Dialysis access is a patient's lifeline, and access failure is one of the leading causes of hospitalization in the haemodialysis population. Duplex surveillance detects failing access before it thromboses, which is the difference between an elective angioplasty and an emergency.

The study has two distinct purposes depending on timing: assessing whether a new fistula is maturing, and surveilling an established access for stenosis.

Anatomy and Access Types

Arteriovenous fistula (AVF) — a direct surgical anastomosis between artery and vein. Preferred access: lower infection and thrombosis rates, longer patency. Needs weeks to months to mature.

Common configurations, in the usual order of surgical preference:

Radiocephalic (Brescia–Cimino) — radial artery to cephalic vein at the wrist
Brachiocephalic — brachial artery to cephalic vein at the antecubital fossa
Brachiobasilic with transposition — basilic vein moved superficially

Arteriovenous graft (AVG) — synthetic conduit between artery and vein. Usable sooner, but higher stenosis and infection rates. The venous anastomosis is the classic site of stenosis.

Technique

Probe: High-frequency linear, 7–15 MHz. A lower-frequency probe may be needed for deep basilic transpositions or obese patients.

Patient position: Arm extended, supported, externally rotated. A warm room genuinely matters — a cold arm vasoconstricts and depresses flow measurements.

Scan the entire circuit in order, because a stenosis anywhere along it affects the whole access:

Inflow artery — proximal to the anastomosis
Anastomosis
Juxta-anastomotic segment — the first few centimetres of outflow vein, the most common stenosis site in an AVF
Body of the fistula or graft — including cannulation zones
Venous outflow — including the graft's venous anastomosis
Central veins — subclavian, brachiocephalic, as far as accessible

Document arm and configuration on every study. Serial comparison is the entire value of surveillance, and it is worthless if the reports do not say what was scanned.

Volume Flow Measurement

The single most important number in access surveillance.

Where to measure: In the brachial artery, roughly 3–5 cm proximal to the anastomosis, where flow is laminar. Measuring within the fistula itself is unreliable — the vessel is tortuous, aneurysmal, and flow is disturbed.

How:

Obtain a clean longitudinal image with the vessel walls parallel
Measure the vessel diameter carefully — volume flow depends on the square of the radius, so a small diameter error produces a large flow error. This is the dominant source of inaccuracy.
Obtain a time-averaged mean velocity (TAMV) with the sample volume covering the full lumen
Keep the Doppler angle at 60° or less

Volume flow (mL/min) = TAMV (cm/s) × cross-sectional area (cm²) × 60

Most machines calculate this automatically once diameter and TAMV are set. Take three measurements and average them.

Interpreting Volume Flow

Volume flow	Interpretation
< 400–500 mL/min (AVF)	Low — high risk of thrombosis
< 600 mL/min (AVG)	Low — commonly used threshold for intervention in grafts
600–1500 mL/min	Typical functioning access
> 2000 mL/min	High-output access; consider cardiac consequences

A fall of more than about 25% from a previous measurement is significant even if the absolute value remains within range. This is why serial studies and consistent technique matter more than any single number.

Maturation Assessment — The Rule of 6s

For a new fistula, a widely used and easily remembered guide:

6 weeks post-creation
Flow greater than 600 mL/min
Diameter greater than 6 mm
Depth less than 6 mm from the skin

Depth is frequently forgotten and is a common reason a fistula with excellent flow still cannot be cannulated — the dialysis nurse cannot reliably needle a vein sitting 12 mm deep. A deep but otherwise mature fistula may need superficialization, so always report depth.

Also assess: absence of significant accessory or competing branches diverting flow, and adequate length of straight, cannulable segment.

Stenosis Criteria

Peak systolic velocity ratio is the primary criterion: PSV at the stenosis divided by PSV in a normal segment 2 cm proximal.

PSV ratio > 2:1 — suggests a haemodynamically significant stenosis (> 50%)
PSV ratio > 3:1 — used by many laboratories as the threshold for a higher-grade stenosis

Supporting findings:

Absolute PSV > 400 cm/s at the stenosis
Focal luminal narrowing on B-mode with a residual lumen under about 2 mm
Post-stenotic turbulence and spectral broadening
Reduced volume flow downstream

Where stenoses occur:

AVF: juxta-anastomotic segment is by far the commonest, followed by cannulation zones and central veins
AVG: the venous anastomosis is classic, plus intra-graft stenosis at cannulation sites

Central venous stenosis deserves specific attention, particularly in patients with prior central lines or pacemakers. Look for loss of respiratory phasicity and loss of cardiac pulsatility in the subclavian and axillary veins — a continuous, non-phasic waveform suggests a more central obstruction even when you cannot image the lesion directly. Arm swelling with a continuous outflow waveform is a strong combination.

Other Complications

True aneurysm: Dilatation involving all vessel wall layers, usually at repeatedly cannulated segments. Measure maximum diameter and assess overlying skin thickness — thin, shiny, or ulcerated skin over an aneurysm is a rupture risk and needs urgent flagging.

Pseudoaneurysm: Contained wall defect, most often at a cannulation site or graft puncture. Look for the yin-yang swirling flow pattern and a demonstrable neck with to-and-fro flow. Measure the sac and the neck.

Thrombus: Echogenic material within the lumen with absent or reduced flow. Distinguish partial from occlusive, and document the extent — how much of the access is involved determines whether it is salvageable.

Steal syndrome: The access diverts so much flow that the distal hand becomes ischaemic. Look for retrograde (reversed) flow in the artery distal to the anastomosis. A useful bedside test: compress the fistula and observe whether distal arterial flow normalizes and the patient's symptoms improve. Correlate with clinical findings — cold, painful, pale hand, and in severe cases digital ulceration.

Infection: Perigraft fluid collection with surrounding hyperaemia. In a graft this is a serious finding warranting prompt communication.

Key Images to Capture

Inflow artery with spectral waveform
Anastomosis, B-mode and colour
Juxta-anastomotic segment with PSV
Volume flow measurement in the brachial artery — diameter, TAMV, calculated flow
Body of fistula or graft with diameter and depth measurements
Any stenosis — B-mode, colour, PSV at and proximal to the lesion, with ratio
Venous outflow and accessible central veins with waveforms
Any aneurysm or pseudoaneurysm — measurements and Doppler
Distal arterial flow direction where steal is suspected

Reporting Language

"Left brachiocephalic AV fistula. Volume flow in the brachial artery 480 mL/min (previous study 890 mL/min, a 46% reduction). Focal narrowing in the juxta-anastomotic segment with residual lumen 1.8 mm, PSV 465 cm/s against 140 cm/s in the adjacent proximal segment, ratio 3.3:1. Post-stenotic turbulence present. Fistula depth 5 mm, diameter 8 mm. Subclavian vein waveform remains phasic. Findings consistent with a high-grade juxta-anastomotic stenosis with significantly reduced access flow; recommend prompt vascular assessment."

SonoBuddy includes volume flow and velocity ratio calculators alongside vascular protocols — offline in the vascular lab.

Dialysis Access Duplex: AV Fistula and Graft Surveillance