Ankle-Brachial Index

Original Editor - Khloud Shreif

Top Contributors - Khloud Shreif, Chloe Waller, Vidya Acharya and Lucinda hampton  

Introduction

PAD

The ankle-brachial index (ABI) is a quick, non-invasive technique used to check vascular status and for peripheral arterial disease (PAD). It was first described by Winsor in 1950.

The ABI is the ratio between the systolic blood pressure of the lower limb (specifically at the ankle) and the upper limb, to assess for narrowing or blockages in the arteries in the legs.

Indications

Symptoms of peripheral artery disease:

• Intermittent claudication or cramping pain in the legs.
• Wounds on the legs that are slow to heal.
• Leg that feels colder than other parts of the body or is a different shade of color.

Those with risk factors for PAD, such as:

• History of tobacco use

• Diabetes

• High blood pressure

• High cholesterol

• Restricted blood flow (atherosclerosis) in other parts of your body. 
• History of stroke or transient ischemic attack.
• Family history of heart disease.
• Over 70 years of age.

Contraindications

• Patients with confirmed or suspected DVT, because of the possibility of breaking of the thrombus.
• Severe leg pain.

Procedures

Tools

• Doppler
• Blood pressure monitor.
• Pressure cuffs

Preparation

ABI measurement

• Rest for 10-20 minutes prior to the test.
• Wear loose, comfortable clothing.
• Avoid nicotine/ smoking for 2 hours prior to the test.
• Supported, comfortable supine position.

Technique

1. Place the cuff around the patient's arm, use the doppler and press against the skin at the level of the brachial artery in the arm. Inflate the cuff to about 20 mmHg above the last audible pulse. The Doppler signal should disappear. Then slowly deflate the cuff, approximately 1 mmHg/sec. When the Doppler signal re-appears, the pressure of the cuff is equal to the brachial systolic pressure. Record the brachial systolic pressure.
2. Place the cuff around the lower leg and locate two arteries:
   1. Dorsalis pedis, which is a branch from the anterior tibial artery, it is located between the tibialis anterior and extensor hallucis longus
   2. Posterior tibial artery, that is behind the medial malleolus.
3. Repeat the same technique as the arm to record the dorsalis pedis and posterior tibial arterty: using the doppler to locate the signal, inflating the cuff and then deflating until the signal re-appears.
4. The ABI value is calculated by taking the higher pressure of the 2 arteries at the ankle, divided by the brachial arterial systolic pressure.
5. Repeat the ankle measurement for the other leg in order to calculate its ABI.

Demonstration video:

Clinical Significance

The normal range is between 0.90 and 1.30. below 0.8 means there is a difficulty of blood flow to the foot and lower limb.

Below 0.7 means severe peripheral artery disease PAD and the patient may develop ulcer and gangrene.

Above the normal level, 1.30 is a sign of calcification of arterial walls that may happen in case of diabetic patients, chronic kidney disease, in this situation you need to measure the ankle pressure at distal digital arteries that is less likely to develop or be calcified

This ABI is not a confirmation for the diagnosis for arteries calcification.

General Considerations

• In normal healthy people, there is a slight decrease in ABI if measured immediately after lower limb exercises due to the decreased ankle systolic pressure because of vasodilation during lower limb exercises, and return to normal within about 3 min after rest.

• There is more decrease in ankle pressure after walking in case of moderate occlusive PAD specifically in the proximal vessels.

• The ABI has a high sensitivity, but these results came from one single study which may be included participants with critical limb ischemia or diabetic patients.

• People with a history of PAD will demonstrate a continuous decrease of ABI over time, about 0.06 over 4.6 years. On the other side, there is a smaller decrease in the general population it's about 0.025 over 5 years.

• If the ankle pressure is less than 50 mm Hg it is associated with a higher risk of avascularity and amputation.

Resources

Ankle-Brachial Index