Ankle-Brachial Index

The Ankle-Brachial Index (ABI) is an indicator of the presence of peripheral artery disease. It is obtained using a sphygmomanometer and a Doppler ultrasound device, measuring blood pressure in the upper and lower limbs and calculating its ratio.

Examen diagnóstico

General Description

The Ankle-Brachial Index (ABI) is a numerical value obtained by measuring and comparing blood pressure in the upper and lower limbs. The purpose of calculating it is to assess the arterial circulation to the lower limbs.

To obtain the ABI, systolic blood pressure in the ankle (in the pedal artery and posterior tibial artery) and systolic blood pressure in the arm (in the brachial artery) must be measured. The index is calculated by dividing the highest value obtained at the ankle by the highest value obtained at the arms. An index is obtained for each side of the body, with the final ABI value being the lowest numerical value of the two.

When is it indicated?

The Ankle-Brachial Index is a simple, non-invasive method for diagnosing obstructive peripheral artery disease (PAD), or peripheral arteriosclerosis, a condition characterized by a decrease or absence of blood flow to the limbs, typically the legs, due to the narrowing or obstruction of peripheral arteries.

Thus, the test is indicated for patients who present early symptoms of PAD, such as leg pain when walking that subsides with rest (intermittent claudication) or a drop in skin temperature in the legs. It is also measured in patients with risk factors for developing peripheral arteriosclerosis:

Atherosclerosis.
Diabetes.
Hypertension.
DyslipidemiaDyslipidemia Dyslipidemia .
Smoking.
Male sex.
Age over 50 years.
Postmenopause.
Dialysis.

Additionally, measuring the ABI is used as a follow-up method for peripheral artery disease after diagnosis and to assess the success of treatment.

How is it performed?

The test to measure the ABI combines the use of a sphygmomanometer (blood pressure cuff) and a Doppler ultrasound device. The Doppler device uses ultrasound to study the dynamic behavior of blood flow, measuring the speed and direction of the flow.

The cuff is placed on the corresponding limb, and after palpating the artery, the ultrasound probe (transducer) is moved across the skin to obtain an adequate arterial sound signal. The cuff is then inflated until the arterial pulse disappears, after which the cuff is slowly deflated. The pressure indicated by the sphygmomanometer when the arterial pulse is detected again by the Doppler device is considered the systolic pressure and used to calculate the index. This procedure is repeated on both arms and both ankles.

If the patient has normal ABI values but exhibits intermittent claudication, an exercise ABI test may be performed. This involves measuring the ABI before and immediately after the patient walks on a treadmill to quantify the drop in blood pressure compared to the resting state and confirm or rule out reduced blood flow. For patients with long-standing diabetes or severely obstructed arteries, it may be necessary to measure the ABI on the big toe to obtain an accurate result.

ABI values obtained are classified into three ranges:

From 1.0 to 1.3: normal values, no obstruction. PAD is unlikely, but if the patient presents symptoms, an exercise ABI test is performed.
From 0.90 to 0.99: values within normal limits, but at the risk threshold. The test is often repeated or an exercise ABI test is performed.
From 0.6 to 0.9: mild or moderate PAD.
Below 0.6: severe PAD.
Below 0.4: critical PAD.

Risks

Obtaining the Ankle-Brachial Index is a simple, non-invasive procedure with no risk to the patient. However, it is contraindicated for patients with blood clots in the veins (thrombosis) of the legs or with infection in the lymphatic vessels (lymphangitis). It is also not recommended for patients with rigid immobilizations in the limbs, such as casts or splints, or with open wounds in the area where the transducer is placed.

What to expect from an Ankle-Brachial Index test

Before starting the procedure, the patient should rest for 5 to 30 minutes. The test is performed with the patient lying on their back on an examination table, after removing their shoes and socks and exposing their arms. The cuff is placed a few centimeters above the elbow bend on the arm, and just above the malleoli (the two bony prominences on the sides of the ankle joint) on the ankle.

After palpating the skin to locate the artery, a conductive ultrasound gel is applied to the skin to facilitate signal transmission. While the cuff inflates, it is normal to feel pressure or even pain, which stops once the cuff deflates.

If an exercise ABI test is performed, after the first measurement, the patient must walk on the treadmill until they feel pain, at which point they lie down and a new measurement is taken.

The ABI test is an outpatient procedure that lasts only a few minutes. Once finished, the patient can resume their usual routine without any issues.

Specialties requesting the Ankle-Brachial Index

The Ankle-Brachial Index is requested in consultations in internal medicine, family medicine, or angiology and vascular surgery.

How to prepare

The ABI test requires no special preparation, though it is recommended to wear comfortable, loose clothing that facilitates the placement of the blood pressure cuff.