Kris Dumont

Influence of microcalcifications on vulnerable plaque mechanics using FSI modeling

Sudden heart attacks remain one of the primary causes of premature death in the developed world. Asymptomatic vulnerable plaques that rupture are believed to prompt such fatal heart attacks and strokes. The role of microcalcifications in the vulnerable plaque rupture mechanics is still debated. Recent studies suggest the microcalcifications increase the plaque vulnerability. In this manuscript we present a numerical study of the role of microcalcifications in plaque vulnerability in an eccentric stenosis model using a transient fluid-structure interaction (FSI) analysis. Two cases are being compared (i) in the absence of a microcalcification (ii) with a microcalcification spot fully embedded in the fibrous cap. Critical plaque stress/strain conditions were affected considerably by the presence of a calcified spot, and were dependent on the timing (phase) during the flow cycle. The vulnerable plaque with the embedded calcification spot presented higher wall stress concentration region in the fibrous cap a bit upstream to the calcified spot, with stress propagating to the deformable parts of the structure around the calcified spot. Following previous studies, this finding supports the hypothesis that microcalcifications increase the plaque vulnerability. Further studies in which the effect of additional microcalcifications and parametric studies of critical plaque cap thickness based on plaque properties and thickness, will help to establish the mechanism by which microcalcifications weaken the plaque and may lead to its rupture.

Comparison of the Hemodynamic and Thrombogenic Performance of Two Bileaflet Mechanical Heart Valves Using a CFD/FSI Model

The hemodynamic and the thrombogenic performance of two commercially available bileaflet mechanical heart valves (MHVs)--the ATS Open Pivot Valve (ATS) and the St. Jude Regent Valve (SJM), was compared using a state of the art computational fluid dynamics-fluid structure interaction (CFD-FSI) methodology. A transient simulation of the ATS and SJM valves was conducted in a three-dimensional model geometry of a straight conduit with sudden expansion distal the valves, including the valve housing and detailed hinge geometry. An aortic flow waveform (60 beats/min, cardiac output 4 l/min) was applied at the inlet. The FSI formulation utilized a fully implicit coupling procedure using a separate solver for the fluid problem (FLUENT) and for the structural problem. Valve leaflet excursion and pressure differences were calculated, as well as shear stress on the leaflets and accumulated shear stress on particles released during both forward and backward flow phases through the open and closed valve, respectively. In contrast to the SJM, the ATS valve opened to less than maximal opening angle. Nevertheless, maximal and mean pressure gradients and velocity patterns through the valve orifices were comparable. Platelet stress accumulation during forward flow indicated that no platelets experienced a stress accumulation higher than 35 dyne x s/cm2, the threshold for platelet activation (Hellums criterion). However, during the regurgitation flow phase, 0.81% of the platelets in the SJM valve experienced a stress accumulation higher than 35 dyne x s/cm2, compared with 0.63% for the ATS valve. The numerical results indicate that the designs of the ATS and SJM valves, which differ mostly in their hinge mechanism, lead to different potential for platelet activation, especially during the regurgitation phase. This numerical methodology can be used to assess the effects of design parameters on the flow induced thrombogenic potential of blood recirculating devices.

Validation of a fluid-structure interaction model of a heart valve using the dynamic mesh method in fluent

Simulations of coupled problems such as fluid–structure interaction (FSI) are becoming more and more important for engineering purposes. This is particularly true when modeling the aortic valve, where the FSI between the blood and the valve determines the valve movement and the valvular hemodynamics. Nevertheless only a few studies are focusing on the opening and closing behavior during the ejection phase (systole). In this paper, we present the validation of a FSI model using the dynamic mesh method of Fluent for the two-dimensional (2D) simulation of mechanical heart valves during the ejection phase of the cardiac cycle. The FSI model is successfully validated by comparing simulation results to experimental data obtained from in vitro studies using a CCD camera.

Intraluminal thrombus and risk of rupture in patient specific abdominal aortic aneurysm - FSI modelling

Recent numerical studies of abdominal aortic aneurysm (AAA) suggest that intraluminal thrombus (ILT) may reduce the stress loading on the aneurysmal wall. Detailed fluid structure interaction (FSI) in the presence and absence of ILT may help predict AAA rupture risk better. Two patients, with varied AAA geometries and ILT structures, were studied and compared in detail. The patient specific 3D geometries were reconstructed from CT scans, and uncoupled FSI approach was applied. Complex flow trajectories within the AAA lumen indicated a viable mechanism for the formation and growth of the ILT. The resulting magnitude and location of the peak wall stresses was dependent on the shape of the AAA, and the ILT appeared to reduce wall stresses for both patients. Accordingly, the inclusion of ILT in stress analysis of AAA is of importance and would likely increase the accuracy of predicting AAA risk of rupture.

Analysis and Stabilization of Fluid-Structure Interaction Algorithm for Rigid-Body Motion

Fluid-structure interaction computations in geometries where different chambers are almost completely separated from each other by a movable rigid body but connected through very small gaps can encounter stability problems when a standard explicit coupling procedure is used for the coupling of the fluid flow and the movement of the rigid body. An example of such kind of flows is the opening and closing of valves, when the valve motion is driven by the flow. A stability analysis is performed for the coupling procedure of the movement of a cylinder in a cylindrical tube, filled with fluid, Between the moving cylinder and the tube, a small gap is present, so that two chambers are formed. It is shown that a standard explicit coupling procedure or an implicit coupling procedure with explicit coupling in the subiterations steps can lead to unstable motion depending on the size of the gaps, the density of the rigid body, and the density of the fluid. It is proven that a reduction of the time-step size cannot stabilize the coupling procedure. An implicit coupling procedure with implicit coupling in the subiterations has to be used. An illustration is given on how such a coupling procedure can be implemented in a commercial computational fluid dynamics (CFD) software package. The CFD package FLUENT (Fluent, Inc.) is used. As an application, the opening and the closing of a prosthetic aortic valve is computed.

Stabilization of a fluid-structure coupling procedure for rigid body motion

Fluid-structure interactions computations in geometries where different chambers are almost completely separated from each other by a movable rigid body but connected through very small gaps, can encounter stability problems when a standard explicit coupling procedure is used for the coupling of the fluid flow and the movement of the rigid body. An example of such kind of flows is the opening and closing of valves, when the valve motion is driven by the flow. In this paper a stability analysis is performed for the coupling procedure of the movement of a cylinder in a cylindrical tube, filled with fluid. Between the moving cylinder and the tube a small gap is present, so that two chambers are formed. It is shown that a standard explicit coupling procedure can lead to unstable motion depending on the size of the gaps, the mass of the rigid body and the density of the fluid. It is proven that a reduction of the time step size can not stabilize the coupling procedure. An implicit coupling procedure has to be used. An illustration is given on how such a coupling procedure can be implemented in a commercial CFD software package. The CFD package Fluent (Fluent Inc.) is used. As an application, the opening and the closing of a prosthetic aortic valve is computed.

Biofluid mechanics and the circulatory system

A fluid is a medium which deforms, or undergoes motion, continuously under the action of a shearing stress and includes liquids and gases. Applying biofluid mechanics to the cardiovascular system requires knowledge of anatomy and geometry, pressure data and blood flow, volume and velocity measurements. A good example is the assessment of the haemodynamics of biological and mechanical heart valves.

Flow induced platelet activation and damage in mechanical heart valves - numerical studies

Computational fluid dynamics (CFD) simulations were used to describe blood flow through mechanical heart valve. Flow calculation results were used to obtain platelet stress and damage accumulation. Numerical simulation of St. Jude medical (SJM) valve implanted in physiologic 3D geometry was conducted. Blood was modeled as non-Newtonian two-phase fluid. Unsteady Reynolds averaged Navier-Stokes (URANS) approach was used with Wilcox komega turbulent model. A new platelet damage model, incorporating damage history, was developed to estimate flow induced platelet activation. Comparison of the thrombogenic potential of two bileaflet MHV geometries was conducted using fluid-structure interaction (FSI) computation. The two geometries, ATS and SJM, are commercially available valves which differ in their hinge design. The thrombogenic potential of the two valves was based on computed wall shear stresses on the leaflets and cumulative shear stress on multiple particles released during forward and backward flow phases. The results of the FSI study indicate the SJM to have higher thrombogenic potential than ATS. Valve generated flow patters are conducive to platelet activation and provide conditions for activated platelets to interact. The new damage model was utilized to estimate the effects of repeated passages and platelet senescence in estimating the thrombogenic potential.

Damage accumulation model, FSI, and multiscale simulations for studying the thrombogenic potential of prosthetic heart valves

3D physiologic geometry of St. Jude Medical (SJM) valve after implantation was simulated with non-Newtonian two-phase blood model. The simulation used the unsteady Reynolds averaged Navier-Stokes (URANS) approach and the Wilcox k-ω turbulent model. Platelet stress accumulation and the resulting platelet damage were calculated from the results.Thrombogenic potential of two bileaflet MHV geometries was conducted using fluid-structure interaction (FSI) computation. Two commercially available valve geometries, SJM and ATS, which differ mostly in their hinge design, were simulated in a straight geometry with sudden expansion downstream of the valve. The thrombogenic potential of the two valves was based on computed wall shear stresses on the leaflets and cumulative shear stress on multiple particles released during forward and reverse flow phases.Platelet stress accumulation along pertinent trajectories from the FSI studies indicated that the SJM valve has a higher thrombogenic potential then the ATS valve.Flow patterns generated by the valve are conducive to platelet activation provide optimal conditions for activated platelets to interact with each other and form aggregates are hypothesized to be the source of thromboemboli formation, increasing the risk for cardioembolic stroke. The new damage model developed was utilized to estimate the effects of repeated passages and platelet senescence on this thrombogenic potential.Flow and pressure effects on a cell like a platelet can be well represented by a continuum mechanics model down to the order of the μm level. However, the molecular effects of adhesion/aggregation bonds are on the order of nm. Thus we also adopt a discrete particles dynamics (DPD) approach in which the macroscopic model provides information about the flow induced stresses that may activate blood cellular constituents. This multiscale modeling approach concentrates on flow regions in prosthetic devices like MHVs and cardiovascular pathologies that have a high propensity to activate platelets and form aggregates. Preliminary simulations of blood flow in simple geometries using this approach, which widely departs from the traditional continuum approach, is successful in generating viscous blood flow velocity distributions in these geometries.© 2007 ASME

Feasibility study of the dynamic mesh model in FLUENT for fluid-structure interaction of a heart valve

A generic engineering example of fluid-structure interaction (FSI) is the motion of thin-walled, leaflet-like structure driven by fluid motion. A standard example in biomechanics is the opening and closing of aortic heart-valves, which is a delicate interaction between blood flow and geometrical and stiffness properties of the heart-valve leaflets. The coupled motion of flow and leaflet motion has been achieved in only a limited number of cases. In this paper a feasibility study of the dynamic mesh model in FLUENT is discussed for simulating heart valves during the ejection (systole). This study resulted in a simplified numerical model of the aortic valve hemodynarnics. Although the results seem very promising, experimental validation is needed to prove the precision of the presented method.