terms of flow patterns, velocity, pressure, and shear stress. The hemodynamic properties and the VAD's potential risk for thrombosis were evaluated in terms of mixing and washout properties, and global estimations of platelet level of activation (LOA). In order to evaluate the role of valves on the flow in the chamber, the flow in a model with bileaflet valves in straight conduits was simulated and compared with the original case. The results showed that in both models a large rotating flow was developed in the chamber during filling. This vortex filled the entire chamber and moved constantly up to the peak ejection phase, resulting in relatively low shear stress (up to 0.4 Pa) and no lasting stagnation regions. Significant shear stresses were found near the valves with higher values near the outlet valve in both models. The configuration of valves and conduits had a large effect on VAD washout and mixing properties, with advantage to the bileaflet model. However, since the bileaflet valves exhibited higher shear stresses, higher LOA were found for the bileaflet model.     

Shear stress related blood damage along the cusp of a tri-leaflet prosthetic valve.

Blood flowing through a prosthetic heart valve can be damaged by flow-induced shear forces. Fluid dynamics variables and geometric factors play an important role in the evaluation of shear-stress-related blood damage. Central-flow prosthetic valves have been considered as an optimal replacement for mechanical and biological valves. Recently it was shown that shear stress distribution along the surface of a polyurethane cusp reaches values that can damage the blood elements. A mathematical model correlating the effects of shear stresses on blood corpuscles with clinical findings was employed in vitro. The model can be applied to the effects of blood-surface interaction and is of clinical relevance.     

Platelet activation due to hemodynamic shear stresses: damage accumulation model and comparison to in vitro measurements

The need to optimize the thrombogenic performance of blood recirculating cardiovascular devices, e.g., prosthetic heart valves (PHV) and ventricular assist devices (VAD), is accentuated by the fact that most of them require lifelong anticoagulation therapy that does not eliminate the risk of thromboembolic complications. The formation of thromboemboli in the flow field of these devices is potentiated by contact with foreign surfaces and regional flow phenomena that stimulate blood clotting, especially platelets. With the lack of appropriate methodology, device manufacturers do not specifically optimize for thrombogenic performance. Such optimization can be facilitated by formulating a robust numerical methodology with predictive capabilities of flow-induced platelet activation. In this study, a phenomenological model for platelet cumulative damage, identified by means of genetic algorithms (GAs), was correlated with in vitro experiments conducted in a Hemodynamic Shearing Device (HSD). Platelets were uniformly exposed to flow shear representing the lower end of the stress levels encountered in devices, and platelet activity state (PAS) was measured in response to six dynamic shear stress waveforms representing repeated passages through a device, and correlated to the predictions of the damage accumulation model. Experimental results demonstrated an increase in PAS with a decrease in "relaxation" time between pulses. The model predictions were in very good agreement with the experimental results.