Computational simulation of flow in the end-to-end anastomosis of a rigid graft and a compliant artery.

Abstract

Implanted vascular grafts often fail because of the development of intimal hyperplasia in the anastomotic region, and compliance mismatch between the host artery and graft exacerbates the problem. This study focused on the effects of radial artery wall motion and phase angle between pressure and flow waves (impedance phase angle [IPA]) on the wall shear rate (WSR) behavior near end-to-end vascular graft anastomoses models connecting rigid grafts and compliant arteries. A finite element model with transient flow and moving boundaries was set up to simulate oscillatory flow through a 16% undersized (mean) diameter graft model. During the simulations, different artery diameter variations (DVs) over a cycle (DV) and IPAs were simulated in the physiologic range for an oscillatory flow (mean Re = 150, peak Re = 300, unsteadiness parameter alpha = 3.9). The results show that for normal physiologic conditions (DV = 6%, IPA = -45 degrees) in a 16% undersized graft, the minimum distal mean WSR is reduced by 60% compared to steady flow at the mean Re; the minimum distal WSR amplitude increases 50% when IPA changes from -5 degrees to -85 degrees, and increases 60% when DV changes from 2% to 10%. This indicates that compliance mismatch induces lower mean WSR and more oscillatory WSR in the distal anastomotic region, which may contribute to intimal hyperplasia. In addition, the convergent-divergent geometry of the 16% undersized graft model can significantly affect the force pattern applied to the local endothelial cell layer near the anastomosis by altering the local phase angle between the flow induced tangential force (synchronous with WSR) and the radial artery expansion induced cyclic hoop strain (synchronous with DV). This local phase angle is decreased by 65 degrees in the distal divergent geometry, while increased by 15 degrees in the proximal convergent geometry.