Using the ankle-brachial index to diagnose peripheral artery disease and assess cardiovascular risk
ABSTRACTThe ankle-brachial index is valuable for screening for peripheral artery disease in patients at risk and for diagnosing the disease in patients who present with lower-extremity symptoms that suggest it. The ankle-brachial index also predicts the risk of cardiovascular events, cerebrovascular events, and even death from any cause. Few other tests provide as much diagnostic accuracy and prognostic information at such low cost and risk.
KEY POINTS
- The ankle-brachial index is the systolic pressure in the ankle (either the dorsalis pedis or the posterior tibial artery, whichever has the higher pressure) divided by the systolic pressure in the arm (either the left or right, whichever is higher). The lower of the two values obtained (left and right) is the patient’s overall ankle-brachial index.
- Most healthy adults have a value greater than 1.0. A value of less than 0.91 indicates significant peripheral artery disease, and a value lower than 0.40 at rest generally indicates severe disease. Values higher than 1.4 indicate stiffened, noncompressible arteries.
- Measuring the ankle-brachial index after exercise can uncover peripheral artery disease in patients with a normal resting ankle-brachial index.
DIFFERING RECOMMENDATIONS
The ankle-brachial index can be used to screen for asymptomatic peripheral artery disease. It can also be used to confirm the diagnosis in patients with symptoms such as intermittent claudication, ischemic pain at rest, or lower extremity ulcers or in patients with signs such as abnormal pulses, bruits, or lower-extremity skin changes. It is also used to reassess the severity of known peripheral artery disease and as a part of a routine surveillance program to assess the patency of bypass grafts and endovascular stents after revascularization procedures.
The complication of peripheral artery disease that patients dread the most is limb loss, but of greater clinical consequence are the alarming rates of cardiovascular events and death in these patients. Epidemiologic studies have shown that fewer than 5% of patients age 55 or older with claudication or asymptomatic peripheral artery disease experience major amputation over a 5-year follow-up period, but 20% of these patients will have a stroke or myocardial infarction and 15% to 30% will die. Of those who die, 75% die of a coronary or cerebrovascular cause.36 Because of the markedly increased risk of death or cardiovascular morbidity in patients with peripheral artery disease, many have advocated screening patients at high risk using the ankle-brachial index. However, there have been conflicting recommendations from national societies and agencies.29,46–48
The United States Preventive Services Task Force (USPSTF) updated its 1996 recommendations on screening for peripheral artery disease in 2005 and recommended against routinely screening for it, giving the practice a “D” recommendation (not recommended). Specifically, it stated that it found “fair evidence that screening asymptomatic adults with the ankle brachial index could lead to some small degree of harm, including false-positive results and unnecessary work-ups,”46 and concluded that “for asymptomatic adults, harms of routine screening for [peripheral artery disease] exceed benefits.”46
This negative recommendation was intensely debated among vascular specialty groups, and a rebuttal was published in 2006.49 The major area of contention was the task force’s assumption that decreased disease-specific morbidity (especially limb loss) is the most important outcome to be prevented by screening for asymptomatic peripheral artery disease, rather than adverse cardiovascular events. The USPSTF has announced plans for an update on screening for peripheral artery disease, anticipated for 2013.50
The American College of Cardiology/American Heart Association task force in 2005 recommended that a history of walking impairment, intermittent claudication, ischemic rest pain, or nonhealing wounds be solicited as part of a standard review of systems for adults age 70 and older or adults age 50 and older who have risk factors for atherosclerosis (class IC recommendation—based only on a consensus opinion of experts, case studies, or standard of care).29 In contrast to the USPSTF recommendations, the joint guidelines further recommended that patients with asymptomatic lower-extremity peripheral artery disease be identified by physical examination, ankle-brachial index, or both, to prevent myocardial infarction, stroke, or death (class IC).29 Patients at risk for lower-extremity peripheral artery disease for whom ankle-brachial index measurement is recommended include those with exertional leg symptoms, those with nonhealing ulcers, those age 70 and older, and those age 50 and older who have a history of moking or diabetes.
The American Diabetes Association and the second Inter-Society Consensus for the Management of Peripheral Arterial Disease (TASC II) issued similar recommendations.48
In 2011, the American College of Cardiology/American Heart Association task force issued a focused update to its 2005 guidelines, broadening the recommendation for testing to include patients age 65 and older on the basis of the getABI study, as well as maintaining the recommendation for testing for those age 50 and older with a history of smoking or diabetes (class IB recommendation).26,41
The task force’s Guideline for the Assessment of Cardiovascular Risk in Asymptomatic Adults says that measuring the ankle-brachial index is reasonable for cardiovascular risk assessment in asymptomatic adults at intermediate risk (class IIA—conflicting evidence or divergence of opinion, from multiple randomized clinical trials).51 Also recommended as risk stratification tools for this patient population are measurement of carotid intima-media thickness and measurement of coronary artery calcium (both class IIA recommendations).
Unlike these tests, however, the ankle-brachial index does not require highly trained technical and medical personnel to perform and interpret. In addition, there is no risk of radiation exposure as is the case in coronary calcium measurement. It is a simpler, lower-cost, and more widely available tool for cardiovascular risk assessment.
LIMITATIONS TO ANKLE-BRACHIAL SCREENING IN PRACTICE
Although this test is relatively simple and noninvasive, it is not widely performed in primary care and cardiovascular medicine. In a study by Mohler and colleagues,52 the most common barriers to its use among primary care providers were the time required to perform it, lack of reimbursement for it, and limited staff availability. Currently, third-party payers do not generally reimburse for an ankle-brachial index examination performed to screen a patient who is asymptomatic but at risk for peripheral artery disease. Unfortunately, this has limited the widespread adoption of a program to detect peripheral artery disease in patients at risk.
Despite these limitations, the ankle-brachial index is an invaluable tool to both screen for peripheral artery disease in the appropriate at-risk patient populations and to diagnose it in patients who present with lower extremity symptoms. There are few diagnostic tests available today that provide such a high degree of diagnostic accuracy with as much prognostic information as the ankle-brachial index and with such little expense and risk to the patient.
