Applicability of an entry flow model of the brachial artery for flow models of the hemodialysis fistula

Abstract

The native arteriovenous fistula creates a shunt that provides the high blood flow that is needed for dialysis. Lumped parameter hemodynamic models of the arteriovenous fistula can be used to predict shear stresses and pressure losses and can be applied to help understand unsolved problems such as the high rate of arteriovenous fistula maturation failure. These models combine together flow components, such as arteries, stenosis, anastomoses, arterial compliance, and blood inertia, and each component must be modeled with an appropriate pressure-flow relationship. Poiseuille flow is generally assumed for straight vessels, but the unique high flow rates within the brachial artery of an arteriovenous fistula are expected to induce entry flow effects that are neglected in this model. To estimate the importance of these effects, brachial artery flow was modeled in a low-resistance network, such as the one that occurs when an arteriovenous fistula is constructed, through the lumped parameter model, and the predicted flow rates and pressures were compared to those predicted by computational fluid dynamics. When Poiseuille flow was assumed, the flow rate from the lumped parameter model was consistently larger than that from computational fluid dynamics, with a cycle-averaged error of 36.8%. When an entry flow model (Shah) was assumed, the lumped parameter-based flow was 6% lower than the computational fluid dynamics model at the peak of the flow waveform, and the cycle-averaged error was reduced to 7.8%. Thus, in a low-resistance (high flow) arteriovenous fistula circuit, an entry flow model can account for steeper near-wall velocity gradients. This result can provide a useful guide for designing engineering models of the arteriovenous fistula.

Comparison of various zero‐dimensional models to analyze the brachial artery blood flow of an arterio‐venous fistula during hemodialysis procedure

“…2 In hemodialysis, blood is circulated to a dialyzer to remove waste and add nutrients, after which it is returned to the venous side at approximately 300-3000 ml/min. [3][4][5] Permanent vascular access is preferred to carry out the procedure of hemodialysis. Of the many forms of vascular access, a matured autogenous arterio-venous fistula (AVF), which is formed by direct connection of a vein to an adjacent artery, has longest survival rates with fewest complications.…”

“…5 The exact mechanism of AVF failure remains unresolved, however, some studies claim that local hemodynamic factors such as fluid shear stress and pressure-flow relationships of the blood, could influence AVF maturation by impairing vein dilatation and arterializations and by promoting stenosis and thrombosis. 4,8 Many of the hemodynamic factors of an AVF cannot be practically obtained during a clinical dialysis session. Caroli et al noted that ultrasound signals are often inadequate for blood flow measurement at the AVF site because blood velocity profiles are non-uniform.…”

“…16 To understand the hemodynamics of the arterial blood flow in the vascular surgery, Marchandise et al applied the RK method to a patient specific WK model. 17 In our previous study, 4 blood flow in the brachial artery of an AVF was modeled with a fifth order WK model (pulsatile) for different flow conditions (i.e., Poiseuille and Shah). The built-in MATLAB adaptive RK function ODE45 was used.…”

“…Variations in the results from those two studies ranged from 6% to 35%, depending upon the flow conditions. 4 Because arterial stiffness is linked to AVF maturation failure, 4 vessel wall elasticity (pulsatility) is crucial in modeling the brachial artery blood flow.…”

“…21 The Shah relationship has closely matched the pressures and flows in brachial artery with an in vitro model of vascular access and with CFD results. 4,12 In the previous study, only laminar flow conditions were considered 4 however, a study by Dixon suggested that the turbulent blood flow might start in the brachial artery, proximal to the anastomosis in an AVF. 5 This claim is further supported by a clinical Doppler ultrasound study, which was conducted by Zamboli et al, where they found blood flow turbulence at the end of the brachial artery in an AVF.…”