Hemodynamics of End-to-End Femoral Bypass Graft

Abstract

In the conventional femoral bypassing operation, side-to-end (STE) configuration at the proximal anastomosis and end-to-side (ETS) configuration at the distal anastomosis are usually employed. With these configurations, blood flow from the bypass graft at the distal anastomosis strongly strikes on the floor of the host artery opposite the anastomosis. This will result in the violent variations of hemodynamics in the vicinity of distal anastomosis, and further bring about anastomotic intimal hyperplasia (IH) and restenosis. Consequently, the effectiveness of bypassing surgery is compromised in the medium and long term by the development of these pathological changes. It is widely accepted that hemodynamics is close correlated to the geometry configuration of femoral bypass graft. It is verified that flow field at the distal junction has more influences on the pathogenesis and its aftereffects are more critical because the development of IH and restenosis is prone to occur in that region and endangers the patency of subsequent arteries. Nonuniform hemodynamics, characterized by nonuniform Wall Shear Stress (WSS) and large sustained Wall Shear Stress Gradients (WSSG), is also commonly considered as one of the most important causes among the numerous complex physiological and biomechanical factors. Purpose of the present study is to investigate an alternative geometry configuration to improve the hemodynamics at the vicinity of distal anastomosis and increase the medium and long term patency rate of bypass graft surgery. According to the clinical observation, the stenosed host artery may become fully stenosed after bypassing surgery and the bypass graft is the only way to restore normal blood flow to ischemic limbs. The authors presented a modified bypassing configuration with an end-to-end (ETE) conjunction at the distal anastomosis. In this new model, the proximal graft is arc-shaped with STE junction and the distal graft is sinusoid-shaped with ETE junction. The bypass graft is of the same diameter of d = 8mm as the host femoral artery, so the graft can be connected with the femoral artery smoothly at the distal junction. The polytetrafluoroethylene (PTFE) is employed as the graft material. The blood is assumed to be an isotropic, homogeneous, incompressible, Newtonian continuum having a constant density and viscosity. The vessel walls are assumed to be rigid and impermeable. The blood flow is assumed to be physiologically pulsatile laminar flow. The mean Reynolds number is Rem = 204.7, Womersley number is α = 6.14. The boundary conditions include: the physiologically pulsatile entrance velocities at the inlet section, the no-slip boundary condition on the wall, the symmetric condition in the centerline plane of femoral and graft, and the outlet pressure condition with a reference pressure P = 0 at the exit section. Three-dimensional idealized femoral bypass graft model is developed and discretized. The blood flow in the proposed model is simulated with computational fluid dynamics (CFD) method using the finite element analysis. The temporal and spatial distributions of hemodynamics such as flow patterns and WSS in the vicinity of distal anastomosis during the cardiac cycle were analyzed. Especially, the emphasis here was on the analysis of WSS, the temporal and spatial WSSG and the Oscillating Shear Index (OSI). The simulation results indicated that: (1) the ETE model is featured with small secondary flow; (2) WSS at the distal anastomosis is uniform, WSSG is small, and OSI of the ETE model has not much changes compared with ETS graft. The present study showed that the femoral bypassing configuration with ETE bypass graft was of more favorable hemodynamics, and it could consequently improve the flow conditions and decrease the probability of IH and restenosis. With the consideration of that numerical simulation was proved to be of great help and guidance meaning for the biofluidmechanics research and the biomedical engineering, the results of the present study can be applied to medical device design and clinical treatment planning in addition to the application of computational methods to cardiovascular disease research.