This test involves measuring varying blood pressures via a ultrasound
transducer at different places on the body's extremities. The goal
of this test is to detect and specifically locate peripheral
arteriosclerotic occlusive disease in the extremities. Blood pressure
cuffs are serially placed on the thigh, calf, and ankle. An ultrasound
conduction gel is typically placed on the area distal (below) the
pressure cuff and the ultrasonographical transducer is applied in the
region. The cuff is inflated to a pressure greater than the systolic
blood pressure and then the pressure is allowed to slowly decrease. By
slowly deflating the blood pressure cuff, the systolic blood pressure
can be accurately measured in that part of the extremity. This is
because the first evidence of blood moving past the pressure cuff can
be accurately detected by the Doppler transducer. The extremly
sensitive Doppler ultrasound detector can recognize the "swishing"
sound of even the most minimal blood flow. Normally, there is a
minimal drop in systolic blood pressure from the arteries of the
arms to those in the legs. If the drop in pressure exceeds 20 mm Hg,
occlusive disease is believed to exist immediately below the area
tested.
After measuring the systolic pressure at the various locations on the
legs, the technologist calculates the Ankle-Brachial Index (ABI). This
index helps to determine if there may be disease in the tested
extremity. To calculate the index, the ankle pressure is divided
by the brachial (arm) pressure. If the ABI is less than 0.85,
significant arterial occlusive disease exists within that extremity.
