Calculating the ankle-brachial index

Advances in Skin & Wound Care, Mar/Apr 2000 by Wills, Sloan H

Before initiating compression therapy; it is important to confirm that the patient's wound is the result of venous, not arterial, insufficiency. Although compression therapy is the mainstay of venous ulcer management, it would be contraindicated in a patient without adequate arterial blood flow.

The ankle-brachial index (ABI) gives a good indication of arterial flow. The following photographs depict the procedure for evaluating the ankle and brachial pulses, which are used in calculating the ABI.

1. A mercury or aneroid sphygmomanometer with cuff, a handheld Doppler device with vascular probe, and conductivity gel compatible with the Doppler device are needed to assess arterial flow.

2. With the patient in the supine position, apply the blood pressure cuff to either of the patient's arms and palpate for the brachial pulse. Apply conductivity gel over the brachial artery.

3. Turn on the Doppler device and place the tip of the probe into the top of the gel at a 45-degree angle. The brachial pulse is indicated by a whooshing sound.

4. Pump up the cuff to the point that the sound disappears, then go 20 mm Hg above that point. Slowly release the air and listen for the return of the whooshing sound. The point at which it is heard is the brachial systolic pressure. Repeat the procedure in the other arm. The higher of the 2 readings will be used as the brachial pressure in the ABI calculation.

5. Place the blood pressure cuff on the patient's leg just above the malleoli. Locate the posterior tibial pulse, then apply conductivity gel over the artery. Repeat the same procedure used to assess the brachial pulse, evaluating the tibial pulses bilaterally. The higher of the 2 readings will be used as the ankle systolic pressure in the ABI calculation.

6. To obtain the ABI, divide the ankle systolic pressure by the brachial systolic pressure. For example, suppose the higher of the 2 brachial pressures is 129 and the ankle systolic pressure is 65. Divide 65 by 129 for an ABI of 0.5.

Interpreting the results

Use the following guideline to interpret the ABI:

0.9 to 1-normal

0.75 to 0.9-moderate disease

0.5 to 0.75-severe disease

below 0.5-limb-threatening disease

In general, compression therapy is contraindicated when the ABI is less than 0.6-arterial insufficiency that severe would put the patient at risk for further ulceration from the pressure of the compression bandage against the skin. Also, keep in mind that calcification of the arterial walls, such as in patients with diabetes, decreases compressibility of the arteries and may falsely elevate the ABI. These patients should be referred to a vascular specialist for additional evaluation to determine if compression therapy would be safe.

Adapted from: Sloan H, Wills EM. Ankle-Brachial Index: Calculating Your Patient's Vascular Risk. Nursing99; 1999:29(10):58-- 9; and Sieggreen MY, Maklebust J. Managing Leg Ulcers. Nursing96;1996:26(12):41-6.