In adults calculating the ankle-brachial index (ABI) is a relatively simple way to confirm the clinical suspicion of lower extremity arterial occlusive disease. In pediatric lower extremity limb injury patients where vascular compromise is a concern (think bad fractures, lawnmower calamities) it may also be a helpful test.

Technique

  • Measure the systolic blood pressure at the ankle

    Place a blood pressure cuff just above the ankle and listen to either the dorsalis pedis or posterior tibial artery signal with a continuous wave Doppler probe. Insufflate the cuff to a pressure above which the audible Doppler signal disappears. Then release the pressure in the cuff just until the pedal signal returns. Record that number as the systolic pressure.

    In adults the other pedal vessel (either the DP or PT, whichever you didn’t use first) on the ipsilateral extremity is then measured. You then repeat on the contralateral extremity – using the highest value.

  • Measure the systolic blood pressure at the brachial artery

    Place the blood pressure cuff around the upper arm and use the Doppler probe in a similar fashion to the lower extremity

    Again, as is the case in adults you should repeat on the contralateral extremity

  • Divide these two numbers

What the numbers mean for vascular disease in adults

  • ABI ≥0.9 to 1.3 is Normal
  • ABI >1.3 suggests the presence of calcified vessels in adults and the need for more testing
  • ABI ≤0.9 is 95% sensitive and 100% specific for occlusive arterial disease in adults with claudication or other signs of ischemia
    • 0.4 to 0.9 suggests a degree of arterial obstruction often associated with claudication
    • ≤0.4 is worrisome for multilevel arterial disease (iliac, femoral or tibial)

The ABI in Trauma

In the emergency or trauma setting, an ABI is useful for the evaluation of a patient who is at increased risk for lower-extremity arterial injury, as follows:

  • An ABI ≤0.90 suggests a need for further vascular imaging. Go with angiography in a stable patient, and operative exploration by a vascular surgeon in an unstable patient.
  • An ABI ≥ 0.90 is associated with a lower likelihood of an arterial injury. You can observe or repeat the ABI later if you are concerned. Immediate arteriography is not warranted.

The above procedure specifies that you should be using both extremities. On a healthy pediatric patient with no peripheral vascular disease you can just perform the ABI on the injured lower extremity.