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From the Heart Lung Institute, Department of Cardiology (J.A.G.M.C., G.P., A.H.S., T.G. van L., C.B.); the Department of Functional Anatomy (B.H.); and Utrecht University Hospital and Interuniversity Cardiology Institute of the Netherlands (G.P., A.H.S., T.G. van L.), Utrecht, the Netherlands.
Correspondence to Gerard Pasterkamp, MD, Heart Lung Institute, Utrecht University Hospital, Heidelberglaan 100, Room G.02-523, 3584 CX, Utrecht, The Netherlands. E-mail g.pasterkamp{at}hli.azu.nl
| Abstract |
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Key Words: remodeling atherosclerosis eccentricity coronary arteries femoral arteries
| Introduction |
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Insight into the mechanism of arterial remodeling in de novo atherosclerosis is of particular interest, since recent studies have demonstrated that failure of enlargement or even shrinkage3 12 may seriously enhance arterial luminal narrowing. To study the postulated role of the disease-free part of the atherosclerotic arterial wall in compensatory enlargement, the degree of remodeling was related to the circumferential extent of the nondiseased vessel wall and to lesion eccentricity in both the coronary artery and the superficial femoral artery.
| Methods |
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5 to
10 cm from their point of origin. The LAD from 1 patient was not
included due to the presence of severe calcifications in the
artery. People had donated their bodies by will according to the Dutch
Anatomy Act. To minimize any influence of anatomic tapering of the femoral artery, segments 10 to 15 cm long were selected from 12 cm proximal of the adductor hiatus to 3 cm distal of the adductor hiatus. In this area no major branches originated. Transverse cross sections were obtained every 2.5 mm from the coronary arteries and every 5.0 mm from the femoral arteries, stained with Lawson's elastic tissue stain, and studied under magnification. The first cross section from a coronary segment was taken 5 mm from the origin. All microscopic images of the cross sections were recorded on VHS videotape with a Sony video camera (3 CCD) for further analysis. A ruler was used for distance calibration.
Image Analysis
The cross sections recorded on videotape were
analyzed with a digital video analyzer as described
previously.13 To determine the degree of remodeling from
each cross section, we measured the luminal cross-sectional area, the
area encompassed by the IEL. For all coronary and femoral cross
sections, we calculated the relative IEL area (ie, degree of
remodeling). For the LAD, LCX, and RCA, we calculated the mean IEL area
at each location of the corresponding arterial segments. To
obtain reference values for the IEL area of cross sections from the
coronary arteries (which values were corrected for
arterial tapering), we performed a linear regression
between the mean IEL area and the distance of the arterial
segment. For each coronary cross section, the relative IEL area
was calculated by using the IEL area on the corresponding regression
line at the same location as the reference value (statistically
expected IEL area).
The relative coronary IEL area was calculated as IELarea/IELexpected areax100. In each femoral segment, the cross section that contained the least amount of plaque was chosen as the reference site. Every 5.0 mm, the cross section was compared with this reference site. The relative femoral IEL area was calculated as IELarea/IELarea at reference sitex100. A relative IEL area >100% indicates that the arterial segment was enlarged with respect to the corresponding value on the regression line (coronary arteries) or with respect to the reference site (femoral arteries); a relative IEL diameter <100% indicates that the arterial wall was shrunken with respect to the corresponding value on the regression line or with respect to the reference site.
To calculate an eccentricity index, we measured the maximal plaque
thickness (Plmax), the minimal plaque thickness (Plmin), the vessel
diameter (D1, the vessel diameter at the location of maximal plaque
thickness [see Fig 1
]) and the vessel
diameter perpendicular to vessel diameter D1 (ie, D2) from each cross
section. For all coronary and femoral artery cross sections, an
eccentricity index
(Fig 1
) was calculated as AC/AB (Equation 1).
Substitution of the measured distances D1, Plmax, and Plmin in Equation
1 yields
![]() | (2) |
![]() | (3) |
1. An eccentricity index of exactly 1 indicates
an arterial segment with the center of the lumen in the
center of the vessel. An eccentricity index >1 indicates an
arterial segment in which the center of the lumen is
shifted from the center of the vessel.
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To determine the extent of plaque-free vessel wall in each cross section, we quantified the circumferential extent of the plaque-free vessel wall, which was expressed semiquantitatively in hours (12 hours=360°=total circumference; 1 hour=30°). The plaque-free vessel wall was defined in one of two ways: (1) no plaque present or (2) plaque thickness <0.5 mm. The plaque cross-sectional area was calculated by subtracting the luminal area from the IEL area.
All cross sections were categorized into three separate groups as described earlier14 : cross sections with a relative IEL area <95% (shrunken lesions), cross sections with a relative IEL area of 95% to 105% (neither shrunken nor enlarged lesions, ie, unremodeled lesions), and cross sections with a relative IEL area >105% (enlarged lesions).
Statistical Analysis
Data are presented as mean±SD. A linear regression
analysis was performed between the relative IEL area and the
eccentricity index, as well as between the relative IEL area and the
plaque-free vessel wall. Vessel diameters D1 and D2, the extent of the
plaque-free vessel wall by both definitions, and the eccentricity index
were calculated for each group of lesions. An ANOVA with post hoc a
Duncan range test was performed for comparison of all values between
the three separate remodeling groups. Differences were considered
significant if P<.05.
| Results |
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Very weak negative relations between the relative IEL area and extent
of the circumference of the plaque-free vessel wall were observed for
the coronary arteries (no plaque:
r2=.13, P<.01; <0.5 mm plaque:
r2=.15, P<.05) and for the femoral
arteries (no plaque: r2=.02, P<.01;
<0.5 mm plaque: r2=.04, P<.01;
Fig 2
). Fig 3
shows the absence of any relation between
the relative IEL area and the eccentricity index (coronary
arteries, r 2=.002, P>.05 and
femoral arteries, r2=.001,
P>.05).
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| Discussion |
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If expansion of the nondiseased part of the arterial wall is responsible for compensatory enlargement,1 3 then enlarged lesions are expected be eccentric. In concentric lesions, on the other hand, enlargement is not expected to occur. In the present study, we hypothesized that enlarged lesions would have a larger circumference of nondiseased arterial wall and be more eccentric than nonenlarged or shrunken lesions. The principal findings are that in both the coronary and femoral artery, the relative IEL area did not correlate with the eccentricity index; relative IEL area was correlated very weakly to the extent of plaque-free vessel wall. The correlation, however, is regarded of little interest because the correlation coefficient was close to zero. In addition, the slope of the regression line appeared to be negative, implying that (in contrast to our hypothesis) enlarged segments have a smaller circumference of nondiseased vessel wall than do shrunken segments. Therefore, the present results do not support the hypothesis that expansion of the nondiseased vessel wall is the mechanism of compensatory enlargement in response to plaque accumulation.
Endothelial dysfunction occurs early in the atherosclerotic process.9 11 15 It may be possible that part of the arterial wall histologically appears nondiseased (ie, plaque-free) but has a dysfunctional endothelial layer. The endothelium contributes to the regulation of vascular tone by the release of endothelium-derived vasodilative substances, such as endothelium derived relaxing factor,16 17 18 and endothelium-derived vasoconstrictive substances, such as endothelin.19 20 So it is conceivable that the plaque-free vessel wall did not play any role in arterial wall remodeling because of impaired endothelial function.
Other mechanisms responsible for enlargement may be considered: enlargement of an arterial segment might be achieved by the degrading effect of the developing plaque on the underlying arterial wall.21 In human atherosclerotic lesions, focal overexpression of activated matrix metalloproteinases (ie, a family of enzymes that degrade extracellular matrix)22 may promote weakening of the underlying arterial wall. Induction of enlargement may be a therapeutic strategy. If failed enlargement or even shrinkage is caused by the altered release of vasodilative substances by the dysfunctional endothelium, then drug induced functional recovery of the affected endothelium may restore the capacity of the arterial wall to expand in response to plaque accumulation.
Limitations and Methodological Considerations
The fixation process may alter vessel size, which might explain
the observed local differences in relative IEL area. However, we
previously3 observed identical relations between the
degree of remodeling and the degree of lumen area stenosis in
cross sections obtained post mortem and cross sections obtained by in
vivo intravascular ultrasound. Therefore, it is unlikely that local
shrinkage during fixation fully explains our observations. In addition,
by relating a cross section to a reference site from the same
arterial segment, interarterial differences due
to the fixation procedure can be excluded.
We related the degree of arterial remodeling to the extent of the nondiseased vessel wall and lesion eccentricity at one moment in time. Although the present study does not support the hypothesis that compensatory enlargement is achieved by expansion of the nondiseased vessel wall, we cannot exclude the possibility that expansion of the nondiseased vessel wall was responsible for enlargement. Serial intravascular ultrasound studies are required to test the hypothesis that enlargement is achieved by expansion of the nondiseased wall.1 3 In de novo atherosclerosis, however, the time interval required between observations is too long to be practical.
The extent of the nondiseased part of the vessel wall was quantified by using Lawson's tissue stain, which provides no information on potential dysfunction of wall layers, in particular the endothelium.9 11 15 Hence, it is conceivable that plaque-free vessel wall in an atherosclerotic vessel should not be classified as nondiseased.
It may be expected that early in the atherosclerotic process, the vessel wall may respond adequately to plaque accumulation by enlargement of the arterial wall. However, the present study does not allow us to draw conclusions regarding the time sequence of both remodeling types: enlargement and shrinkage. To determine whether compensatory enlargement is a phenomenon that occurs early and/or late in the atherosclerotic process, serial intravascular ultrasound studies are necessary.
We considered eccentricity index as a continuous variable; however,
it is possible to consider eccentric versus concentric lesions in a
dichotomous approach. To categorize all lesions as eccentric or
concentric, a cutoff point in the eccentricity index is needed. It may
be questionable, however, whether a lesion with an eccentricity index
of 1.5 is a concentric or eccentric lesion. From our results, no
correlation was found between the degree of remodeling and the
eccentricity index. Therefore, no difference in the degree of
remodeling between the two artificial categories of concentric and
eccentric lesions may be expected. When concentric lesions were defined
as those with an eccentricity index <x and eccentric
lesions as lesions with an eccentricity index >x, then no
significant differences in the relative IEL area between concentric and
eccentric lesions were observed for either the coronary
(P>.05) or femoral (P>.05) arteries if
x
1.5.
Conclusion
Atherosclerotic arterial wall remodeling was related
to neither the extent of the nondiseased part of the vessel wall nor
lesion eccentricity. This study provides no support for the hypothesis
that expansion of the nondiseased vessel wall is responsible for
arterial compensatory enlargement in response to plaque
accumulation.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received February 19, 1997; accepted July 3, 1997.
| References |
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