C OMPUTATIONAL F LUID D YNAMIC M ODEL OF A RTERIOVENOUS F ISTULA A NASTOMOSIS TO S TUDY THE E FFECTS...
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Transcript of C OMPUTATIONAL F LUID D YNAMIC M ODEL OF A RTERIOVENOUS F ISTULA A NASTOMOSIS TO S TUDY THE E FFECTS...
COMPUTATIONAL FLUID DYNAMIC MODEL OF ARTERIOVENOUS FISTULA ANASTOMOSIS TO STUDY THE EFFECTS OF VESSEL SIZE AND PRESSURE GRADIENT ON THE PRESENCE OF CLINICAL STEAL AND A
THRILLNicole Varble BS1, Dan Phillips Ph.D. 1, Risa Robinson Ph.D. 1, Karl Illig, MD2, Ankur Chandra2
1 Rochester Institute of Technology, Rochester, NY2 University of Rochester, Rochester, NY
IntroductionClinical steal, decreased or retrograde blood flow to the hand from the brachial artery, can occur after implementation of an arteriovenous fistula (AVF) anastomosis. Having the ability to predict the onset of steal has the potential to improve operating room procedures and treatment of steal. With the use of Computational Fluid Dynamics (CFD) software, an experimental model has been created which can predict the onset of retrograde flow in the distal brachial artery based on the outlet pressure of the AVF and the relationship of the AVF and distal brachial artery diameters.
ObjectivesPart I: Discover a relationship between the outlet pressures and the onset of steal. The magnitude and direction of flow in the distal brachial artery relative to the inflow at the proximal brachial artery was monitored under several different AVF outlet pressure conditions. Ultimately, a pressure threshold at which retrograde flow occurs was found and quantified in terms of differential pressure between the two outlets.
Part II: Discover a relationship between the ratio of the fistula and distal brachial artery diameter and the onset of steal. The magnitude and direction of flow in the distal brachial artery relative to the inflow at the proximal brachial artery was monitored under several different AVF sizes. The goal was to determine a threshold at which steal occurs based on the AVF diameter and develop a relationship between the ratio of the brachial artery and the AVF diameter.
Methods-Assumptions
•Non- pulsatile flow •Blood vessels are idealized as perfect cylinders•Initial diameters were based on the average size of blood vessels complied from the current literature•Inlet and outlet pressures and flows are based on average pressures and flows in the vessels and blood and as found in current literature•Working fluid is considered a Newtonian fluid with an average density, ρ = 1060 kg/m3, and dynamic viscosity, µ = 0.005 Ns/m2.
-Mesh and Numerical Methods
Quad elements were pave on the bifurcation (edge) and at the inflow and outflow faces. Tet/hybrid elements were used in the model volume. The model was tested for grid independence. A total of 153,024 elements were used. Scheme- simple, gradient- least squares cell based, pressure- standard, momentum- first order upwind. Residuals for mass conservation and momentum converged at 1e-6.
Methods-Geometry and Boundary Conditions
The geometry was created in Gambit (Ansys Inc.) and imported into the fluid dynamics solver, Fluent (Ansys Inc.).
A small portion of the arm vasculature was modeled which focused on the bifurcation of the brachial artery and AVF.
Geometric properties and boundary conditions include:Proximal Brachial Artery: Diameter = 4.4 mm, Length = 13 cmDistal Brachial Artery: Diameter = 4.4 mm, Length = 13 cmAVF: Diameter = 1.1 - 6.16 mm (typical 5.5 mm), Length = 10 cmInlet Velocity of Brachial Artery = 570 mL/minOutlet Pressure of Brachial Artery = 67 mmHg Outlet Pressure of AVF = 47 - 67 mmHg
All fluid entered the system at the proximal brachial artery and flowed either through the distal brachial artery or through the access vessel. Flow was pressure driven, but given an initial velocity.
Results-Velocity Magnitude Plots
Using velocity magnitude plots, areas of maximum velocity were identified (as shown red). This may give insight into locations of possible eddies and a resulting thrill.
Results-Pressure Magnitude Plot
The pressure magnitude plots gave insight towhich vessel, the distal brachial artery or theAVF, acted as a low pressure vessel. Flow willpreferentially travel through the low pressure vessel.
-Velocity Vector Plots
The differential pressure, dP, of the outlets (pressure outlet 1- pressure outlet 2 were varied until antegrade flow was observed. As shown in Figures 11 and 12 a dP of 20 mmHgresulted in retrograde flow, where a dP of 0 mmHg yielded antegrade flow. Areas of turbulence were also identified.
-Prediction of Steal
Compiling the results of 7 pressure differential situations and 7 vessel diameter ratios, thresholds were determined to be the following:
Minimum pressure differential betweenPbrachial and Pfistula to ensure antegrade flow: dP > 10 mmHg
Minimum ratio of fistula to brachial diameter to ensure antegrade flow: Dfistula: Dbrachial > 0.80 -or- Dbrachial: Dfistula > 1.25
ConclusionsPart I: In all cases, the access acts as a low pressure vessel and flow preferentially travels through it. When the differential pressure between the outlet of the brachial artery and the outlet of the access is limited to 10 mmHg, antegrade flow can still be preserved in the distal brachial artery.
Part II: In order to ensure that antegrade flow is achieved, the ratio of the diameter of the fistula to the diameter of the brachial artery must be greater than 0.80.
Future Work
Improvements such as the assumptions such as blood being non- Newtonian and the vessel walls being rigid can be eliminated. An investigation into the eddies and turbulent flow patterns at bifurcations can further enhance the understanding of the onset of retrograde flow in the brachial artery and examine the presence of a “thrill”.
Meshed Bifurcation
Antegrade
Antegrade
Retrograde
Retrograde
Figure 2: The 3D geometry created in Gambit
Figure 3: Schematic of the modeled area
Figure 1: The meshed edge (left) and the entire meshed geometry (right)
Figure 5: Pressure magnitude plots [Pa] identify the low pressure vessel (blue).
Figure 6: Velocity vector plots yield both the magnitude in m/s (high magnitude is red) and direction of flow (arrows).
Results
Conclusions
Future Work
Introduction
Objectives
Methods
Model
Results
dP = 20 mmHg
Low Pressure Vessel
9.76e+03
6.26e+03
dP = 0 mmHg
Antegrade Flow
9.10e-01
8.36e-03
dP = 20 mmHg
Retrograde Flow
1.43e+00
8.73e-03
Turbulent Regions
Figure 4: An investigation of the effects of differential pressure (dP, figures to the left) and ratio of brachial diameter to fistula diameter (right) , the location of the maximum flow [m/s] was identified through velocity magnitude plots.
dP = 5 mmHg
Maximum Flow
9.10e-01
0.00e+00
df = 6.16 mmdb = 4.4 mmratio: 1.4
1.17e+00
0.00e+00
df = 2.2 mmdb = 4.4 mmratio: 0.5
7.18e-01
0.00e+00
Brachial Artery
AVF
Wall, no slip
Blood Flow
Brachial Artery
Access Vessel
Inlet, vo
Outlet, P2
Outlet, P1
df
L2L1
L3
db
Maximum Flow
dP = 20 mmHg1.41e+00
0.00e+00
Brachial Artery
AVF