Ankle brachial index

Ankle brachial index (ABI) is a means of detecting and quantifying peripheral arterial disease (PAD). It can be performed in conjunction with ultrasound for better results.

Indications

Many (20-50%) patients with PAD may be asymptomatic but they may also present with

• limb pain / claudication
• critical limb ischemia
• chest pain

Procedure

Equipment

• blood pressure cuff
• pulse volume recording (PVR) Doppler ultrasound
• grayscale duplex Doppler can be useful to map the arterial disease and problem solve 
  • for mapping, evaluate the iliac arteries, common femoral arteries, the proximal, mid, and distal femoral arteries, popliteal artery, and the tibial and peroneal artery runoffs
  • evaluated acceleration times, velocities, and waveforms in the segments

Technique

A blood pressure cuff or ultrasound is used to evaluate the pressure in the brachial artery in both arms and the anterior and posterior tibial arteries in both legs.

The higher of the two brachial artery pressures is used for the index.

The higher of the anterior or posterior tibial artery pressures is used for the index.

Radiographic features

The index is a ratio of the pressure in the highest ankle artery / the highest brachial artery

• 1.0-1.4: normal
• 0.91-0.99 borderline
• ≤0.9: abnormal (i.e. PAD)
  • 0.4-0.9: mild-to-moderate PAD
  • <0.4: suggestive of severe PAD

There is a special category for an index 1.3-1.4 or above. These vessels are considered "non-compressible", and this is either because the arteries are very healthy or because there is heavy arteriolosclerotic or atherosclerotic calcification in the artery, preventing compression. The ultrasound waveform of the artery helps differentiate between these possibilities.

Outcomes

ABI values correlate with morbidity and mortality of not only the affected lower limb but also with cardiovascular risk :

• ABI <0.90: 10% chance of a cardiac event in 5 years
• ABI <0.70: ~20% chance of a cardiac event in 5 years