What is Ankle Brachial Pressure Index and How is it Measured?

An Ankle Brachial Pressure Index (ABPI) is a common, quick, simple, non-invasive diagnostic test for measuring the effect that arterial disease is having on the blood pressure in the lower leg. It's also known as the ankle-to-arm systolic pressure index and is good for determining peripheral arterial disease.

If there is arterial disease in the arteries of the leg the blood pressure at the foot is reduced. The degree of reduction of the blood pressure is a measure of the severity of the arterial disease. ABPI is measured the same way as blood pressure is measured in the arm using a Pressure Manometer cuff that is wrapped around the leg and inflated until the pressure in the cuff is the same as the pressure in the artery just under the skin. The only difference is that a special probe is used to listen to blood flow in the artery rather than a stethoscope. Normally, the pressure in the leg is compared with the equivalent pressure in the arm to see what proportion of pressure is lost by the damaged arteries. Systolic pressure at the ankle can be found using a handheld continuous wave Doppler unit. It is then divided by the systolic pressure in the brachial (arm) artery. This ratio of ankle blood pressure to arm blood pressure is the "ankle brachial pressure index".

Is it Reliable? What's Normal?

An ABPI greater than 0.9 (90%) is considered normal. An ABPI between 0.5 and 0.9 (50% to 90%) could be a symptom of a problem. An ABPI of less than 0.5 (50%) is often considered a severe symptom and could indicate Critical Ischaemia. But these decimals and percentages are merely guidelines and insufficient on their own to absolutely evaluate blood flow. They must be taken in context of and correlated with other tests done at the same time.

In most patients, Ankle Brachial Pressure Index is a very reliable way to measure the severity of arterial disease. However, there are some situations where the ABPI may not be so reliable, such as in:

• Diabetic patients who have severe disease in the smaller arteries of their leg (e.g.; Monckeberg’s Sclerosis aka medial wall calcification). The test may actually reflect the ability of the blood vessel wall to resist compression rather than truly indicating blood flow through and the pressure within it.; and
• Patients with significant disease in the arteries of the pelvis.

With these types of patients, the accuracy of the test can be greatly improved by adjusting it slightly and measuring the:

• Pressure in the toe of diabetics instead of the ankle; and
• ABPI immediately after exercise in patients with pelvic arterial disease.

How is an Ankle Brachial Pressure Index Performed?

Priot to the test the patient must rest at least five minutes. The person conducting the ABPI will begin by holding the Doppler probe at about a 50 degree angle to the artery and then move it around over the artery to find the loudest signal and highest possible blood velocity. The pressure in both arms will be measured and the higher pressure used for calculations.

The cuff is inflated until it's greater than arm systolic pressures and completely collapses the dorsalis pedis and posterior tibial arteries. It is then deflated slowly to accurately determine the pressure at which blood flow returns.

Who Should Get an Ankle Brachial Pressure Index?

Any patient with diabetes or a history of organic occlusion, ulceration, or ischaemic pain should ask their physician whether an ABPI is advisable. Particularly good candidates for this test are persons who have been diabetic more than 20 years and are undergoing other physical exams as well as diabetics with:

• leg pain of unknown causes;
• foot ulcers;
• femoral bruits;
• decreased pulse;
• Type 1 diabetes mellitus patients over 35 years old who are undergoing other exams;
• Type 2 diabetes mellitus aged 40 years or older who are undergoing other exams; and