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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Am Coll Cardiol. Author manuscript; available in PMC 2009 February 20.
Published in final edited form as:
PMCID: PMC2645658
NIHMSID: NIHMS45550

Baseline Functional Performance Predicts the Rate of Mobility Loss in Persons With Peripheral Arterial Disease

Mary M. McDermott, MD,* Jack M. Guralnik, MD, PhD, Lu Tian, ScD, Luigi Ferrucci, MD, PhD,§ Kiang Liu, PhD, Yihua Liao, MS, and Michael H. Criqui, MD, MPH

Abstract

Objectives

We compared rates of mobility loss among persons with versus without peripheral arterial disease (PAD). Associations between baseline functional performance and mobility loss in persons with and without PAD were studied.

Background

Persons with PAD have poorer functional performance than persons without PAD. The prognostic value of poorer performance in persons with PAD is unknown.

Methods

Participants were 398 persons with and 240 without PAD who were free of mobility impairment at baseline. Participants were followed for a median of 50 months. Baseline measures included the 6-min walk and the Short Physical Performance Battery score. Mobility status, assessed annually, was defined as the self-reported loss of the ability to walk one-quarter mile or walk up and down one flight of stairs without assistance.

Results

Adjusting for age and gender, we found that PAD participants had a greater rate of mobility loss than persons without PAD (hazard ratio [HR] 1.63; 95% confidence interval [CI] 1.03 to 2.56). This difference was not statistically significant after additional adjustment for baseline performance. Among PAD participants, risk of mobility loss in the lowest versus the 2 highest quartiles of baseline performance were as follows: HR 9.65 (95% CI 3.35 to 27.77, p < 0.001) for the 6-min walk and HR 12.84 (95% CI 4.64 to 35.55, p < 0.001) for the Short Physical Performance Battery when adjusting for confounders.

Conclusions

Persons with PAD experience greater mobility loss than persons without PAD. This association was explained by poorer baseline functional performance among participants with PAD. Poorer lower extremity performance predicts increased mobility loss in persons with and without PAD.

Lower-extremity peripheral arterial disease (PAD) is common in outpatient settings (1,2). The prevalence of PAD was 29% in a national study of outpatient medical practices, in which ankle-brachial index (ABI) screening was performed among men and women ages 70 and older or age 50 to 69 with a history of diabetes or cigarette smoking (1). In a separate study, the prevalence of PAD was 25% among men and women age 55 and older who were screened (2). In cross-sectional analyses, men and women with PAD have poorer performance on objective measures of lower-extremity functioning than persons without PAD (3,4). However, the clinical consequences and prognostic significance of functional impairment in persons with PAD are unknown.

A critical factor in an older person's ability to function independently in the community is mobility, defined as the ability to walk or climb stairs without assistance (5). Older people who lose mobility are less likely to remain in the community; have greater rates of morbidity, mortality, and hospitalizations; and experience a poorer quality of life (5,6). However, the ability of persons with PAD to maintain mobility in the community over time, compared with those without PAD, is unknown. Few studies of the natural history of PAD have been performed to measure disease progression (7). Studying associations of PAD with mobility loss is important to better inform clinicians about the clinical consequences of PAD.

In a prospective observational study, we studied during 50-month follow-up whether persons with PAD have a greater rate of mobility loss than persons without PAD. We hypothesized that persons with PAD would experience greater rates of mobility loss than persons without PAD. Second, we studied whether measures of lower-extremity performance predict risk of mobility loss in participants with and without PAD. We hypothesized that participants with and without PAD who have poorer lower-extremity performance at baseline would have a greater risk of mobility loss. Third, we studied whether associations of baseline lower-extremity performance with mobility loss are similar between participants with versus without PAD. Fourth, we tested the hypothesis that associations of PAD with greater mobility loss compared with persons without PAD are explained by differences in baseline lower-extremity performance between persons with versus without PAD. Finally, we determined whether baseline functional performance measures also predict objectively assessed functional decline by studying associations of baseline functional performance with becoming unable to walk continuously for 6 min without stopping among persons with and without PAD, respectively.

Methods

Study overview

The funding source for this study played no role in the design, conduct, reporting of the study, or decision to submit the manuscript. The institutional review boards of Northwestern University and Catholic Health Partners Hospital approved the protocol. Participants gave written informed consent. Participants were part of the WALCS (Walking and Leg Circulation Study), a prospective, observational study designed to identify predictors of functional decline in persons with and without PAD (3,4,8). Participants underwent baseline assessment and returned for annual follow-up visits. Participants unable to return for follow-up because they were ill or had moved away were interviewed by telephone. Mean follow-up was 50 months.

Participant identification

Participants with PAD were identified from among consecutive patients age 55 years and older diagnosed with PAD in 3 Chicago-area noninvasive vascular laboratories. Half of the participants without PAD were identified from persons with normal lower-extremity arterial studies at the 3 noninvasive vascular laboratories, and one-half were identified from among patients with appointments in a large general internal medicine practice at Northwestern. A few PAD participants were those recruited from general internal medicine with a low ABI at their study visit. Exclusion criteria for the WALCS have been reported and are briefly summarized here (8). Exclusion criteria included dementia, recent major surgery, above- or below-knee amputations, nursing home residence, confined to a wheelchair, and ABI >1.50. Non–English-speaking patients were excluded because investigators were not fluent in non-English languages. Individuals with PAD diagnosed in the noninvasive vascular laboratory were excluded if their baseline visit ABI indicated absence of PAD. Patients with a normal ABI with prior lower-extremity revascularization were excluded (n = 16) because they could not clearly be classified as PAD or non-PAD. Participants who were not free of mobility impairment at baseline were excluded.

ABI measurement

A handheld Doppler probe (Nicolet Vascular Pocket Dop II, Nicolet Biomedical Inc., Golden, Colorado) was used to obtain systolic pressures in the right and left brachial, dorsalis pedis, and posterior tibial arteries (9,10). Each pressure was measured twice: in the order listed and in reverse order. The ABI was calculated by dividing the mean of the dorsalis pedis and posterior tibial pressures in each leg by the mean of the 4 brachial pressures (3,4,8,9). Zero values for the dorsalis pedis and posterior tibial pulses were set to missing for the ABI calculation. Average brachial pressures in the arm with highest pressure were used when 1 brachial pressure was higher than the opposite brachial pressure in both measurement sets and the 2 brachial pressures differed by 10 mm Hg or more in at least one measurement set, since in such cases subclavian stenosis was possible (11). The lowest leg ABI was used in analyses.

Comorbidities

Comorbidities assessed were diabetes, angina, myocardial infarction, heart failure, cancer, chronic lung disease, lower-extremity arthritis, spinal stenosis, spinal disk disease, and stroke. Disease-specific algorithms that combine data from patient report, medical record review, medications, laboratory values, and a questionnaire completed by the participant's primary care physician were used to verify and document baseline comorbidities, based on previously developed criteria (12). The American College of Rheumatology criteria were used to diagnose knee and hip osteoarthritis (13,14).

Exertional leg symptoms

Leg symptoms were classified based on responses to the San Diego Claudication Questionnaire, according to prior studies (3,4,15).

Other measures

Body mass index (BMI) was calculated as weight (kilograms)/(height [meters])2. Cigarette smoking history was determined with patient report.

Functional measures

6-MIN WALK

Following a standardized protocol, participants walked up and down a 100-ft hallway for 6 min after instructions to cover as much distance as possible (16,17). Research staff recorded whether or not the participant stopped to rest during this test. The 6-min walk test was repeated at each annual follow-up visit.

Repeated Chair Rises

Participants sit in a straight-backed chair with arms folded across their chest and stand 5 times consecutively as quickly as possible. Time to complete 5 chair rises was measured.

Standing Balance

Participants were asked to hold 3 increasingly difficult standing positions for 10 s each: standing with feet together side-by-side and parallel (side-by-side stand), standing with feet parallel with the toes of one foot adjacent to and touching the heel of the opposite foot (semi-tandem stand), and standing with one foot directly in front of the other (tandem stand) (18,19).

4-M Walking Velocity

Walking velocity was measured with a 4-m walk performed at “usual” and “fastest” pace. For the “usual” paced walk, participants were instructed to walk at their usual pace, “as if going down the street to the store.” Each walk was performed twice. The faster walk in each pair was used in analyses (19,20).

Short Physical Performance Battery (Sppb)

The SPPB combines data from the usual paced 4-m walking velocity, time to rise from a seated position 5 times, and standing balance. Individuals receive a 0 score for each task they are unable to complete. Scores of 1 to 4 are assigned for remaining tasks, based upon quartiles of performance for >6,000 participants in the Established Populations for the Epidemiologic Study of the Elderly (18,19). Scores are summed to obtain the SPPB, ranging from 0 to 12.

Physical activity and walking exercise frequency

Participants were asked to report the number of times they went walking for exercise during the previous week. Participants were categorized according to whether they went walking for exercise 0 times per week, 1 to 2 times per week, or ≥3 times per week (20). For physical activity, participants were asked, “During the last week, how many city blocks or their equivalent did you walk? Let 12 city blocks equal 1 mile” (21). This measure of patient-reported physical activity is highly correlated with vertical accelerometer-measured physical activity (21) and is significantly associated with baseline functional performance (data not shown).

Mobility measures

At each follow-up visit, participants were asked whether they were able to walk a quarter mile and whether they could climb up and down one flight of stairs, selecting one of the following 3 response options for each measure: a) “yes, on my own”; b) “yes, with assistance”; or c) “no, not at all.” Mobility loss was defined as becoming unable to walk a quarter mile or walk up and down one flight of stairs without assistance (18,19).

Statistical analyses

Baseline characteristics of participants with versus without PAD were compared using general linear models for continuous variables and chi-square tests for categorical variables, adjusting for age and gender. For Cox regression analyses, person-time for each participant was calculated as the number of months from the baseline visit to the date of the most recent visit (last seen) or the date of the visit during which mobility loss was first reported, whichever came first. Participants who died before mobility loss or who underwent lower-extremity revascularization during follow-up were censored at the time of their last interview. Similar methods were used for calculating time to loss of the ability to walk for 6 min continuously. Cox proportional hazards survival model analyses were performed to determine the hazard ratio for mobility loss between PAD and non-PAD participants, adjusting for age and gender. These analyses were repeated with additional adjustment for baseline performance on each measure of functioning.

For PAD and non-PAD participants combined, performance on the 6-min walk test, 4-m walking speed (usual pace), and 4-m walking speed (fast pace) was categorized into quartiles. The fourth quartile represented the best performance level and the first quartile represented the poorest performance level. Because of low rates of mobility loss among participants in the third and fourth quartiles of performance at baseline, these 2 quartiles were combined. No participant had an SPPB score of 0 at baseline. A priori, the SPPB score was categorized as follows: Category 1: score = 1 to 8; Category 2: score = 9 to 10; Category 3: score = 11 to 12. Cox proportional hazards analyses were used to compare differences in mobility loss and becoming unable to walk for 6 min continuously across categories of baseline performance for participants with and without PAD. The first set of Cox proportional hazard analyses adjusted for age, gender, race, BMI, pack-years of cigarette smoking, walking exercise frequency, and comorbidities (Model 1). Among participants with PAD, Model 1 additionally adjusted for the ABI and leg symptoms. Model 2 adjusted for covariates in Model 1 in addition to physical activity. Analyses were repeated within the entire cohort, including a test for an interaction between PAD status and performance measures in their association with mobility loss. We tested the proportional hazards assumption for mobility loss using martingale residuals based methods, and we did not find any evidence of significant deviation from the proportional hazards assumption (22). Analyses were performed using SAS statistical software (version 9.1, SAS Institute Inc., Cary, North Carolina).

Results

Among 723 WALCS participants eligible for these analyses, including 460 with ABI <0.90, 4 PAD participants without baseline functional performance data were excluded. An additional 42 participants (33 with PAD) were excluded because follow-up data on mobility were not available. Of the remaining 423 PAD and 254 non-PAD participants, 398 (94.1%) with PAD and 240 (94.5%) without PAD reported no mobility loss at baseline and were included in analyses.

Average ages of PAD and non-PAD participants were 71.7 ± 8.4 years and 69.3 ± 8.0 years, respectively. The proportions of women among PAD and non-PAD participants were 39.7% and 47.1%. Among PAD participants, 31.9% had classic symptoms of claudication, and 19.1% reported no exertional leg symptoms. The remainder had exertional leg symptoms other than claudication.

Table 1 shows characteristics of PAD and non-PAD participants, adjusting for age and gender. Participants with PAD had a greater pack-year history of cigarette smoking, greater prevalences of diabetes and cardiac or cerebrovascular diseases, and a lower prevalence of spinal stenosis compared with participants without PAD. Participants with PAD walked fewer blocks during the prior week and had significantly poorer performance on measures of functioning, compared to those without PAD (Table 1).

Table 1
Age- and Gender-Adjusted Characteristics of Study Participants With and Without PAD

Adjusting for age and gender, the hazard ratio for mobility loss among PAD vs. non-PAD participants was 1.63 (95% confidence interval 1.03 to 2.56, p = 0.036). After additional adjustment for each performance measure, the association between PAD and mobility loss was attenuated and no longer statistically significant (Table 2).

Table 2
Adjusted Associations of Presence of Peripheral Arterial Disease and Loss of Mobility or the Ability to Walk Continuously for 6 Min at Follow-Up (n = 638)

Figure 1 shows the associations between baseline lower-extremity performance and mobility loss at 50-month follow-up among participants with PAD. Adjusting for age, gender, race, ABI, BMI, cigarette smoking, leg symptoms, comorbidities, and walking exercise frequency (Model 1), poorer performance on each measure of lower-extremity performance at baseline was associated with significantly increased rates of mobility loss at follow-up (Fig. 1). These findings were not substantially changed after additional adjustment for physical activity (Model 2, Fig. 1). Participants in the poorest quartile of the 6-min walk performance at baseline had a hazard ratio of 9.65 (95% confidence interval 3.35 to 27.77, p < 0.001) for mobility loss during follow-up, compared with participants in the highest 2 quartiles for baseline performance, adjusting for confounders (Model 2, Fig. 1). Similarly, participants in the lowest baseline quartiles of performance for the usual paced and fast-paced 4-m walking speed and the SPPB, respectively, had significantly increased rates of mobility loss during follow-up, compared with participants in the highest 2 quartiles for these measures (Model 2, Fig. 1). Peripheral artery disease participants in the second quartile of performance for each functional measure had significantly greater rates of mobility loss compared to PAD participants in the highest quartiles of performance at baseline (Model 2, Fig. 1).

Figure 1
Hazard Ratios for Mobility Loss at 50-Month Follow-Up Among Men and Women With PAD, According to Baseline Functional Performance

Figure 2 shows associations between baseline performance and mobility loss among participants without PAD. Adjusting for age, gender, race, BMI, cigarette smoking, comorbidities, and frequency of walking exercise, poorer performance on each baseline measure of lower extremity functioning was associated with higher risk of mobility loss among participants without PAD (Model 1, Fig. 2). These associations were not substantially changed after additional adjustment for physical activity (Model 2, Fig. 2).

Figure 2
Hazard Ratios for Mobility Loss at 50-Month Follow-Up Among Men and Women Without PAD, According to Baseline Functional Performance

We observed no significant interactions between PAD status and functional performance in their associations with mobility loss (data not shown). Figure 3 compares rates of mobility loss between PAD and non-PAD participants according to categories of baseline performance. Within the lowest and highest quartiles of performance, rates of mobility loss were comparable between PAD and non-PAD participants. Within the second quartile of baseline performance, rates of mobility loss were higher among PAD compared with non-PAD participants (Fig. 3). Data in Table 2 and Figure 3 suggest that differences in mobility loss between PAD and non-PAD participants over time are attributable in part to differences in baseline performance between PAD and non-PAD participants.

Figure 3
Rates of Mobility Loss for Participants With and Without PAD, According to Baseline Functional Performance Categories Defined for the Entire Cohort

At baseline, 509 participants (284 with PAD) completed the 6-min walk test without stopping. At 42.8 months follow-up, 71 PAD participants (25%) and 32 non-PAD participants (14%) became unable to walk for 6 min continuously (p = 0.0026). Among participants with PAD, poorer baseline 6-min walk performance and slower fast-paced 4-m walking velocity were associated independently with increased risk of becoming unable to walk for 6 min continuously when adjusting for confounders (Table 3). Among participants without PAD, baseline 6-min walk performance, normal-paced 4-m walking velocity, and SPPB scores were each associated independently with increased risk of becoming unable to walk for 6 min continuously, adjusting for confounders (Table 3). Adjusting for age and gender, the hazard ratio for loss of the ability to walk continuously for 6 min among PAD versus non-PAD participants was 2.17 (95% confidence interval 1.42 to 3.32, p = 0.0004). After additional adjustment for baseline 6-min walk performance, the association between PAD and mobility loss was attenuated and no longer statistically significant (Table 2). However, the association between PAD and loss of the ability to walk continuously for 6 min remained statistically significant even after additional adjustment for usual-paced 4-m walking speed, fast-paced 4-m walking speed, and the SPPB (Table 2).

Table 3
Adjusted Associations of Baseline Functional Performance Measures With Loss of the Ability to Walk Continuously for 6 Min Without Stopping (n = 509)*

Discussion

Our data demonstrate that persons with PAD have an increased rate of mobility loss at 4-year follow-up compared with persons without PAD. Because mobility maintenance is integral to maintenance of functional independence, social interactions, and activities of daily living, the increased rate of mobility loss in persons with PAD has potentially important implications for maintaining functional independence in individuals with PAD.

Results presented here demonstrate that simple, objective measures of lower-extremity performance can be used to identify persons with PAD who are at highest risk for mobility loss. Although all participants were free of mobility impairment at baseline, a wide range of baseline functional performance was observed. The objective measures of performance detected subclinical deficits among persons with PAD that were associated with mobility loss later. Measures of lower-extremity performance are reliable and require minimal time for administration. These results are expected to be useful to clinicians, who can use these simple tests to identify PAD patients who are at increased risk of mobility loss.

Previous study demonstrated that baseline measures of functional performance predict mobility loss, loss of the ability to perform activities of daily living, and mortality among community dwelling older men and women without PAD (18,19). Our study confirms these findings in persons without PAD and provides new information about the ability of performance based measures to predict mobility loss in persons with PAD. Results presented here also suggest that the increased mobility loss in persons with PAD is largely attributable to the poorer baseline functional performance among persons with PAD compared with those without PAD. The greater incidence of mobility loss for PAD participants compared with those without PAD was no longer statistically significant after adjusting for differences in baseline lower-extremity performance between participants with versus without PAD. Further study is needed to determine whether performance based measures also predict mortality and hospitalization in persons with PAD.

Findings regarding baseline functional performance and loss of the ability to walk continuously for 6 min without stopping differed from those with mobility loss. Among persons with and without PAD, performance on some, but not all, baseline functional performance measures predicted loss of the ability to walk for 6 min continuously without stopping. The higher rate of losing the ability to walk for 6 min continuously among participants with PAD compared with those without PAD remained statistically significant, even after adjusting for usual-paced 4-m walking speed, fast-paced 4-m walking speed, and the SPPB. Patient-reported mobility loss and objectively measured loss of the ability to walk continuously for 6 min are distinct outcomes. A potential explanation for lack of full concordance in findings between these 2 outcomes is that patients with the greatest disability at follow-up may have provided data on mobility loss in a telephone interview, but may have been too ill to return for a 6-min walk test.

As indicated in recently published clinical practice guidelines, few data are available to document the natural history of lower-extremity outcomes in persons with PAD (7). On the basis of available data, current clinical practice guidelines report that claudication symptoms in persons with PAD typically remain stable and do not worsen at a rapid rate (7). However, stabilization of leg symptoms over time may not be indicative of mobility preservation because some persons with PAD may restrict their physical activity to avoid leg symptoms. Further study is needed to determine whether improving lower-extremity functioning with interventions such as exercise, medical therapy, or revascularization can reduce rates of mobility loss among persons with PAD. Data presented here suggest that these interventions could be targeted toward PAD patients with poorer performance on objective measures of functioning.

Study limitations

Our study has limitations. First, participants were identified from academic medical centers, and it is unclear whether our findings are generalizable to individuals outside of academic medical centers. However, there is no reason to believe that the relationships presented here might differ for persons outside academic medical centers. Second, this study was observational. Associations between functional impairment and mobility loss cannot be construed as causal.

Conclusions

Persons with PAD have a greater incidence of mobility loss compared with individuals without PAD. Differences in rates of mobility loss between persons with versus without PAD appear to be primarily related to greater baseline functional impairment in persons with versus without PAD. Simple, objective measures of functional performance, which can be readily performed in the office setting, can be used to identify PAD persons at highest risk for mobility loss.

Acknowledgments

Supported by grants #R01-HL58099, R01-HL64739, and R01-HL071223 from the National Heart, Lung, and Blood Institute and by grant #RR-00048 from the National Center for Research Resources, NIH. Supported in part by the Intramural Research Program, National Institute on Aging, NIH.

Abbreviations and Acronyms

ABI
ankle-brachial index
BMI
body mass index
PAD
peripheral arterial disease
SPPB
Short Physical Performance Battery
WALCS
Walking and Leg Circulation Study

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