Multidirectional near-wall flow in arteries and its spatial correlation with atherosclerosis

Abstract

Atherosclerotic lesions develop non-uniformly throughout the arterial system and their distribution around branches changes with age. This thesis investigates whether multidirectional blood flow, characterised by the transverse wall shear stress metric (transWSS), is related to the initiation of atherosclerosis. The spectral/hp element method was used to simulate pulsatile blood flow in immature and mature rabbit aortic geometries obtained by microCT of vascular corrosion casts. Bootstrapping was used to quantitatively correlate spatial maps around intercostal branch ostia, and found that the low/oscillatory shear theory did not correlate with disease, and could not account for age-related changes in macromolecule uptake. However, evidence for the multidirectional theory was found: transWSS related strongly and positively with disease at both ages and with wall permeability in young rabbits. The pulsatility of the blood is necessary for the existence of multidirectionality, yet the precise nature of the cardiac waveform did not dominantly influence both the large- and small-scale pattern of transWSS. Simulations in partially-idealised vessels revealed that geometry instead was crucial. Vessel curvature was responsible for a Dean vortex pair, whose changing strength over the cardiac cycle created changes in the shear direction at the wall, resulting in two large-scale axial streaks of high transWSS. Vessel torsion determined the asymmetry in their strengths and spatial locations over the descending aorta. Two distributions of transWSS resembling the age-dependent lesion patterns were identified at the individual-branch level, but which arose equally in both age groups. The small-scale patterns were found to depend on the branch's location with respect to the large-scale streaks, and therefore primarily on torsion, for which no statistically significant difference between age groups was found. Multidirectional flow relates well to disease at the aggregate-level and may explain age-dependent lesion patterns. The importance of geometry was highlighted, and calls into question how accurately it is captured.