The Ankle Brachial Index Is Associated with Leg Function and Physical Activity: The Walking and Leg Circulation Study

Context The ankle brachial index (ABI) is a simple, noninvasive measure of peripheral arterial disease. The ABI is readily available and can be used to broaden understanding of typical clinical findings in peripheral arterial disease. Contribution This large study from three academic medical centers found that only 33% of adults with ABIs indicative of peripheral arterial disease had intermittent claudication. Adults with abnormal ABIs had worse lower-extremity function, including an inability to walk continuously, shorter distance walked for 6 minutes, slower velocity, and inability to maintain a tandem stand. Implications Peripheral arterial disease is associated with impaired lower-extremity function, and many patients with peripheral arterial disease do not have classic claudication symptoms. The Editors Lower-extremity peripheral arterial disease is common, both in community studies of unselected men and women and in medical care settings. In a study of community-dwelling men and women 65 years of age and older, 12% had peripheral arterial disease (1). In primary care medical practices, the prevalence of peripheral arterial disease is 18% to 23% among patients 55 years of age and older (2, 3). Intermittent claudication is considered the most typical symptom of peripheral arterial disease (4-6). However, peripheral arterial disease is frequently asymptomatic or accompanied by exertional leg symptoms other than intermittent claudication (7-10). Because physical examination is insensitive for peripheral arterial disease, detection often requires testing with the ankle brachial index (ABI) (11-14). The ABI is noninvasive and simple to measure. An ABI less than 0.90 is 95% sensitive and 99% specific for peripheral arterial disease (15). Ankle brachial index values are strong, independent predictors of cardiovascular morbidity and mortality (16-19). Nonetheless, the ABI seems to be used rarely, and available data suggest that peripheral arterial disease is largely unrecognized in medical practice (20, 21). In a recent survey, 79% of practicing internists in southern Illinois reported performing the ABI 0% to 25% of the time when pedal pulses were absent (22). Lower-extremity function is an important predictor of future disability, mobility loss, and nursing home placement (23-25). However, the association between ABI and objectively measured lower-extremity function has not been well studied. We examined the association between the ABI and comprehensive, objective measures of lower-extremity function. We tested the hypothesis that in a heterogeneous group of persons with and without peripheral arterial disease, the ABI would be an independent marker of objectively measured impaired lower-extremity function. If ABI is associated with lower-extremity function (23-25), ABI may also identify persons whose high risk for mobility loss and nursing home placement might otherwise go unrecognized. Interventions to prevent mobility loss and nursing home placement might then be appropriate for persons with low ABI. Methods Participants The institutional review boards of the Feinberg School of Medicine at Northwestern University Medical School and Catholic Health Partners Hospitals approved the study protocol. All participants gave informed consent and were unaware of study hypotheses. Participants were identified from three academic medical centers in the Chicago, Illinois, area. We identified most participants with peripheral arterial disease through lists of consecutive men and women who received a diagnosis of peripheral arterial disease at each institution's noninvasive vascular laboratory. A few participants with peripheral arterial disease were patients seen in a general internal medicine practice who had a low ABI at their study visit. Half of the participants without peripheral arterial disease were identified by using lists of men and women with normal lower-extremity arterial values at these three noninvasive vascular laboratories, and half were identified from among patients with appointments in a large general internal medicine practice at Northwestern Medical Faculty Foundation. Exclusion Criteria Figure 1 shows the number of excluded persons and the reasons for exclusion. We excluded persons who had received a diagnosis of peripheral arterial disease at one of the noninvasive vascular laboratories but were found at the study visit to have a normal ABI, indicating an absence of peripheral arterial disease. We also excluded persons who were determined not to have peripheral arterial disease at the noninvasive vascular laboratory but had an ABI value consistent with peripheral arterial disease at their study visit. Nursing home residents, wheelchair-bound patients, and patients with foot or leg amputations were excluded because their function is uniquely impaired. Non-Englishspeaking patients were excluded because the study investigators were not fluent in non-English languages. Patients with ABI values of 1.50 or higher were excluded. An ABI of 1.50 or higher is consistent with poorly compressible leg arteries and an inability to gauge arterial perfusion accurately (15). Figure 1. Description of identified potential study participants. * Ankle Brachial Index Measurement Tests to measure ABI were usually performed before functional measures. When we were unable to schedule ABI measurements immediately before the functional measures, more than 30 minutes lapsed between completion of functional measures and ABI measurement. In all cases, participants rested in the supine position for 5 minutes before ABI measurement. Appropriately sized blood pressure cuffs were placed over each brachial artery and above each malleolus. The cuffs were rapidly inflated to 20 mm Hg above the audible systolic pressure in each arm and were deflated at a rate of 2 mm/s. Using a hand-held 5-MHz Doppler probe (Nicolet Vascular Pocket Dop II, Golden, Colorado), we measured systolic pressures twice, first in the following order and then in the reverse order: in the right brachial artery, in the right dorsalis pedis and posterior tibial arteries, in the left dorsalis pedis and posterior tibial arteries, and in the left brachial artery. The systolic pressure was recorded as the pressure at which the first sustained systolic pressure was audible. A previous study indicated that the association between ABI and lower-extremity function was strongest when ABI was calculated by averaging the values from the dorsalis pedis and posterior tibial arteries (26) Therefore, we calculated the ABI for each leg by averaging the two arterial pressures. In most instances, we used the average of all four brachial arterial pressures as the brachial arterial pressure for the ABI calculation. When one brachial arterial pressure was greater than the opposite brachial arterial pressure in both measurement sets and the right and left brachial arterial pressures differed by at least 10 mm Hg in one of the two measurement sets, we suspected subclavian stenosis and used the values from the arm with highest pressures to calculate the ABI. For each participant, we used the measurement from the leg with the lower ABI. We classified participants with an ABI less than 0.90 as having peripheral arterial disease and classified participants with an ABI of 0.90 or higher as not having peripheral arterial disease. Measures of Lower-Extremity Function The research assistant who assessed function was blinded to participants' ABI values. Participants received their ABI results after all functional measures were obtained, except for 7-day physical activity. 6-Minute Walk The 6-minute walk is a highly reliable test in patients with intermittent claudication (27). We followed a standardized protocol based on previous studies (27, 28). For this test, participants were instructed to walk up and down a 100-ft hallway for 6 minutes, covering as much distance as possible during that time (28). Accelerometer-Measured Physical Activity Physical activity levels over 7 days were measured by using the Caltrac accelerometer (Morey Corp., Woodridge, Illinois), which is the size of a pager and is worn at the waist below the axilla. The accelerometer calculates kilocalories of physical activity on the basis of vertical acceleration during movement, factoring in patient age, sex, height, and weight. On the basis of previous studies (29-33), we programmed the accelerometers using identical weight, height, age, and sex for each participant so that activity levels could be compared specifically among all study participants. When programmed in this manner, the accelerometer does not measure kilocalories expended by each person but instead measures activity units. Participants were asked to wear the accelerometer continuously for 7 days, removing it only for bathing or sleeping. Because we had few accelerometers, they were distributed to participants when available. The 47% of participants who wore an accelerometer did not differ significantly from those who did not in terms of age, sex, or ABI (data not shown). Repeated Chair Rises The test of repeated chair rises measures leg strength, balance, and endurance. For this test, we asked participants to sit in a straight-backed chair, with their arms folded across their chests, and to stand up five times consecutively as quickly as possible without using their arms (23, 24). We recorded the time to complete five chair rises. Standing Balance Participants were asked to hold three increasingly difficult standing positions for 10 seconds each: 1) a side-by-side stand [standing with both feet together, side by side], 2) a semitandem stand [standing with feet parallel, with the toes of one foot adjacent to the heel of the other foot], and 3) a tandem stand (standing with one foot directly in front of the other) (23, 24). 4-Meter Walking Velocity A 4-m distance was marked out in a hallway. Participants stood with both feet together at the starting line. Timing b