Session A26: Biological Fluid Dynamics: Physiological Valvular Flows

High resolution numerical simulations of a novel Trileaflet Mechanical Heart Valve

Bileaflet Mechanical Heart Valves (BMHVs) are susceptible to cause platelet activation, induced by non-physiological blood flow patterns generated by their current designs, characterized by two leaflets. This study investigates the hemodynamics of a novel Trileaflet Mechanical Heart Valve (TMHV) having three leaflets, similar to the native aortic valve. Numerical simulations are carried out with the well-validated Curvilinear Immersed Boundary (CURVIB) solver, strongly coupled to a fluid-structure interaction (FSI) algorithm under aortic flow conditions. The TMHV leaflets open synchronously during early systole and start to asynchronously close during forward deceleration phase of the cardiac cycle, fully closing by early regurgitation. The flow field is mainly characterized by a strong central jet with triangular cross-section. Shear stresses exceeding 10 Pa in magnitude are only observed in regions where platelets would be advected with high velocity blood stream. The TMHV flow is closer to the native valve flow compared to the BMHV and may circumvent their biocompatibility issues, which needs further investigation.   

Quantifying drivers of valve failure in bicuspid aortic valves: Raphe-dependent strain concentration

Bicuspid aortic valve (BAV), the most common congenital heart disease, is associated with a high surgical disease burden. BAVs fail with a fusion of two aortic valve leaflets, creating raphe, which contributes to further leaflet calcification (strongly correlated with localized strain concentration), thickening, and immobility. Leveraging our computational platform that considers 2-way fluid-structure interactions, we simulated the mechanical environment of BAVs with varying extents of the raphe. A 3-parameter Mooney-Rivlin hyper-elastic material model was employed and input pressure waveforms were measured using an in vitro experimental setup with a bioprosthetic valve module. Leaflets sutured to several extents mimicked the congenital raphe. Simulation results revealed that the raphe in BAV modulates strain concentration, amplifying the strain value in selected regions ̶ a determinant factor and potential predictive metric of aortic valve calcification. Though more extensive studies are warranted to couple in silico predictions to in vivo events, engineering tools akin to the one presented herein could provide a robust and reliable means for a mechanistic understanding of pathophysiology to inform the timing of interventional planning and design of emerging prosthetic valves.   

Assessing the Risk of Leaflet Thrombosis following Transcatheter Aortic Valve-in-Valve Procedures: A Simulation Study

Transcatheter aortic valve-in-valve (ViV) implantation is a safe and effective treatment for patients with failed surgical or transcatheter bioprostheses. Recently concerns have been raised about the occurrence of subclinical leaflet thrombosis following the ViV procedures. The aim of this study was to assess the likelihood of leaflet thrombosis post-ViV after the use of transcatheter aortic valves with intra-annular and supra-annular design. In the study, we developed a fluid-solid interaction modeling approach to quantify blood stasis on the leaflets of 26-mm Edwards SAPIEN 3 and 26-mm Medtronic CoreValve. Three-dimensional flow fields of the two valves were obtained using computational simulations. The simulation results were also validated by experimental testing (particle imaging velocimetry) in a pulse duplicator system. The results showed a significantly longer blood stasis on the leaflets of the SAPIEN 3 compared to the CoreValve. A good agreement was also observed between the simulation results and the experimental data. The study suggests that the use of transcatheter aortic valves with supra-annular design reduces blood stasis on the leaflets and consequently has the potential to reduce the likelihood of leaflet thrombosis following the ViV procedures.   

Aortic Hemodynamics due to Valve Leaflet Stiffness and Valve size

The hemodynamics inside the aorta are heavily affected by the aortic valve that is situated between the aorta and the left ventricle. We have previously showed how altering individual leaflet stiffnesses can dramatically affect Wall Shear Stress (WSS) and residence time inside the aorta. This year we will present how different sizes of aortic valves can affect downstream hemodynamics. These findings are significant in a clinical sense because surgeons typically insert the largest valve possible during valve replacement surgeries. This is done because larger valves have lower pressure drops (the pressure drop across a valve is one of the most common measures of the health of a given valve). However, little attention is given to how oversized valves can block coronary arteries and create regions of slow moving flow that increase the residence time and increase the propensity of thrombosis near the valve.    

A contact model based on coefficient of restitution for simulations of tissue heart valves in an immersed boundary-thin shell finite element framework

A simple and efficient contact model is introduced for inter-leaflet penetration prevention of tissue heart valves and added to a rotation-free, high-deformation thin shell finite element (FE) framework. The proposed method applies the impenetrability constraints and momentum exchange between the impacting bodies separately using coefficient of restitution. The contact method is verified and validated against several dynamic benchmark problems. Additionally, the strain distribution of a statically loaded bio-prosthetic heart valve (BHV) with an anisotropic and nonlinear material model is compared against experimental results. Then, the dynamic performance of the BHV for two fiber orientations is analyzed using a physiological pressure waveform for a complete cycle. Finally, the FE framework incorporating the new contact model is coupled with the sharp-interface curvilinear immersed boundary (CURVIB) incompressible Navier-Stokes flow solver. The validity of the CURVIB-FE framework for fluid-structure interaction simulations is established by comparing the results of an inverted elastic flag with experiments. Finally, the capabilities of the framework are demonstrated by simulating the complex cardiovascular flow of a BHV for both opening and closing phases of the cardiac cycle.   

An in-vitro study of the flow past a transcatheter aortic valve using time-resolved 3D particle tracking

The performance of a prosthetic heart valve can be evaluated by analyzing the complex flow structures downstream of it. Particle residence time, a measure of flow stasis, can be used to determine the potential risk of thrombosis. In our past studies, such information was obtained by pseudo particle tracking schemes applied to 2D PIV data. However, since the flow past a heart valve is predominantly 3D, 2D methods could not capture the whole picture. In this work, time-resolved 3D particle tracking velocimetry (Shake-The-Box 3D-PTV) was used to study the flow past a transcatheter aortic valve in an idealized aortic root model under physiological flow conditions. The acquired flow structures and 3D time-resolved Lagrangian particle tracks allowed direct assessments of the particle residence time in the aorta and sinuses of Valsalva. Vorticity distributions and turbulence statistics were obtained by projecting the particle tracks onto a Cartesian grid. During the systolic phase, the flow was dominated by the aortic jet and a shear layer between this fast-moving jet and the ambient fluid. At the end of systole, as large amounts of fluid reentered the flow field during valve closure, a circumferential flow pattern was observed in the sinuses, enhancing the particle washout there.   

Mitral Valve Regurgitation Murmurs – Insights from Hemoacoustic Computational Modeling

Mitral regurgitation (MR) is the backflow of the blood through the mitral valve from the left ventricle into the left atrium during systole and the incidence of MR is 2% worldwide (Theal et al., Can J Cardiol., 20, 5, 2004). MR results in decreased ejection fraction and may progress to heart failure (Carabello, Mod Concepts Cardiovasc Dis., 57, 10, 1988). Thus, methods that can enable, cheap, non-invasive and accurate diagnosis detection and classification) of MR could have a significant impact on the management of this condition. MR results in a systolic murmur which can serve as an indicator of this condition. In the current study, we employ hemoacoustic simulations to quantify the characteristics of these murmurs for various MR severities and regurgitant jet directions. Blood flow is simulated by solving the incompressible Navier Stokes equations using the immersed-boundary code ViCar3D. The resultant pressure fluctuations on the wall surface serve as the source of the murmur and the propagation of the murmur through the thorax is modeled as a 3D elastic wave in a linear viscoelastic material. The murmur intensity and frequency characteristics are correlated with the degree of MR as well as the direction of the MR jet, and implications for automated auscultation-based diagnosis of MR are discussed.   

Numerical investigation of the turbulent flow features past different mechanical aortic prostheses

Bilefalet Mechanical Valves (BMVs) are routinely implanted as permanent replacement for dysfunctional or diseased aortic valves. Compared with bioprosthetic valves,BMVs are more durable and insusceptible to tearing and calcification, neverthelessthey provide non-physiological hemodynamics which might lead to platelet activationand mechanical hemolysis. Previous studieshave related the blood damaging to augmented levels of turbulent stress downstreamof BMVs, compared to bioprosthetic devices. In this scenario we investigate two emerging technologies for reducing the detrimental effects of hemodynamics past mechanical prostheses, such as a trileaflet configuration and a bileaflet valve with vortex generators(VGs). The simulations are carried out by means of a second-order accurate, finite difference flow solver with direct immersed boundary forcing. When compared to the baseline design, the trileaflet configurationwas found to produce a flow dominated by the core jet, where mixing effects produce lower turbulent kinetic energy peaks. The presence of VGsprovides additional instabilitiesinducing lower vorticity magnitude in the mixing regions, instead.   

The interplay between paravalvular leakage and coronary artery diseases following transcatheter aortic valve replacement

Transcatheter aortic valve replacement (TAVR) has surpassed the traditional surgical aortic valve replacement for patients who suffer from aortic stenosis. While TAVR continues to gain traction in the field of cardiology, postoperative complications such as paravalvular leakage (PVL) become necessary to tackle. Although coronary artery disease (CAD) is present in approximately 50% of the TAVR population, the correlation between PVL and CAD has not been understood yet. Despite the prevalence of PVL, the quantitative understanding of the interplay between pre-existing valvular pathologies, PVL, CAD and post-TAVR recovery is inadequate. In this study we developed a Doppler-based and patient-specific lumped-parameter model as well as a fluid-solid interaction modeling framework, that takes interactions of the valves, left ventricle, and arterial system into account, to estimate the LV workload (global hemodynamics) and 3-D blood flow dynamics inside the coronary arteries (local hemodynamics) non-invasively. Based on our present findings, PVL limits the benefits of TAVR and restricts coronary perfusion due to the lack of sufficient coronary blood flow. Moreover, PVL may increase the LV load (ventricular volume overload) and decrease the coronary wall shear stress (low wall shear stress promotes atherosclerosis development or plaque progression through loss of the physiological flow-oriented alignment of the endothelial cells).