Influences of Geometric Configurations of Bypass Grafts on Hemodynamics in End-to-Side Anastomosis

BACKGROUND: Although considerable efforts have been made to improve the graft patency in coronary artery bypass surgery, the role of biomechanical factors remains underrecognized. The aim of this study is to investigate the influences of geometric configurations of the bypass graft on hemodynamic characteristics in relation to anastomosis. MATERIALS AND METHODS: The Numerical analysis focuses on understanding the flow patterns for different values of inlet and distal diameters and graft angles. The Blood flow field is treated as a two-dimensional incompressible laminar flow. A finite volume method is adopted for discretization of the governing equations. The Carreau model is employed as a constitutive equation for blood. In an attempt to obtain the optimal aorto-coronary bypass conditions, the blood flow characteristics are analyzed using in vitro models of the end-to-side anastomotic angles of 45degrees, 60degrees and 90degrees. To find the optimal graft configurations, the mass flow rates at the outlets of the four models are compared quantitatively. RESULTS: This study finds that Model 3, whose bypass diameter is the same as the inlet diameter of the stenosed coronary artery, delivers the largest amount of blood and the least pressure drop along the arteries. CONCLUSION: Biomechanical factors are speculated to contribute to the graft patency in coronary artery bypass grafting.ope

Computational Simulations of Magnetic Particle Capture in Arterial Flows

The aim of Magnetic Drug Targeting (MDT) is to concentrate drugs, attached to magnetic particles, in a specific part of the human body by applying a magnetic field. Computational simulations are performed of blood flow and magnetic particle motion in a left coronary artery and a carotid artery, using the properties of presently available magnetic carriers and strong superconducting magnets (up to B ≈ 2 T). For simple tube geometries it is deduced theoretically that the particle capture efficiency scales as \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\eta \sim \sqrt{{Mn}_{\rm p}}$$\end{document}, with Mnp the characteristic ratio of the particle magnetization force and the drag force. This relation is found to hold quite well for the carotid artery. For the coronary artery, the presence of side branches and domain curvature causes deviations from this scaling rule, viz. η ∼ Mnpβ, with β > 1/2. The simulations demonstrate that approximately a quarter of the inserted 4 μm particles can be captured from the bloodstream of the left coronary artery, when the magnet is placed at a distance of 4.25 cm. When the same magnet is placed at a distance of 1 cm from a carotid artery, almost all of the inserted 4 μm particles are captured. The performed simulations, therefore, reveal significant potential for the application of MDT to the treatment of atherosclerosis

The role of vascular wall mechanics in the failure of surgical bypass grafts

grantor: University of TorontoThe long term patency of surgical bypass grafts is reduced by the development of distal anastomotic intimal hyperplasia (DAIH), which can lead to stenosis or thrombosis of the graft-artery junction. The non-physiological structural mechanics of the vascular reconstruction are thought to promote DAIH, however, the exact mechanism of DAIH formation has not yet been elucidated, and is the subject of this thesis. Even though arterial behaviour is nonlinear and anisotropic, and arteries are subject to large strains and deformations 'in vivo', most analyses of vascular mechanics in complex three dimensional geometries use linear, isotropic, small strain models. In the present study, a strain energy density function (SEDF) r0W=12aI 2E-bIIE is used to describe the large strain mechanics of blood vessels. This model is shown to predict the nonlinear stress stiffening behaviour of blood vessels as well as the role of residual stresses in reducing stress gradients through the vessel wall. The applicability of this model to describing the behaviour of blood vessels subjected to physiological axial stretches, inflation pressures and torsion is also demonstrated. The above SEDF is incorporated into an efficient finite element model of vascular mechanics so that the stress distribution in complex 3-D graft-artery anastomoses can be calculated. Stress analyses of graft-artery junctions, which include suture-induced stress concentrations, are presented. These analyses demonstrate that, in the first few months following implantation, large stress concentrations exist around the suture-line. Histological examination of coronary artery bypass grafts (CABG) shows that during this time, DAIH develops mostly around the suture-line. Together, these findings suggest that early hyperplasia development in CABGs is promoted by high suture-induced stresses. A further analysis of the influence of graft-artery compliance mismatch on suture-induced stresses demonstrates that increased compliance mismatch at end-to-end anastomoses ' does not' increase anastomotic stress concentrations, however, in end-to-side anastomoses, it 'does' lead to a substantial increase in anastomotic stresses. This combination of results explains previous experimental findings that increased compliance mismatch 'does not' lead to increased DAIH in end-to-end anastomoses, but in end-to-side anastomoses, it 'does' lead to a profound increase in DAIH. Finally, since high suture-line stresses are found in both end-to-end and end-to-side junctions, elevated intramural stresses may be an important proliferative stimulus for DAIH in all vascular reconstructions.Ph.D

Computational Study of Traveling Wave Solutions of Isothermal Chemical Systems

This article studies propagating traveling waves in a class of reaction-diffusion systems which model isothermal autocatalytic chemical reactions as well as microbial growth and competition in a flow reactor. In the context of isothermal autocatalytic systems, two different cases will be studied. The first is autocatalytic chemical reaction of order m without decay. The second is chemical reaction of order m with a decay of order n, where m and n are positive integers and m\u3en≥1. A typical system in autocatalysis is A+2B→3B and B→C involving two chemical species, a reactant A and an auto-catalyst B and C an inert chemical species. The numerical computation gives more accurate estimates on minimum speed of traveling waves for autocatalytic reaction without decay, providing useful insight in the study of stability of traveling waves. For autocatalytic reaction of order m = 2 with linear decay n = 1, which has a particular important role in chemical waves, it is shown numerically that there exist multiple traveling waves with 1, 2 and 3 peaks with certain choices of parameters