Mitigation of the turbulence within an arteriovenous fistula with a stent implantation

The transitional flow which initiates within the junction (anastomosis) of an arteriovenous fistula (AVF) is known to be a contributing factor in the onset of vascular disease. A novel treatment method involving the implantation of a flexible stent across the anastomosis has enabled the retention of a large proportion of functioning AVFs, despite the propensity for stent malapposition to occur at the sharp inner curve of the anastomosis. Large eddy simulations of a single patient-specific AVF with and without the presence of a stent captured oscillatory flow behavior emanating from the interface of the two inlet flows in the stent-absent case, however, these oscillatory features were subdued in the stented case. The stent-absent case generally had higher turbulent kinetic energy (TKE) in the anastomosis which led to larger cycle-to-cycle variations in wall shear stress (WSS). The significantly lower TKE generated at the heel of the stented AVF was contained within the malapposed stent, thereby resulting in lower WSS fluctuations. However, a slight increase in turbulence downstream of the malapposed stent edge was noted. This detailed study reveals a significant decrease in turbulence within the AVF in the presence of the stent, thereby providing a level of understanding underpinning the success of the treatment strategy (in this patient case) from a fluid dynamic perspective.

Figure 1

Geometry and boundary conditions of the AVF models. The stented (top left) and stent-absent (top right) AVF geometries have been annotated with key regions of the vasculature. The inlet flow rate profiles applied at the proximal artery and distal artery are displayed at the bottom left.

Figure 2

Detailed view of cells at the vessel and stent wall boundaries near the anastomosis as annotated by the red box in the left subfigure. The average cell edge length on the vessel wall (middle) was $103\mu \mathrm{m}$ and the average cell edge length on the stent wall was $32.5\mu \mathrm{m}$.

Figure 3

Streamlines colored with the phase-averaged velocity magnitude at the maximum (top) and minimum (bottom) time points. The streamlines are illustrated for both stented (left) and stent-absent (right) cases.

Figure 4

Spectral decomposition of the velocity magnitude time-traces at monitor points along the vein for the stented and stent-absent cases. The darker lines correspond to the stent-absent case while the lighter lines correspond to the stented case. The dotted black line indicates the ‘-5/3’ slope. The monitor point locations ($0\text{mm}$, $11\text{mm}$, $21\text{mm}$, $50\text{mm}$, and $79\text{mm}$ away from the anastomosis along the centerline) shown in the subfigure at the bottom right, link to the spectral plots by color.

Figure 5

Contour plots of instantaneous velocity magnitude on the central plane of the anastomosis. The distribution is illustrated near the maximum time point with a time separation $\mathrm{\Delta }t=0.009\text{s}$. Annotations in the stent-absent geometry highlight the oscillations of the flow initiating from the interface of the two inlets.

Figure 6

Contour plots of turbulent kinetic energy (TKE) on the central plane of the anastomosis. The distribution of the TKE is illustrated at the maximum (top) and deceleration (bottom) inlet flow time points for both stented (left) and stent-absent (right) cases.

Figure 7

Contour plots of vorticity magnitude (left) and turbulent kinetic energy (right) on the central plane at the edge of the stent. The distributions have been plotted at the maximum (top) and deceleration (bottom) inlet flow time points for the stented case.

Figure 8

Example of unwrapping the 3D vessel wall locations onto a 2D space using local vectors constructed from centerline node locations. The color bar of the cross-sectional slices assist with understanding the 3D location of the wall element on the corresponding 2D plots.

Figure 9

Wall shear stress metrics on vessel wall at the distal artery.

Figure 10

Wall shear stress metrics on vessel wall at the proximal vein.

Figure 11

Turbulent wall shear stress distribution at the maximum (top) and deceleration (bottom) time points across the proximal vein.