Norm of Ankle-Brachial Index (ABI)
|0.75–0.85||Mild occlusive disease|
|0.30–0.50||Severe disease: rest pain may occur; pregangrenous state|
|0.20–0.30||Poor probability for tissue healing or limb viability unless compensation by collateral blood flow occurs|
|<0.20||Ischemic or gangrenous extremities|
Usage of Ankle-Brachial Index (ABI)
Assessment of arterial blood flow in clients with peripheral vascular disease; monitoring postoperative flow in the lower extremities after vascular surgery such as femoral bypass or after aortofemoral bypass from iliac occlusion; assessment of severity of peripheral vascular disease; predicting carotid artery stenosis. Cilostazol (Pletal) increases ABI at rest.
Description of Ankle-Brachial Index (ABI)
The ABI is a mathematically calculated ratio of the systolic pressure at a pulse point in a lower extremity with peripheral vascular disease as compared to the systolic pressure of the brachial artery. The index provides a quick, noninvasive assessment of how much arterial blood is perfusing the extremity. Typically an ABI that increases by at least 0.15 (15%) after vascular surgery indicates that the surgery was successful. A baseline in women with an ABI of <0.60 indicates significantly higher probability of developing severe disability for walking specific outcomes (such as walking a quarter of a mile).
Professional Considerations of Ankle-Brachial Index (ABI)
Consent form NOT required.
- Obtain a dual-frequency Doppler ultrasonograph, a marker, two sphygmomanometers, and ultrasonic gel.
- Client is positioned supine.
- The femoral, popliteal, dorsalis pedis, and posterior tibial pulse points in both lower extremities are palpated and identified with a marker.
- The sphygmomanometer cuff is placed proximally to the marked site. If the flow is being assessed at the knee, the cuff is placed proximally to the popliteal pulse. If the flow is being assessed at the ankle, the cuff is placed proximally to the ankle.
- Ultrasonic gel is placed over the marked site (popliteal, posterior tibial, or dorsalis pedis), and the Doppler flow signal is identified.
- With the Doppler in place, the sphygmomanometer cuff is inflated until the Doppler flow signal disappears.
- The cuff is slowly deflated, and the pressure at which the Doppler tone is again audible is noted and recorded.
- The brachial systolic blood pressure in both arms is measured with a Doppler scanner, and the highest pressure is selected for use in the ABI calculation.
- The ABI ratio is calculated with the following equation:
- Wipe the ultrasonic gel from the skin and remove the sphygmomanometer cuff.
- If performing serial ABI measurements postoperatively, notify the physician for a decrease in ABI of at least 0.15 (15%) or for the loss of a previously palpable pulse or audible Doppler tone.
Client and Family Teaching
- This test is painless.
- This measurement helps estimate how much blood is flowing to the leg and foot.
Factors That Affect Results
- Values may be inconsistent if the same arm is not used for every brachial pressure measurement.
- Immediate postoperative hypotension and low body temperature may necessitate use of a Doppler scanner to locate pulse tones because pulses may not be palpable.
- The ABI is a good predictor of survival in clients with peripheral vascular disease. Those with ABIs less than 0.30 have significantly poorer survival than clients with ABIs of 0.31–0.91.
- The transfer function index (TFI) has been shown to be superior to ABI in detecting vascular grafts at risk for failing. See Pulse volume recording of peripheral vasculature.
- See also Doppler ultrasonographic flow studies.