A mature fistula follows the "rule of 6s" (≥ 6 mm diameter, ≤ 6 mm deep, ≥ 600 mL/min flow); dysfunction is a stenosis with a peak velocity ratio of about 2–3 or more, most often at the juxta-anastomotic segment.
Orient first
- Radiocephalic and brachiocephalic fistulas are the common native accesses; grafts stenose at the venous anastomosis.
- Poor maturation is usually a juxta-anastomotic stenosis or competing accessory veins.
- Steal syndrome: reversed flow in the distal radial artery with hand ischaemia.
Acquire the study
- High-frequency linear probe with colour and spectral Doppler along the inflow artery, anastomosis and outflow vein to the central veins.
The manoeuvre
- Brachial artery volume flow in mL/min (angle-corrected, at a straight segment).
- Anastomosis and juxta-anastomotic vein: diameter in mm and peak systolic velocity.
- Outflow vein: diameter, depth from skin in mm, accessory branches, stenoses — velocity ratio across each.
- Thrombus and aneurysmal segments with wall thickness in mm.
- Distal radial artery flow direction (steal).
What confirms it
- A stenosis with a velocity ratio above threshold and reduced access flow; maturation criteria met or not.
What licenses you to exclude it
- Adequate volume flow and no velocity step-up along the circuit exclude a haemodynamically significant stenosis.
The classic misread
- Measuring volume flow in a tortuous or aneurysmal segment.
- Missing a central venous stenosis that needs venography.