Arterial tree taper and bifurcations: implications on wave reflection and pressure at the aortic root using a one-dimensional computational model

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonCardiovascular diseases are the prominent cause of mortality across the globe, and numbers are anticipated to grow with late industrialisation and urbanisation of the developing countries. Therefore, advancing techniques for the diagnosis of cardiovascular diseases is both desired and needed. Understanding the behaviour of the flow in the arterial system in normal and pathological conditions may aid in improving the diagnosis and treatment of cardiovascular diseases. The existence of the wave reflections and their effect on the increase of pressure at the aortic root is well established, however, the distance that a reflected wave can travel from the periphery to reach the aortic root has not been investigated. This project aims to investigate the distance travelled and the amplitude of reflected waves originating from the periphery and addresses this objective by investigating wave propagation in simple structures first, then focusing on studying the main geometrical features of large arterial vessels. A one-dimensional formulation of blood flow is used for replicating pulse wave propagation within compliant vessels. Initially, the influence of bifurcations is studied and used as building blocks for explaining the behaviour of reflections waves in a system. Another fundamental feature of the arterial tree which has not been widely studied is the arterial tapering. In this thesis, different computational models are used to reveal the direct impact of tapering and the tapering angle on the amplitude of pressure and wave speed. We further expand our understanding of the impact of tapering on the reflections by using a 55-segment arterial tree computational model. The results show that with a high number of bifurcations, single reflections originating from a distal site may not be discernible due to re-reflection and entrapment of the waves between the bifurcations. This work shows that if there is a reflection generated from distal limbs, only 1 per cent of the pressure amplitude of the wave reaches to the aortic root. Further, tapering in the arterial tree causes a higher dissipation of wave compared to an arterial tree without tapering. However, the bifurcations may be the predominant cause of reflection in the arterial tree. Furthermore, it is shown that increasing the tapering angle is associated with an increase in pulse pressure and peak pressure, as it can be seen in the ageing descending aorta. Finally, it can be concluded that determining a location for wave reflections can be an oversimplification of the matter and might overlook the fact that some reflections are an amalgamation of multiple reflections or re-reflections. The outcome of this project can provide a better understanding of the response of a healthy arterial tree to wave reflections