What is the Arteriovenous (AV) fistula functioning test?

Arteriovenous (AV) Fistula Functioning Test

Duplex Doppler ultrasound is the preferred method for AV fistula functioning assessment, providing both morphological and functional data essential for optimal AVF outcomes. 1

Key Parameters Assessed During AV Fistula Functioning Tests

Flow Parameters

• Blood flow rate measurement (most critical parameter):
  • Optimal functioning AVF: 700-1,300 mL/min 2
  • Warning signs of dysfunction:
    • Flow <500 mL/min (predictor of access dysfunction) 2
    • Flow <300 mL/min (predictor of pending thrombosis) 2
    • Reduction of >25% in flow volume over 1-4 months in previously stable access with flow >1,000 mL/min 2

Anatomical Parameters

• Vein diameter: Minimum 6 mm for functional access 2
• Access depth: Should be <6 mm from skin surface 2
• Vessel margins: Should be readily identifiable 2
• Straight segment length: At least 10 cm to support two dialysis needles 2

Stenosis Assessment

• Peak systolic velocity ratios to identify stenosis:
  • Duplex Doppler has 95.5% sensitivity and 57.1% specificity for stenoses >50% 2
  • Hemodynamically significant stenosis defined as 50% narrowing with at least one abnormal clinical or hemodynamic indicator 2

Testing Methods

Duplex Doppler Ultrasound (First-line)

• Advantages:

  • Non-invasive
  • Provides both structural and functional assessment
  • Can differentiate structural stenoses from transient vasospasm 2
  • High sensitivity (87%) and specificity (96%) for detecting AVF dysfunction 3
  • Can be performed at bedside
  • Avoids contrast exposure

• Parameters measured:

  • Blood flow volume (mL/min)
  • Vein diameter
  • Access depth
  • Presence of stenosis
  • Peak systolic velocities

Fistulography (Second-line)

• Indications:

  • When Doppler ultrasound suggests abnormalities
  • When limitations to routine dialysis arise
  • For definitive diagnosis before intervention 2

• Advantages:

  • Gold standard for detecting stenosis 3
  • Allows simultaneous diagnosis and intervention
  • Comprehensive imaging of entire access circuit from arterial anastomosis to right atrium 2

• Limitations:

  • Invasive procedure
  • Requires contrast exposure
  • May not detect hemodynamically significant stenoses that are occult on imaging 2

Integrated Surveillance Protocol

• Combination of clinical examination with Doppler ultrasound shows optimal sensitivity (100%) and specificity (89%) compared to fistulogram 3
• Recommended surveillance sequence:
  1. Regular clinical examination (look for changes in thrill/bruit, prolonged bleeding after decannulation, arm swelling)
  2. Doppler ultrasound for objective measurements
  3. Fistulogram if dysfunction is suspected based on clinical and ultrasound findings

Automated Assessment Technologies

• Newer technologies like automated 3D ultrasound systems enable non-expert personnel to measure AVF parameters with comparable accuracy to conventional duplex performed by trained sonographers 4
• These systems may improve compliance with regular surveillance by making testing more accessible

Clinical Application

• Regular surveillance using these testing methods can detect early dysfunction and allow for timely intervention, which prolongs access patency
• The KDOQI guidelines define a mature and functional fistula as one that dependably provides prescribed dialysis using 2 needles for greater than two-thirds of dialysis sessions over 4 consecutive weeks 2

AV fistula functioning tests are essential for early detection of access dysfunction, allowing for timely intervention and improved patient outcomes by avoiding emergency procedures and prolonged catheter dependence.