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Ankle-Brachial Index Test
Background
The ankle-brachial index (ABI) is a common noninvasive test used to determine if there is occlusive disease of the peripheral vasculature. It uses the brachial arterial systolic blood pressure as the control, assuming it to be nonocclusive, and compares it to either that of the posterior tibial (PT) or dorsalis pedis (DP) arteries. The ABI is calculated in each foot by dividing the higher systolic blood pressure reading of either of the PT or DP arteries by the higher systolic blood pressure reading of either the left or right brachial artery. This index suggests occlusive peripheral artery disease (PAD) when it is ≤0.9. Furthermore, the location of the occlusion may be inferred by using segmental blood pressure readings and supportive data such as pulse volume recordings (PVRs).
How to Use It
This test is ordered when there is a suspicion of obstructive vascular pathology, most commonly due to obstructive atherosclerosis of the lower extremities, but may also include obstruction due to vasculitis or thromboembolic disease. It is indicated in patients who are asymptomatic but have risk factors for atherosclerotic cardiovascular disease (ASCVD), who have symptoms of occlusive PAD (such as claudication or erectile dysfunction), or who have physical signs of limb ischemia such as poor wound healing.
How It Is Done
This test can be performed as either an inpatient or outpatient test, but is most commonly performed in the outpatient setting. A plethysmographic cuff is placed around the left and right arms, and the left and right lower calves just above the ankle. Segmental pressure readings require additional plethysmographic cuffs to be placed around the upper thighs, lower thighs, and upper calves. PVRs are used to help interpret the results of ABIs and may be obtained by the use of pneumoplethysmographic cuffs or alternatively obtained using continuous-wave Doppler. Testing is usually performed at rest but can also be performed post exercise using a standardized exercise protocol if there is clinical suspicion of obstructive PAD despite a normal resting ABI. In this case, the patient is exercised according to an institution’s protocol and, shortly after, ABIs are obtained at regular intervals. When there is suspicion of obstructive PAD in patients with calcified vessels (which often produces an ABI >1.4), a toe-brachial index may be performed by placing a small plethysmographic cuff usually around the great toe.
HOW IT IS INTERPRETED
Compared with the brachial artery, systolic arterial pressure should be similar or slightly higher in the more distal arteries. As such, the normal ABI is 1.0–1.4. An abnormal ABI is ≤0.9 and is suggestive of occlusive PAD. Borderline normal ABI is 0.91–0.99. An ABI >1.4 suggests noncompressible calcified vessels that may or may not be obstructed. An ABI >1.4 is an abnormal finding and is associated with an increased risk of stroke, heart failure, and a higher incidence of occlusive PAD compared with normal ABI.
Further testing in patients with ABI >1.4 using a toe-brachial index is indicated, as the small arterial vessels in the toes are rarely calcified. A normal toe-brachial index is >0.7. If the toe-brachial index is ≤0.7, it is abnormal and diagnostic of occlusive PAD. In addition, when there is further clinical suspicion of occlusive PAD in the setting of a normal or borderline normal ABI, an exercise ABI is indicated.
Segmental plethysmographic readings may help localize disease to various areas of the lower extremity such as the aortoiliac, femoropopliteal, or infrapopliteal regions. If blood pressure decreases from one contiguous segment to the next by more than 20 mmHg, it can be inferred that there is a focal arterial occlusion between those two segments.
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