How to calculate the ankle brachial index

Written by julie revel
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How to calculate the ankle brachial index
Measure the systolic pressure of the brachial and ankle arteries. (Hemera Technologies/PhotoObjects.net/Getty Images)

The ankle-brachial index (ABI) is a fundamental tool used to assess the risk of peripheral vascular disease. The American Heart Association estimates that over 8 million Americans have vascular disease, which can lead to an increased incidence of heart attack, stroke, aneurysms and chronic venous disease. The ABI helps to detect early signs of arterial stress and to establish risk factors for cardiovascular disease. It is a calculated ratio of systolic blood pressure in the ankle compared to the arm.

Skill level:
Moderately Challenging

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Things you need

  • Patient exam table
  • Sphygmomanometer
  • Doppler ultrasound
  • Pencil
  • Paper
  • Calculator

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Instructions

  1. 1

    Place the patient in a supine position and rest for 10 minutes.

  2. 2

    Record the systolic blood pressure of both arms. Wrap the sphygmomanometer around the arm about 1 inch above the elbow. Use the ultrasound to locate the pulse of the brachial artery, located in the middle of the upper arm. Inflate the cuff of the sphygmomanometer to approximately 160mm Hg or at least 20mm Hg higher than when the pulse disappears. Slowly release the air of the cuff until you hear the pulse return for two consecutive beats. Record this number as the systolic pressure.

  3. 3

    Wait two minutes and repeat the measure. Average the two left recordings together and average the two right recordings together. The higher pressure between the left and right arm will be the brachial pressure used.

  4. 4

    Record the systolic blood pressure for the right posterior tibial and dorsal pedis arteries. The posterior tibial artery is located on the inside of the leg between the bony protrusion of the ankle and the Achilles tendon. The dorsal pedis artery is on the top of the foot below the ankle. Wait two minutes and repeat the measure. Average the two recordings to obtain the average pressure for the right posterior tibial and right and dorsal pedis arteries. The higher pressure between the tibial and dorsal arteries will be the ankle pressure used for the right ankle.

  5. 5

    Record the systolic blood pressure for the left posterior tibial and dorsal pedis arteries. Wait two minutes and repeat the measure. Average the two recordings to obtain the pressure for the left posterior tibial and left and dorsal pedis arteries. The higher pressure between the tibial and dorsal arteries will be the ankle pressure used for the left ankle.

  6. 6

    Calculate the right ABI, which is equal to the highest right average ankle pressure (dorsal or tibial) divided by the brachial pressure.

  7. 7

    Calculate the left ABI, which is equal to the highest right average ankle pressure (dorsal or tibial) divided by the brachial pressure.

Tips and warnings

  • An ABI of less than 0.40 indicates severe peripheral artery disease. Between 0.3 to 0.8 displays mild to moderate signs of disease, while 0.95 to 1.2 is considered normal.
  • Peripheral artery disease is a serious medical condition. An ABI should be performed by a knowledgeable provider who can properly assess cardiovascular health.

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