Ankle-Brachial Index

Aka: Ankle-Brachial Index, Ankle-Brachial Ratio, Toe-Brachial Ratio, Toe-Brachial Index

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II. Indications

  1. Claudication
  2. Peripheral Arterial Disease Diagnosis and Monitoring
  3. Screening for Peripheral Arterial Disease
    1. See Peripheral Arterial Disease for screening indications
  4. Vascular Injury Evaluation
    1. Leg Trauma
    2. Posterior Knee Dislocation

III. Contraindications

  1. Known or suspected Deep Vein Thrombosis
  2. Severe Leg Pain (e.g. leg wound, tissue ischemia, Fracture)
  3. Arterial Injury Hard Signs
    1. Active Hemorrhage
    2. Distal pulses absent
    3. Pulsatile or absent Hematoma
    4. Acute Limb Ischemia signs

IV. Efficacy

  1. Test Sensitivity: 90-97%
  2. Test Specificity: 98%

V. Technique

  1. Precautions
    1. ABI may be difficult to obtain in obese, hypothermic or Peripheral Arterial Disease patients
    2. Consider increasing cuff size
    3. Consider repeating exam when well hydrated, warm and after injury stabilization
  2. Measure highest systolic reading in both arms
    1. Record first doppler sound as cuff is deflated
    2. Record at the radial pulse
    3. Use highest of the two arm pressures
      1. Repeat the first arm Blood Pressure if there is a discrepancy between arms >10 mmHg
  3. Measure systolic readings in both legs
    1. Cuff applied to calf, just above the malleoli
      1. Avoid applying over a distal bypass site (risk of bypass thrombosis)
    2. Inflate cuff to 20 mmHg after doppler signal disappears
    3. Record first doppler sound as cuff is deflated
    4. Use Doppler Ultrasound device (8 MHz)
      1. Record dorsalis pedis pressure (absent in 2-3% of normal patients)
      2. Record posterior tibial pressure
    5. Use highest ankle pressure (DP or PT) for each leg
  4. Calculate ratio of each ankle to brachial pressure
    1. Divide each ankle pressure by highest brachial pressure

VI. Interpretation

  1. Absolute ankle pressure <50 mmHg is abnormal
  2. Ankle-Brachial Ratio >=1.4: Non-compressable vessels (see False Negatives below)
  3. Ankle-Brachial Ratio >0.90: Normal
  4. Ankle-Brachial Ratio <0.90: Peripheral Vascular Disease
  5. Ankle-Brachial Ratio <0.6: Intermittent Claudication
  6. Ankle-Brachial Ratio <0.5: Multi-level disease
  7. Ankle-Brachial Ratio <0.3: Limb Threatening Ischemia (requires emergent intervention)
  8. Ankle-Brachial Ratio <0.26: Resting ischemic pain
  9. Ankle-Brachial Ratio <0.2: Gangrenous extremity

VII. Interpretation: False Negative Test - Non-compressable vessels (ABI >=1.4)

  1. Results in falsely elevated ankle pressure
  2. Conditions where vessels are non-compressable or small vessel disease
    1. Diabetes Mellitus
    2. End-stage renal disease (ESRD)
    3. Diffuse arterial calcifications (as in severe or diffuse PAD)
  3. Alternative studies
    1. Toe-Brachial Ratio
      1. Typically 0.7 to 0.8
      2. Abnormal <0.7 (severe if <0.4)
    2. Exercise ABI Testing
      1. Obtain ABI immediately after walking 5 minutes on treadmill at 12% grade and 2.0 miles/h OR
      2. Symptoms require patient to stop

VIII. Management: Abnormal Ankle-Brachial Index (ABI < 0.9 or ABI>1.4)

  1. Segmental Arterial Pressure
  2. Other options
    1. Arterial Duplex Ultrasound
    2. Standard angiography
    3. CT angiography (also consider in Trauma, e.g. posterior Knee Dislocation)
    4. Magnetic resonance angiography

IX. References

  1. Hirsch (2002) Lecture, Fairview Lakes RMC
  2. Warrington (2020) Crit Dec Emerg Med 34(11): 11
  3. Aboyans (2012) Circulation 126: 2890-909 [PubMed]
  4. Carman (2000) Am Fam Physician 61(4):1027-32 [PubMed]

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