New insights into the assessment of the prosthetic valve performance in the presence of subaortic stenosis through a fluid-structure interaction model

Fluid-structure interaction simulation of mechanical aortic valves: a narrative review exploring its role in total product life cycle

Over the last years computer modelling and simulation has emerged as an effective tool to support the total product life cycle of cardiovascular devices, particularly in the device preclinical evaluation and post-market assessment. Computational modelling is particularly relevant for heart valve prostheses, which require an extensive assessment of their hydrodynamic performance and of risks of hemolysis and thromboembolic complications associated with mechanically-induced blood damage. These biomechanical aspects are typically evaluated through a fluid-structure interaction (FSI) approach, which enables valve fluid dynamics evaluation accounting for leaflets movement. In this context, the present narrative review focuses on the computational modelling of bileaflet mechanical aortic valves through FSI approach, aiming to foster and guide the use of simulations in device total product life cycle. The state of the art of FSI simulation of heart valve prostheses is reviewed to highlight the variety of modelling strategies adopted in the literature. Furthermore, the integration of FSI simulations in the total product life cycle of bileaflet aortic valves is discussed, with particular emphasis on the role of simulations in complementing and potentially replacing the experimental tests suggested by international standards. Simulations credibility assessment is also discussed in the light of recently published guidelines, thus paving the way for a broader inclusion of in silico evidence in regulatory submissions. The present narrative review highlights that FSI simulations can be successfully framed within the total product life cycle of bileaflet mechanical aortic valves, emphasizing that credible in silico models evaluating the performance of implantable devices can (at least) partially replace preclinical in vitro experimentation and support post-market biomechanical evaluation, leading to a reduction in both time and cost required for device development.

Correction: Chen et al. Hemodynamic Effects of Subaortic Stenosis on Blood Flow Characteristics of a Mechanical Heart Valve Based on OpenFOAM Simulation. Bioengineering 2023, 10, 312

In the original publication [...].

Simulation of Mechanical Heart Valve Dysfunction and the Non-Newtonian Blood Model Approach

The mechanical heart valve (MHV) is commonly used for the treatment of cardiovascular diseases. Nonphysiological hemodynamic in the MHV may cause hemolysis, platelet activation, and an increased risk of thromboembolism. Thromboembolism may cause severe complications and valve dysfunction. This paper thoroughly reviewed the simulation of physical quantities (velocity distribution, vortex formation, and shear stress) in healthy and dysfunctional MHV and reviewed the non-Newtonian blood flow characteristics in MHV. In the MHV numerical study, the dysfunction will affect the simulation results, increase the pressure gradient and shear stress, and change the blood flow patterns, increasing the risks of hemolysis and platelet activation. The blood flow passes downstream and has obvious recirculation and stagnation region with the increased dysfunction severity. Due to the complex structure of the MHV, the non-Newtonian shear-thinning viscosity blood characteristics become apparent in MHV simulations. The comparative study between Newtonian and non-Newtonian always shows the difference. The shear-thinning blood viscosity model is the basics to build the blood, also the blood exhibiting viscoelastic properties. More details are needed to establish a complete and more realistic simulation.

Computational Assessment of Valvular Dysfunction in Discrete Subaortic Stenosis: A Parametric Study

PURPOSE

Discrete subaortic stenosis (DSS) is a left-ventricular outflow tract (LVOT) obstruction caused by a membranous lesion. DSS is associated with steep aortoseptal angles (AoSAs) and is a risk factor for aortic regurgitation (AR). However, the etiology of AR secondary to DSS remains unknown. This study aimed at quantifying computationally the impact of AoSA steepening and DSS on aortic valve (AV) hemodynamics and AR.

METHODS

An LV geometry reconstructed from cine-MRI data was connected to an AV geometry to generate a unified 2D LV-AV model. Six geometrical variants were considered: unobstructed (CTRL) and DSS-obstructed LVOT (DSS), each reflecting three AoSA variations (110°, 120°, 130°). Fluid-structure interaction simulations were run to compute LVOT flow, AV leaflet dynamics, and regurgitant fraction (RF).

RESULTS

AoSA steepening and DSS generated vortex dynamics alterations and stenotic flow conditions. While the CTRL-110° model generated the highest degree of leaflet opening asymmetry, DSS preferentially altered superior leaflet kinematics, and caused leaflet-dependent alterations in systolic fluttering. LVOT steepening and DSS subjected the leaflets to increasing WSS overloads (up to 94% increase in temporal shear magnitude), while DSS also increased WSS bidirectionality on the inferior leaflet belly (+ 0.30-point in oscillatory shear index). Although AoSA steepening and DSS increased diastolic transvalvular backflow, regurgitant fractions (RF < 7%) remained below the threshold defining clinical mild AR.

CONCLUSIONS

The mechanical interactions between AV leaflets and LVOT steepening/DSS hemodynamic derangements do not cause AR. However, the leaflet WSS abnormalities predicted in those anatomies provide new support to a mechanobiological etiology of AR secondary to DSS.

Cavitation Suppression of Bileaflet Mechanical Heart Valves

PURPOSE

Mechanical heart valves (MHVs) are widely used to replace diseased heart valves, but it may suffer from cavitation due to the rapid closing velocity of the leaflets, resulting in the damage of red blood cells and platelets. The aim of this study is to apply computational fluid dynamics (CFD) method to investigate the cavitation in bileaflets mechanical heart valves (BMHVs) and discuss the effects of the conduit and leaflet geometries on cavitation intensity.

METHODS

Firstly, CFD method together with moving-grid technology were applied and validated by comparing with experimental results obtained from other literature. Then the leaflets movement and the flow rate of BMHVs with different conduit geometries and leaflet geometries are compared. At last, the duration time of the saturated vapor pressure and the closing velocity of leaflets at the instant of valve closure were used to represent the cavitation intensity.

RESULTS

Larger closing velocity of leaflets at the instant of valve closure means higher cavitation intensity. For BMHVs with different conduit geometries, the conduit with Valsalva sinuses has the maximum cavitation intensity and the straight conduit has the minimum cavitation intensity, but the leaflets cannot reach the fully opened state in a straight conduit. For BMHVs with different leaflet geometries, in order to minimize the cavitation intensity, the leaflets are better to have a large thickness and a small rotational radius.

CONCLUSION

CFD method is a promising method to deal with cavitation in BMHVs, and the closing velocity of leaflets has the same trend with the cavitation intensity. By using CFD method, the effects of the conduit geometry and the leaflet geometry on cavitaion in BMHVs are found out.

Hemodynamic Performance of Dysfunctional Prosthetic Heart Valve with the Concomitant Presence of Subaortic Stenosis: In Silico Study

The prosthetic heart valve is vulnerable to dysfunction after surgery, thus a frequent assessment is required. Doppler electrocardiography and its quantitative parameters are commonly used to assess the performance of the prosthetic heart valves and provide detailed information on the interaction between the heart chambers and related prosthetic valves, allowing early detection of complications. However, in the case of the presence of subaortic stenosis, the accuracy of Doppler has not been fully investigated in previous studies and guidelines. Therefore, it is important to evaluate the accuracy of the parameters in such cases to get early detection, and a proper treatment plan for the patient, at the right time. In the current study, a CFD simulation was performed for the blood flow through a Bileaflet Mechanical Heart Valve (BMHV) with concomitant obstruction in the Left Ventricle Outflow Tract (LVOT). The current study explores the impact of the presence of the subaortic on flow patterns. It also investigates the accuracy of (BMHV) evaluation using Doppler parameters, as proposed in the American Society of Echocardiography (ASE) guidelines.

Integrated strategy for in vitro characterization of a bileaflet mechanical aortic valve

Background

Haemodynamic performance of heart valve prosthesis can be defined as its ability to fully open and completely close during the cardiac cycle, neither overloading heart work nor damaging blood particles when passing through the valve. In this perspective, global and local flow parameters, valve dynamics and blood damage safety of the prosthesis, as well as their mutual interactions, have all to be accounted for when assessing the device functionality. Even though all these issues have been and continue to be widely investigated, they are not usually studied through an integrated approach yet, i.e. by analyzing them simultaneously and highlighting their connections.

Results

An in vitro test campaign of flow through a bileaflet mechanical heart valve (Sorin Slimline 25 mm) was performed in a suitably arranged pulsatile mock loop able to reproduce human systemic pressure and flow curves. The valve was placed in an elastic, transparent, and anatomically accurate model of healthy aorta, and tested under several pulsatile flow conditions. Global and local hydrodynamics measurements and leaflet dynamics were analysed focusing on correlations between flow characteristics and valve motion. The haemolysis index due to the valve was estimated according to a literature power law model and related to hydrodynamic conditions, and a correlation between the spatial distribution of experimental shear stress and pannus/thrombotic deposits on mechanical valves was suggested. As main and general result, this study validates the potential of the integrated strategy for performance assessment of any prosthetic valve thanks to its capability of highlighting the complex interaction between the different physical mechanisms that govern transvalvular haemodynamics.

Conclusions

We have defined an in vitro procedure for a comprehensive analysis of aortic valve prosthesis performance; the rationale for this study was the belief that a proper and overall characterization of the device should be based on the simultaneous measurement of all different quantities of interest for haemodynamic performance and the analysis of their mutual interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12938-017-0314-2) contains supplementary material, which is available to authorized users.

The upstream boundary condition influences the leaflet opening dynamics in the numerical FSI simulation of an aortic BMHV

In this paper, the influence of the upstream boundary condition in the numerical simulation of an aortic bileaflet mechanical heart valve (BMHV) is studied. Three three-dimensional cases with different upstream boundary conditions are compared. The first case consists of a rigid straight tube with a velocity profile at its inlet. In the second case, the upstream geometry is a contracting left ventricle (LV), positioned symmetrically with respect to the valve. In the last case, the LV is positioned asymmetrical with respect to the valve. The cases are used to simulate the same three-dimensional BMHV. The change in time of the LV volume is calculated such that the flow rate through the valve is identical in each case. The opening dynamics of the BMHV are modelled using fluid-structure interaction. The simulations show that differences occur in the leaflet movement of the three cases. In particular, with the asymmetric LV, one of the leaflets impacts the blocking mechanism at its open position with a 34% higher velocity than when using the velocity profile, and with an 88% higher velocity than in the symmetric LV case. Therefore, when simulating such an impact, the upstream boundary condition needs to be chosen carefully.

FSI simulation of asymmetric mitral valve dynamics during diastolic filling

In this article, we present a fluid-structure interaction algorithm accounting for the mutual interaction between two rigid bodies. The algorithm was used to perform a numerical simulation of mitral valve (MV) dynamics during diastolic filling. In numerical simulations of intraventricular flow and MV motion, the asymmetry of the leaflets is often neglected. In this study the MV was rendered as two rigid, asymmetric leaflets. The 2D simulations incorporated the dynamic interaction of blood flow and leaflet motion and an imposed subject-specific, transient left ventricular wall movement obtained from ultrasound recordings. By including the full Jacobian matrix in the algorithm, the speed of the simulation was enhanced by more than 20% compared to using a diagonal Jacobian matrix. Furthermore, our results indicate that important features of the flow field may not be predicted by the use of symmetric leaflets or in the absence of an adequate model for the left atrium.

A coupled sharp-interface immersed boundary-finite-element method for flow-structure interaction with application to human phonation

A new flow-structure interaction method is presented, which couples a sharp-interface immersed boundary method flow solver with a finite-element method based solid dynamics solver. The coupled method provides robust and high-fidelity solution for complex flow-structure interaction (FSI) problems such as those involving three-dimensional flow and viscoelastic solids. The FSI solver is used to simulate flow-induced vibrations of the vocal folds during phonation. Both two- and three-dimensional models have been examined and qualitative, as well as quantitative comparisons, have been made with established results in order to validate the solver. The solver is used to study the onset of phonation in a two-dimensional laryngeal model and the dynamics of the glottal jet in a three-dimensional model and results from these studies are also presented.

Fluid-Structure Interactions of the Mitral Valve and Left Heart: Comprehensive Strategies, Past, Present and Future

The remodeling that occurs after a posterolateral myocardial infarction can alter mitral valve function by creating conformational abnormalities in the mitral annulus and in the posteromedial papillary muscle, leading to mitral regurgitation (MR). It is generally assumed that this remodeling is caused by a volume load and is mediated by an increase in diastolic wall stress. Thus, mitral regurgitation can be both the cause and effect of an abnormal cardiac stress environment. Computational modeling of ischemic MR and its surgical correction is attractive because it enables an examination of whether a given intervention addresses the correction of regurgitation (fluid-flow) at the cost of abnormal tissue stress. This is significant because the negative effects of an increased wall stress due to the intervention will only be evident over time. However, a meaningful fluid-structure interaction model of the left heart is not trivial; it requires a careful characterization of the in-vivo cardiac geometry, tissue parameterization though inverse analysis, a robust coupled solver that handles collapsing Lagrangian interfaces, automatic grid-generation algorithms that are capable of accurately discretizing the cardiac geometry, innovations in image analysis, competent and efficient constitutive models and an understanding of the spatial organization of tissue microstructure. In this manuscript, we profile our work toward a comprehensive fluid-structure interaction model of the left heart by reviewing our early work, presenting our current work and laying out our future work in four broad categories: data collection, geometry, fluid-structure interaction and validation.

Computational analysis of an aortic valve jet with Lagrangian coherent structures

Important progress has been achieved in recent years in simulating the fluid-structure interaction around cardiac valves. An important step in making these computational tools useful to clinical practice is the development of postprocessing techniques to extract clinically relevant information from these simulations. This work focuses on flow through the aortic valve and illustrates how the computation of Lagrangian coherent structures can be used to improve insight into the transport mechanics of the flow downstream of the valve, toward the goal of aiding clinical decision making and the understanding of pathophysiology.

Numerical analysis on the hemodynamics and leaflet dynamics in a bileaflet mechanical heart valve using a fluid-structure interaction method

Bileaflet mechanical heart valves (BMHVs) are widely implanted to replace diseased heart valves but still suffer from complications such as hemolysis and platelet activation. These complications are closely related to both flow characteristics through the valves and leaflet dynamics. In this study, a fluid-structure interaction (FSI) simulation is performed to investigate the characteristics of physiological flow interacting with moving leaflets in a BMHV. The present FSI model uses both a finite volume computational fluid dynamics code and a finite element structure dynamics code to solve the governing equations for fluid flow and leaflet dynamics. In addition, a structural analysis is performed with the forces acting on the leaflet surfaces. From the analysis, detailed flow information and leaflet behavior are quantified for a cardiac cycle. The results show that the present FSI model performs well at predicting the overall flow patterns interacting with the moving leaflets and leaflet behavior in the BMHV.

Flow through a defective mechanical heart valve: a steady flow analysis

Approximately 250,000 valve replacement operations occur annually around the world and more than two thirds of these operations use mechanical heart valves (MHV). These valves are subject to complications such: pannus and/or thrombus formation. Another potential complication is a malfunction in one of the valve leaflets. Although the occurrence of such malfunctions is low, they are life-threatening events that require emergency surgery. It is, therefore, important to develop parameters that will allow an early non-invasive diagnosis of such valve malfunction. In the present study, we performed numerical simulations of the flow through a defective mechanical valve under several flow and malfunction severity conditions. Our results show that the flow upstream and downstream of the defective valve is highly influenced by malfunction severity and this resulted in a misleading improvement in the correlation between simulated Doppler echocardiographic and catheter transvalvular pressure gradients. In this study, we were also able to propose and test two potential non-invasive parameters, using Doppler echocardiography and phase contrast magnetic resonance imaging, for an early detection of mechanical heart valve malfunction. Finally, we showed that valve malfunction has a significant impact on platelet activation and therefore on thrombus formation.