Ankle-Brachial Pressure Index
When stenotic lesions are present in major arteries of the leg (except the deep femoral and internal iliac arteries), the systolic pressure measured at the ankle will be reduced relative to the systolic pressure measured in the brachial artery. The ratio of ankle systolic pressure to brachial artery systolic pressure is termed the ankle-brachial pressure index (ABI) and is normally about 1.1. (The normal ratio is greater than 1.0 because the shape of the arterial pressure pulse changes from the ascending aorta to the periphery with the systolic pressure increasing and the diastolic pressure falling at peripheral sites.) An ABI of <0.9 indicates the presence of arterial stenosis within the limb. These patients typically experience claudication (leg pain when walking). Severe stenosis is associated with resting leg pain and an ABI <0.5. An abnormally elevated ABI (>1.3) indicates "stiff" arterial vessels that may be found in diabetic and elderly patients.
During exercise, increased flow across the stenotic lesion increases the pressure drop across the lesion resulting in a fall in distal pressures. Therefore, the ankle-brachial pressure index decreases during exercise when there is a proximal stenotic lesion.
An excellent video resource from Stanford School of Medicine showing how ABI is measured in a patient is shown below.