Using the ankle-brachial index to diagnose peripheral artery disease and assess cardiovascular risk

INTERPRETING THE ANKLE-BRACHIAL INDEX

Diagnostic criteria for the ankle-brachial index were standardized in 2011 (Table 1).²⁶ Most healthy adults have a value greater than 1.0. A value of less than 0.91 is consistent with significant peripheral artery disease, and a value lower than 0.40 at rest generally indicates severe disease. A value between 0.91 and 0.99 is borderline abnormal and does not rule out peripheral artery disease. A value greater than 1.40 reflects noncompressibility of the leg arteries and is not diagnostic (see below).

The ankle-brachial index after exercise. In patients strongly suspected of having peripheral artery disease but who have a normal ankle-brachial index at rest, and especially if the resting value is borderline (ie, 0.91–0.99), the measurement should be repeated after exercise, the better to detect “mild” peripheral artery disease.¹⁵ With exercise, increased flow across a fixed stenosis leads to a significant fall in ankle pressure and a lower ankle-brachial index. In one study,²⁷ the ankle-brachial index fell below 0.9 after exercise in 31% of outpatients with symptoms who had initially tested normal.

The exercise is optimally done on a motorized treadmill set at an incline. A number of exercise protocols are in use; at our institution, we use a fixed workload protocol. The ankle-brachial index and ankle pulse-volume recordings are recorded on both sides at rest, after which the patient generally walks for 5 minutes at a 12% grade at 2.0 mph or until symptoms force the patient to stop. The advantage of treadmill testing is the ability to assess functional capacity by measuring the time to the onset of pain and the total walking time.

Alternatively, active pedal plantar flexion maneuvers (heel raises) or corridor walking to the point at which limiting symptoms occur can be done if a treadmill is not available, though this is not the favored approach and does not qualify as formal exercise testing for reimbursement purposes. The patient is asked to do heel raises as high and as fast as possible for 30 seconds or until limiting pain symptoms occur. Results with this maneuver have been shown to correlate well with those of treadmill exercise testing.²⁸

Immediately after any exercise maneuver, arm and ankle pressures are remeasured and bilateral ankle-brachial indices are recalculated. A fall in ankle pressure or the ankle-brachial index after exercise (generally, a fall of more than 20%) supports the diagnosis of peripheral artery disease. If the patient develops leg symptoms during exercise while his or her ankle-brachial index falls significantly, this also supports the vasculogenic nature of the leg symptoms.

An ankle-brachial index greater than 1.40 means that the pedal arteries are stiff and cannot be compressed by the blood pressure cuff. This is considered abnormal, though not necessarily diagnostic of peripheral artery disease. Noncompressible leg arteries are common among patients with long-standing diabetes mellitus or end-stage renal disease, and also can be found in obese patients.

Because toe arteries are usually compressible even when the pedal arteries are not, a toe-brachial index can be obtained to confirm the diagnosis of peripheral artery disease in these cases. This is calculated by measuring the blood pressure in the great toe using a small digital blood pressure cuff and a Doppler probe or a plethysmographic flow sensor. The toe-brachial index is calculated by dividing the toe blood pressure by the higher of the two brachial artery pressures; a value of 0.7 or less generally indicates peripheral artery disease.

WHAT SHOULD BE DONE WITH AN ABNORMAL RESULT?

An abnormal ankle-brachial index establishes the diagnosis of peripheral artery disease, and in many cases no additional diagnostic testing is necessary.

Care of patients with peripheral artery disease has three elements:

• Cardiovascular risk factor assessment and reduction to prevent myocardial infarction, stroke, and death
• Assessment and treatment of leg symptoms to improve function and quality of life
• Foot care to prevent ulcers and amputation.

Risk factor reduction. Because they have a markedly greater risk of cardiovascular disease and death, all patients with peripheral artery disease should undergo aggressive cardiovascular risk factor modification,^26,29 including:

• Antiplatelet therapy in the form of aspirin 75–325 mg daily or clopidogrel 75 mg daily as an alternative to aspirin
• Counseling and therapy for immediate smoking cessation if the patient smokes
• Treatment of hypertension to Seventh Joint National Committee goals³⁰
• Treatment of lipids to Adult Treatment Panel III goals³¹ (generally to a goal low-density lipoprotein cholesterol of less than 100 mg/dL, and less than 70 mg/dL if possible)
• Treatment of diabetes to a goal hemoglobin A_1c of less than 7% (in the absence of contraindications).³²

Exercise and anticlaudication medication. Patients with an abnormal ankle-brachial index and intermittent claudication may benefit from a supervised exercise program, a trial of drug therapy for claudication, or both. All patients with peripheral artery disease, regardless of symptoms, should be advised to incorporate aerobic exercise (ideally, walking) into their daily routine.

Cilostazol (Pletal), a phosphodiesterase inhibitor, has been given a class IA recommendation in the American College of Cardiology/American Heart Association guidelines for the treatment of intermittent claudication. The dose is generally 100 mg by mouth twice daily.²⁹

Revascularization. Patients with an abnormal ankle-brachial index and lifestyle-limiting claudication that has failed to improve with medical therapy or a course of supervised exercise training should be referred to a vascular specialist for evaluation for revascularization (endovascular therapy or surgical bypass). ²⁹ Endovascular therapy is particularly attractive for patients with claudication and evidence of aortoiliac disease (suspected in patients with gluteal or thigh claudication, diminution of the femoral pulse, or a bruit over the femoral artery on examination and confirmed by noninvasive vascular laboratory testing).

Patients who have ischemic pain at rest, gangrene, or a nonhealing lower-extremity wound that has been present for at least 2 weeks should be referred for revascularization on an urgent basis, given the risk of impending limb loss associated with critical limb ischemia.

A detailed review of the medical, endovascular, and surgical management of peripheral artery disease can be found in a supplement to the Cleveland Clinic Journal of Medicine published in 2006³³ and in comprehensive multi-society guidelines.^26,29