Optimization of a Y Graft Design for Improved Hepatic Flow Distribution in the Fontan Circulation

Abstract

Single ventricle heart defects are among the most serious congenital heart diseases, and are uniformly fatal if left untreated. Typically, a threestaged surgical course, consisting of the Norwood, Glenn, and Fontan surgeries is performed, after which the superior vena cava (SVC) and inferior vena cava (IVC) are directly connected to the pulmonary arteries (PA). In an attempt to improve hemodynamic performance and hepatic flow distribution (HFD) of Fontan patients, a novel Yshaped graft has recently been proposed to replace the traditional tubeshaped extracardiac grafts. Previous studies have demonstrated that the Ygraft is a promising design with the potential to reduce energy loss and improve HFD. However these studies also found suboptimal Ygraft performance in some patient models. The goal of this work is to determine whether performance can be improved in these models through further design optimization. Geometric and hemodynamic factors that influence the HFD have not been sufficiently investigated in previous work, particularly for the Ygraft. In this work, we couple Lagrangian particle tracking to an optimal design framework to study the effects of boundary conditions and geometry on HFD. Specifically, we investigate the potential of using a Ygraft design with unequal branch diameters to improve hepatic distribution under a highly uneven RPA/LPA flow split. As expected, the resulting optimal Ygraft geometry largely depends on the pulmonary flow split for a particular patient. The unequal branch design is demonstrated to be unnecessary under most conditions, as it is possible to achieve the same or better performance with equalsized branches. Two patientspecific examples show that optimizationderived Ygrafts effectively improve the HFD, compared to initial nonoptimized designs using equal branch diameters. An instance of constrained optimization shows that energy efficiency slightly increases with increasing branch size for the Ygraft, but that a smaller branch size is preferred when a proximal anastomosis is needed to achieve optimal HFD.