title: Fluid-structure interaction models of mitral valve and left atrium
Creators: name: Feng, Liuyang
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Fluid-structure interaction, Mitral valve, Left atrium

description: Mitral valve (MV), together with left atrium (LA) form an important coupled structure inside human heart. Mitral valve dysfunction including mitral stenosis, prolapse and regurgitation, is one of the most common valvular heart disease and hence has attracted significant research interest. Left atrium dysfunction such as atrial fibrillation, fibrosis and dilation, etc., also greatly affects the normal heart performance and leads to serious consequences. Computational modelling of human MV and LA function can improve our understanding of the biomechanics and flow details, which are important for improving surgical procedures and medical therapies. More importantly, it provides a tool for isolating the effects of different physiological and anatomical changes that occur under pathological conditions. However, because of the challenges of modelling the highly complex MV and LA structures and their interaction with the blood flow, only limited progress in multi-physics modelling of MV and LA has been made to date. Therefore, this thesis aims to develop computational models for MV and LA that include detailed geometry information and the fluid-structure interaction (FSI) behaviour as well as to investigate various factors that affect their normal function. First, we present a FSI model of the mitral valve that uses an anatomically and physiologically realistic description of the MV leaflets and chordae tendineae. Three different chordae models --- complex, `pseudo-fibre', and simplified chordae --- are compared to determine how different chordae representations affect the dynamics of the MV. Interesting flow patterns and vortex formulation are observed in all three cases. Additionally, results show that the complex and pseudo-fibre chordae models yield good valve closure during systole, but that the simplified chordae model leads to poorer leaflet coaptation and an unrealistic bulge in the anterior leaflet belly. An energy budget analysis shows that the MV models with complex and pseudo-fibre chordae have similar energy distribution patterns, but the MV model with the simplified chordae consumes more energy, especially during valve closing and opening. To conclude, we find that the complex chordae and pseudo-fibre chordae have similar impact on the overall MV function, but that the simplified chordae representation is less accurate. Following the MV modelling work, we further develop a coupled left atrium - mitral valve model based on coronary computed tomography angiography (CTA). The LA model incorporates patient-specific LA geometry and detailed fibre structure defined by an atlas-based method. It is equipped with a transversely isotropic material model. Effects of pathological conditions, e.g. mitral valve regurgitation and atrial fibrillation, and geometric and structural variations, namely, uniform vs non-uniform atrial wall thickness, atlas-based vs rule-based fibre architectures, on the system are investigated. We show that in the case of atrial fibrillation, the reversal pulmonary venous flow at late diastole disappear and also the filling waves at left atrial appendage orifice during systole have reduced magnitude. In the case of mitral regurgitation, a higher atrial pressure and disturbed flows are seen, especially during systole where a large regurgitant jet can be found with the suppressed pulmonary venous flow. We also show that both the rule-based and atlas-based fibre defining methods lead to similar flow fields and atrial wall deformations. However, the changes in wall thickness from non-uniform to uniform tend to underestimate the atrial deformation. Using a uniform but thickened wall also lowers the overall strain level. The flow velocity within the left atrial appendage, which is important in terms of appendage thrombosis, increases with the thickness of the left atrial wall. Energy analysis shows that the kinetic and dissipation energies of the flow within the left atrium are altered differently under conditions of atrial fibrillation and mitral valve regurgitation, providing a useful indication of the atrial performance in pathological situations. The future development of the coupled model involves applying patient-specific pressure and flow boundary conditions, introducing physiological LV models and modelling the process of blood clotting, etc.