Numerical Simulation of Transitional Flow in an Arteriovenous Graft
Goal: Determine the role of biomechanical mechanism in the failure of arteriovenous grafts.

Hemodialysis vascular access dysfunction has the most significant impact on morbidity in end stage renal disease patients. A major cause of this failure is occlusive venous intimal hyperplasia (VIH) near the venous anastomosis, which is followed by thrombosis. VIH in an arteriovenous (AV) graft progresses more rapidly compared to arterial intimal hyperplasia, which occurs in bypass grafts. While the natural healing response from surgical injury causes some intimal thickening, biomechanical forces such as wall shear stress (WSS) and pressure have been implicated to be responsible. High flow rates in AV grafts are necessary for efficient hemodialysis performance. This is achieved by bypassing the high-resistance vessels (arterioles and capillaries), which creates a unique hemodynamic environment compared with normal venous circulation. An audible thrill is commonly detected in AV grafts and it appears to be caused by turbulence-induced vein-wall vibration (VWV). The study will investigate the fluid physics of transitional to weakly turbulent flow using a direct numerical simulation (DNS) technique based on boundary conditions obtained in vivo. These results will be correlated to biological measurements to help understand the biomechanical mechanism involved in the development of VIH in an AV graft. The insights gained in this study will lead to significant improvements in the durability of AV grafts.

Hemodynamics in an Autologous Vein Graft and Prediction of Its Patency
Goal: Find out the relationship between the longitudinal impedance and the patency of vein graft

Approximately 60,000,000 Americans currently have vascular occlusive disease, including 1,000,000 who require surgical and/or endovascular procedures each year. For patients with long-segment stenosis or occlusion, interposition grafting with autologous vein has become the treatment of choice. Procuring acceptable vein is frequently problematic, however, as veins vary widely with respect to length, diameter, quality, and surgical accessibility. Despite the widespread use of the autologous vein in cardiovascular surgery, there is surprisingly little information on what qualities may favor long-term patency. The purpose of this analysis was to critically examine the contribution of diameter to vein graft resistance using the concept of longitudinal impedance (ZL). This allows for estimation of the minimal graft diameter allowable for a given length, as well as the possible hydraulic consequences of the inevitable taper that accompanies long vein grafts. For the flow conditions for numerical computations, proximal and distal graft pressure, pressure gradient (dP), and blood flow rate(Q) were measured intraoperatively in a 100 cm bypass graft and digitally recorded for 10 s at 200 Hz.