In silico and in vitro analysis of calcific and rheumatic aortic stenosis
Aortic stenosis (AS) is a valvular heart disease characterised by the narrowing of the aortic valve opening that restricts blood flow from the left ventricle to the aorta. AS is caused by different pathologies that each result in a specific valve opening morphology. The most common types of AS are degenerative or calcific AS caused by the calcification of the valve leaflets, rheumatic AS caused by rheumatic heart disease that results in commissural fusion of the leaflets, and congenital or bicuspid AS where the valve has two leaflets instead of three. The severity of AS is classified by the degree of valve obstruction and the resulting haemodynamic environment in terms of, amongst other parameters, the mean transvalvular pressure gradient (TPG) and the peak aortic jet velocity. The gold standard for determining these parameters is through non-invasive echocardiography with Doppler interrogation where the jet velocity is measured and the TPG estimated using the simplified Bernoulli equation. The simplified Bernoulli equation is derived from the Bernoulli equation and relates the pressure gradient across the valve to the velocity of the blood flowing through the valve. As the simplified Bernoulli equation is insensitive to the morphology of the valve, the effect that different AS pathologies have on the haemodynamic environment is not well understood, and the inherent limitations of the simplified Bernoulli equation in estimating the TPG in different AS pathologies are not well characterised. In this study, the haemodynamic environment of calcific and rheumatic AS is investigated in silico through computational fluid dynamics (CFD) and fluid-structure interaction (FSI) modelling techniques to determine the accuracy of the simplified Bernoulli equation in estimating the TPG in these pathologies. The FSI simulated results are validated against in vitro experimental data using a cardiac pulse duplicator (CPD) system designed in accordance with ISO 5840 for in vitro testing of heart valve prostheses. The results show that the simplified Bernoulli equation both over- and underestimates the TPG depending on the type and severity of AS. The results further show that the mechanism of flow obstruction and the pressure loss across the valve differ between calcific and rheumatic AS due to the different valve morphologies. The clinical implication of using a morphology-independent correlation to estimate the TPG is analysed for the different valve morphologies by evaluating the correlation between the estimated TPG and the haemodynamic parameters that can be measured non-invasively. A preliminary FSI-informed TPG correlation is developed for calcific and rheumatic AS that accounts for the morphology of the valve and the severity of the disease. The correlations are compared to in vivo clinical data to assess their potential for clinical application.
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