USMLE® Step 2 CK Question of the Day: Ankle-Brachial Index

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We’re back with a USMLE® Step 2 CK Question of the Day! Today’s case involves a 65-year-old man with pain in his left leg. The pain appears during walking and goes away with rest. Which of the following studies is the most appropriate next step in management?

A 65-year-old man comes to the clinic complaining of left leg pain for three months. The pain is characterized by continuous cramping in the left calf that starts after walking two blocks and goes away with rest. The patient has a history of hypertension and type II diabetes mellitus. His family history includes a pulmonary embolism experienced by his mother at age 55. The patient quit smoking 5 years ago. The patient’s temperature is 37.0°C (98.6°F), pulse is 80/min and regular, respirations are 20/min, and left arm blood pressure is 140/85 mmHg. Left ankle blood pressure is 210/150. Physical exam shows symmetric legs without swelling, redness, or skin breakdown. Which of the following studies is the most appropriate next step in management?

A. Toe-brachial index

B. Exercise ankle-brachial index

C. Doppler ultrasound

D. CT angiogram

E. MR arteriography

Scroll down for the correct answer!

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The correct answer to today’s USMLE® Step 2 CK Question is…

A. Toe-brachial index

Before we get to the Main Explanation, let’s see why the answer wasn’t B, C, D or E. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

Today’s incorrect answers are…

B. Exercise ankle-brachial index

Incorrect: Exercise ankle-brachial index (ABI) is reserved for patients with equivocal resting ABI values between 0.9 and 1.0. A decrease in exercising ABI of 15 to 20% is considered diagnostic for peripheral vascular disease.

C. Doppler ultrasound

Incorrect: Doppler ultrasound employs the Doppler effect to measure the velocity of body fluids in relation to the ultrasound probe. Patients with a pathologic positive ankle-brachial index (less than 0.9) should undergo duplex ultrasonography of their leg to identify the location and extent of the stenosis.

D. CT angiogram

Incorrect: CT angiogram is reserved for patients in whom ABI and Doppler ultrasound are equivocal for peripheral artery disease, and may play a role in preoperative revascularization surgery planning.

E. MR arteriography

Incorrect: MR arteriography is reserved for patients in whom ABI and Doppler ultrasound are equivocal for peripheral artery disease, and may play a role in preoperative revascularization surgery planning.

Main Explanation

This patient has symptoms and risk factors concerning for peripheral vascular disease (PVD). The leg pain that worsens with walking and resolves with rest is consistent with claudication, which is pain caused by ischemia when the working extremity fails to get adequate blood supply. Atherosclerotic blockages in peripheral arteries along the legs begin to cause symptoms when the vessel’s internal diameter is decreased by 70%.

When there is suspicion of peripheral artery disease (PAD), an ankle-brachial index test (ABI) is performed. ABI is the ratio of ankle systolic blood pressure to brachial systolic blood pressure. Normally, both pressures are about equal, so the ratio is between 1.0 and 1.4. In individuals with intermittent claudication (mild to moderate disease), the ABI usually lies between 0.4 and 0.9, since blood pressure in the ankle is decreased. In individuals with resting pain (severe PAD), the ABI is less than 0.4.

An ABI greater than 1.4 can indicate vessel wall stiffness caused by calcification, as in this patient with an ABI of 1.5. To prevent false negatives, patients with an ABI greater than 1.4 should have a toe-brachial index (TBI) performed. In general, a TBI below 0.5 is considered diagnostic of peripheral vascular disease.Individuals with suspected PVD with normal or borderline ABIs (between 0.9 and 1.0) can undergo exercise testing. In this test, a baseline ABI is obtained prior to exercise, and the individual is then asked to exercise until claudication pain occurs; the ABI is then re-measured. A decrease in ABI of 15 to 20% is diagnostic for PVD. If PVD is suspected based on the ABI or TBI, the next step is to perform a lower limb Doppler ultrasound to determine the location and severity of arterial obstruction.

Major Takeaway

The diagnosis of peripheral vascular disease is made by measuring the ankle-brachial index (ABI), which is the ratio of the arm to ankle systolic blood pressures. ABI values greater than 1.4 suggest calcified arteries and should be referenced against the toe-brachial index (TBI).

References

Creager, M. A., & Loscalzo, J. (2018). Arterial Diseases of the Extremities. In J. L. Jameson, A. S. Fauci, D. L. Kasper, S. L. Hauser, D. L. Longo, & J. Loscalzo (Eds.), Harrison’s Principles of Internal Medicine (20th ed.). McGraw-Hill Education

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