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Ankle-brachial index
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The ankle-brachial index is a simple measurement: blood pressure at the ankle is compared with blood pressure in the arm, and those two numbers are turned into a ratio. The pressure used is the systolic blood pressure, the pressure at the moment the heart pushes.
What that ratio reflects is the resistance blood meets as it moves through the vessels, and one of the things that matters most in that resistance is diameter, meaning how wide the vessel is on the inside. A vessel narrows from the inside because of plaque built up on its wall or because of a tear in the intima, its innermost layer. It is like comparing pressure at the end of a pipe with pressure at the start.
The pressure is measured in three arteries: the brachial artery, in the arm, and at the ankle either the dorsalis pedis or the posterior tibial. A low value points to narrowing; a high one usually points to a stiffened vessel.
In brief
The ankle-brachial index is a noninvasive test of vascular status, comparing systolic pressure at the ankle with that in the arm.
The ratio reflects vessel resistance, and one of its main factors is diameter, reduced by plaque or a tear in the intima.
It is like comparing pressure at the end of a pipe with pressure at the start.
Three arteries are used: the brachial in the arm, the dorsalis pedis or posterior tibial at the ankle; a low value points to narrowing, a high one usually to stiffening.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingAnkle Brachial Index, StatPearlsPublished
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