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(Circulation. 1999;99:36-43.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
Functional Recovery of Subepicardial Myocardial Tissue in Transmural Myocardial Infarction After Successful Reperfusion
An Important Contribution to the Improvement of Regional and Global Left Ventricular Function
Presented in part at the 69th Scientific Sessions of the American Heart Association, New Orleans, La, November 10–13, 1996, and published in abstract form (Circulation. 1996;94[suppl I]:I-244).
From the Departments of Radiology (J.B., H.B., G.M.), Nuclear Medicine (A.M., J.N., L.M.), and Cardiology (F.V.d.W., M.-C.H., W.D., F.E.R.), Gasthuisberg University Hospital, Leuven, Belgium.
Correspondence to Frank Rademakers, MD, PhD, Department of Cardiology, Gasthuisberg University Hospital, University of Leuven, Herestraat 49, B-3000 Leuven, Belgium.
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Abstract |
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 Top Abstract IntroductionMethodsResultsDiscussionReferences |
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Background—The transmural extent of myocardial necrosis after an acute coronary artery occlusion can vary considerably. The contribution of residual subepicardial viable myocardium to global left ventricular function is largely unknown.
Methods and Results—We studied 12 patients with single-vessel disease 1 week after successful reperfusion of a first transmural anterior myocardial infarction (MI). With PET, myocardial blood flow (MBF) and glucose metabolism were measured regionally, and the viability was graded as normal, mismatch, or match with severely (<50% of normal) or intermediately (50% to 80% of normal) impaired MBF. Magnetic resonance tagging was used to regionally quantify fiber strains, wall thickening, and ejection fraction in patients 1 week and 3 months after the MI and in age-matched healthy volunteers. From 1 week to 3 months, subepicardial fiber shortening improved significantly in the match region (MBF <50%, -5.1±7.0% to -9.9±8.7%; MBF of 50% to 80%, -7.1±7.6% to -14.9±7.9%). This was associated with an improvement in regional ejection fraction in the infarcted myocardium (29.6±21.8% to 43.5±15.5%, P<0.0001) and in normal regions (54.3±15.1% to 56.5±13.1%, P=0.013), contributing to an increase in global ejection fraction from 44.2±22.2% to 49.3±17.9% (P<0.0001).
Conclusions—Functional recovery of viable subepicardial regions is a mechanism of late improvement in regional and global ejection fraction after a so-called transmural MI.
Key Words: myocardial infarction • magnetic resonance imaging • tomography • reperfusion
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Introduction |
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Several studies have demonstrated the presence of viable but ischemically compromised tissue in infarcted myocardium.1 In canine studies, the subendocardial lateral boundaries of a myocardial infarction (MI) are established within the first 40 minutes, whereas the MI enlarges in a transmural wave front over a period of 3 to 6 hours.2 In patients, the final lateral boundaries of the infarcted area closely correspond to the myocardium at risk, whereas a variable degree of transmural progression determines the final extent of necrosis.3 Islands of viable tissue, especially in the subepicardial layers, remain intermixed with necrotic cells, and if antegrade flow in the infarct-related artery is restored, a late improvement in myocardial perfusion and metabolism in this nonviable myocardium has been observed in humans.4 However, a possible concomitant functional improvement in these subepicardial layers of a transmural MI has not been studied in humans.
The purpose of the present study was to investigate transmural differences in functional recovery in relation to the degree of viability in patients with a first transmural MI.
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Methods |
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Patient Selection
Twelve patients with a first anterior MI were selected for the study; in all patients, the left anterior descending coronary artery could be successfully reperfused by either thrombolytic therapy or rescue angioplasty, and no significant lesions were present in other coronary arteries.
For comparison, we also studied a control group of 31 age-matched volunteers (age, 59.5±7.1 years) without evidence of cardiac disease.
Study Protocol
Both PET and magnetic resonance (MR) tagging studies were
performed at 5±2 days (range, 2 to 10 days) after the acute event
(1-week study). A repeated MR tagging study was performed after 3
months (range, 82 to 96 days; 3-month study).
Measurement of Regional Myocardial Viability With PET
All patients were studied by use of the
hyperinsulinemic euglycemic clamp
technique.5 Serial images were acquired during infusion of
20 mCi of [13N]NH3 in a
whole-body tomograph. Fifty minutes later, 10 mCi of
[18F]deoxyglucose (18FDG)
was injected, and serial images were recorded for 70 minutes.
A 3-dimensional (3D) delineation of the left ventricular (LV) wall was used to construct a polar map (33 regions: 1 apical region and 4 rings of 8 regions each) for every frame of the dynamic study.6
A flow index was calculated as the ratio of
[13N]NH3 uptake in each
region over the [13N]NH3
uptake in the region with the highest uptake (reference region). The
same anatomic region was used as the reference region for
18FDG. A metabolic index was defined
as the ratio of the glucose use in each region over that in the
reference zone. Regions with a flow index of >80% were considered
normal. In the remaining regions, a flow-metabolism
mismatch pattern was assumed if the ratio of metabolism to
flow was >1.2 and a match pattern if this ratio was
1.2.7 A mismatch pattern was considered viable
myocardium, whereas a match pattern on PET was considered
infarcted myocardium.8 The
myocardium with a match pattern was further divided into
regions with a severely depressed myocardial blood flow (MBF) (MBF
<50% of normal) or an intermediately depressed MBF (MBF between 50%
and 80% of normal).9
Measurement of Regional Myocardial Function With MR
Tagging
All MR tagging studies were performed on a 1-T MR unit with a
segmented k-space FLASH gradient-recalled echo sequence with
acquisition of 3 k-lines per heart beat (repetition time, 14 ms;
echo time, 8 ms; flip angle, variable; field of view, 400 mm;
matrix, 180x256; slice thickness, 8 mm).
End-diastolic and end-systolic time points were
acquired.
MRI with tagging has been used to mark sites in the
myocardium noninvasively, to subsequently image the LV at
different times during the cardiac cycle,10 and to
calculate the normal and shear strains of the
myocardium.11 A 3D deformation
analysis of the myocardium can be performed with
the use of perpendicular short- and long-axis images.12
The LV myocardium was thus divided into 32 small cuboids
encompassing the entire LV except for the apex (Figure 1
).
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Strain Computations
The images were processed by a dedicated contouring system.
After identification of the intersections between the tags and the
epicardial and endocardial contours, the long- and short-axis
coordinates were merged to obtain 1 unique set of xyz
coordinates for each time point. Next, translation of xyz
data was performed to a local fiber coordinate system. The new axes
were radial (R) using the perpendicular direction to the wall; fiber
(F), tangent to the surface and parallel to the local fiber orientation
at either the epicardium or endocardium; or cross fiber (X), tangent to
the surface and perpendicular to F. Fiber directions were obtained from
histological fiber angle data in cadaver
studies.13 14 15 Normal and shear strains were
computed from the displacements from end diastole to end
systole. Positive radial strains represented wall
thickening; negative strains, wall thinning; and negative fiber
strains, quantified shortening of the myocardium along the
local direction of the actively contracting fibers. Positive strains
represented fiber lengthening (usually not present in
normal myocardium); cross-fiber strains were
representative of the deformation of the
myocardium perpendicular to the fiber orientation and were
related to interaction with fibers at a distance.12
Regional and Global Ejection Fraction
The regional ejection fraction quantified the amount of
intracavitary blood ejected by each cuboid during systole by use of the
intracavitary volume delimited by the converging tag lines and each
cuboid. Furthermore, a global LV ejection fraction was obtained by a
summation of the regional ejection fraction in each of the 32 cuboids
(thereby excluding the apex) and from the 4-chamber long-axis images by
use of the area-length method as is done in
echocardiography.
Myocardial Wall Thickness
The true myocardial wall thickness was obtained in a 3D
fashion by adjusting the tag length for wall curvature in the
longitudinal direction.
Matching the Data From PET and MR Tagging
Regional analysis of myocardial viability, strain,
and function was performed similarly. The lateral walls were divided
into 4 levels, each consisting of 8 segments. Matching of the MR
tagging and PET results was accomplished by use of anatomic landmarks.
PET results for the LV apex were omitted because MR tagging did not
cover this region.
Reproducibility of Repeated MR Tagging Studies
To assess the degree of reproducibility between repeated MR
tagging studies, 2 MR tagging studies were performed on consecutive
days in 5 healthy volunteers. The percentage of variability, calculated
as the absolute value of the difference between 2 measurements divided
by the mean value, and SD were used to study the interstudy
variability.
Statistical Analysis
All data are expressed as mean±SD. All data were normally
distributed except regional ejection fraction, which showed a slight
skew. Comparisons were performed with Student's t test for
paired or unpaired comparisons, a Wilcoxon signed rank, or a
multiple ANOVA with Scheffé's test when appropriate.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Patient Characteristics and Angiographic Findings
Twelve patients, 10 men and 2 women (age, 62±8 years [mean±SD]) were studied (Table 1
).
Between the 1-week and 3-month studies, there were no significant
differences in heart rate or blood pressure (72±13 bpm and
146±13/84±8 mm Hg at the 1- week study versus 70±11 bpm and
147±13/83±7 mm Hg at the 3-month study; P=NS).
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All patients received thrombolysis within 6 hours after
the onset of symptoms. In 3 patients, a rescue angioplasty was
performed because of failed thrombolysis. Evolution of
ECG and enzymatic parameters were indicative of successful
reperfusion in all patients. Nevertheless, all patients developed new Q
waves in the anterior leads and had significant positive cardiac
enzymes (Table 1
).
In 6 patients, an elective angioplasty of the residual stenosis
was performed at 1 to 5 days after initial reperfusion but before the
MR tagging and PET studies (Table 1).
Regional Viability on PET
For a total of 32 regions, a normal flow pattern was found in
18.4±4.7 regions, a mismatch pattern was found in 2.5±2.1 regions,
and a match pattern with an MBF between 50% and 80% was seen in
6.8±2.6 regions and with an MBF <50% in 4.3±4.5 regions (Table 2).
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Regional LV Function at 1 Week
At 1 week, all strain parameters were significantly
more impaired in areas with a match pattern than in areas with a
mismatch pattern or in the normal, remote myocardium
(P<0.0001 for all) (Table 3). In the matched
myocardium, the functional damage of the
myocardium was similar for all layers. Subepicardial
cross-fiber strain was the most impaired, with mean positive values
indicating substantial systolic bulging; negative values of
thickening correspond to regional wall thinning during systole. The
result was a significant decrease in ejection fraction most pronounced
for the matched myocardium (MBF <50%, 20.9±18.9%; MBF
of 50% to 80%, 29.6±21.8%), gradually improving over the mismatch
to the normal flow regions. Compared with control subjects, however,
even these remote normal regions showed a diminished function
(54.3±15.1% in the "normal" myocardium versus
65.5±10.0% in control subjects; P<0.0001).
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Compared with healthy control subjects, a significant increase in LV
end-diastolic and end-systolic volumes was found
(103±30 and 62±27 mL in patients with acute MI [Table 4] versus 84±16 and 30±10 mL in
control subjects; P=0.0077 and P<0.0001,
respectively). In the first week after the acute event, the LV
end-diastolic wall thickness was significantly larger in
all regions compared with healthy volunteers (P<0.0001 for
all), possibly because of a preexisting hypertrophy or an
early hypertrophic remodeling.
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Functional Recovery at 3 Months
Fiber strains in the matched myocardium showed more
recovery than those in the mismatched and normal regions. This was most
pronounced for the subepicardial region in which both fiber and
cross-fiber strains improved significantly; the increase was most
striking in the region with an MBF between 50% and 80%. In the
subendocardial layers, improvement was also present for the matched
myocardium but only in the region with an MBF between 50%
and 80%. In the normally perfused, remote areas, subendocardial fiber
shortening and cross-fiber shortening increased significantly (Figure 2). Wall thickening did not change
significantly between 1 week and 3 months in either region, but the
increased fiber contractions at the subepicardium resulted in more
epicardial inward motion and a very significant increase in regional
ejection fraction from 29.6±21.8% to 43.5±15.5%
(P<0.0001) (Figure 3). The
other regions manifested no or only a small increase.
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Wall thickness at end diastole decreased significantly in all regions from 1 week to 3 months but remained higher than in the control population.
Global LV ejection fraction (summation of all the cuboids) was 44.2±22.2% at 1 week (compared with 46±8% from LV angiography), increased to 49.3±17.9% at 3 months (P<0.0001), but remained significantly lower than in healthy control subjects (65.5±10.0%; P<0.0001 for both). The values obtained with the area-length method showed similar trends (41±11% to 49±7%; P<0.05). The LV end-diastolic volume increased from 103±30 to 114±27 mL (P=0.0085), whereas no significant changes were found for LV end-systolic volume (62±27 versus 59±18 mL; P=0.34).
Reproducibility of Repeated MR Tagging Studies
No statistically significant differences were found between the 2
measurements. The interstudy variability for the ejection fraction was
4.7±1.9% and varied between 3.7±1.8% and 10.7±2.6% for the fiber
strains.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Combined PET-MR Tagging Approach
Several approaches have been proposed to detect the degree of myocardial viability and functional recovery after MI.4 16 17 18 19 We investigated patients with a first anterior transmural MI, combining PET and MR tagging, to compare regional function with the level of flow and metabolism.
Early Findings
Our findings at 1 week show that contractile function of the
infarcted region was significantly impaired. In addition, in the
mismatch and normal areas, a depressed function compared with control
subjects was present. These findings confirm previous research
showing a functional impairment of noninfarcted
myocardium,20 which is very likely related to
increased wall stress,21 although a vasomotor dysfunction
may also contribute.22
Late Recovery
Recovery at 3 months is present mainly in the match
region with an MBF of 50% to 80% and can be related
to improved fiber contraction in the subepicardial layers of this part
of the infarct territory. All infarctions in this study could be
defined as transmural infarctions. Nevertheless, a functional recovery
was clearly demonstrated even in the matched myocardium
with severely reduced MBF. Because we looked at deformation of the
myocardium in the direction of the actively contracting
fibers, this study strongly suggests the presence of viable myocardial
tissue in the subepicardial layers of a transmural MI, not detectable
by PET, which gradually recovers after the acute event. In the matched
myocardium with a severe reduction in MBF, functional
recovery in the subepicardial layers, however, is insufficient to
improve regional ejection fraction; this can also be inferred from the
limited increase in subendocardial cross-fiber shortening. These
regions very likely correspond to the myocardium showing
the largest infarct "transmurality," whereas matched regions with
intermediately reduced MBF are representative of less
severe transmural infarct extensions. Although the results of this
study could not be compared with patients in whom reperfusion failed,
restoration of blood flow with salvage of the subepicardial fibers is
the most likely mechanism underlying this functional recovery, and even
when regional ejection fraction is not increased, improvement in
subepicardial contraction could limit infarct expansion and development
of an aneurysm.
Detection of the most marked cross-fiber recovery in the subendocardium of the matched region with an MBF of 50% to 80% indirectly supports the finding of a larger improvement in active fiber contraction in the subepicardium of the same region caused by myocardial tethering.12 23 The smaller but significant recovery of the subendocardial fiber shortening in the matched myocardium with an MBF between 50% and 80% suggests the presence of subendocardial islands of viable myocardial tissue.
The apparent contradiction of recovery of regional ejection fraction without an improvement in wall thickening in the infarct region suggests that the mechanism for ejection relies on both wall thickening and epicardial inward motion. If epicardial function recovers with increased deformation and inward motion, the epicardium can push the more endocardially located layers toward the cavity without a significant increase in wall thickening but nevertheless with an improved regional ejection fraction.
Absence of a significant functional recovery between 1 week and 3 months in myocardial regions showing a mismatch pattern at 1 week is unexpected. These regions are the least represented in these patients with an aggressive reperfusion strategy and at the 1-week study already show a function comparable to the normal, remote areas, which leaves little room for further improvement. They could contain both normal and ischemic or necrotic tissue in various proportions. Also, increases in wall stress in these mismatch areas at the edge of the infarction could explain the lack of recovery of these load-dependent functional parameters. Further study in patients with larger areas of mismatch myocardium is needed.
In the normal myocardium, a small but significant recovery in regional ejection fraction was demonstrated as a consequence of an improved subendocardial fiber and cross-fiber shortening. Because this region has normal myocardial flow, the most likely mechanism is an improved stress-strain relation as a consequence of the functional recovery in the infarct region. Lowering the wall stress in noninfarcted myocardium represents a favorable remodeling after thrombolytic therapy. Compensatory hyperkinesis in the noninfarcted region seems less likely because this region has significantly lower myocardial strains compared with a control group.
Study Limitations
Number of Patients
The number of patients included in this study is small, mainly
because of the elaborate study protocol. By analyzing the changes in
strains in every segment from 1 week to 3 months, we obtained
statistically significant results for those regions that contain a
sufficient number of segments and are equally distributed among
patients. This is clearly the case for the normal and match regions
with MBF of 50% to 80%, less so for the match regions with MBF
<50%, and not so for the mismatch regions (Table 2). The
results obtained in the mismatch regions, therefore, have to be
interpreted with caution. When segments are unevenly spread among
patients, the magnitude of the results could also be influenced.
Reliability of MR Myocardial Tagging
The accuracy of MR myocardial tagging to study myocardial
deformation has been validated by means of solid and deformable phantom
models.24 25 The technique allows precise quantification
of complex 3D motion and deformation patterns, and the results
correspond very well with those obtained by invasive methods that use
metallic markers sewn in the
myocardium.12 26 27 Variability percentages
were small and comparable with previous reports that used MR imaging to
quantify LV parameters.28 So although there is
ample evidence to support the intrinsic accuracy of the technique, the
limitations of the present study are introduced by the need to
match MRI and PET data and the imprecision of matching serial MRI
studies. Although matching was optimized using the same distribution of
regional segments and aligning anatomic landmarks, some degree of
malalignment cannot be discarded, affecting primarily the border
zones.
Matching serial MRI studies with different LV volumes is another problem. Because dilatation of the ventricle usually is not homogeneous and the tag distribution is, we could not completely be certain that we had matched regions between the first and second MRI examinations. The mismatch error is in principle limited to regions in which changes in volume or shape are very localized, but we saw no large aneurysms on control MRI or echo studies.
Quantification of LV Ejection Fraction
Global ejection fraction was rather high for patients with a
transmural anteroapical infarction. This is very likely due to
exclusion of the LV apex. When the volumetric or angiographic data were
used for ejection fraction calculation, consistently smaller LV
ejection fractions were obtained.
Use of Fiber Strains
Although the calculation of fiber and cross-fiber shortening could
suffer from the use of cadaver measurements rather than actually
measured pathological fiber angles, use of fiber strains was preferred
over principal or local cardiac strains because this greatly enhances
the understanding of underlying mechanical phenomena.15
Changes in principal and local cardiac strains showed completely
consistent results with changes primarily in the
subepicardium.
Conclusions
This study for the first time relates regional functional
impairment of a first transmural anterior MI to the degree of viability
and shows that recovery of subepicardial fibers of a transmural infarct
region significantly contributes to the late improvement in regional
and global LV function. Whereas early reperfusion is undoubtedly
extremely important for limiting overall infarct size, restoration of
flow in the infarct-related vessel can also preserve fibers in the
subepicardial and lateral border zone of a transmural infarction.
Although similar studies in patients in whom reperfusion failed are
needed, we can speculate that the absence of reperfusion and subsequent
recovery of the subepicardial region of a transmural infarct region
will lead to infarct expansion, ventricular remodeling, and
possibly aneurysm formation.
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Acknowledgments |
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This study was supported by a grant from the Fonds voor Wetenschappelijk Onderzoek (No. G 3132.94). We furthermore would like to thank the Interdisciplinary Research Unit for Radiological Imaging (P. Suetens, PhD, chairman).
Received February 2, 1998; revision received September 4, 1998; accepted September 16, 1998.
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J. B. Selvanayagam, A. Kardos, J. M. Francis, F. Wiesmann, S. E. Petersen, D. P. Taggart, and S. Neubauer Value of Delayed-Enhancement Cardiovascular Magnetic Resonance Imaging in Predicting Myocardial Viability After Surgical Revascularization Circulation, September 21, 2004; 110(12): 1535 - 1541. [Abstract] [Full Text] [PDF]
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P. Colonna, R. Montisci, L. Galiuto, L. Meloni, and S. Iliceto Effects of Acute Myocardial Ischemia on Intramyocardial Contraction Heterogeneity : A Study Performed with Ultrasound Integrated Backscatter During Transesophageal Atrial Pacing Circulation, October 26, 1999; 100(17): 1770 - 1776. [Abstract] [Full Text] [PDF]
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G. K. Lund, C. B. Higgins, M. F. Wendland, N. Watzinger, H.-J. Weinmann, and M. Saeed Assessment of Nicorandil Therapy in Ischemic Myocardial Injury by Using Contrast-enhanced and Functional MR Imaging Radiology, December 1, 2001; 221(3): 676 - 682. [Abstract] [Full Text] [PDF]
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