RVT Dialysis Access Evaluation (AV Fistula and Graft Assessment) — Questions and Answers
Question 1: According to KDOQI clinical guidelines, what minimum access flow volume (Qa) is generally considered adequate for a functioning AV fistula to support hemodialysis?
- 100–200 mL/min
- 200–300 mL/min
- 400–500 mL/min (Correct answer)
- 800–1000 mL/min
Correct answer: 400–500 mL/min
KDOQI guidelines recommend a minimum Qa of approximately 400–500 mL/min for AV fistulas to sustain adequate dialysis clearance. Values below this threshold, particularly when declining on serial measurements, indicate a hemodynamically significant stenosis and trigger further evaluation or intervention.
Question 2: Which duplex ultrasound finding is MOST suggestive of a hemodynamically significant stenosis in an AV fistula requiring intervention?
- PSV of 180 cm/s at the venous anastomosis with a normal Qa
- PSV ratio >2:1 across a focal stenosis with access flow <400 mL/min (Correct answer)
- Mild color-flow aliasing throughout the entire fistula body
- Pulsatile waveform pattern in the outflow vein
Correct answer: PSV ratio >2:1 across a focal stenosis with access flow <400 mL/min
A PSV ratio >2:1 (stenotic segment vs. adjacent normal segment) combined with a reduced access flow (<400 mL/min) meets the threshold for intervention in most clinical guidelines. Isolated velocity elevation without flow reduction may not be hemodynamically significant; both criteria together confirm clinically meaningful stenosis.
Question 3: What is the most common anatomic location and cause of AV graft thrombosis?
- Arterial inflow stenosis at the graft-artery anastomosis
- Venous outflow stenosis, most often at the graft-vein anastomosis (Correct answer)
- Mid-graft kinking due to patient positioning
- Infection-related thrombosis at needle puncture sites
Correct answer: Venous outflow stenosis, most often at the graft-vein anastomosis
The venous outflow — particularly the graft-to-vein anastomosis — is the most common site of stenosis in AV grafts because neo-intimal hyperplasia preferentially develops at this venous anastomosis. The resulting outflow obstruction raises circuit pressure, reduces flow, and ultimately leads to thrombosis if untreated.
Question 4: Pre-operative vein mapping for AV fistula creation shows a cephalic vein with a diameter of 1.6 mm that does not distend when a tourniquet is applied. This finding indicates the vein is:
- Adequate — any vein ≥1.5 mm is acceptable for fistula creation
- Unsuitable — the diameter is below the 2 mm threshold and the vein lacks distensibility (Correct answer)
- Suitable if the patient maintains adequate hydration before surgery
- Borderline — a repeat mapping after hand exercises should be performed
Correct answer: Unsuitable — the diameter is below the 2 mm threshold and the vein lacks distensibility
Guidelines recommend a minimum cephalic vein diameter of ≥2 mm (with many centers preferring ≥2.5 mm) for successful fistula maturation. Poor distensibility indicates a sclerosed or calcified vein that will not enlarge adequately after arterialization, predicting fistula failure.
Question 5: A patient with a forearm AV fistula reports pain, coldness, and numbness in the hand that worsens during dialysis. Duplex reveals reversal of flow in the radial artery distal to the fistula anastomosis. This presentation is MOST consistent with:
- Central venous stenosis causing venous hypertension
- Dialysis-associated steal syndrome (Correct answer)
- Arterial inflow stenosis proximal to the anastomosis
- Venous outflow obstruction causing elevated access pressures
Correct answer: Dialysis-associated steal syndrome
Steal syndrome occurs when the low-resistance AV fistula 'steals' blood from the distal limb. The reversed (retrograde) flow in the radial artery distal to the anastomosis is the pathognomonic duplex finding — blood that should perfuse the hand is instead diverted into the fistula, causing ischemic symptoms that worsen during high-flow dialysis sessions.
Question 6: When evaluating AV fistula maturation readiness for cannulation ('rule of 6s'), which duplex criterion best supports adequate maturation?
- Outflow vein diameter ≥2 mm and depth ≤1 cm below skin surface
- Outflow vein diameter ≥6 mm, depth ≤6 mm below skin surface, and access flow ≥600 mL/min (Correct answer)
- Access flow ≥200 mL/min with any vein diameter
- Vein pulsatility index <1.0 and diameter ≥4 mm
Correct answer: Outflow vein diameter ≥6 mm, depth ≤6 mm below skin surface, and access flow ≥600 mL/min
The 'rule of 6s' for fistula maturation states the vein should be ≥6 mm in diameter, ≤6 mm deep (accessible for cannulation), and achieve ≥600 mL/min flow, ideally within 6 weeks. Duplex ultrasound is the primary tool used to objectively assess each of these criteria before clinical cannulation begins.
RVT Dialysis Access Evaluation (AV Fistula and Graft Assessment)
According to KDOQI clinical guidelines, what minimum access flow volume (Qa) is generally considered adequate for a functioning AV fistula to support hemodialysis?