The Ankle-Brachial Index

The currently acceptable method of determining the presence of PAD consists of a historical review of patient symptoms and atherosclerotic risk factors, a physical examination, and noninvasive vascular testing. The first and most important noninvasive test for PAD is the ankle-brachial index (ABI). This test may be performed in the physician's office and has only 4 requirements:

1. basic understanding of how to perform an ABI.
2. basic knowledge of arterial anatomy.
3. continuous-wave Doppler ultrasonic probe.
4. sphygmomanometer.

The ABI compares the blood pressure obtained with the Doppler in the dorsalis pedis and posterior tibial artery. An ABI range that is generally considered normal is .95 to 1.2. <0.95 to > 0.40 reflects mild to moderate PAD, and </=0.40 suggests severe arterial occlusive disease. More than 1.3 reflects calcified tibial artery.

The ABI has emerged as one of the most potent markers of diffuse atherosclerosis, CV risk, and overall survival in various patient populations; an abnormal ABI indicates a 3-fold CV risk.

Limitations of the ABI include

. A normal ABI in the face of abnormal peripheral arterial circulation (ie, a false-negative result). In elderly patients or patients with end-stage renal disease or, more commonly, diabetes mellitus, the ankle arteries may have calcification in the medial layer. Therefore, when the physician compresses the sphygmomanometer and listens with the Doppler probe, the Doppler signal does not disappear even at a pressure of >/=250 mm Hg. This reading does not translate into a normal ABI but instead indicates vessel calcification.

. For patients with such rigid ankle blood vessels, toe pressure measurements may be taken since toe arteries are rarely rigid. This examination is called a toe brachial index (TBI) and is a calculation based on the systolic blood pressures of the arm and the systolic blood pressures of the toes. The examination is similar to the ABI except that it is performed with a photoplethysmograph (PPG) infrared light sensor and a very small blood pressure cuff placed around the toe. A TBI of .8 or greater is considered normal. Also when the absolute toe pressure is more than 70 mmHg, then it is considered normal. A toe pressure of less than 30 mmHg considered very severe.

. A normal ABI in patients with classic symptoms suggesting intermittent claudication and PAD. Patients with moderate disease of the infrarenal aorta or iliac arteries may have normal arterial circulation at rest but when exercised demonstrate a decrease in ankle pressure. Therefore, a resting study is inadequate for patients with exertional symptoms of intermittent claudication. In this situation, an exercise arterial study should be performed to determine the true etiology of exertional limb pain.


Segmental Limb Pressures and Pulse-Volume Recordings

Once the ABI has been performed, which provides objective evidence of the presence and overall severity of PAD in a limb, more specific information can be obtained in the vascular laboratory. In the laboratory, segmental limb pressure measurement can aid in localizing stenosis or occlusions. Limb pressure cuffs are placed on the thigh (some laboratories prefer high- and low-thigh cuffs), calf, ankle, trans meta tarsal region of the foot, and digit. The ABI is calculated and then the pressure is inflated sequentially in each cuff to ~20 to 30 mm Hg above systolic pressure. With a continuous-wave Doppler probe placed at a pedal vessel, the pressure in the cuff is released gradually, and the pressure at each segment is measured. A decrease in pressure between 2 consecutive levels of >30 mm Hg suggests arterial occlusive disease of the artery proximal to the cuff. In comparing the 2 limbs, a 20- to 30-mm Hg discrepancy from one limb to the other at the same cuff level also suggests a significant arterial stenosis or occlusion proximal to the cuff.

Pulse-volume recordings (PVRs) or Pressure Cuff Recording (PCR) are plethysmographic tracings that detect changes in the volume of blood flowing through a limb. Using equipment similar to the segmental limb pressure technique, pressure cuffs are inflated to 65 mm Hg, and a plethysmographic tracing is recorded at various levels. A normal PVR is similar to a normal arterial pulse wave tracing and consists of a rapid systolic upstroke and a rapid downstroke with a prominent dicrotic notch. With increasing severity of PAD, the waveforms become more attenuated with a wide downslope and, ultimately, virtually absent waveforms.

The ABI, segmental limb pressure measurement, and PVRs are useful noninvasive tests for evaluating patients with suspected PAD or limb discomfort without an obvious cause. These tests are inexpensive, painless, and reproducible, and the equipment required to perform them is significantly less expensive than modern color-flow duplex ultrasonography.