Increased Inlet Blood Flow Velocity Predicts Low Wall Shear Stress in the Cephalic Arch of Patients with Brachiocephalic Fistula Access

Abstract

BackgroundAn autogenous arteriovenous fistula is the optimal vascular access for hemodialysis. In the case of brachiocephalic fistula, cephalic arch stenosis commonly develops leading to access failure. We have hypothesized that a contribution to fistula failure is low wall shear stress resulting from post-fistula creation hemodynamic changes that occur in the cephalic arch.MethodsTwenty-two subjects with advanced renal failure had brachiocephalic fistulae placed. The following procedures were performed at mapping (pre-operative) and at fistula maturation (8–32 weeks post-operative): venogram, Doppler to measure venous blood flow velocity, and whole blood viscosity. Geometric and computational modeling was performed to determine wall shear stress and other geometric parameters. The relationship between hemodynamic parameters and clinical findings was examined using univariate analysis and linear regression.ResultsThe percent low wall shear stress was linearly related to the increase in blood flow velocity (p < 0.01). This relationship was more significant in non-diabetic patients (p < 0.01) than diabetic patients. The change in global measures of arch curvature and asymmetry also evolve with time to maturation (p < 0.05).ConclusionsThe curvature and hemodynamic changes during fistula maturation increase the percentage of low wall shear stress regions within the cephalic arch. Low wall shear stress may contribute to subsequent neointimal hyperplasia and resultant cephalic arch stenosis. If this hypothesis remains tenable with further studies, ways of protecting the arch through control of blood flow velocity may need to be developed.

A predictive framework to elucidate venous stenosis: CFD & shape optimization

“…In a parallel study, we have statistically and computationally identified a physiologic range of WSS in the cephalic vein pre-surgery. This range was determined to be 0.076 ≤ τ w ≤ 0.76 Pa [61] , which is consistent with the range given in previous literature [22] .…”

“…The flow is assumed to be Newtonian. Pulsatility and non-Newtonian effects were both attended to previously and were found to be insignificant in our patient cohort [61,62,63,64,77,75,76] . WBV for the CFD simulation is taken to be the patient-specific high-shear rate asymptotic viscosity values from the viscometry tests [62,78] .…”

“…The alternative would be to consider patient-specific cardiac waveforms and, instead, consider the TAWSS distribution in the domain, which is computationally very costly. In this specific application, as we showed before [61,62,63] , the WSS distribution is found to be insensitive to pulsatility and implementation of the non-Newtonian constitutive relations [61] .…”

“…Moreover, the simulations discussed thus far are all stationary and therefore, it stands to reason that no turbulent scales are present. We reiterate that the inlet pressure is time-independent [61,63] . The alternative would be to consider patient-specific cardiac waveforms and, instead, consider the TAWSS distribution in the domain, which is computationally very costly.…”

Suppressing unsteady flow in arterio-venous fistulae

“…25 More recent work, undertaken specifically in the context of vascular fluid mechanics, includes studies of stenotic flows, 26 pulsatile flow in curved pipes, [27][28][29][30] flow in helical pipes, [31][32][33][34] and flow at junctions. [35][36][37] In terms of simulating flow in AVF configurations, there have been a range of previous Computational Fluid Dynamics (CFD) studies, including 1D simulations, [38][39][40] 2D simulations, 41 3D simulations with rigid walls in idealised geometries, [42][43][44][45][46][47] 3D simulations with rigid walls in more realistic geometries, [48][49][50][51][52][53][54][55] and simulations with distensible walls. 56 Of these studies, several investigated flow unsteadiness.…”