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Ankle-brachial index

The ankle-brachial index (ABI) is essential in the evaluation of peripheral arterial disease (PAD). It’s the ratio of systolic ankle blood pressure to systolic brachial blood pressure. This shows the relative blood pressure in the feet compared to the arms, which is a good estimation of blood flow. Normally, the ABI = 1 (BP in feet and hands is the same), but in case of ABI < 1, the blood flow to the feet is decreased, which is a consequence of PAD.

The ABI is related to the toe-brachial index.

Procedure

To calculate the ABI, the systolic BP is first measured in both arms like usual. Then, the systolic BP is measured for the ankles. This is done with the help of a continuous wave Doppler probe, a small device which uses Doppler ultrasound to amplify the sound of arterial blood flow. This is needed because the sound of arterial blood flow in the arteries of the ankles are too small to be heard with a stethoscope. The Doppler probe is used to find the dorsalis pedis and posterior tibialis arteries. A blood pressure cuff is applied to the ankle (just above the malleolus) and inflated while the Doppler probe is left in place. The cuff is inflated to a pressure which is guaranteed higher than the systolic blood pressure (like 150+ mmHg), at which point the sound of arterial blood flow from the Doppler probe will disappear. The pressure in the cuff is slowly released until the sound of blood flow returns. The pressure in the cuff at this point is the systolic blood pressure of that artery. This is then repeated for the other artery of the foot.

The ankle-branchial index is then calculated like this: Ankle-brachial index = systolic blood pressure of ankle / systolic blood pressure of arm on the same side.

Interpretation

In case of elderly, chronic diabetics, and ESRD patients, the arteries may be sclerotic. This makes them harder to compress and gives a falsely elevated systolic blood pressure of the ankle, which gives a falsely elevated ABI. As such, the ABI should be interpreted like this:

  • ABI > 1,3 = sclerotic arteries
  • ABI 1 – 1,30 = normal value
  • ABI 0,90 – 1 = borderline value. PAD may be present but is not clinically relevant (yet)
  • ABI 0,40 – 0,90 = diagnostic for mild to moderate PAD
  • ABI < 0,40 = diagnostic for severe PAD, critical limb ischaemia

If the ABI is normal but PAD is still highly suspected, it may be repeated after simple exercise. If the ABI decreases by > 20% after exercise, this is diagnostic for PAD.

If arterial sclerosis is suspected, the toe-brachial index should be measured.