Indications
Preoperative vessel mapping before fistula creation, assessment of maturation at 4–6 weeks, routine surveillance of a functioning access, and suspected dysfunction (difficult cannulation, reduced dialysis blood flow, recirculation, high venous pressures on the machine, limb swelling, aneurysm or pseudoaneurysm, and suspected hand ischaemia from vascular steal).
Equipment and patient position
- Linear array transducer, high frequency, 7–15 MHz: the fistula vessels are superficial and a high frequency maximises spatial resolution.
- Patient seated or supine with the limb externally rotated and abducted, resting on a firm surface; the limb should be examined at a comfortable room temperature, since cold causes vasoconstriction and artificially reduces the measured flow.
- The examination is always performed at rest, without recent compression of the site (avoid scanning immediately after haemostasis following dialysis needling).
Systematic scanning
3.1 · Inflow artery
The arterial segment proximal to the anastomosis (typically radial or brachial) is assessed for calibre, patency and flow profile, which in a mature fistula physiologically becomes low-resistance with persistent diastolic flow, unlike the triphasic pattern of a native peripheral artery.
3.2 · Anastomosis
The commonest site of early stenosis (within the first year). Document calibre and any narrowing, and sample with pulsed-wave Doppler carefully, since a high-velocity jet and some turbulence just beyond the anastomosis are physiological, even without pathological stenosis.
3.3 · Outflow vein (fistula body)
The whole course is followed to the central veins, documenting calibre, depth from the skin (relevant to cannulation), patency, any stenosis (juxta-anastomotic, mid-segment, or at sites of repeated needling) and any aneurysmal or pseudoaneurysmal dilatation.
Volume flow measurement (Qa)
The fistula volume flow (Qa) is calculated with the formula:
It is ideally measured on a straight segment of the inflow artery or of the fistula body, away from the anastomosis, bends and stenoses, with an insonation angle ≤ 60° and sampling of the whole lumen (a sample volume covering the full vessel diameter, so as to include the true velocity profile and not only the central peak). Most ultrasound machines calculate Qa automatically once the vessel diameter and the time-averaged mean velocity (TAMV) envelope have been traced.
Stenosis criteria
As in other arterial territories, the velocity ratio between the site of suspected stenosis and the normal segment immediately proximal is used: a PSV ratio ≥ 2–3, together with post-stenotic turbulence on colour Doppler and a fall in overall Qa, is the most widely used combined criterion for a haemodynamically significant stenosis warranting correction (angioplasty or surgical revision).
Steal syndrome
Where there are symptoms of distal ischaemia (pain, pallor, paraesthesiae, digital ulceration), the artery distal to the anastomosis is assessed (for example the distal radial artery in a radiocephalic fistula, or the arteries of the hand): retrograde flow, directed from the periphery towards the anastomosis rather than towards the fingers, confirms that the fistula is "stealing" flow from the hand. The extent is also documented (retrograde flow throughout the cycle versus only in one phase) and always correlated with examination of the hand (colour, temperature, capillary refill).
Maturation criteria
At 4–6 weeks after creation, assessment of maturation for suitability for cannulation is classically based on the "rule of 6", which may be adapted locally:
- Volume flow (Qa) ≥ 600 mL/min.
- Outflow vein diameter ≥ 6 mm.
- Depth from the skin ≤ 6 mm.
A fistula that does not reach these parameters within the expected time should be classified as at risk of failure to mature and assessed for possible correction (for example ligation of competing collaterals, or dilatation of a juxta-anastomotic stenosis).
What to document
- Calibre and patency of the inflow artery, the anastomosis and the outflow vein.
- Volume flow (Qa) and, on serial studies, the percentage change from the previous measurement.
- Site, degree (PSV ratio) and extent of any stenosis.
- Presence and size of aneurysms or pseudoaneurysms, with the condition of the overlying skin.
- The result of the assessment for steal syndrome, where requested.
- Maturation parameters (Qa, diameter, depth), where the examination is performed for this purpose.