Fluid structure interaction modelling of aortic valve stenosis: Effects of valve calcification on coronary artery flow and aortic root hemodynamics

Highlights

• The effect of aortic valve stenosis on hemodynamic parameters inside aortic root including transvalvular pressure gradient, valve orifice diameter, maximum jet velocity along the aortic valve have been investigated.

• The effect of calcification of the aortic valve on wall shear stress on leaflets has been studied. The results showed that the difference between the wall shear stress of the fibrosa and ventricularis layers can be a reason for calcification progression.

• The flow features inside sinuses because of the aortic valve stenosis have been investigated. The results revealed that calcification changes the flow behaviour in sinuses and this could be a reason for calcium deposition on the leaflets.

• The influence of aortic valve stenosis on coronary flow and its hemodynamics has been investigated. Calcification significantly changes the hemodynamic parameters inside coronary arteries and might be a factor for initiation of the coronary artery diseases.

Abstract

Background and objective

Coronary artery diseases and aortic valve stenosis are two of the main causes of mortality and morbidity worldwide. Stenosis of the aortic valve develops due to calcium deposition on the aortic valve leaflets during the cardiac cycle. Clinical investigations have demonstrated that aortic valve stenosis not only affects hemodynamic parameters inside the aortic root but also has a significant influence on the coronary artery hemodynamics and leads to the initiation of coronary artery disease. The aim of this study is to investigate the effect of calcification of the aortic valve on the variation of hemodynamic parameters in the aortic root and coronary arteries in order to find potential locations for initiation of the coronary stenoses.

Methods

Fluid structure interaction modelling methodology was used to simulate aortic valve hemodynamics in the presence of coronary artery flow. A 2-D model of the aortic valve leaflets was developed in ANSYS Fluent based on the available echocardiography images in literature. The k-ω SST turbulence model was utilised to model the turbulent flow downstream of the leaflets.

Results

The effects of calcification of the aortic valve on aortic root hemodynamics including transvalvular pressure gradient, valve orifice dimeter, vorticity magnitude in the sinuses and wall shear stress on the ventricularis and fibrosa layers of the leaflets were studied. Results revealed that the transvalvular pressure gradient increases from 792 Pa (∼ 6 mmHg) for a healthy aortic valve to 2885 Pa (∼ 22 mmHg) for a severely calcified one. Furthermore, the influence of the calcification of the aortic valve leaflets on the velocity profile and the wall shear stress in the coronary arteries was investigated and used for identification of potential locations of initiation of the coronary stenoses. Obtained results show that the maximum velocity inside the coronary arteries at early diastole decreases from 1 m/s for the healthy valve to 0.45 m/s for the severely calcified case.

Conclusions

Calcification significantly decreases the wall shear stress of the coronary arteries. This reduction in the wall shear stress can be a main reason for initiation of the coronary atherosclerosis process and eventually results in coronary stenoses.

Introduction

Aortic valve disease is the most common form of valvular heart disease (VHD) in the elderly and frequently coexists with coronary artery disease [38]. Contemporary registries suggest the prevalence of coronary artery disease increases with age and the presence of aortic stenosis such that over half of all patients require simultaneous coronary bypass during valve surgery [23]. Aortic valve disease of the elderly occurs when calcium deposits on aortic valve leaflets over time, thereby changing the geometry and material properties of the aortic valve leaflets [10,34,41]. Invasive studies demonstrate that changes in the geometry and material properties of the aortic valve leaflets not only affect hemodynamics within the aortic root but also have a significant impact on coronary blood flow [7,9,33]. The hemodynamic variations in coronary arteries due to calcification may lead to accelerated atherosclerosis and eventually result in coronary stenosis [16,31].

Clinical investigations [5] show that the wall shear stress plays a significant role in the initiation of atherosclerosis within the coronary arterial wall and the progression of the calcium deposition on the leaflets. Calcification and atherosclerosis are, however, complex multifactorial processes which are yet to be completely understood [9,13,28]. It is believed that blood flow-induced shear stress affects the endothelial cells, leading to endothelial dysfunction, inflammatory responses, oxidative stress which remodels the artery wall and valve structure, and eventually results in progression of calcification and initiation of atherosclerosis [17,18,40]. For example, a recent study carried out by Hatoum et al. [17] shows that the recirculation zone in the sinuses for a calcified aortic valve varies in terms of strength and number of vortices with that of for the healthy valve. They also demonstrated that the calcified aortic valve leaflet experiences a smaller range of shear stress with higher shear stress probabilities during the systole compared to that of a healthy valve.

During the past decades, researchers have investigated pathologies of aortic valve and coronary arteries using numerical [3,24], clinical (in-vivo), and experimental (in-vitro) methods [6,9,18,30,39]. In numerical modelling, finite element analysis (FEM) and computational fluid dynamics (CFD) have been used to investigate valve hemodynamics [12,36,37,42]. The FEM studies [12,22] considered only the structural domain and did not account for the effect of fluid in their simulations. Similarly, the CFD simulations are not able to capture the effect of the structural domain on the fluid one.

To overcome the limitations of the aforementioned works not being able to model the fluid and structure domain simultaneously, fluid structure interaction (FSI) modelling has been employed by researchers [14,15] to study the influence of the valve structure and the coronary wall on the fluid domain. For example, Weinberg et al. [43] developed a FSI model of the aortic valve with nonlinear, anisotropic material properties and investigated the effect of material properties on leaflet strain during the cardiac cycle, comparing the impact of trileaflet and bicuspid valve morphology. They found that nonlinearity in the material properties of the bicuspid aortic valve has a significant effect on leaflet strain compared to that of the trileaflet valve. Katayama et al. [19] developed a model of the aortic valve to investigate the impact of the presence of sinuses on hemodynamic parameters inside the aortic root, and found that the presence of the sinuses affect the opening time of the valve. These aforementioned studies have not considered coronary arteries in their model, although the presence of coronary flow is able to change the hemodynamic parameters inside the aortic root [4,32].

To consider the effect of coronary arteries, Kim et al. [21] developed an FSI model of the aortic root including coronary arteries. However, the model did not incorporate the leaflets and sinuses. Nobari et al. [32] extended the model of Kim et al. [21] by adding right and left coronary arteries. They also utilised the ALE approach to simulate an FSI model of the aortic valve. Mohammadi et al. [29] improved the previous model by considering the tapered shape and branches of the coronary arteries. They used the combined FD and ALE approaches to simulate the model of the aortic valve. They investigated the effect of narrowing of the coronary arteries on the wall shear stress inside the coronary arteries based on the explicit finite element method using LS-DYNA software. Cao et al. [4] presented a FSI model of the aortic valve with coronary arteries and investigated the impact of having coronary artery on leaflet wall shear stress. They found that vortex development in sinus due to the presence of coronary arteries can, in turn, changes the magnitude of the leaflet wall shear stress. However, the impact of aortic valve stenosis on the hemodynamic parameters inside the aortic root, flow features in sinuses, and wall shear stress on the leaflets were neglected.

In this paper, the effect of valve stenosis on the aortic root and coronary hemodynamic parameters as well as the shape of the vortices inside the sinuses and the aortic root hemodynamics were investigated. To do this, 2D models of the healthy, calcified and severely calcified aortic valves were developed in ANSYS Fluent based on the echocardiography images available in the literature. In order to capture the turbulent nature of the flow downstream of the leaflets, the k-ω SST turbulence model was used. The influence of the calcification of the aortic-valve leaflets on hemodynamic parameters inside the aortic root, such as transvalvular pressure gradient, valve orifice diameter, maximum jet velocity along the valve orifice, wall shear stress on the fibrosa and ventricularis layers of the leaflets, was investigated. The impact of calcification of the aortic valve leaflets on the flow pattern in the sinuses was also studied; the vorticity magnitude corresponding to different points behind the leaflets was calculated in order to show how strong vortices can become as a result of the stiffening of the aortic valve leaflets. Furthermore, the flow features such as velocity magnitude, and the wall shear stress on the coronary walls inside the coronary arteries were studied in order to show the significance of the calcification on coronary hemodynamics. Based on the calculated hemodynamic parameters, susceptible locations for the initiation of coronary artery disease were determined.