(Circulation. 1996;93:1818-1825.)
© 1996 American Heart Association, Inc.
Articles |
From the Heart Lung Institute (G.P., C.B., M.J.P.), Department of Functional Anatomy (P.J.W.W., B.H.) and Department of Radiology (W.P.T.M.M.), Utrecht (the Netherlands) University Hospital; the Thoraxcentre, Erasmus University (E.J.G.), Rotterdam, the Netherlands; and the Interuniversity Cardiology Institute of the Netherlands (G.P., M.J.P., E.J.G.), Utrecht.
Correspondence to Cornelius Borst, MD, PhD, Professor of Experimental Cardiology, Heart Lung Institute, Utrecht University Hospital, Heidelberglaan 100, Room G02.523, 3584 CX Utrecht, Netherlands.
| Abstract |
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Methods and Results Twenty patients were studied by 30-MHz intravascular ultrasound (IVUS) before balloon angioplasty of the superficial femoral artery (370 cross sections), and 25 femoral artery segments were studied postmortem (551 cross sections). In each cross section, the area surrounded by the internal elastic lamina (IEL area) and the plaque area were measured. The IEL area was larger in the cross section with the largest plaque area than in the cross section with the smallest plaque area (32.5±13.0 and 32.0±11.5 mm2 versus 28.9±9.7 [P=NS] and 26.7±10.1 [P<.05] mm2 for IVUS and histology, respectively [mean±SD]). A significant positive correlation was found between plaque area and IEL area for the pooled data (r=.61 and r=.47 and slope=1.07 and 0.90 for IVUS and histology, respectively; both P<.001). In 12 of 20 and 16 of 25 individual arterial segments, however, no significant correlation was observed between plaque area and IEL area for IVUS and histology, respectively. A large variation was found in the correlation of the regression of plaque to IEL area (IVUS, r=-.40 to .89; histology, r=-.13 to .91) and slope (IVUS, -0.28 to 1.29; histology, -0.18 to 1.32).
Conclusions In the majority of atherosclerotic femoral arteries, significant compensatory enlargement could not be determined. It is inferred that arterial remodeling in response to plaque formation may vary among individuals.
Key Words: atherosclerosis remodeling ultrasonics peripheral vascular disease stenosis
| Introduction |
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To acquire insight into individual variability of local arterial remodeling of de novo lesions, we assessed local changes in arterial size in response to focal plaque formation in 10- to 15-cm atherosclerotic femoral artery segments using in vivo intravascular ultrasound (IVUS) and postmortem histology.
| Methods |
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To minimize any influence of anatomic tapering of the artery, segments 10 to 15 cm long were selected from 12 cm proximal to the adductor hiatus to 3 cm distal to the adductor hiatus. No major side branches originated in this area. The femoral arteries were decalcified with EDTA 10% in 5 days and dehydrated in an alcohol sequence ranging from 70% to 100%. From 7 arterial segments embedded in Technovit 7100 hydroxyethylmethacrylate, histological cross sections were obtained every 0.5 cm and stained with Verhoeff's elastic tissue stain. Eighteen arterial segments were embedded in a mixture of liquefied polyethylene glycol 1000 and polyethylene glycol 400 in a 4:1 ratio, cut into 0.5-cm-long parts, stained with Lawson's elastic tissue stain, and studied under magnification. All microscopic images of the cross sections were recorded on super VHS videotape with a Sony video camera (3 CCD) for further image analysis. A ruler was used for distance calibration. A total of 580 histological cross sections were obtained. Twenty-nine cross sections were excluded because of cutting artifacts. Thus, quantitative analysis was performed on 551 histological cross sections.
Intravascular Ultrasound
Twenty patients (14 men, 6 women; age, 65.4±8.2) were studied
by IVUS before routine balloon angioplasty of the superficial femoral
artery to treat disabling claudication. Six patients did not take any
medication before hospitalization. In the remaining 14 patients, the
following medications were taken: carbasalaatcalcium (n=6),
acenocoumarol (n=6), ß-blockade (metoprolol [n=5] or bisoprolol
[n=1]), calcium antagonists (amlodipine [n=1],
nifedipine [n=1], and diltiazem [n=1]), ACE
inhibitor (captopril [n=2]), and antidiabetic treatment
(insulin [n=1] and glibenclamide [n=1]). Only de novo
atherosclerotic lesions were studied. Informed consent was obtained
from all patients. A 4.1F IVUS mechanical catheter was used (30-MHz
transducer rotating at 1000 rpm; axial resolution, 100 µm; lateral
resolution, 200 µm; DuMED). The resulting images were displayed on a
monitor by means of a video-scanned memory and recorded on
super VHS videotape.
In all patients, a series of cross-sectional images was recorded during pullback of the IVUS catheter. To localize the IVUS catheter, a radiopaque ruler and a distance sensing device8 9 were used as reference during fluoroscopy in 17 and 3 patients, respectively. When vasospasm was suspected angiographically, 0.2 mg nitroglycerin was administered through the introducer sheath (n=4). Nitroglycerin was not administered routinely. All stenosis sites visualized with IVUS were detected previously with duplex measurements. Therefore, it was unlikely that these stenoses were caused by vasospasm elicited by the IVUS catheter.
To study individual differences in the relation between plaque area and
the area encompassed by the echo-lucent media, ultrasound images
were selected every 0.5 cm in arterial segments of
10 cm
length. A total of 400 IVUS images were obtained from the stenosed
femoral segments. Thirty IVUS images were excluded for further
analysis owing to the presence of side branches or excessive
calcification. Thus, quantitative analysis was performed on 370
IVUS cross-sectional images.
Image Analysis
Histological sections and IVUS images
recorded on videotape were analyzed with a digital video
analyzer as described previously.10 In short,
during image acquisition, video signals from a VHS videotape were
converted with a frame grabber (Brand) to 512x512x8-bit digital image
data and stored on a personal computer. In the IVUS image, we traced
the lumen area and the IEL area. The latter is the area encompassed by
the interface between the echo-dense intimal layer and the
echo-lucent media. The circumferential outlines were processed to
produce a smoothed and closed contour. The cross-sectional lumen
area, IEL area, and plaque area were determined automatically after the
two contours were completed. Plaque area was calculated by subtracting
the lumen area from the IEL area. Intraobserver and interobserver
variabilities in lumen area and plaque area measurements were
assessed in 22 arteries. No significant observer bias was present
(paired differences were not significantly different from zero). In
lumen area measurements, observer variation (SD of the paired
differences) was smaller than in lesion area measurements (0.6 and 1.1
mm2 and 1.2 and 1.9 mm2 for intraobserver and
interobserver variations for lumen and lesion areas, respectively).
Intraobserver and interobserver variations in measurements of percent
area obstruction were 2.6% and 5.6%, respectively.
Statistical Analysis
All measured values are presented as mean±SD. The
relation between plaque area and IEL area was studied by linear
regression analysis, first on the pooled data and then
separately on each individual arterial segment. Paired
Student's t test was used to compare measured values in the
cross section with the largest plaque area and the cross section with
the least amount of plaque obtained from the same arterial
segment. A value of P<.05 was considered statistically
significant.
| Results |
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Individual Regression Analysis
In the individual artery, a large variation in correlation (IVUS,
r=-.40 to .89; histology, r=-.13 to
.91) and slope (IVUS, -0.28 to 1.29; histology: -0.18 to
1.32) between plaque area and IEL area was observed (Table 2
). In only 8 of 20 and 9 of 25 arterial
segments, the correlation between plaque area and IEL area was
significant (P<.05) for IVUS and histology, respectively
(Fig 2
, top). In the remaining arterial
segments, no significant correlation was found (P>.05; Fig 2
, bottom).
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The linear regression lines of the individual arterial
segments (Table 2
) and the regression line calculated for the pooled
data (Fig 1
) are shown together in Fig 3
. This graph
illustrates that in 19 and 21 arterial segments (95% and
84%, IVUS and histology, respectively), the slope of the individual
artery linear regression was smaller than that calculated for the
pooled data. Furthermore, Fig 3
illustrates that in the cross sections
with the least amount of plaque (0.0 to 11.0 and 1.2 to 13.3
mm2 [Table 2
]), the IEL area increased by 2.88 and 1.53
mm2 for each 1-mm2 increase in plaque area for
IVUS and histology, respectively.
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In the cross sections with the least amount of plaque (Table 1
), the
IEL areas were classified into the two groups listed in Table 3
. The groups consist of cross sections obtained from
arterial segments with and without a significant positive
correlation between plaque and IEL area. In arterial
segments with a significant positive correlation, the IEL area (ie,
artery size) was smaller than in arterial segments without
a significant correlation (P=.03 and P=.02 for
IVUS and histology, respectively).
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| Discussion |
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In addition to compensatory enlargement of the coronary1 2 3 4 5 and femoral6 7 arteries in response to plaque formation, we recently observed in the femoral artery that paradoxical arterial shrinkage may contribute to atherosclerotic luminal narrowing.7 Compensatory enlargement will retard but arterial wall shrinkage will accelerate luminal narrowing by plaque formation. In hemodynamically significant stenoses (>50% diameter stenosis), arterial shrinkage dominated compensatory enlargement.7
Compensatory enlargement has previously been inferred from pooled data based on one cross section per arterial segment.1 2 6 In the present study, we focused on individual arterial segments analyzed at 0.5-cm intervals. From a set of 55 arterial segments reported on earlier,7 we selected 45 segments on the basis of the large variation in plaque load over the length of the segment. The principal findings are as follows: (1) A significant positive correlation between plaque area and IEL area was observed in only 17 of 45 (39%) of the superficial femoral arteries; (2) in 40 of 45 arterial segments (89%), the slope of the individual regression line between plaque area and IEL area was smaller than the slope calculated from the pooled data; and (3) segments with a significant positive correlation between plaque area and IEL area were smaller than segments without a significant correlation.
Relation Between Plaque Area and IEL Area
From coronary postmortem measurements, Glagov et
al1 inferred a model for compensatory enlargement in which
the artery enlarges until, on average,
40% of the IEL area is
occupied by plaque. In the early phase, even luminal overcompensation
may be observed. When the plaque exceeds
40% of the IEL area,
however, further plaque growth will encroach on the lumen.
Glagov et al1 and other investigators2 6
based their inference of compensatory enlargement on pooled data. In
the present study, in which the IEL area was plotted versus plaque
area, individual segments also were analyzed. For the pooled
data, a slope of 1.07 and 0.88 was found between plaque area and IEL
area for IVUS and histology, respectively. This is consistent
with the slopes observed in previous studies.1 2 6 It is
unlikely, however, that a slope of 1.07 for the IVUS data, obtained
from patients to be treated for claudication, is based solely on an
increase in the IEL area in response to plaque increase because it
implies that on average, compensatory enlargement was >100%
effective. Such effective compensatory enlargement would be
incompatible with symptomatic disease. We hypothesized that
pooling data from all subjects may mask an individual variation in the
susceptibility of the artery to undergo compensatory enlargement and
any relation between artery size and minimum plaque load. Fig 3
illustrates both aspects. First, in 19 of 20 and 21 of 25
arterial segments, the slope of the regression line between
plaque area and IEL area was smaller than for the pooled data, implying
that the extent to which the artery enlarges in response to plaque
formation differs for individual segments. Second, slopes of 2.88 and
1.53 were observed for the relation between plaque area and IEL area in
the cross sections that contained the least amount of plaque for IVUS
and histology, respectively, implying a large variation in vessel size.
The cross section with the least amount of plaque is probably least
affected by arterial remodeling and therefore best
approximates the original lumen. Thus, two independent variables
account for the positive relation between plaque area and IEL area in a
pooled data set: compensatory enlargement and individual variation in
vessel size. Therefore, a positive relation between plaque area and IEL
area with a large slope cannot be attributed to compensatory
enlargement solely without correction for differences in individual
arterial size (see "Appendix").
The IVUS studies were performed on patients with advanced, symptomatic atherosclerotic disease. It may be argued that the response of compensatory enlargement was overwhelmed by atherogenesis, implying the disappearance of any correlation between plaque area and IEL area. However, for the IVUS data, 222 of 370 images (60%) demonstrated <50% histological stenosis; ie, <50% of the IEL area was occupied by plaque. For the histological data, only 382 of 551 images (69%) demonstrated <50% histological stenosis. Thus, the early and advanced stages of the atherosclerotic process have been investigated in the present study.
Losordo et al6 compared the IEL areas of cross sections obtained from an atherosclerotic femoral artery segment and a nondiseased, proximally obtained cross section and found an increase of the IEL area at the atherosclerotic cross section compared with the nondiseased site. In our IVUS data set, no significant increase was observed between the IEL area of a proximally or distally located cross section with the least amount of plaque and the cross section with the largest plaque load. An explanation for these conflicting results may be that in the present study, the IEL area of a reference site, ie, the cross section with the least amount of plaque, was compared with the IEL area in the cross section with the largest plaque load and a substantial luminal narrowing. In contrast, Losordo et al6 did not specifically use the cross section with maximal plaque load or maximal luminal narrowing for comparison. When a cross section with maximal plaque load and significant luminal narrowing is used, it is to be expected that in addition to compensatory enlargement, paradoxical shrinkage may be observed.7 Thus, the group of cross sections with the largest plaque load is apparently comprised of two subgroups of cross sections with different types of remodeling: compensatory enlarged and shrunken.
Individual Variability
The present results demonstrate that in individual femoral
artery segments, a local increase of the IEL area may be observed in
response to plaque accumulation. However, in only 17 of 45 segments
(39%), a significant correlation between plaque area and IEL area was
observed, implying that both local and systemic factors may play an
important role in the process of arterial enlargement.
Thus, local variation in arterial wall remodeling (ie,
compensatory enlargement or paradoxical shrinkage) may be observed
despite the fact that on average, the artery enlarges or fails to
enlarge. In some arterial segments, a slope of the
regression line was calculated with a large 95% CI, suggesting that
either a large variation in arterial size or a nonlinear
relation was present. From the individual data point plots, we
inferred that the former was generally the case.
It has been suggested that eccentric lesions, with a partly disease-free wall, are more prone to compensatory enlargement.1 In contrast to concentric lesions, the disease-free wall in an eccentric lesion can still show endothelium-dependent dilation11 in response to a flow increase.12 However, it is still unknown whether a relation exists between the type and degree of arterial remodeling and lesion eccentricity.
A difference in IEL area was observed between
arterial segments that demonstrated a significant
correlation between plaque area and IEL area and those that did not.
The reason for the difference in arterial size among groups
is unknown. Hemodynamic changes may occur sooner in
small vessels compared with large vessels exposed to the same amount of
plaque. Therefore, the compensatory adaptive mechanism of the vessel to
enlarge may be enhanced in small vessels. Our findings are supported by
Zarins et al,2 who observed that compensatory enlargement
is more pronounced in the small distal coronary artery compared
with the large proximal parts of the coronary artery. Another
explanation is that the group without a significant correlation may
comprise arterial segments in a later stage of
atherosclerosis that may subsequently be maximally
enlarged. However, for the cross sections with the smallest plaque area
and IEL area, the percentage of IEL area occupied by plaque did not
differ between arterial segments with and without a
significant correlation between plaque and IEL area (Table 3
). This
implies that the arterial segments were in the same stage
of the atherosclerotic process.
Study Limitations
A number of potential limitations of the present study need to
be addressed. Only 4 of the postmortem arteries were pressure fixed. As
a result, the lumen area and IEL area may be underestimated in 21
nonperfusion-fixed arterial segments. However,
the results obtained with histology are corroborated by the results
obtained with IVUS studies.
Arterial tapering might influence the differences in IEL area between cross sections in one segment. However, the IEL area in the most proximal and most distal cross sections did not differ significantly (30.0±11.9 versus 28.9±10.4 mm2 and 27.8±12.2 versus 26.4±10.8 mm2 for IVUS and histology, respectively).
The present study was performed in the superficial femoral artery. Therefore, these findings may not be generalizable, for example, to the coronary circulation.
Arterial remodeling occurs over time. In the present study, the IEL area was related to the plaque area through the use of different points in the same artery with the assumption that the different stages of the atherosclerotic process within one artery are representative for changes in vessel size over time. Future serial studies are needed to determine whether this assumption is correct.
Clinical Implications
Understanding of the mechanisms responsible for local and
individual variabilities of arterial wall remodeling may
aid in the development of new therapeutic strategies to prevent luminal
narrowing by de novo atherosclerotic lesions. The remarkable local and
individual variabilities in arterial remodeling in response
to plaque accumulation may have a bearing on the early and late results
of angioplasty.
Conclusions
A significant positive correlation between plaque area and IEL
area was observed in the minority of the superficial femoral arteries.
Arterial remodeling in response to plaque accumulation
appears to vary among individuals. The mechanisms of local
atherosclerotic arterial remodeling remain to be
investigated.
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| Acknowledgments |
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| Appendix 1 |
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From Fig 4C
, it might be inferred that in the plaque area range of 5.5
to 8.5 mm2, the artery expands by 5.2
mm2 for every 1-mm2 increase in plaque area.
This may be interpreted incorrectly as being a result of gross
overcompensation in artery size in response to plaque accumulation.
With more plaque accumulation, the slope of the regression line
decreases and reaches 1.0 when total occlusion occurs without any
change in IEL area. The slope of the regression line of IEL area versus
plaque area becomes <1.0 if and only if plaque accumulates without
100% compensatory enlargement in vessels of the same original size.
The influence of (over)compensatory enlargement is due to the
substantial bias introduced by pooling single measurements from
different vessels with originally different sizes (ie, without plaque).
Implicit to the current interpretation of the regression line of IEL
area versus plaque area by use of pooled data1 is the
assumption that the arteries originally (before the onset of
atherosclerosis) had the same size. This assumption
probably is unfounded. The bias introduced by arteries of originally
different size tends to amplify or introduce (Fig 4C
) compensatory
enlargement, as inferred from the plot of IEL area versus plaque
area.
A second approach was chosen to illustrate that the regression of
plaque area versus IEL area is confounded by the arterial
size. In addition to plaque area, two measures of arterial
size were added as variable in a multivariate
regression model: the mean IEL area of all cross sections of one artery
and the IEL area of the reference cross section of that artery. These
variables were determined for each artery and alternatively put
into the multivariate regression model. For both the
IVUS and histology data, we performed four subsequent regression
analyses: (1) multivariate regression with
plaque area and mean IEL area, (2) regression with plaque area, (3)
multivariate regression with plaque area and reference
IEL area, and (4) regression with plaque area, with the reference cross
section excluded. For each regression, the F statistic was
calculated. The difference between the F statistic from the
multivariate and (plaque area) monovariate models
quantifies the influence of arterial size on the
regression. Table 4
shows the results of these multiple
regression analyses. For all four multivariate
regression analyses, F statistics of plaque area and
mean IEL area or reference IEL area were larger than for plaque area
(P<.0001). Thus, arterial size as defined by
the average IEL area per artery or by the IEL area of the reference
cross section had more impact on the regression of IEL area on plaque
area than plaque area itself.
Received November 13, 1995; revision received December 8, 1995; accepted December 19, 1995.
| References |
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