ABC of Vascular Disease

Ankle Brachial Pressure Index (ABPI)

1. What is an ABPI?
An ABPI is a quick, simple, non-invasive method of measuring the effect that arterial disease is having on the blood pressure in the lower leg.

2. How is the ABPI measured?
If there is arterial disease in the arteries of the leg the blood pressure at the foot is reduced (see Haemodynamics). The degree of reduction of the blood pressure is a measure of the severity of the arterial disease. The ABPI is measured in just the same way as the blood pressure is measured in the arm: with a 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. This ratio of ankle blood pressure to brachial (arm) blood pressure pressure is called the ankle brachial pressure index or ABPI.

3. What is a normal and an abnormal ABPI?
An ABPI of greater than 0.9 (90%) is considered to be normal. An ABPI of beween 0.5 and 0.9 (50% to 90%) is consistent with symptoms of Intermittent Claudication. An ABPI of less than 0.5 (50%) is usually associated with more severe symptoms, even Critical Ischaemia.

4. How reliable is the ABPI measurement?
In most patients it is a very reliable measure of the severity of arterial disease. There are a number of situations where the ABPI is less reliable:

	the ABPI may be falsely raised in diabetic patients who have severe disease in the smaller arteries in the leg.
	the ABPI may appear normal at rest in patients with significant disease the the arteries in the pelvis.

In these cases modifications to the technique may be required such as:

	measuring the pressure in the toe rather than at the ankle in diabetics
	measuring the ABPI immediately after exercise in patients with pelvic arterial disease.

4. How is the ABPI measurement used?
The ABPI is used as a simple, objective method of measuring the severity of arterial disease and monitoring the response to treatment. A change in the ABPI usually reflects a change in the severity of the symptoms. An ABPI can also be used to exclude arterial disease. If a patient describes leg pain on exercise and the ABPI is normal at rest and after exercise, it is very unlikely that the leg pain is due to arterial disease and another cause should be looked for.

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