Post on 24-Feb-2016
description
AN ARTERIOVENOUS MODEL OF THE ARM CIRCULATION, AN ARTERIOVENOUS FISTULA AND DISTAL REVASCULARIZATION AND INTERVAL LIGATION
Nicole VarbleBS/MS Mechanical Engineering
Advisor: Dr. Steven Day
Introduction
Objectives
Results Continued
Conclusions
Future Work
Introduction
Methods
Methods Continued
Results
An arteriovenous fistula (AVF) is a vessel which bridges the arterial to the venous blood flow. An AVF is put in place as an access point for hemodialysis as an alternative to a catheter.
Clinical 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.
A common treatment for malfunctioning AVFs is a procedure called Distal Revascularization and Interval Ligation (DRIL), where the AVF is bypassed on the arterial side.
Little is known about the hemodynamics surrounding an AVF or DRIL procedure, therefore, using mechanical engineering techniques, combined with vascular surgery expertise, a thorough investigation of these procedures was performed.
Physical ModelA physical model using tubes and plastic connectors and driven by the hemodynamic simulator, was built to model the entire arm vasculature. Variations to model include:Blood Pressure, AVF Flow, Diameter and Length of AVF, Position of AVF, and features include:
Analytical AnalysisOnce the experimental resistances were calculated, a comparison of several analytical approaches were used and their validity was assessed in regards to the physical model where pulsatile flow through compliant tubing was present. The analytical theories are shown below:
Physical Model
0 1 2 30
50
100
150
P2: CC In-Line with Flow
Pre
ssur
e (m
mH
g)
0 1 2 30
50
100
150
P12: CC In-Line with Flow
Pre
ssur
e(m
mH
g)
0 1 2 30
50
100
150
P2: CC Side Stream
Pre
ssur
e (m
mH
g)
0 1 2 30
50
100
150
P12: CC Side Stream
Pre
ssur
e(m
mH
g)
0 1 2 30
50
100
150
P2: No CC
Pre
ssur
e (m
mH
g)
time (s)0 1 2 3
0
50
100
150
P12: No CC
Pre
ssur
e(m
mH
g)
time (s)
7.6 7.8 8 8.2 8.4 8.675
80
85
90
95
100
105
110
115
time (s)
Arte
rial B
lood
Pre
ssre
(mm
Hg)
1 2 3 4 5 6 7 8 9 10200
250
300
350
400
450
500
550
600
650
700
AVF Diameter (mm)
Han
d Fl
ow (m
L/m
in)
Hollow Tygon tubingTubing length and thickness match vessel compliance and anatomy and includes venous return
Glycerin and Water40/60 Glycerin/Water mix was run through the physical model to match blood viscosity
Connectors with Pressure TapsNon- Compliant tubing, capable of acquiring pressure measurements at each junction through pressure transducers, one-way valve included in venous return
Hand Compliance ChamberColumn of water below column of pressurized air, accurately mimics compliance and resistance of hand capillary bed
Hemodynamic SimulatorVentricular and Venous Compliance chamber, ventricular and buffing chamber, two artificial valves, driven by Servo motor, outputs pulsatile flow
Mathematical ModelUsing electrical resistors and capacitors to simulate the equivalent resistance and capacitance of a blood vessel, a model was created to compare the physical model results to this mathematical model
Computational Fluid Dynamics ModelA simpleCFD model simulated the AVF and brachial artery bifurcation. The outlet pressure of the AVF and to AVF diameter were altered to determine a threshold at which steal may occur.
Brachial Artery
AVF
Computational Fluid Dynamics Model ResultsPressure difference between the outlet of the AVF and the outlet of the brachial artery must not exceed 10 mmHg for antegrade flow to occur. Additionally, the ratio of the brachial artery diameter to the fistula diameter must not exceed 1.25 for antegrade flow to occur.
Mathematical Model ResultsAn accurate physiological waveform was produced by dampening a sinusoidal curve with two diodes. Using the Impedance Model, a second order relationship is seen between the radius and the flow through a vessel.
Analytical Analysis ResultsIt was found that the Womersley’s Impedance method most accurately predicts the resistance of a vessel. Poiseuille’s Law overestimates the increase in resistance as a function of vessel diameter.
DRIL
AortaVena Cava
Subclavian
Axillary
PROX Brachial
DIST Brachial
Ulnar
Radial Hand Compliance
DIST Vein
PROX Vein
AVF
Collateral
0 0.5 1 1.5 2 2.5 34
4.5
5
5.5
6
6.5
time (s)A
ortic
Flo
w (L
/min
)
AVF Flow AVF PositionNative Circulation, AVF,
and DRIL
•The physical model accurately represents the native circulation•Clinical steal is a result of decreased brachial artery pressure•Moving the AVF proximal and lengthening the AVF marginally increases the distal flow and pressure•Decreasing the AVF diameter greatly increases distal flow and distal pressure•High blood pressure increases distal flow and distal pressure•Collateral blood supply increases distal flow and distal pressure•Distal revascularization increases both distal flow and distal pressure and is further improved with interval ligation
Improvements to physical model may include, a more accurate representation of stenosis of the arteries, an investigation into the patency of the pressure taps, and more physiologic accurate connectors.
The CFD model can be expanded immensely to include flexible walls, an expanded arterial and venous blood flow, non-Newtonian fluid, and an investigation of turbulent regions of the bifurcation.
The mathematical model can be expanded to more accurately mimic the physical model.
dP = 20 mmHg
Retrograde Flow
1.43e+00
8.73e-03
df = 6.16 mmdb = 4.4 mmratio: 0.7
1.17e+00
0.00e+00
Figure 1- The AVF (white) moves blood from the arterial to the venous side. The arterial bypass
(DRIL) passes over the AVF and is revascularized at a distal location of the arterial blood flow. The arterial ligature (interval ligation) is just
distal to the AVF [1].
[1] "ACS Surgery," Decker Intellectual Properties, 2010.
Figure 2- The subclavian artery made with clear Tygon tubing and equipped with the
ME PXL10 flow probe and pressure transducer.
Figure 3- The brachial- collateral bifurcation, adapted with a pressure tap (left). The brachial- AVF
bifurcation adapted with a pressure tap and shown with the leur lock assembly (right) to easily connect
to pressure transducers.
Figure 4- The unfilled compliance chamber
after the radial and ulnar arteries converge.
Figure 5- Schematic of RIT MSD P09026 Hemodynamic Simulator (image created to Matthew DeCapua)
Figure 6- The brachial artery/AVF bifurcation modeled and meshed in ANSYS,
a CFD software
Figure 7- The sin wave driving the mathematical model with two damping diodes (left). The brachial artery modeled in Simulink using
two resistors and a capacitor. (right).
Figure 8- The complete artierovenous physical model capable of producing physiologic pressure and flow waveforms.
0 1 2 30
50
100
150
P2: CC In-Line with Flow
Pre
ssur
e (m
mH
g)
0 1 2 30
50
100
150
P12: CC In-Line with Flow
Pre
ssur
e(m
mH
g)
0 1 2 30
50
100
150
P2: CC Side Stream
Pre
ssur
e (m
mH
g)
0 1 2 30
50
100
150
P12: CC Side Stream
Pre
ssur
e(m
mH
g)
0 1 2 30
50
100
150
P2: No CC
Pre
ssur
e (m
mH
g)
time (s)0 1 2 3
0
50
100
150
P12: No CC
Pre
ssur
e(m
mH
g)
time (s)
Mean Arterial Pressure: 92 mmHg
Mean Venous Pressure: 27 mmHg
Mean Aortic Flow: 5.45 L/min
Figure 9- The effects on the distal (hand) flow and pressure when the flow through the AVF is altered (left), when the position is altered (center), and when the DRIL bypass procedure is implemented (right).
Figure 10- A velocity vector plot of retrograde flow occurring when the difference between the AVF and brachial artery outlet pressure is 20 mmHg (left), and a velocity magnitude plot of the an enlarged AVF where maximum velocity is shown in red (right).
Figure 11- The pulsatile waveform generated in Simulink (left), and the relationship between AVF diameter and distal (hand) flow when the Impedance model is used (right).
Figure 12- The experimentally found resistances as compared to the analytical impedance method is shown to
closely predict the resistances in a pulsatile vessel.