Comparison of two endovascular treatments of a stenosed arteriovenous fistula: balloon-angioplasty with and without stenting.

PURPOSE: Arteriovenous fistulas (AVFs) are created in patients to enable a permanent vascular access for hemodialysis. The AVF causes changes in the hemodynamic conditions leading to possible complications, stenoses being the most common one. Our objective was to compare the effect of treating the stenosed AVF by balloon-angioplasty, whether followed or not with stenting. METHODS: We considered an AVF presenting an 60% arterial stenosis and simulated the two endovascular treatments using an implicit approach. We then simulated the fluid-structure interactions (FSI) within (i) the patient-specific stenosed AVF, (ii) the AVF after angioplasty, and (iii) the AVF after angioplasty plus stenting with ANSYS Workbench. RESULTS: We show that a self-expandable stent does not modify the curvature of the vessel after angioplasty; it only increases the local Young modulus of the stented wall by an order of magnitude. The results of the FSI simulations indicate that the two treatments induce the same hemodynamic conditions: they both reduce the pressure difference across the stenosis, while maintaining the flow distribution downstream of the stenosis. The venous flow rate that has to be guaranteed for hemodialysis is unaltered. Thanks to its large axial flexibility, the self-expandable stent causes at maximum a three-fold increase in the internal wall stresses at peak systole as compared to angioplasty alone. CONCLUSIONS: By maintaining the vessel lumen shape over time, the stent is likely to reduce the risk of restenosis that can otherwise occur after balloon-angioplasty because of the viscoelastic recoil of the vessel.

Numerical simulation of the fluid structure interactions in a compliant patient-specific arteriovenous fistula.

The objective of the study is to investigate numerically the fluid-structure interactions (FSI) in a patient-specific arteriovenous fistula (AVF) and analyze the degree of complexity that such a numerical simulation requires to provide clinically relevant information. The reference FSI simulation takes into account the non-Newtonian behavior of blood, as well as the variation in mechanical properties of the vascular walls along the AVF. We have explored whether less comprehensive versions of the simulation could still provide relevant results. The non-Newtonian blood model is necessary to predict the hemodynamics in the AVF because of the predominance of low shear rates in the vein. An uncoupled fluid simulation provides informative qualitative maps of the hemodynamic conditions in the AVF; quantitatively, the hemodynamic parameters are accurate within 20% maximum. Conversely, an uncoupled structural simulation with non-uniform wall properties along the vasculature provides the accurate distribution of internal wall stresses, but only at one instant of time within the cardiac cycle. The FSI simulation advantageously provides the time-evolution of both the hemodynamic and structural stresses. However, the higher computational cost renders a clinical use still difficult in routine.

Numerical and experimental study of blood flow through a patient-specific arteriovenous fistula used for hemodialysis.

Arteriovenous fistula (AVF) pathologies related to blood flow necessitate valid calculation tools for local velocity and wall shear stress determination to overcome the clinical diagnostic limits. To illustrate this issue, a reconstructed patient-specific AVF was investigated, using computational fluid dynamics (CFDs) and particle image velocimetry (PIV). The aim of this study was to validate the methodology from medical images to numerical simulations of an AVF by comparing numerical and experimental data. Two numerical grids were presented with a refinement difference of a factor of four. A mold of the same volume was created and mounted on an experimental bench with similar boundary conditions. The patient's acquired echo D006Fppler flow waveform was injected at the arterial inlet. Experimental and numerical velocity vector cartography qualitatively produced similar flow fields. Quantification with a point-to-point approach thoroughly investigated the velocity profiles using the mean difference between both results. The finest mesh generated CFD results with a mean percentage of the difference in velocity magnitude, taking the PIV as reference, did not exceed 10%. At specific zones, the coarse mesh required adaptive meshing to improve fitting with experimental data. Meshing refinement was necessary to improve velocity accuracy at wide diameters and wall shear stress at narrow diameters. Provided that these criteria were properly respected, we show through this difficult example the validity of using CFD to properly describe flow patterns in image-based reconstructed blood vessels.