Effects of Stenosis Geometry on Flow in Arteriovenous Fistula Patients

This focus of this study is to understand the relationship between the fluid properties present in and the geometric parameters of stenoses developed in end-stage renal disease (ESRD) patients, after creation of an arteriovenous fistula (AVF). Stenosis is the leading cause of failure in AVF creation and maturation. A fistula is meant to provide an access point for hemodialysis treatment necessary for ESRD patients, but large failure rates in fistula creation and maturation cause reoccurring problems for patients and a disproportionately high amount of spending on ESRD patient care. In the United States alone, ESRD patients account for 1% of the Medicare patient population, but the Centers for Medicare & Medicaid Services spent $35.4 billion, 7.2% of the 2016 Medicare budget on their treatment (United States Renal Data System, 2018 Annual Report). This study uses CFD to simulate blood flowing through venous stenoses of varying lengths and initial flow conditions. Computational modeling allows for specific control of geometric conditions as well as simple generation of resulting properties, such as wall shear stress, that are difficult to acquire in vivo. For this study, five different geometric models were constructed to represent straight vascular segments with varying lengths of stenosis. Each vessel was 4-millimeters in diameter with a 2-millimeter diameter stenosis. The lengths of the inlet and outlet vessel segments adjacent to the stenosis were each four times the vessel diameter. Stenosis lengths of 5, 15, 30, 45 and 60-millimeters were used. Vessels were treated as rigid tubes, and the geometries were created using PTC Creo Parametric (PTC Inc., Needham, MA), a commercially available CAD software. CFD analysis of the flow through the vessel segments was performed using ANSYS Fluent (ANSYS, Inc., Canonsburg, PA) for each geometric model with a range of boundary conditions. The working fluid was blood, treated as a Newtonian fluid for the shear rates present, with dynamic viscosity of 2.55 × 10−3 kg/m-s and density of 1060 kg/m3. To model the range of pressure experienced by vessels during the cardiac cycle, simulations were performed using a range of pressure values at the vessel inlet. The boundary condition used at the inlet was a static pressure ranging from 50 to 160 mmHg in increments of 10 mmHg for each geometric model. Outflow pressure values of 10, 15, and 20mmHg were used on the outlet boundary. As expected, flow rate through the system was found to increase linearly with inlet pressure for each geometry and outlet pressure. Flow rate decreased logarithmically as stenosis length increased for each inlet and outlet pressure. Flow rate through the system also decreased as outflow pressure increased, as it would in the presence of further downstream blockages in patients. The data collected here shows under which flow conditions different stenosis geometries can result in a failed fistula, as well as under which conditions the stenosis alone will not prevent the fistula from providing the required flow for dialysis treatment.