Dialysis AV Fistula
Renal failure requires one of 3 supplementary therapies:
- Haemodialysis (hemodialysis)
- Peritoneal dialysis
- Renal transplant.
This section deals with hemodialysis and the assessment of an arterio-venous fistula.
IMPORTANTLY, ALWAYS LIAISE WITH YOUR RENAL TEAM AND FOLLOW DEPARTMENTAL PROTOCOL.
Below is a guide/proposed protocol.
Role of Ultrasound
- Pre-operative assessment.
- Pre-op Mapping.
- Post operative assessment.
In 1943 Dr Willem Kollf designed and built the first dialysis machine using sausage skins, juice tins and an old washing machine.
Dr Willem Kolff
One of the original renal dialysis machines.
Simplified schematic of how dialisys works:
Arterial blood from the patient enters the dialyser in many small tubes.
In the dialyser the blood is in many fine tubes and surrounded by a dialysate liquid that will ‘draw’ the impurities/waste chemicals from the blood via a semipermeable membrane and a pressure gradient between the blood and dialysate.
The ‘cleaned’ blood is returned to the venous system.
The dialysate is refreshed throughout.
A haemodialysis machine.
Image courtesy of:
Types of Fistulae and Anastomoses
- To facilitate adequate flow volumes, an arterio-venous fistula is surgically created.
- These can be in either arm, in the legs and may involve grafts or just native vessels.
- An anastomosis is the junction between vessels or graft to vessel.
Different methods of creating an arterio-venous fistula.
The side artery – end vein is most common.
There are several places and methods used to create the AVF.
(Arm vessels are used in these protocols, however it can also be applied to leg assessment)
- Always assess the non-dominant arm (unless otherwise clinically directed).
- If the measurements are not favourable, assess the other arm.
The objective is to confirm:
- Vessel patency and flow quality.
- Vessel size.
-Distal axillary artery to the wrist.
- Assess for patency and disease.
- Document: B mode, colour + spectral trace of all vessels.
- Measure luminal diameter of:
– Radial Artery – proximal mid and distal
– Brachial artery – proximal & antecubital.
Ideal minimum arterial luminal diameter: 2.0 mm.
(Many surgeons will accept 1.6 mm)
Arm arterial anatomy diagram.
Transverse ultrasound of the distal radial artery (red)
and veins (blue).
Subclavian vein to the wrist.
- Assess for variants and thrombosis.
- Measure luminal diameter of:
– Cephalic vein – proximal, mid and distal
– Brachial vein – proximal & antecubital.
– Basilic vein – distal upper arm
- By applying a tourniquet (or BP cuff) to the upper arm, more veins of a suitable size may become visible.
Ideal minimum vein diameter:
- 2.0 mm without tourniquet.
- 2.5 mm with tourniquet.
Deep veins anatomy of the arm.
(note this shows the possible variation of dual Brachial veins)
Superficial veins anatomy of the arm.
(there can be numerous variations)
It is important to show the crisp echogenic line of the intima. This indicates the probe is perpendicular to the vessel.
Upper Arm Vein Anatomy
Ultrasound image- Cephalic vein demonstrating the correct transluminal diameter and the incorrect external diameter
Allen’s test of the deep palmar arch.
- There is a risk of compromise to distal radial arterial flow after creation of the AV fistula.
- It is common to have post AVF retrograde arterial flow (Radial Steal). This is usually assymptomatic.
- The hand and fingers are supplied via the palmar arch by both the radial and ulnar arteries. If one is compromised, the other will still supply the arch. However if there is disease in the deep palmar arch, loss of flow in the radial artery may result in ischaemia of segments of the hand/fingers.
To assess the integrity of the deep palmar arch:
- Identify arterial flow in the thenar eminence.
- Occlude the radial artery with firm pressure at the wrist.
- Reassess the arterial flow. It should be present and retrograde (as it is now supplied by the ulnar artery. Absence of flow indicates deep palmar arch pathology.
Flow feeds the palmar arch from both the ulnar and radial arteries.
Pressure is applied, occluding the radial artery.
Flow feeds the palmar arch from only the ulnar artery.
Failed Allen’s test.
No flow distal to the occluded radial artery.
Indicates deep palmar arch compromise.
Use a china-graph pencil or permanent marker
Explain to the patient the importance of not washing the markings off.
- Place the probe longitudinally on the vessel.
- Place a skin mark at each end of the probe.
- Continue along the vessel for the desired length
- Join-the-dots (a preferred method is a solid-line for venous and a dashed-line for arterial).
Distal lateral forearm:
- Radial artery (dashed line)
- Cephalic vein (solid line)
Antecubital fossa (elbow):
- Brachial artery (dashed line)
- Cepahlic vein (medially and Basilic vein (laterally) with the common bridging vein all as solid lines.
Post operative assessment and Follow-up
- The AVF should mature to a useful state by 3 months post operative creation.
- Post operative assessment may be for:
- A post operative complication.
- A failure to mature (ie Lack of development of vein dilatation).
- Complications with using the mature AVF in dialysis.
- Routine follow-up may be requested by some renal units.
Most immediate complications can be resolved with repeat surgery, angioplasty of the stenosis or ligation of decompressing veins.
- Accessory veins
If the vein is >6mm deep to the skin, it may not be palpable and therefore seem clinically immature so make note of the depth.
- Thrombosis (most common complication)
- Infection (high risk of morbidity)
- Ischaemic radial steal
- Pseudoaneurysms (often related to inadequate puncture site rotation)
- Aneurysm (may indicate a proximal stenosis)
The Magic 6's for a good AVF
>6mm diameter vein
<6mm from the skin surface
What To Assess
- Is there a collection?
- Is the arterial inflow patent?
- Is the arterial inflow, low resistance biphasic?
- Is the fistula anastamosis patent?
- Is there a stenosis at the fistula?
- Is the artery distal to the anastamosis antegrade or retrograde?
- Is the vein patent with high velocity, high volume, low resistant flow?
- Is the vein 6mm in diam?
- Is there any venous thrombosis?
- What are the flow volumes in the artery and vein 5cm from the anastamosis (and further proximal in the vein.
Ultrasound image- Normal arterial inflow.
Low resistance, high diastolic flow
Ultrasound image- Normal venous outflow.
High velocity, turbulent, low resistance, high diastolic flow.
Ultrasound image- Normal dialysis AV fistula anastamosis.
Side artery, end vein type.
Ultrasound image- Typical kaleidoscope colour doppler at the fistula anastamosis.
This can be improved by increasing the colour doppler PRF (scale)
- Volumetric analysis of the venous outflow and arterial inflow.
- Most high end machines have the capability of performing this.
- Must be beyond the turbulence of the anastomosis. (usually >5cm).
- >500mL/minute suggests adequate flow.
Take a doppler trace with the sample gate opened to the luminal diameter of the vein.
Ultrasound image- A trace with average velocity must be obtained.
Some equipment uses the sample gate to calculate the area, some require an independant area measurement.
If you do grade stenoses, use:
- Direct b-mode area reduction measurement if possible.
- Arterial velocity 3:1=50%
- Vein or graft velocity 2:1 = 50%
Failure to mature.
The vein diameter was <6mm due to a venous stenosis at the anastamosis (hypoechoic plaque at the arrow).
ref: W Coulls/A. Russell
Ultrasound image- A stent in the basilic vein at the anterior elbow as it crosses the brachial artery.
Thrombus is present mid stent (this did not affect overall flow volumes).
Cephalic vein Aneurysm
Ultrasound image- High velocity jet into the venous pseudoaneursm proximal to the AVF anastamosis.
Ultrasound image- Cephalic vein pseudo aneurysm.
Likely cannulation trauma.
ref: W Coulls/A. Russell
Ultrasound image-The end of the stent demonstrating the end of the stenotic segment.
Ultrasound image- The typical ‘dotted-wall’ appearance of an intraluminal stent.
This contains hypoechoic plaque creating a stenosis.
The length and diameter of the stenosis is measured
Cephalic vein Aneurysm
Ultrasound image- A curved stent.
Ultrasound image- Occluded inflow radial artery.
The fistula flow was via the retrograde distal radial artery resulting in poor venous flow volumes.
Ultrasound image- Typical parallel lines of a graft.
The hypoechoic stenosis is measured.
Ultrasound image- Transverse view of the stenosed graft.
The residual lumen highlighted with colour doppler.
Ultrasound of a dialysis AV fistula - Protocol
- Overlying dressings, particularly immediately post operative or on a dialysis day.
- If possible, coordinate the scan booking prior to dialysing or on a different day.
- If the scan is to be done after dialysis cannulation, allow several hours to be able to remove dressings.
- Seated comfortably.
Either on the side of the bed with their arm on a pillow on their lap.
Opposite side of the bed with arm resting, extended towards you on the bed.
Linear array probe. Ideally 2.
- Mid range frequency 6-10MHz
- High frequency 12-18MHz
This is because, whilst superficial, the high velocities often require a high PRF doppler setting which may not be achievable with the higher frequency probe.
Begin at the palpable fistula (thrill).
- Perform a sweep through the anatomy in transverse without and then with colour doppler to assess the anatomy.
Basic Hardcopy Imaging
- Fistula anastamosis B-mode and colour.
- Fistula anastamosis diameter.
- Spectral doppler trace velocity and volume in the feeding artery.
- Spectral doppler trace velocity and volume in the draining vein. 5cm from the anastamosis. At least 2.
- Document flow velocity and volume in the draining vein proximally in the arm.
- Document flow presence and direction in the artery distal to the anastamosis.
- Document any anatomical variants encountered..
- Document all pathology in B-mode and colour.
- A diagramatic worksheet is essential.
- A simple diagram with diameters, flow volumes and any pathology.