No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

This research forms part of an interdisciplinary project that aims to improve the understanding of haemodynamics and vascular mechanics in arteriovenous shunting. To achieve the high flow rates that enable patients with renal disease to receive haemodialysis, a fistula is created between an artery and a vein. The patency rate of fistulas, especially those located in the upper arm, is low. The approach adopted here makes use of new magnetic resonance image (MRI) technology and computational modelling of blood flow, with a view to improving therapeutic strategies of disease requiring vascular interventions. This thesis presents the construction and development of a 3D finite element model of the fluid-structure interaction in a brachial–cephalic patient–specific fistula. An overview of the mathematical models that describe the vessel wall and fluid behaviour as well their interaction with each other is given. An Arbitrary Lagrangian- Eulerian (ALE) framework is used together with a transversely isotropic hyperelastic constitutive model for the vessel walls, while blood flow is modelled as a Newtonian fluid. A three-element Windkessel model is used to allow the fluid to move through the outlets of the computational domain without causing non–physical reflections. Flow data acquired from MRI is used to prescribe the flow at the inlet. The parameters of the Windkessel-model at the two outlets are calibrated to resemble the flow acquired from the 2D MRI. The model is validated against the flow patterns acquired from the 4D MRI. The flow patterns of the blood, and stress present in the vessel are investigated. Of special significance are the flow and wall shear stress at the anastomosis. An area of very high velocity in the anastomosis is followed by an area of recirculation and low velocity. The propagation of pressure waves and their reflection at the anastomosis are studied. Areas that are subjected to low wall shear stress, high oscillatory wall shear stress or flow circulation are identified as areas where intimal hyperplasia may develop. The flow results from the simulation show good qualitative agreement with the MRI data.

The development of efficient mathematical models and numerical methods for the study of haemodynamics is becoming increasingly prominent in the analysis of pathological states of the cardiovascular system. Computational models contribute to medical diagnosis processes, reducing the need of classical invasive medical techniques, which are not risk-free for patients and generate high healthcare costs. The first part of this thesis focuses on the modeling and simulation of coronary blood flow, with emphasis on stable Coronary Artery Disease (CAD), a pathological condition that occurs when an abnormal narrowing builds inside coronary vessel walls. Our goal is to develop a CCTA-based Fractional Flow Reserve (FFR) model which incorporates clinical imaging and patient-specific characteristics to predict the haemodynamic behavior and properties of individuals, reducing the need for invasive measurements. A novel aspect of the proposed methodology is the inclusion of the pressure guidewire, used in clinical settings, and the assessment of its impact on local fluid dynamics and FFR predictions. Thereafter, the second part of this dissertation is devoted to the development of numerical methods for the simulation of incompressible flows with particular emphasis on the simulation of cardiovascular haemodynamics. A novel implicit hybrid finite volume/finite element methodology for the efficient simulation of blood flow is proposed and validated. The implicit discretization of the transport-diffusion equations making use of an inexact Newton-Krylov method with an SGS preconditioner yields to an efficient scheme avoiding the severe CFL condition arising in explicit or semi-implicit methods for blood flow dynamics. Besides, the Ducros flux function employed for the nonlinear convective terms leads to a provably kinetic energy stable scheme of the advection terms. In addition, a staggered semi-implicit method for the simulation of incompressible flows in one-dimensional elastic and viscoelastic vessels is proposed. The convective stage is treated explicitly in time, while the diffusive and pressure stage are handled implicitly to avoid strict bounds on the time steps. The one-dimensional methodology is then extended to networks of vessels by introducing a local three-dimensional representation of the junction.

No description available.

Rock climbing is a multi-dimensional sport encompassing physiological, psychological, bio-mechanical and skill components. Interpretation of data in current investigations is limited by the lack of knowledge regarding the extent of the potential interaction of pre-climb anxieties with the physiological responses during an ascent. This thesis attempts to delineate the psychological and physiological contributions of on-sight top rope and lead climbing in multiple ability groups of rock climbers. Furthermore, the thesis goes on to gain an understanding of the de-oxygenation and re-oxygenation profiles in two forearm flexors during sustained and intermittent contractions-to-failure, as well as during the subsequent recovery period. In study one, intermediate, advanced and elite rock climbers were asked to on-sight a route at the top of their respective best self-reported on-sight grade. There were no ability group or ascent style differences for any pre-climb measures of anxiety. However, elite rock climbers had significantly higher oxygen consumption, heart rate (HR) and cortisol (physiological component) responses compared to lower ability groups. Furthermore, the elite climbers spent a significantly greater percentage of their static time resting during the ascent compared to all lower ability groups. As there appears to be no differences in the anxiety based interaction with the physiological response, study one suggests that ability group and ascent style differences may be attributed mainly to the changes in the physical demands of the route. Furthermore, it would appear the higher level rock climbers may have a greater reliance on the aerobic metabolism during an on-sight ascent. Study two investigated the haemodynamic responses to sustained and intermittent handgrip contractions which are seen during rock climbing ascents. Intermediate, advanced and elite climbers as well as a control group were asked to perform sustained and intermittent contractions (10s) at 40% of maximal volitional capacity until exhaustion. Oxygen saturation, blood flow (BF) and HR were measured pre, during and post contractions. Elite and advanced climbers were able to de-oxygenate both the flexor digitorum profundus and the flexor carpi radialis significantly more than the intermediate climbers, and the control group. During the intermittent test to failure, relative re-oxygenation during the rest period (3s) (re-oxygenation which takes into account the amount of de-oxygenation during the previous contraction), may be an important determinant of the force time integral. During the intermittent test, the increase in Δ BF, release HR and Δ HR during the rest periods suggest that vessel occlusion in elite and advanced rock climbers may not be as prominant as previously speculated upon. Furthermore, elite rock climbers appear to have a significantly faster time to half recovery after both sustained and intermittent contractions-to-failure. In conclusion, it would appear that the psychological responses assessed pre on-sight rock climbing may not be different between ability groups or ascent styles. Instead, ability group differences may be due to physiological adaptations caused in part by the significantly greater amount of training. Furthermore, elite rock climbers appear to be able to de-oxygenate and re-oxygenate faster and to a greater extent than lower ability level climbers due to an increased Δ BF and Δ HR during intermittent rest periods, as well as post-exercise. Further investigation focusing on aerobic/anaerobic contribution, determination of capillary density and muscle fiber type would aid in gaining a greater understanding of rock climbing performance.