Wall Shear Stresses Remain Elevated in Mature Arteriovenous Fistulas: A Case Study

Abstract

Maintaining vascular access (VA) patency continues to be the greatest challenge for dialysis patients. VA dysfunction, primarily due to venous neointimal hyperplasia development and stenotic lesion formation, is mainly attributed to complex hemodynamics within the arteriovenous fistula (AVF). The effect of VA creation and the subsequent geometrical remodeling on the hemodynamics and shear forces within a mature patient-specific AVF is investigated. A 3D reconstructed geometry of a healthy vein and a fully mature patient-specific AVF was developed from a series of 2D magnetic resonance image scans. A previously validated thresholding technique for region segmentation and lumen cross section contour creation was conducted in MIMICS 10.01, allowing for the creation of a 3D reconstructed geometry. The healthy vein and AVF computational models were built, subdivided, and meshed in GAMBIT 2.3. The computational fluid dynamic (CFD) code FLUENT 6.3.2 (Fluent Inc., Lebanon, NH) was employed as the finite volume solver to determine the hemodynamics and shear forces within the healthy vein and patient-specific AVF. Geometrical alterations were evaluated and a CFD analysis was conducted. Substantial geometrical remodeling was observed, following VA creation with an increase in cross-sectional area, out of plane curvature (maximum angle of curvature in AVF=30 deg), and angle of blood flow entry. The mean flow velocity entering the vein of the AVF is dramatically increased. These factors result in complex three-dimensional hemodynamics within VA junction (VAJ) and efferent vein of the AVF. Complex flow patterns were observed and the maximum and mean wall shear stress (WSS) magnitudes are significantly elevated. Flow reversal was found within the VAJ and efferent vein. Extensive geometrical remodeling during AVF maturation does not restore physiological hemodynamics to the VAJ and venous conduit of the AVF, and high WSS and WSS gradients, and flow reversal persist. It is theorized that the vessel remodelling and the continued non-physiological hemodynamics within the AVF compound to result in stenotic lesion development.

Incomplete Restoration of Homeostatic Shear Stress Within Arteriovenous Fistulae

“…Although it is generally accepted that the vessel remodeling after fistula creation is a process of mechanical homeostasis [9], increasing evidence is questioning the traditional view that there is a complete restoration of a normal homeostatic shear stress at the end of the normal maturation process [13,30]. Instead, it has been suggested that there is only a partial restoration of homeostatic shear stress [31].…”

“…It has been suggested by Carroll et al [13] that high shear (exceeding 15-20 Pa) can initiate aggressive intimal hyperplasia and patency threatening stenoses, based on computational observations that high shear in stenosis-free vessels is localized at sites which are generally prone to lesion formation. It was hypothesized in Ref.…”

“…It was hypothesized in Ref. [13] that high shear stress either mechanically damages the endothelium and/or alters the endothelial phenotype, which might transform endothelial cells into a proliferative state leading to intimal hyperplasia. Additionally, in vitro experiments by Huynh et al [32] reported that endothelial cells show increased apoptosis and increased denudation under turbulent flow conditions similar to those of the venous outflow in a fistula.…”

“…Even though blood flow is thought to play an important role in fistula remodeling, the characterization of the hemodynamic stresses occurring within fistulae remain ambiguous [12], even within functioning accesses [13]. Characterizing the fistula hemodynamics has been hindered, in part, due to the relative complexity of the flow, including separation, vortex shedding, and swirling flows [14].…”

Novel Paradigms for Dialysis Vascular Access

“…Among the upstream events that may contribute to neointimal hyperplasia formation, the hemodynamic shear stress at the AVF anastomosis was investigated in several CFD studies in the last decade (40)(41)(42)(43). Numerical analysis of flow velocity has been validated with established experimental data (44), even for high values of the Reynolds number, an index of flow instability or turbulence.…”