| Summary: | This thesis contributes to knowledge by assessing the link between the aortic valve and pathology of the thoracic aorta from both a biomechanical and computational fluid dynamics viewpoint. For many years, international guidelines for intervention on the aorta have concentrated on size alone. However, aortic size alone does not distinguish between different pathological processes which vary in their risk of acute complications. Large registries indicate acute aortic dissection or rupture can occur when the aortic size is below intervention criteria. There is strong evidence for a link between aortic valve morphology and aortopathy. What is missing is a functional assessment of the aorta in order to fully investigate the haemodynamic causes of aortopathy. We have carried out computational fluid dynamics (CFD) of the thoracic aorta in a patient- specific manner in order to measure flow indices and wall biomechanics of the thoracic aorta. To achieve this, we have devised a novel method of acquiring patient-specific velocity profiles above the aortic valve from MRI image data. This has allowed us to run patient-specific CFD simulations. We have compared this novel method with simple “idealised” velocity profiles traditionally used in CFD simulations of the cardiovascular system. We have shown that the traditional “idealised” inflow conditions oversimplify flow in the thoracic aorta, and do not exhibit the complex fluid dynamics encountered (Study 1). Using this methodology, we have performed an “in-vitro” comparison of aortic haemodynamics between tricuspid and bicuspid aortic valves (BAV) using a phantom heart and aorta model (Study 2). This has shown bicuspid valves to create eccentric and asymmetrical flow patterns, with higher levels of wall shear stress in the greater curvature of the ascending aorta. This leads on to the “in-vivo” study comparing aortic haemodynamics in 45 subjects separated into 5 groups (Study 3): healthy tricuspid aortic valve, tricuspid aortic valve with stenosis, tricuspid aortic valve with regurgitation, right-left fusion bicuspid aortic valve with stenosis, and right-non fusion bicuspid aortic valve with stenosis. Results show increased velocity jets at the periphery of the aorta in BAV patients. Velocity streamlines show that these narrow jets impact on the greater curvature of the ascending aorta, and subsequently spiral around the ascending aorta and arch. They cause increased wall shear stress and reduced oscillatory shear index at the greater curvature, corresponding to larger mid-ascending aorta diameters. We further investigated the effects of aortic root intervention on haemodynamics of the aorta (Study 4). Our results showed that valve-sparing aortic root replacement, using the remodelling technique, leads to a reduction of wall shear stress in the native preserved aortic root (interleaflet triangles and commissures) and greater curvature of the ascending aorta. There is increased axial velocity, and reduced radial velocity, indicating enhanced forward flow. However, this was observed alongside a small increase in wall shear stress in the arch and descending aorta. The outcomes in aortic haemodynamics from this work may relate to a potential explanation for the increased incidence of aortopathy in BAV patients. This has highlighted the need to develop a functional assessment of the thoracic aorta in order to understand the haemodynamic causes for aortopathy, as well as a means of better predicting complications. CFD, if carried out in a patient-specific manner, provides a potential method of acquiring this functional assessment. It may help in assessment of the adequacy of current management and imaging guidelines of the aortic valve and aorta.
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