Dialysis Access Grafts
One common method to achieve a high flow rate of blood during dialysis (in order to make the procedure practical) is to graft an artery to a vein, bypassing the hydraulic resistance of the capillaries. However, this creates non-physiological fluid dynamics conditions in the vein. The vein wall remodels over time in order to adjust to the arterial flow conditions. The remodeling oftentimes progresses in an uncontrolled fashion leading to stenoses and occlusions which require additional surgical corrections to keep the access site patent. The patency rate for dialysis access sites is as low as 50% after only one year. As a result, dialysis access revisions account for up to 50% of all hospital costs and 17% of the total spending for hemodialysis. The cost in the United States per year are over $1 billion.
Our current research goal is to develop a methodology for the numerical simulation of cardiovascular fluid/structure interaction. With the development of more accurate simulation capabilities, a surgeon might be able to use a computational tool to better predict the physiological conditions in a vascular graft after surgery. This would allow the surgeon to simulate several "what-if" graft configurations, allowing the surgeon to optimize the graft geometry and increase graft patency. This capability is known as "predictive surgery".
Viscous wall shear stress magnitude (in Pa) acting on the lumen of a patient specific simulation of an end-to-side anastomosis
Although the key biomechanical factors causing graft wall remodeling are not wholly understood, the viscous shear stress exerted on the vessel wall by the blood has been identified as an important parameter in the process. It is believed that the cells within the wall of the blood vessel (endothelial cells) react to the viscous shear stress and pulsatile wall stretch. The cells then convert the mechanical stimulus into biochemical activity through a process known as mechanotransduction. These activated chemical pathways then cause an abnormal proliferation of the vascular cells which leads to an undesirable thickening of the vein wall (intimal hyperplasia).
We can use numerical simulation to calculate shear stress characteristics from patient specific geometries. The numerical simulation of these flows is complicated by additional factors such as: unstable and highly vortical flows, transitional and weakly turbulent flow, irregular 3-D patient vascular geometries, and complicated stress/strain relationships describing the vessel wall mechanics.
We use the commercial computational fluid dynamics (CFD) code Ansys/Fluent to perform vascular simulations. We also utilize the Mechanical Engineering Department's Linux Beowulf cluster for high performance execution of simulations.
Cycle-averaged viscous wall shear stress magnitude (in Pa) acting on the lumen of a patient specific simulation of an side-to-side anastomosis
Cycle-averaged viscous wall shear stress magnitude (in Pa) acting on the lumen of a patient specific simulation of an side-to-side anastomosis
Contact Patrick at pmcgah@u.washington.edu for more information on the dialysis access research.