Effect of anastomosis angle on the localization of disturbed flow in ‘side-to-end’ fistulae for haemodialysis access.

Background - Early failure of vascular access (VA) for haemodialysis (HD) after the surgery of radial-cephalic arteriovenous fistula (AVF) occurs mainly due to a juxta-anastomotic stenosis. Even if the elevated blood flow induces high wall shear stresses (WSS), we have recently shown that disturbed flow, characterized by low and reciprocating flow, may develop in zones of the AVF that locate well the sites of future stenosis. The present study was aimed at investigating whether the anastomosis angle influences disturbed flow in radial-cephalic end-to-side AVF. Methods - By means of a parametric AVF model we created four equivalent meshes having the anastomotic angle of 30°, 45°, 60° and 90°, respectively. We then performed transient, non-Newtonian computational fluid dynamics (CFD) simulations using as boundary conditions previously measured blood volume flow and division ratio in subjects requiring primary access. The relative residence time (RRT), a robust indicator of disturbed flow, was calculated for the overall wall surface and disturbed flow was localized by areas having RRT > 1. Quantitative characterization and statistical tests were employed to assess the difference in RRT medians between the four anastomosis angle cases. Results - Disturbed flow was located in all AVF models in the same areas where flow recirculation and stagnation occurs, on the inner wall of the swing segment (SS) and on the arterial wall at the anastomosis floor (AF). Smaller angle AVF had smaller disturbed flow areas with lower RRT peak values, either on the vein or the arterial limb. There were significant differences in the RRT medians on the SS and on the AF between sharper (30° and 45°) and wider (60° or 90°) angles. Conclusions - We have found that in end-to-side radial-cephalic AVF for HD the anastomosis angle does impact on the local disturbed flow patterns. Among the four geometries we considered in this study, the smaller angle (30°) would be the preferred choice that minimizes development of neointima. Clinicians should consider this at the time of AVF creation because anastomosis angle is in part amenable to surgical manipulation.