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Research Papers

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

[+] Author and Article Information
G. T. Carroll

Centre for Applied Biomedical Engineering Research (CABER), Department of Mechanical and Aeronautical Engineering, and the Materials and Surface Science Institute, University of Limerick, Limerick, Irelandgrainne.carroll@ul.ie

T. M. McGloughlin

Centre for Applied Biomedical Engineering Research (CABER), Department of Mechanical and Aeronautical Engineering, and the Materials and Surface Science Institute, University of Limerick, Limerick, Irelandtim.mcgloughlin@ul.ie

P. E. Burke

Department of Vascular Surgery, HSE Midwestern Regional Hospital, Limerick, Ireland; St John’s Hospital, Limerick, Ireland

M. Egan, F. Wallis

Department of Radiology, HSE Midwestern Regional Hospital, Limerick, Ireland

M. T. Walsh

Centre for Applied Biomedical Engineering Research (CABER), Department of Mechanical and Aeronautical Engineering, and the Materials and Surface Science Institute, University of Limerick, Limerick, Irelandmichael.walsh@ul.ie

J Biomech Eng 133(2), 021003 (Jan 24, 2011) (9 pages) doi:10.1115/1.4003310 History: Received October 01, 2009; Revised December 20, 2010; Posted January 03, 2011; Published January 24, 2011; Online January 24, 2011

## 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.

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## Figures

Figure 1

Diagrams of (a) the healthy vein geometry and (b) the patient-specific radiocephalic AV fistula geometry. A 3D image of both models is presented. Both geometries are also exhibited in the x-y and x-z planes and locations of the velocity inlet and outflow are highlighted.

Figure 2

2D images acquired from MRI scans. The series of 2D create 3D images: (a) healthy vein 2D slice and 3D image and (b) radiocephalic AV fistula 2D slice and 3D image.

Figure 3

The resting velocity inlet boundary condition used in the numerical models of the healthy vein and the radiocephalic AVF. The blood flow times of interest are also displayed, namely, the maximum acceleration velocity, the maximum velocity, the maximum deceleration velocity, and the minimum velocity.

Figure 4

(a) Healthy vein geometry. Region A: the first area of out of plane curvature. Region B: the straight section of the vein model. Region C: the second location of out of plane curvature. (b) Cross-sectional (C-S) slices illustrating velocity contours and in-plane secondary flow vectors of the healthy vein geometry within the aforementioned three regions of interest.

Figure 5

(a) Schematic of AV fistula geometry detailing specific locations of interest and of each cross-sectional slice, (b) axial velocity contours in the AV fistula, (c) velocity path-lines in the AV fistula, and (d) velocity contours and vectors at each cross-sectional slice of AVF geometry

Figure 6

(a) Graphed circumferential-WSS profiles at each of the regions of interest detailed in Fig. 4. (b) Overview of all WSS magnitudes at each circumferential point along the length of the vein geometry.

Figure 7

[(a) and (b)] The axial and circumferential direction venous floor center-line WSSs: (c) NWSS and (d) WSSG results for the AV fistula model at four time-points during the pulse cycle: t=0.8 s refers to the point of maximum acceleration and t=0.27 s is the peak velocity of the pulse. t=0.97 s and t=0.99 s refer to the maximum deceleration and minimum velocity phases of the AVF pulse cycle, respectively. (e) The fifth graph exhibits the degree to which the WSS within the VA junction of the AVF deviates from the healthy vein WSS environment over the duration of the pulse.

Figure 8

The temporal-spatial WSS distribution as a function of surface area within the patient-specific AVF model

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