Ankle-Brachial Index (ABI): The ankle brachial index is a simple non-invasive screening test utilizing doppler ultrasound to help detect reduced arterial blood flow to an extremity.  The test, abbreviated ABI, is used to compare the systolic blood pressure at the distal leg (ankle) to that of the arm (brachial).  The ABI provides an effective method of screening for peripheral arterial disease (PAD).  It is used to follow the progression of the disease process and evaluate the effectiveness of a treatment plan.

Progressive arterial narrowing results in a corresponding fall of systolic blood pressure below the site or sites (if multiple sites of occlusion exists within the limb(s) or pelvis) of intra-arterial obstruction or narrowing.  The extent to which the systolic pressure falls is dependent upon the extent of involvement.

To perform the study a pneumatic blood pressure cuff is applied to the limb to be tested and the sensing ultrasound probe is placed over a designated artery below the level of the cuff.  The cuff is rapidly inflated above systolic pressures, subsequently obliterating blood flow to the part under study.  As the pressure in the cuff is gradually deflated, the point at which flow is resumed is recorded as the opening or the systolic pressure.

ABI findings are usually correlated with corresponding Doppler or pulse volume waveform analysis.  A significant drop in arterial systolic pressure in one limb indicates reduced blood flow secondary to obstruction of one or more blood vessels.  The presence of asymptomatic arterial obstructive disease may be detected by performing exercise testing.

Patients, such as diabetics, with calcified vessels may show falsely elevated ankle pressures. In this case, waveform assessment is critical.  Pressure measurements can be taken from toes using Photo Plethysmography (PPG) for more accurate results.  Diagnostic information is obtained both from the waveform of the arterial flow when displayed on a chart recorder and from using the PPG sensor to determine the blood pressure in the digit.