Author + information
- Received July 26, 2012
- Revision received October 11, 2012
- Accepted October 18, 2012
- Published online June 1, 2013.
- Mary M. McDermott, MD⁎,†,⁎ (, )
- Timothy J. Carroll, PhD‡,§,
- Melina Kibbe, MD∥,¶,
- Christopher M. Kramer, MD#,
- Kiang Liu, PhD†,
- Jack M. Guralnik, MD, PhD⁎⁎,
- Aoife N. Keeling, MD††,
- Michael H. Criqui, MD, MPH‡‡,
- Luigi Ferrucci, MD, PhD§§,
- Chun Yuan, PhD∥∥,¶¶,
- Lu Tian, ScD##,
- Yihua Liao, MS†,
- Jarett Berry, MD⁎⁎⁎,
- Lihui Zhao, PhD† and
- James Carr, MD‡
- ↵⁎Reprint requests and correspondence:
Dr. Mary McDermott, Northwestern University's Feinberg School of Medicine, Department of Medicine and Department of Preventive Medicine, 750 North Lake Shore Drive, 10th floor, Chicago, Illinois 60611
Objectives We studied associations of magnetic resonance imaging (MRI)-measured superficial femoral artery (SFA) occlusions with functional performance, leg symptoms, and collateral vessel number in peripheral artery disease (PAD). We studied associations of collateral vessel number with functional performance in PAD.
Background Associations of MRI-detected SFA occlusion and collateral vessel number with functional performance among individuals with PAD have not been reported.
Methods A total of 457 participants with an ankle brachial index (ABI) <1.00 had MRI measurement of the proximal SFA with 12 consecutive 2.5-μm cross-sectional images. An occluded SFA was defined as an SFA in which at least 1 segment was occluded. A nonoccluded SFA was defined as absence of any occluded slices. Collateral vessels were visualized with magnetic resonance angiography. Lower extremity functional performance was measured with the 6-min walk, 4-m walking velocity at usual and fastest pace, and the Short Physical Performance Battery (SPPB) (0 to 12 scale, 12 = best).
Results Adjusting for age, sex, race, comorbidities, and other confounders, the presence of an SFA occlusion was associated with poorer 6-min walk performance (1,031 vs. 1,169 feet, p = 0.006), slower fast-paced walking velocity (1.15 vs. 1.22 m/s, p = 0.042), and lower SPPB score (9.07 vs. 9.75, p = 0.038) compared with the absence of an SFA occlusion. More numerous collateral vessels were associated with better 6-min walk performance (0 to 3 collaterals—1,064 feet, 4 to 7 collaterals—1,165 feet, ≥8 collaterals—1,246 feet, p trend = 0.007), faster usual-paced walking speed (0 to 3 collaterals—0.84 m/s, 4 to 7 collaterals—0.88 m/s, ≥8 collaterals—0.91 m/s, p trend = 0.029), and faster rapid-paced walking speed (0 to 3 collaterals—1.17 m/s, 4 to 7 collaterals—1.22 m/s, ≥8 collaterals—1.29 m/s, p trend = 0.002), adjusting for age, sex, race, comorbidities, ABI, and other confounders.
Conclusions Among PAD participants, MRI-visualized occlusions in the proximal SFA are associated with poorer functional performance, whereas more numerous collaterals are associated with better functional performance. (Magnetic Resonance Imaging to Identify Characteristics of Plaque Build-Up in People With Peripheral Arterial Disease; NCT00520312)
Data from humans and animals demonstrate that pre-existing collateral vessels enlarge in response to shear stress following lower extremity arterial occlusions (1–3). Collateral vessels serve as a natural bypass system, are thought to protect against critical limb ischemia, and may reduce leg symptoms in people with peripheral artery disease (PAD) (1–3). Occlusion of a major lower extremity artery is a primary stimulus to the enlargement of pre-existing collateral vessels, and the superficial femoral artery (SFA) is the most common site of lower extremity arterial occlusions (4). However, these collateral vessels typically do not restore lower extremity perfusion to normal (2). Whether larger or more numerous lower extremity collateral vessels protect against functional impairment and functional decline in people with PAD is unknown.
Among individuals with PAD, we studied associations of occlusions in the SFA with leg symptoms, functional performance, and collateral vessel characteristics. We hypothesized that occlusions in the SFA would be associated with more leg symptoms and greater functional impairment. However, we also hypothesized that larger and more numerous lower extremity collateral vessels would mitigate against PAD-related functional impairment. Specifically, we hypothesized that for a given level of PAD severity, more numerous and larger collateral vessels would be associated with better functional performance in PAD.
Participants were those in the WALCS (Walking and Leg Circulation Study) III cohort (5). Participants were identified from among consecutive PAD patients in 4 Chicago-area medical centers. Participants were also identified from among lists of consecutive patients with a diagnosis of PAD in the vascular surgery, cardiology, endocrinology, general medicine, and geriatric practices at Northwestern Medical Faculty Foundation and in the vascular surgery practice at the Jesse Brown Veterans Administration Medical Center. A small number of participants were identified from among men and women age 70 years and older in Northwestern's largest general internal medicine practice who were screened with the ankle brachial index (ABI) and found to have an ABI <1.00. The protocol was institutional review board–approved by Northwestern University and all participating sites. Participants gave written informed consent. Rates of participation and reasons for exclusion among those contacted for WALCS III participation have been reported (5).
The inclusion criterion was an ABI <1.00. This criterion was selected because normal ABI values are 1.10 to 1.40 (6).
Potential participants with dementia or a mini-mental status examination score <23 (7) were excluded. Nursing home residents, wheelchair-bound patients, and patients with foot or leg amputations were excluded because of severely impaired functioning. Non–English-speaking patients were excluded because investigators were not fluent in non-English languages. Individuals who required oxygen therapy, had contraindications to magnetic resonance imaging (MRI) testing, stopped the 6-min walk test due to shortness of breath, had recent major surgery, or had severe knee osteoarthritis were excluded (8). Potential participants with bilateral SFA stents were excluded, because stents interfere with plaque imaging.
After participants rested supine for 5 min, a hand-held Doppler probe (Nicolet Vascular Pocket Dop II, Golden, Colorado) was used to measure systolic pressures in the right brachial, dorsalis pedis, and posterior tibial arteries and left dorsalis pedis, posterior tibial, and brachial arteries. Pressures were repeated in reverse order. The ABI was calculated in each leg by dividing the average pressures in each leg by the average of the 4 brachial pressures (9). 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 or more mm Hg in at least 1 measurement set.
Magnetic resonance imaging
We imaged the SFA because it is the most common site of lower extremity atherosclerosis (10) and because it supplies the calf muscle, which is typically symptomatic in patients with PAD. The leg with lowest ABI was imaged. If the leg with lowest ABI had an SFA stent, the opposite leg was imaged. MRI data were obtained with a 1.5-T (Siemens Medical Solutions, Erlangen, Germany) platform using 4-element phased-array surface coils. The bifurcation of the common femoral artery served as the reference point. Twelve consecutive 2.5-mm cross-sectional images were obtained, moving distally from the most proximal point of the SFA, using 2-dimensional bright-blood time-of-flight and proton density–weighted images. Fat suppression was applied in black-blood sequences to improve image quality. This method has excellent test-retest reliability (5).
CASCADE software (University of Washington Vascular Imaging Laboratory, Seattle, Washington) was used by 2 physician reviewers to trace the outer boundary and the lumen of each cross-sectional image of the SFA. A nonoccluded SFA was defined as the absence of any occluded segments in the imaged proximal SFA. An occluded SFA was defined as the presence of at least 1 occluded segment.
Images for each participant were assigned to 1 primary reviewer, and arterial tracings were reviewed by the second reviewer to ensure accuracy. A 6% subsample of participants returned on a second day for test-retest reliability assessment of MRI measurements. The coefficient of variation percent values for these test-retest reliability assessments were 5.7 for mean plaque area, 8.9 for maximum plaque area, 8.0 for mean percent lumen area, and 12.9 for minimum percent lumen area (5).
Magnetic resonance angiography
A 1.5-T Siemens Espree (Siemens Medical Solutions) MRI scanner was employed for magnetic resonance angiography (MRA) image acquisition. A 12-channel surface array coil (Siemens Medical Solutions) was used for signal reception. Dynamic MRA images from 1 station, the groin to the knee, were acquired from both legs, including the common femoral artery, the profunda femoris, the SFA, and the popliteal artery. Dynamic images were acquired with the TWIST (time-resolved angiography with interleaved stochastic trajectories) pulse sequence (11). Parallel imaging with generalized autocalibrating partially parallel acquisitions (12) image reconstruction provided a 2-fold increase in frame rate. Following a 3-plane localizer image, the TWIST sequence was applied in the coronal orientation. A 10-ml bolus of nondiluted gadopentetate dimeglumine (0.05 mmol/kg, Magnevist; Berlex, Montville, New Jersey) was administered intravenously at 2 ml/s. Imaging parameters were repetition time/echo time/Flip angle = 3.3 ms/TE, 1.3 ms/25°; rectangular field of view (rFOV), 246 × 375 mm; matrix, 210 × 320; 88 partitions; voxel size after zero interpolation, 1.2 × 1.2 × 1.2 mm3 (true voxel size, 1.2 × 1.2 × 2.0 mm3); acceleration factor: 2.
MRA image analysis
A single radiologist (A.N.K.), blinded to all other participant characteristics, read all MRA images on a workstation (GE Healthcare, Milwaukee, Wisconsin). A validated scoring system was used to grade the number and size of collateral vessels, based on previous studies (13,14). This method has excellent intrarater reliability (mean 85.7% agreement) (14). On the basis of previously reported methods (13,14), small collaterals were defined as those occupying <25% of the length of the imaged thigh and <50% of the diameter of the SFA. Large collaterals were defined as occupying more than 25% of the length of the imaged thigh and >50% of the diameter of the SFA. Categories were used to classify the number and size of collateral vessels, based on previous studies (13,14). For collateral vessel size, Grade 1 was defined as ≤5 small collateral vessels, Grade 2 was defined as >5 small collateral vessels, Grade 3 was defined as ≤5 large ± small collateral vessels, and Grade 4 was defined as >5 large ± small collateral vessels (13). For collateral vessel number, Category 1 was defined as 0 to 3 collateral vessels, Category 2 was defined as 4 to 7 collateral vessels, and Category 3 was defined as ≥8 collateral vessels (14). Our MRA analyses did not include identifying proximal SFA occlusions.
Participants walked up and down a 100-foot hallway for 6 min after instructions to cover as much distance as possible (5,15).
4-m walking velocity
Walking velocity was measured with a 4-m walk performed at “usual” and “fastest” pace, based on previous studies (5,15). Each walk was performed twice. The faster walk in each pair was used in analyses.
Short Physical Performance Battery
The Short Physical Performance Battery (SPPB) combines data from usual-paced 4-m walking velocity, time to rise from a seated position 5 times, and standing balance (16). Individuals receive a 0 score for each task they are unable to complete. Scores of 1 to 4 are assigned for remaining tasks, according to established methods. Scores are summed to obtain the SPPB, ranging from 0 to 12 (16).
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 is measured (16).
Participants are asked to hold 3 increasingly difficult standing positions for 10 s each: the side-by-side stand, semi-tandem stand (standing with feet parallel and the heel of 1 foot touching the base of the first toe of the opposite foot), and the full-tandem stand (standing with 1 foot directly in front of the other) (16). Scores range from 0 (unable to hold the side-by-side stand for 10 s to 4 (able to hold the full-tandem stand for 10 s) (16).
Algorithms developed for the Women's Health and Aging Study and the Cardiovascular Health Study were used to document comorbidities (17). These algorithms combine data from patient report, physical examination, medical record review, medications, laboratory values, and a primary care physician questionnaire (17). Comorbidities assessed were angina pectoris, diabetes mellitus, myocardial infarction, stroke, heart failure, pulmonary disease, cancer, spinal stenosis, and disk disease. Criteria from the American College of Rheumatology were used to diagnose knee and hip osteoarthritis (8,18).
Leg symptoms were classified using the San Diego claudication questionnaire (19). Intermittent claudication was defined as exertional calf pain that does not begin at rest, causes the participant to stop walking, and resolves within 10 min of rest (15,19). Participants without any exertional leg pain were classified as asymptomatic (15,19). Participants with exertional leg pain who walked through their pain were classified as “leg pain carry on,” those with exertional leg pain that sometimes begins at rest were classified as “pain on exertion and rest.” Remaining participants with exertional leg symptoms who did not meet criteria for the aforementioned leg pain categories were classified as “atypical exertional leg pain” (15,19).
Height and weight were measured at the study visit. Body mass index (BMI) was calculated as weight (in kilograms)/(height [in meters])2. Cigarette smoking history was measured with self-report.
Clinical characteristics were compared between PAD participants with versus without an occluded SFA using chi-square tests for categorical variables and analyses of variance for continuous variables. Clinical characteristics and functional performance measures were compared between participants with versus without an occluded SFA using analyses of covariance, adjusting for age, sex, and race (Model 1). Analyses involving nominal variables were performed using logistic regression. Functional performance analyses were repeated with additional adjustment for comorbidities, BMI, and smoking (Model 2). Finally, Model 2 was additionally adjusted for the ABI (Model 3).
Functional performance measures were compared across collateral vessel size (grade) and collateral vessel number (category), adjusting for age, sex, race, smoking, diabetes, BMI, comorbidities, and ABI. These analyses were repeated separately among participants with versus without SFA occlusions. Because of small sample size in the 0 to 3 collateral vessel group among participants with an SFA occlusion, participants with an SFA occlusion and 0 to 3 collateral vessels were combined with those with 4 to 7 collateral vessels.
Of 473 PAD participants in the WALCS III cohort with SFA imaging, 457 had good quality images and are included in this report. Of these, 59 (12.9%) had an occluded SFA. Mean ages were 68.3 ± 9.1 years and 69.4 ± 10.3 years (p = 0.409), the prevalence of men was 72.9% versus 64.3% (p = 0.197), and the prevalence of African Americans was 35.6% versus 32.9% (p = 0.684) among participants with versus without 1 or more occlusions in the proximal SFA, respectively. Figure 1 shows an occluded SFA with corresponding collateral vessels.
After adjusting for age, race, and sex, participants with 1 or more occlusions in the proximal SFA had a lower ABI, a lower prevalence of asymptomatic PAD, a higher prevalence of leg pain/carry-on, higher mean plaque area, and lower mean lumen area, compared with participants without any occlusions (Table 1).
Among the 457 participants with PAD, 291 were eligible for and consented to an MRA. Participants who underwent MRA were younger (68.5 ± 10.4 years vs. 70.7 ± 9.6 years, p = 0.025), included a higher proportion of men (70.5% vs. 56.6%, p = 0.003), and included a higher proportion of current smokers (28.6% vs. 17.5%, p = 0.008) compared with those who did not undergo MRA. Adjusting for age, sex, and race, participants with an occlusion in the proximal SFA had a higher prevalence of large collateral vessels (i.e., Grade 3 and Grade 4) and a lower prevalence of smaller collateral vessels (i.e., Grade 1), compared with participants without occlusions (Table 1). After adjusting for age, sex, and race, participants with an occlusion in the proximal SFA had a lower prevalence of 0 to 3 collateral vessels and a higher prevalence of ≥8 collateral vessels, compared with participants without proximal SFA occlusions (Table 1).
Presence of an occlusion in the proximal SFA was associated with shorter 6-min walk distance, compared with individuals without any occlusions, after adjusting for age, sex, and race (Model 1, Table 2). After additional statistical adjustment for comorbidities, BMI, and cigarette smoking (Model 2), participants with an occlusion in the proximal SFA had poorer 6-min walk performance, slower fast-paced 4-m walking velocity, and lower SPPB scores compared with participants without any occlusions (Table 2, Model 2). When the Model 2 results in Table 2 were additionally adjusted for the ABI, there were no significant associations of an occluded SFA with functional performance (data not shown). Thus, after taking into account differences in ABI values between participants with versus without SFA occlusions, there were no significant associations of SFA occlusions with functional performance.
Within the entire cohort, more numerous collateral vessels were associated with better 6-min walk performance and faster walking velocity at usual and fastest pace, adjusting for age, sex, race, smoking, comorbidities, ABI, and BMI (Table 3). Similar findings were observed among participants without SFA occlusions (Table 3). Among PAD participants with an SFA occlusion, more numerous collateral vessels were associated with better 6-min walk performance (953 feet for participants with <8 collateral vessels vs. 1,335 feet for participants with ≥8 collateral vessels, p = 0.027) and faster rapid-paced 4-m walking velocity (1.07 m/s for participants with <8 collateral vessels vs. 1.43 m/s for participants with ≥8 collateral vessels, p < 0.001), adjusting for age, sex, race, comorbidities, smoking, ABI, and BMI. There were no associations of collateral vessel number with usual-paced 4-m walking velocity (data not shown) or the SPPB score (9.17 vs. 10.82, p = 0.082) among participants with an SFA occlusion.
There were no significant associations of collateral size (grade) with functional impairment (data not shown).
Among 457 participants in the WALCS III cohort with ABI <1.00, 13% had 1 or more occlusions in the proximal SFA of the leg with lowest ABI. There were no differences in age, race, sex, or prevalence of smoking or diabetes mellitus between PAD participants with an occlusion and those without any occlusions. PAD participants with an occlusion in the proximal SFA had poorer 6-min walk performance, slower fast-paced walking velocity, and a lower SPPB score, compared with PAD participants without any proximal SFA occlusions after adjusting for age, sex, race, BMI, smoking, diabetes, comorbidities, and other confounders. Our results also show that the presence of more numerous collateral vessels is associated with better functional performance compared with fewer collateral vessels among participants with PAD. By contrast, we found no significant associations of collateral size (grade) with functional performance.
Occlusions are relatively common in lower extremity arteries and are an important stimulus to the development of collateral vessels. Yet to our knowledge, no prior studies have evaluated associations of MRI-visualized occlusions or collateral vessels with functional performance measures in individuals with PAD. Collaterals develop from pre-existing arterioles that enlarge in response to sheer stress forces precipitated by arterial occlusion (20). Sheer stress activates mechanosensors, resulting in molecular signaling, modulation of gene expression, and cytokine release, including monocyte chemoattractant protein-1, granulocyte-macrophage colony-stimulating factor, and cellular adhesion molecules (21–23). Influx of inflammatory cells is followed by release of vascular growth factors, remodeling of the vessel wall, enlargement of vessel diameter, and migration of smooth muscle cells into the artery wall (23,24). Animal evidence suggests that following arterial occlusion, vessels first increase rapidly in number and later enlarge in diameter (23). In animal models, exercise training is associated with collateral vessel enlargement and increased collateral vessel compliance (2). In animals, maximal collateral vessel enlargement occurs during exercise (3). This phenomenon may explain our finding that greater collateral vessel number, but not collateral vessel size (i.e., diameter), was associated with better functional performance in PAD, because our images were obtained during rest and thus may not have captured maximal vessel diameter.
First, data are cross-sectional. Associations reported cannot be construed as causal. Second, we imaged a short segment of the SFA. We did not image atherosclerotic plaque in the aortoiliac or femoropopliteal arterial segments. Nonetheless, we found that the presence of any occluded segments in the proximal SFA was associated with greater functional impairment than absence of occluded segments in the proximal SFA. Third, our collateral vessel data are limited to those who were eligible for and consented to MRA. Our findings regarding collateral vessels may not be generalizable to individuals who did not undergo MRA testing. Fourth, a relatively small proportion of participants had occlusions of the proximal SFA, limiting statistical power to detect meaningful differences between participants with versus without occlusions in the proximal SFA. Fifth, MRA has lower spatial resolution than computed tomographic angiography or digital subtraction angiography. Some collateral vessels may be too small for identification on MRA (25). Sixth, lack of collateral vessel measurement during exercise may have limited our ability to identify associations of vessel size with functional performance.
Our results demonstrate that PAD participants with 1 or more occluded segments in the proximal SFA have greater functional impairment than PAD participants without proximal SFA occlusions. More numerous collateral vessels are associated with better functional performance in PAD. Our findings are important in part because of current interest in identifying therapies to increase lower extremity perfusion in individuals with PAD.
Supported by the National Heart Lung and Blood Institute (R01-HL083064) and (R01-HL109244), the Intramural Research Program of the National Institute on Aging, and the Jesse Brown VA Medical Center. Dr. Kramer receives research support from Siemens Healthcare. Dr. Yuan receives research support from VP Diagnostics and from Philips Healthcare; and is a consultant for ImagePace. Dr. Berry is on the Speakers' Bureau of Merck. Dr. Carr receives research support from Siemens and Lantheus Medical Imaging. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.
- Abbreviations and Acronyms
- ankle brachial index
- body mass index
- magnetic resonance angiography
- magnetic resonance imaging
- peripheral artery disease
- superficial femoral artery
- Short Physical Performance Battery
- Received July 26, 2012.
- Revision received October 11, 2012.
- Accepted October 18, 2012.
- American College of Cardiology Foundation
- Schoop W.
- ↵Prior BM, Lloyd PG, Ren J, et al. Time course of changes in collateral blood flow and isolated vessel size and gene expression after femoral artery occlusion in rats. Am J Physiol Heart Circ Physiol 287:H2434–47.
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