Ankle Brachial Index (ABI) For PAD Risk

The ABI method explained

The Ankle Brachial Index (ABI) checks the systolic pressure values to diagnose or rule out peripheral arterial disease (PAD). It may also be used to stratify patient risk for other cardiac events.

This method has been found to be both specific (98%) and sensitive (90%) in detecting stenosis in subsequent studies.

It is now recommended as routine examination for all patients over 70 years and for patients over 50 if smoking or diabetic. It is also used to monitor medical treatments or recovery after angioplasty.

The ABI formula is the systolic pressure at the ankle divided by the systolic pressure at the arm.

The assessment is non-invasive and is performed on the patient placed in supine position. The pressure cuff is used subsequently for every arm and the right and left systolic pressures are recorded.

The upper arm procedure differs from classic pressure measuring because it also involves the use of ultrasound gel in the antecubital fossa over the brachial pulse.

An effect of intensity maximization is obtained by using the transducer of the handheld Doppler on that area.

The cuff is then inflated at above 20 mmHg higher than the expected blood pressure or to the value where the signal on the Doppler disappears.

The recommendation for deflating the cuff is of approximately 11 mmHg/s. While the gradual deflation happens, the Doppler is monitored to register the brachial systolic pressure.

The ankle pressure measuring is performed by placing the cuff proximal to the malleoli. The ultrasound gel is then applied to the skin overlying the dorsalis pedis (DP) and posterior tibial (PT) arteries.

The inflation and deflation of the cuff are both done while monitoring the Dopper signal.

In some cases, tests may be performed twice with a treadmill test of around 6 minutes, in-between. This is because sometimes, ABI test performed at rest may be insensitive to mild PAD.

Regarding the calculation process there are 6 measurements to be taken, 2 for arms and 4 for feet arteries.

Inter-arm difference in the systolic brachial should be less than 10 mmHg, in case it is higher (even closer to 20 mmHg), this is indicative of subclavian or axillary arterial stenosis.

The highest of the measured values is taken in consideration and for each feet, the highest from dorsalis pedis (DP) and posterior tibial (PT) is considered.

The two formulas used are:

Right ABI = Highest pressure in right foot / Highest pressure in both arms

Left ABI = Highest pressure in left foot / Highest pressure in both arms

ABI interpretation

The normal values of the ABI test range between 1.0 and 1.4 because ankle pressure tends to be higher than brachial pressure.

This is an indicator that no narrowing or blockage has occurred along the vessels. This doesn’t mean that PAD can be ruled out in the case of patients with PAD risk factors.

Values that are above the normal range (>1.4) indicate that the vessel is non-compressible and calcified as it is often the case with elderly patients and diabetics.

Values of normal range and below it are discussed in the table below:

Table source: Stanford Medicine 25

Peripheral vascular disease guidelines

The American College of Cardiology (ACC) and the American Heart Association (AHA) have developed PAD management guidelines which regulate the use of ABI tests.

The most common symptoms of peripheral vascular disease include:

■ Claudication (pain with walking);

■ Lack of palpable pulse of DP and/or PT;

■ Pallor of distal extremities due to reduction or cessation of blood flow.

In the latter stages of PAD, severe ischemia is accompanied by paralysis and paraesthesia.

PAD is diagnosed through clinical tests, medical history and physical examination and in some cases magnetic resonance angiogram (MRA), arteriograms and blood tests for PAD risk factors such as diabetes or high cholesterol.

Patients with PAD are at a higher risk of coronary heart disease, cardiac arrest, transient ischemic attack (TIA) or stroke.

References

1. Aboyans V, Criqui MH, Abraham P, Allison MA, American Heart Association Council on Peripheral Vascular Disease; Council on Epidemiology and Prevention; Council on Clinical Cardiology; Council on Cardiovascular Nursing; Council on Cardiovascular Radiology and Intervention, and Council on Cardiovascular Surgery and Anesthesia et al. Measurement and interpretation of the ankle-brachial index: a scientific statement from the American Heart Association. Circulation. 2012; 126(24):2890-909.

2. Stone NJ, Robinson JG, Lichtenstein AH, Bairey Merz CN, American College of Cardiology/American Heart Association Task Force on Practice Guidelines et al. 2013 ACC/AHA guideline on the treatment of blood cholesterol to reduce atherosclerotic cardiovascular risk in adults: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 2014; 63(25 Pt B):2889-934.

3. Allison MA, Hiatt WR, Hirsch AT, Coll JR, Criqui MH. A high ankle-brachial index is associated with increased cardiovascular disease morbidity and lower quality of life. J Am Coll Cardiol. 2008; 51(13):1292-8.