Analysis of Helical Grafts in Steady and Unsteady Flow: Development of a Novel Bypass Graft

Abstract

Helical secondary flow has been shown to be beneficial as it has improved bypass graft patency in revascularization through more uniform wall shear stress and improved mixing. An unfavorable by-product of generating helical flow is the proportional increase in pressure drop, which is a critical limiting factor as it constrains the amount of beneficial helicity that can be generated. A validated CFD methodology was used to simulate the development of secondary flow in multiple helical bypass grafts with Newtonian and non-Newtonian rheology. These simulations revealed that the secondary flow is fully developed by the second pitch of a helical geometry for physiologically realistic, unsteady flows, indicating the potential for maximizing secondary flows while at the same time minimizing the induced pressure drops through optimization studies. Building on this, a novel Hybrid Graft Geometry (HGG) was developed which resulted in a 390% increase in cycle-averaged helical intensity while maintaining a mere 2% increase in cycle-averaged pressure drop when compared to graft geometries in the literature. The helical effectiveness he, defined as the ratio of helical intensity to the induced pressure drop, is a newly created parameter designed to quantify the performance of the helical grafts. The cycle-averaged he clearly reveals the superior performance of the HGG, which is up to 3.6 times higher than other helical grafts tested. For the first time in the open literature, this study presents the proper basis for future optimization studies through he, which should be maximized to improve graft patency.