Short communicationDisturbed flow in a patient-specific arteriovenous fistula for hemodialysis: Multidirectional and reciprocating near-wall flow patterns
Introduction
A well-functioning vascular access (VA) serves as lifeline for the patients on hemodialysis. There is general consensus in the literature on the superiority of autogenous arteriovenous fistulae (AVF) over arteriovenous grafts (AVG) and central venous catheters regarding VA survival, related complications and costs (Leermakers et al., 2013, Vassalotti et al., 2012). Despite the existence of clinical guidelines (NKF/KDOQI, 2006) recommending well-defined criteria to create AVF, a high failure rate has been reported due to the formation of juxta-anastomotic stenoses. In studies performed between 1977 and 2002 where VA was provided by AVF (Allon and Robbin, 2002), the mean early failure rate was 25% (range 2–53%) while the mean one-year patency rate was 70% (42–90%).
Since the 1990s computational fluid dynamics (CFD) applied to blood vessels was intensively used to assess the wall shear stress (WSS) in the study of the link between hemodynamics and cardiovascular disease. Beside characterization of the general flow field, many patient-specific CFD studies have focused on the assessment of the so-called “disturbed flow” acting near wall. The pattern of disturbed flow is irregular, it features secondary and recirculation eddies that may change in direction with time and space, and hence it exerts low and oscillating WSS on the endothelial layer (Davies, 2009). Localization of atherosclerosis within specific sites in branch points or curvatures of the arterial tree, in humans and in experimental animals (Chiu and Chien, 2011), led to the concept that the disturbed flow is related to the vascular lesions. Also in VA, recent findings about the localization of these sites matching areas of disturbed flow (Remuzzi and Ene-Iordache, 2013) may add new insights into the mechanism of pathogenesis of neointimal hyperplasia (NH) after the surgical creation of the anastomosis.
By using CFD we have shown that disturbed flow may develop in focal sites of radial-cephalic models of AVF, either in side-to-end or end-to-end configuration, at least in idealized geometry with flow conditions resembling the initial days after surgery (Ene-Iordache and Remuzzi, 2012). In that study, we speculated on a local remodeling mechanism for the neointima formation induced by the local disturbed flow. The present study was aimed at investigating whether disturbed flow occurs also in a patient-specific AVF model, which would confirm the above hypothesis on the hemodynamics-related mechanism of local development of stenosis.
Section snippets
Patient-specific data and AVF model
The subject was a 48 year old male, who participated in a prospective clinical trial (Caroli et al., 2013). As per the study protocol (Bode et al., 2011), the patient had blood sample, ultrasound (US) and magnetic resonance angiography (MRA) investigations of the left arm vessels, pre-operatively and after six weeks post-operatively. Patient-specific flow rate waveforms derived from US in the arteries, namely the proximal artery (PA) and the distal artery (DA) are shown in Fig. 1a. Details on
Results
The patterns of disturbed flow in this patient-specific AVF are presented in Fig. 2. Reciprocating shear disturbed flow zones revealed by high OSI (Fig. 2a), are located on the inner wall of the SS, after the vein curvature, and on the DA near the anastomosis floor. Multidirectional flow, as characterized by medium-to-high transWSS (>10 dyne/cm2, Fig. 2b) is located on the anastomosis floor, the whole SS and, in a lesser extent more distally, after the vein curvature. Such patterns of transWSS
Discussion
While the mechanism of vessel wall pathophysiology has been the subject of considerable research, the idea of the link between disturbed flow and NH in VA is relatively new (Remuzzi and Ene-Iordache, 2013). In the present study we employed image-based CFD in a realistic model of side-to-end radial-cephalic AVF, showing development of disturbed flow. The working hypothesis regarding existence of disturbed flow zones that may trigger the local remodeling mechanism (Ene-Iordache and Remuzzi, 2012
Conflict of interest
All the authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials
Acknowledgments
Part of this study was presented at the 7th World Congress of Biomechanics held in Boston in July 2014. The authors acknowledge their collaborators from the ARCH-Consortium (Project FP7-ICT-2007-2-224390) for patient-data gathering.
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