Ankle:brachial systolic blood pressure index (ABI)

Most commonly used noninvasive test in the diagnosis of peripheral arterial occlusive disease.

Defined as the ratio of the ankle systolic blood pressure to the brachial artery systolic blood pressure.

The systolic blood pressure increases distally in the arterial circulation and is higher in the lower than in the upper limb.

With hemodynamically significant stenosis, there is a decline in the systolic pressure distal to the obstruction that is proportional to its severity.

The ratio of systolic pressure measured at the brachial artery and at the ankle arteries has been adopted to diagnosed peripheral arterial disease.

ABI  is performed by measuring a Doppler recorded systolic pressure in each brachial artery and in the dorsalis pedis and posterior tibial arteries in each leg.

The ABI is typically calculated by dividing the highest or the mean dorsalis pedis and posterior tibial artery pressures in each leg by the highest or the mean of the two brachial artery pressures.

In the absence of arterial obstruction, systolic pressure increase with greater distance from the heart: without lower extremity atherosclerotic obstruction there is an ABI of 1.1 -1.30.

A normal supine resting ABI is 1.0, with values of 0.8 to 0.9, 0.5 to 0.8 and <0.5 correlating with mild, moderate and severe peripheral arterial occlusive disease.

Guidelines for PAD recognize in ABI range of 1-1.4 as normal and 0.91-0.99 as borderline.

Typically, and ABI of less than 0.9 define significant PAD with arterial stenosis of 50% or greater.

An ABI of 0.9-1.09 may indicate mild lower extremity atherosclerosis but is associated with the higher rate of cardiovascular mortality and mobility loss and a greater functional impairment compared with tat of people of 1.1- 1.30.

An independent predictor of cardiovascular mortality and cerebrovascular events.

A normal resting ABI does not rule out peripheral arterial disease and an exercise ABI should be obtained if claudication is suspected.

Some patients who symptomatic PAD have a normal or borderline resting ABI and and ABI obtained immediately after exercise increases sensitivity of ABI testing.

In patients without PAD, treadmill exercise increases blood flow to the legs, but if there is lower extremity stenosis the pressure beyond the lower extremity stenosis substantially decreases during exercise compared with the brachial artery pressure and this lowered ABI immediately after exercise compared with the resting ABI before exercise improves sensitivity of the test.

Post exercise ABI has a 97% sensitivity and a 96% specificity  for differentiating between diseased  and normal limbs.

A low ABI is associated with concomitant coronary and cerebrovascular disease, and in healthy persons an increased risk of vascular events, independent of cardiovascular risk factors(Fowkes FG).

A meta-analysis of 16 studies of healthy individuals, the 10 year risk of major coronary events in men with an ABI of 0.90 or less was 27% compared with 9% in those with an ABI in the normal range(Fowkes FG).

A meta-analysis of 16 studies of healthy individuals, the 10 year risk of major coronary events in women with an ABI of 0.90 or less was 19% compared with 6% in those with an ABI in the normal range(Fowkes FG).

The Aspirin for Asymptomatic Atherosclerosis trial involving 28980 men and women age 50-75 years free of clinical cardiovascular disease, with 3350 patients with low ABI treated with 100 mg of daily aspirin compared to placebo: no difference was noted in vascular events.

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