Structure And Mechanics of Healing Mi
-
Upload
rick-green -
Category
Documents
-
view
219 -
download
0
Transcript of Structure And Mechanics of Healing Mi
-
7/31/2019 Structure And Mechanics of Healing Mi
1/32
Annu. Rev. Biomed. Eng. 2005. 7:22353doi: 10.1146/annurev.bioeng.7.060804.100453
Copyright c 2005 by Annual Reviews. All rights reservedFirst published online as a Review in Advance on February 22, 2005
STRUCTURE AND MECHANICS OF HEALINGMYOCARDIAL INFARCTS
Jeffrey W. HolmesDepartment of Biomedical Engineering, Columbia University, New York, NY 10027;
email: [email protected]
Thomas K. BorgDepartment of Cell and Developmental Biology and Anatomy, University of South
Carolina, Columbia, South Carolina 29208; email: [email protected]
James W. CovellDepartments of Medicine and Bioengineering, University of California San Diego,
La Jolla, California 92093; email: [email protected]
Key Words collagen, constitutive properties, cross-linking, deformation, edema,
scar, strain, stress, necrosis, ventricular function Abstract Therapies for myocardial infarction have historically been developedby trial and error, rather than from an understanding of the structure and function of thehealing infarct. With exciting new bioengineering therapies for myocardial infarctionon the horizon, we have reviewed the time course of structural and mechanical changesin the healing infarct in an attempt to identify key structural determinants of mechanicsat several stages of healing. Based on temporal correlation, we hypothesize that normalpassive material properties dominate the mechanics during acute ischemia, edemaduring the subsequent necrotic phase, large collagen fiber structure during the fibroticphase, and cross-linking of collagen during the long-term remodeling phase. We hopethese hypotheses will stimulate further research on infarct mechanics, particularlystudies that integrate material testing, in vivo mechanics, and quantitative structuralanalysis.
CONTENTS
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
IMPACT OF INFARCT MECHANICAL PROPERTIES ON
VENTRICULAR FUNCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
ACUTE ISCHEMIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227Structural Changes During Acute Ischemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
Changes in Mechanical Properties During Acute Ischemia . . . . . . . . . . . . . . . . . . . 228
Determinants of Mechanics During Acute Ischemia . . . . . . . . . . . . . . . . . . . . . . . . 230
Ventricular Function During Acute Ischemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
2/32
224 HOLMES BORG COVELL
THE NECROTIC PHASE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
Structural Changes During the Necrotic Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
Changes in Mechanical Properties During the Necrotic Phase . . . . . . . . . . . . . . . . 236
Determinants of Infarct Mechanics During the Necrotic Phase . . . . . . . . . . . . . . . . 237
Ventricular Function During the Necrotic Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
THE FIBROTIC PHASE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
Structural Changes During the Fibrotic Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
Changes in Mechanical Properties During the Fibrotic Phase . . . . . . . . . . . . . . . . . 240
Determinants of Infarct Mechanics During the Fibrotic Phase . . . . . . . . . . . . . . . . 242
Ventricular Function During the Fibrotic Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
THE REMODELING PHASE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243
Structural Remodeling of Myocardial Scar Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . 243
Changes in Mechanical Properties During the Remodeling Phase . . . . . . . . . . . . . 243
Determinants of Infarct Mechanics During the Remodeling Phase. . . . . . . . . . . . .
244Ventricular Function During the Remodeling Phase . . . . . . . . . . . . . . . . . . . . . . . . 244
SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
INTRODUCTION
Each year, approximately 565,000 Americans experience a new myocardial in-
farction; of these, 75% of men and 62% of women survive for at least one year (1).
In addition, each year nearly 300,000 Americans experience a recurrent infarction
(1). As a result, a large portion of the practice of clinical cardiology is currentlydevoted to management of patients with a healing or healed myocardial infarct.
Excellent progress has been made, particularly in the areas of revascularization
during the first hours following infarction (2, 3) and pharmacologic therapy to limit
adverse geometric remodeling of the left ventricle (LV) and progression to dilated
heart failure (46). Even more dramatic therapies are on the horizon. Direct stem
cell transplantation into the healing infarct is already in use as an experimental
therapy (710), and tissue-engineered replacement patches of myocardium may
not be far behind (11, 12).
However, these therapies continue to be developed primarily on a trial-and-errorbasis rather than from an understanding of the mechanical properties of the healing
infarct and its coupling to the LV. This trial-and-error approach has led not only
to some dramatic successes but also to some catastrophic failures. For example,
preliminary evidence that steroid administration limits postinfarction necrosis led
to a trial of postinfarction steroid therapy in which high-dose steroid adminis-
tration caused dramatic increases in infarct size and the incidence of ventricular
arrhythmias, and in which 5 of 12 patients in the high-dose group died (13).
This review, therefore, has two primary goals. The first goal is to review what is
known about the evolving structure and mechanics of healing myocardial infarcts.The second goal is to temporally correlate structural and mechanical information
from a range of studies to formulate hypotheses about which specific structural
features are the primary determinants of infarct mechanics during each temporal
phase of infarct healing. It is our hope that this new analysis of the temporal
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
3/32
INFARCT STRUCTURE AND MECHANICS 225
course of infarct healing in terms of key structural determinants will stimulate new
research on the mechanics of healing infarcts and provide a conceptual platform
for improved rational design of postinfarction therapies.
This review focuses on the structure and mechanics of healing infarcts followinga single, nonreperfused myocardial infarction, and is organized as follows. First,
we outline the different mechanisms by which the presence of a myocardial infarct
may impair ventricular function. This list includes many of the potential adverse
consequences of myocardial infarction, including rupture, infarct expansion, ven-
tricular remodeling, hypertrophy, and heart failure, the occurrence and severity of
which all depend on the mechanical properties of the healing infarct. The next
four sections address different temporal phases of healing and each has the same
general format: a review of the composition and structure of healing infarcts at
that time point, a review of available data on the mechanics of healing infarcts atthat time point, hypotheses regarding structural determinants of infarct mechanics
based on temporal correlation of the structural and mechanical data, and finally a
brief discussion of which of the mechanisms of functional impairment are most
relevant at each temporal stage of healing. General conclusions and challenges for
future work are addressed in the final section.
IMPACT OF INFARCT MECHANICAL PROPERTIES ON
VENTRICULAR FUNCTION
Below we list and briefly explain six different ways in which the presence of a
healing myocardial infarct can impair overall pump function of the LV. In each
case, the size and mechanical properties of the healing infarct determine the de-
gree of impairment of LV function. Therefore, it follows that an understanding of
the mechanical properties of the healing infarct is essential to understanding, pre-
dicting, and ultimately modifying the short- and long-term changes in ventricular
function that occur following myocardial infarction.
1. An infarct may fail catastrophically (rupture). Infarct rupture accounts
for 15%30% of deaths in the first week after infarction (14, 15). Rupture
obviously represents the most catastrophic way in which the presence of an
infarct can impair ventricular function. Although the exact mechanical prop-
erties most related to rupture have not been identified, the balance between
the mechanical properties of the infarct and the stresses placed on it clearly
determines whether rupture occurs (1618).
2. Infarct bulging or stretching wastes energy generated by healthy
myocardium. Because lost myocardium is replaced by scar tissue ratherthan by regenerated muscle, clinical studies have shown that once 40% of
the LV myocardium has been lost, either through a single large infarction
or a combination of smaller ones, the LV is at risk of pump failure (19,
20). Although it is tempting to attribute this finding simply to a reduction in
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
4/32
226 HOLMES BORG COVELL
Figure 1 Effects of large infarcts on systolic and diastolic pressure-volume relation-
ships predicted by a model of Bogen et al. (21). Data are estimated from figure 8 in
Bogen et al. for healing infarcts corresponding to the phases of healing defined in
this review: control (C), acutely ischemic (I), necrotic (N), and fibrotic (F), assuming
an unstressed volume of 30 ml. Very compliant infarcts (acutely ischemic, I) primar-
ily depress systolic function, whereas very stiff infarcts (fibrotic, F) primarily restrict
diastolic function.
the amount of healthy myocardium contributing to ejection, model studies
have found that the degree of systolic impairment is directly related to the
compliance of the infarct (Figure 1). For very stiff infarcts, little systolic
dysfunction is predicted (21). For compliant infarcts, much of the work of
the remaining myocardium is wasted stretching the infarct, reducing systolic
pump function dramatically (2124).
3. Infarct stiffness may limit diastolic function of the remaining healthy
myocardium. Model studies have also shown an important disadvantage to
an overly noncompliant infarct. Bogen et al. predicted that whereas com-pliant infarcts primarily disrupt systolic mechanics, the presence of a large
noncompliant infarct severely limits ventricular function by impairing di-
astolic filling (Figure 1) (21). The presence of the very stiff infarct impairs
diastolic function by increasing overall chamber stiffness (25) and limiting
the ability of remaining healthy myocardium to utilize the Frank-Starling
mechanism to adjust ventricular output (26).
4. Infarct expansion and cavity dilation increase wall stress throughout
the LV. One common postinfarction complication is infarct expansion, a
remodeling process characterized by rearrangement of material within theinfarct to yield a thinner infarct with increased endocardial surface area (27).
This dilatation and thinning clearly increases the wall stress within the infarct
at any cavity pressure, potentially worsening problems already mentioned,
such as systolic stretching and the risk of rupture. The resulting increase
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
5/32
INFARCT STRUCTURE AND MECHANICS 227
in cavity size also increases wall stress in the remainder of the ventricle,
forcing noninfarcted myocardium to generate higher stresses to achieve the
same systolic cavity pressure (27, 28).
5. Coupling to the infarct may limit deformation of adjacent myocardium.The arguments outlined above regarding infarct compliance appear to sug-
gest that in terms of ventricular function, the stiffer the healing infarct the
better, except in the limit of a healing infarct large and stiff enough to impair
diastolic filling. However, all the reasoning to this point has been one-
dimensional (infarcts are either stiff or compliant) and global (consider-
ing two-compartment models with infarcted and normal segments). In
fact, healing infarcts are anisotropic (29, 30) and coupled locally to adjacent
noninfarcted myocardium. During acute ischemia, coupling to the compliant
infarct creates a functional border zone where deformation is reduced de-spite normal perfusion (31). Later in healing, we have argued that stiff infarcts
may also restrict the deformation of adjacent noninfarcted myocardium (30).
For example, high circumferential stiffness may limit systolic stretching of
the infarct, but high radial stiffness would limit radial thickening of adjacent
myocardium tethered to the infarct (30).
6. The infarct sets boundary conditions for ventricular hypertrophy and
remodeling. Over the long term, the presence of an infarct may also impair
ventricular function indirectly by triggering adverse ventricular remodeling
that increases wall stress throughout the remodeled ventricle. This remodel-ing has been described as a volume-overload hypertrophy of the surviving
myocardium and is characterized by lengthening and thinning of the ven-
tricular wall and overall cavity dilation (32). Although the specific stimuli
that drive volume-overload hypertrophy are still incompletely understood
(33), in the postinfarction setting the values of most of the likely mechanical
candidates (stress, strain, work) in the noninfarcted myocardium, and hence
the resulting pattern of hypertrophy and remodeling, are determined largely
by the material properties and remodeling of the healing infarct. As with in-
farct expansion, increases in wall stress associated with cavity dilation placenoninfarcted myocardium at a mechanical disadvantage and may lead to a
downward spiral into dilated heart failure.
ACUTE ISCHEMIA
During the first minutes to hours after infarction, the balance between oxygen
supply and demand is a dynamic one, and the final size of the infarct can be
influenced by changes in loading conditions and by pharmacologic agents (3437). During this period, the mechanics of the infarct region are dominated by the
conversion of the infarcted myocardium from an active, force-generating material
to a passive, viscoelastic material. Initially, the material properties of the infarct
appear to change little; by 6 h after permanent coronary occlusion the infarct
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
6/32
228 HOLMES BORG COVELL
clearly begins to stiffen (38, 39). We therefore define acute ischemia from the
point of view of infarct mechanics as beginning with the experimental or natural
occlusion of the coronary artery supplying the infarct and ending when stiffening
becomes evident, 46 h after infarction in large animal models. Reperfusion duringthis period may dramatically alter many or all aspects of the subsequent healing
process. Owing to space limitations, we have limited the discussion throughout
this review to nonreperfused infarcts, taking this as the simplest starting point
for understanding the subsequent effects of a variety of interventions, including
reperfusion.
Structural Changes During Acute Ischemia
Excellent descriptive studies of the time course of changes in pathologic appear-
ance have been published for healing rat (40) and human (41, 42) infarcts. Cardiacmyocytes are attached by integrins at specific sites near the Z band to an inter-
connected collagen network containing other mechanically and biologically active
extracellular matrix (ECM) components, including glycoproteins, proteoglycans,
growth factors, cytokines, and proteases (4346). During cardiac remodeling and
wound healing, any change to this network may alter mechanical properties, in-
cluding changes within the myocytes, remodeling of myocyte attachments to the
ECM (47, 48), changes in ECM content (49), and remodeling of ECM organization
and structure (50). In general, postinfarction changes in active myocyte properties
and in ECM content have received the most attention, whereas much less atten-tion has been paid to myocyte-ECM coupling, other ECM components, and ECM
organization.
Within hours after infarction, theinfarctedmuscle loses its striations andchanges
its staining properties (42). Breakdown of matrix-associated glycoproteins has
been reported as early as 40 min after infarction and damage to collagen and
elastin fibers has been demonstrated 2 h after coronary ligation (51, 52); one study
reported a 50% drop in infarct collagen content after 3 h (53). This time course of
matrix damage is consistent with the recent finding that matrix metalloproteinase
(MMP) activity is significantly increased 1 h after infarction, with measurablerelease of soluble MMPs after 2 h (54).
Changes in Mechanical Properties During Acute Ischemia
The most important change in mechanical properties in acutely ischemic my-
ocardium is that throughout the first few minutes of ischemia, the myocardium
gradually loses its ability to generate systolic force. The ischemic myocardium
then behaves as a passive elastic material throughout the cardiac cycle, display-
ing in-plane stretching and thinning during filling and isovolumic systole, thenrecoiling passively during ejection and isovolumic relaxation (5557). The central
question with regard to the mechanical properties of acutely ischemic myocardium
is whether it simply behaves as passive myocardium or whether its constitu-
tive properties are altered by ischemia. Surprisingly, although acute ischemia has
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
7/32
INFARCT STRUCTURE AND MECHANICS 229
received far more attention in the literature than later phases of healing, it is still
not possible to definitively answer this question. As outlined below, there is wide
agreement that passive pressure-segment length curves shift rightward within min-
utes after infarction, so that in-plane lengths at any pressure are greater than control.There is also solid evidence that by several hours after infarction, the infarct region
begins to stiffen. However, the relative contributions of changes in local geome-
try and stresses versus changes in material properties to the reported mechanical
behavior in the first hours after infarction are still largely unresolved.
The LV is more compliant than normal 1 h after experimental coronary ligation
(58), but becomes less compliant than normal within a few hours after infarc-
tion (59, 60). Tracking of segment lengths in the ischemic region using strain
gauges and ultrasonic crystals showed that within 30 s after experimental coro-
nary occlusion, systolic shortening of acutely ischemic myocardium is replacedby systolic stretching (55, 56), which gradually increases in magnitude over the
first 5 min (56, 61). The passive nature of the ischemic segment deformation was
demonstrated convincingly by Tyberg et al., who constructed pressure-length loops
throughout the cardiac cycle and showed that the ischemic segments convert from
a counterclockwise loop, indicating work being performed by the segment prior
to occlusion, to a clockwise loop, indicating work being performed on the seg-
ment by adjacent myocardium, 5 min after experimental occlusion (56). Akaishi
showed that the ischemic region operates on a highly nonlinear tension-length
curve, with the amount of systolic stretching much higher at low end-diastolicpressures (EDP), when the segment starts from a relatively flat part of the curve,
than at high EDP, when the segment operates on a very steep portion of the same
curve (62).
Many of these early studies also compared diastolic pressure-segment length
curves before and after coronary occlusion to assess possible changes in ischemic
region compliance. Although all studies agreed that the diastolic pressure-length
curves shift rightward (greater segment lengths at a given diastolic pressure) (38,
39, 56, 61, 63), there was disagreement over whether the slope of the pressure-
length curves increased (61, 63) or decreased (38, 56) during acute ischemia. Therewere a number of methodological differences among these studies, including the
transmural location and orientation of the segments, the use of closed- or open-
chest animals, and the definition of slope directly from the curves versus from
log-transformed plots, but on close review none of these factors can completely
resolve the discrepancy. In any case, it seems clear that stiffness of the ischemic
region begins to increase during the next few hours after infarction. Vokonas et al.
reported a gradual decrease in systolic stretching of an ischemic segment beginning
15 min after and continuing throughout the first 6 h following infarction, without
concurrent changes in the EDP or end-diastolic segment length (39). Pirzada et al.reported a similar time course for diminishing systolic stretching of the ischemic
segment and found that the slope of the diastolic pressure-segment length rela-
tionship increased in parallel over the same time period (38). Theroux, working
in closed-chest animals at much higher diastolic pressures, saw very little systolic
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
8/32
230 HOLMES BORG COVELL
stretching at 5 min or 2 h, but found that the slope of the diastolic pressure-length
relationship doubled between these time points (63). Analysis of regional wall
motion using echocardiography revealed a slightly different time course in the
same animal model. Both the circumferential extent and the severity of regionalwall motion abnormalities increased during the first 30 min, then remained stable
up to 6 h after coronary ligation (64).
Subsequent two- and three-dimensional analyses of the mechanics of acutely
ischemic myocardium have added detail but still have not clearly resolved the
question of whether the constitutive properties of ischemic myocardium differ
from those of normal passive myocardium. Using a three-dimensional array of im-
planted markers, Villarreal confirmed that 5 min of experimental ischemia in dog
converted the normal pattern of systolic circumferential and longitudinal shorten-
ing and radial thickening to circumferential and longitudinal stretching and radialthinning as expected (57). They also found that while the magnitude of normal sys-
tolic strains typically increases from epicardium to endocardium, during ischemia
the systolic strains were transmurally uniform. An increase in EDP from 2.3
1.5 mm Hg at control to 4.6 1.0 mm Hg after 10 min of ischemia produced a
small transmurally uniform stretch (remodeling strain
-
7/31/2019 Structure And Mechanics of Healing Mi
9/32
INFARCT STRUCTURE AND MECHANICS 231
Figure 2 Data from Gupta et al. on the evolution of anisotropy in healing ovine
infarcts (29). Graph is based on data in table 3 of Gupta et al. and reflects stresses
in the circumferential and longitudinal directions during 15% equibiaxial extension of
excised full-thickness infarcts. Stresses peak at 12 weeks, and the direction of greatest
stress switches from longitudinal to circumferential between 1 and 6 weeks. Each time
point corresponds to one phase of healing as defined in this review: control (C), acutely
ischemic (I), necrotic (N), fibrotic (F), and remodeling (R).
first hour after infarction could be explained by local geometric changes resultingin increased stresses in the infarct at any given cavity pressure, without postulating
a change in material properties; this possibility is discussed first below. Then, we
briefly consider mechanisms that would act to decrease infarct stiffness: strain
softening, disruption of key structural proteins, and loss of coronary perfusion
pressure. We omit ischemic contracture because it would shift pressure-segment
length curves leftward in contrast to experimental observations and therefore does
not appear to dominate the mechanics of acute ischemia. We consider it likely
that the stiffening of the infarct reported to begin hours after infarction is due to
edema, and therefore take this time as the break point between acute ischemia andthe necrotic phase discussed later in this review.
CONSTITUTIVE PROPERTIES OF PASSIVE MYOCARDIUM It is clear that acutely is-
chemic myocardium is stretched in the circumferential and longitudinal directions
and thinned in the radial direction at end-diastole compared to preinfarction control
(57). Although some of this diastolic remodeling reflects increased EDP during
ischemia, diastolic remodeling is more pronounced in the ischemic region than in
remote regions of the same heart (39, 61, 65). This disproportionate local stretching
and thinning in the ischemic region would be expected to result in higher circumfer-ential and longitudinal stresses in the ischemic region than in remote nonischemic
myocardium at any given pressure. Therefore, even if the constitutive properties
of the ischemic and nonischemic regions were identical, pressure-segment length
curves in the ischemic region would shift rightward, with a given cavity pressure
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
10/32
-
7/31/2019 Structure And Mechanics of Healing Mi
11/32
INFARCT STRUCTURE AND MECHANICS 233
of 30 mm Hg or 120 mm Hg (67, 68). This effect certainly seems relevant to acutely
ischemic myocardium, which is exposed to new maximum stresses and stretches
during systole once active force generation ceases. The pressure-strain curves pub-
lished by Emery closely resemble the pressure-dimension curves published by anumber of investigators during acute ischemia, with a near-parallel rightward shift
at pressures above 10 mm Hg, a decreased slope at lower pressures, and no change
in zero-pressure lengths (67). Kirton et al. recently reported that strain softening
occurs only in nonviable (i.e., incapable of generating a twitch in response to elec-
trical stimulation) isolated cardiac trabeculae, suggesting that elevated stress and
stretch alone are not sufficient to induce softening in myocardium unless other
damage has occurred (66). Although this finding does not rule out a role for strain
softening in acutely ischemic myocardium, at least two studies suggest that strain
softening alone cannot explain observed changes in mechanics during ischemia.First, Summerour et al. could not reproduce the changes in opening angle that
occur following 30 min of left coronary occlusion in the rat by inducing global
strain-softening in nonischemic rat hearts (69). Second, Paulus et al. demonstrated
that strain softening is not required to obtain the right-shifted passive pressure-
length curves typical of ischemic myocardium. They induced relative ischemia
by pacing tachycardia in dogs with coronary stenoses and found that segments
with well-preserved systolic function during ischemia had left-shifted diastolic
pressure-segment length curves compared to control, whereas segments with de-
pressed systolic function had right-shifted pressure-segment length curves similarto those observed following coronary occlusion (70). Because systolic stretch was
not required to produce a rightward shift of the diastolic pressure-segment length
curves, strain softening was not responsible for the shift in this study.
DISRUPTION OF STRUCTURAL PROTEINS Most of the passive stiffness of normal
myocardium appears to reside in two structural proteins: titin determines stiffness
at lower sarcomere lengths, whereas collagen is the primary determinant at the
higher end of the working sarcomere length range (71). Therefore, disruption of
either of these proteins during acute ischemia could result in changes in mechanicsof the ischemic region. Titin is a particularly appealing candidate because increased
compliance at low stresses (owing to titin disruption) with preserved properties
at higher stresses (owing to intact collagen) would appear as a rightward shift
in pressure-segment length curves at the relatively high end-diastolic pressures
typical of acute ischemia. However, structural studies identifying damage to the
myocardial collagen network early in ischemia suggest that collagen disruption
may also play a role. Support for this idea comes from a study by MacKenna et al.,
in which bacterial collagenase treatment of perfused isolated arrested rat LVs
caused a rightward shift in passive pressure-volume and pressure-strain curves(72). MacKennas pressure-strain data resembled data from acute ischemia in that
the rightward shifts occurred with little change in slope and the largest shifts were
in circumferential strain. However, the changes in this study were faster than col-
lagenase normally degrades collagen, and this experimental preparation becomes
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
12/32
234 HOLMES BORG COVELL
rapidly edematous, so it is difficult to separate changes owing to disruption of col-
lagen or collagen-myocyte attachments from changes owing to edema. Although a
discussion of myocardial stunning is beyond the scope of this review, experiments
on stunning have also provided evidence linking collagen damage and increasedcompliance of passive myocardium (73, 74).
LOSS OF PERFUSION PRESSURE Perfusion of isolated arrested hearts is associated
with decreased LV compliance and left-shifted passive pressure-strain curves com-
pared to the unperfused state, raising the possibility that the increased LV com-
pliance and rightward shift in pressure-dimension curves reported during acute
ischemia could be explained, in part, by a loss of coronary perfusion pressure in
the occluded vessel. Allaart et al. found that perfusion increased axial stiffness and
unstressed length in papillary muscles owing to an increase in axial stiffness of theperfused blood vessels (75). From their data, loss of perfusion might be expected
to decrease both stiffness and unstressed length, but changes in unstressed length
have not been reported during acute ischemia. May-Newman and coworkers found
that perfusion decreased longitudinal, cross-fiber, and radial strains during passive
inflation of isolated arrested hearts and increased local tissue volume, especially at
the endocardium (76). Because circumferential and fiber strains were not signifi-
cantly altered by perfusion, loss of perfusion would not completelyexplain reported
data for ischemic myocardium, where circumferential remodeling is prominent.
However, the large radial changes reported by May-Newman could account forall of the thinning reported by Villarreal in acutely ischemic myocardium (57),
and thereby for rightward shifting of pressure-dimension curves through locally
increased stresses.
Ventricular Function During Acute Ischemia
Three of the mechanisms by which the presence of an infarct depresses LV function
are relevant to acute ischemia: energy loss through stretching of the infarct (mech-
anism 2), elevated wall stresses owing to infarct and LV dilation (mechanism 4)
and impaired function of adjacent myocardium owing to physical coupling with
the infarct (mechanism 5). Systolic stretching of the ischemic region is apparent
experimentally as a parallel rightward shift of the end-systolic pressure-volume
relationship (ESPVR) (77), which can be explained using simple compartmental
(22, 77) or spherical membrane (21) models (Figure 1). The key to the response is
the exponential passive stress-strain behavior of ischemic myocardium. Although
the ischemic region may be relatively extensible at low pressures, at the much
higher pressures and wall stresses typical of end-systole, the ischemic region is
stretched onto a portion of its stress-strain curve so steep it is essentially inextensi-
ble. Compared to normally activated systolic myocardium (which has contracted
rather than stretched relative to its end-diastolic configuration), the ischemic region
therefore contains roughly the same volume of extra blood at any physiologic sys-
tolic pressure, accounting for the rightward shift of the ESPVR. Systolic stretching
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
13/32
INFARCT STRUCTURE AND MECHANICS 235
of the ischemic region also depresses global ventricular function through a second
mechanism not explicitly incorporated in simple compartmental models. Reduced
systolic ejection eventually leads to a new steady state in which systolic and dias-
tolic volumes are increased relative to control and ejection fraction is depressed,in other words, to global ventricular dilation (78). Dilation places the noninfarcted
myocardium at a mechanical disadvantage, with higher systolic stresses required
to eject against a given pressure.
In addition to the impact of systolic stretching of the infarct region, studies of
regional function during acute ischemia have indicated that the extent of regional
dysfunction extends beyond the region of reduced blood flow, creating a functional
border zone (31). Recently, a combination of modeling and experimental studies
have shown that border zone dysfunction can be explained by physical coupling
to the ischemic region (79) and elevated border zone stresses (8083), withoutpostulating reduced contractility.
THE NECROTIC PHASE
During the first few days after infarction, the dominant pathologic processes are
inflammation and necrosis. We define the necrotic phase as beginning within a
few hours, when the infarct begins to stiffen, and ending when the number of
fibroblasts and amount of new collagen begin to increase rapidly in the healinginfarct [approximately 7 days after infarction in the human (41) and 5 days after
infarction in the rat (40) (Figure 4)]. Infarct rupture is most common during this
period (14, 15). Given that the infarcted muscle is dead and undergoing necrosis,
and significant new collagen has not yet been deposited, it is perhaps surprising
Figure 4 Comparative diagram of the temporal course of the phases of healing de-
fined in this review for various animal models. Time course for other large animal
models is similar to that for dog. Please see text for definition of phases and primary
references for various models.
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
14/32
236 HOLMES BORG COVELL
that every infarct does not rupture during this phase. Infarct mechanics during this
critical period are still poorly understood. In this section, we attempt to identify
structural features responsible for infarct mechanical properties and maintenance
of infarct integrity during the necrotic phase.
Structural Changes During the Necrotic Phase
Within hours after infarction, the infarcted muscle loses its striations and changes
its staining properties (42). Within 24 h, 94% of human infarcts have wavy fibers,
indicating intercellular edema, and 90% have clear necrosis characterized by al-
tered staining properties and nuclear pyknosis or karyolysis (41). By the fourth or
fifth day, removal of dead muscle is clearly observed (41, 42). Collagenase and
gelatinase activity of MMP-1, MMP-2, and MMP-9 is elevated during the necrotic
phase of infarct healing (84, 85), and disruption of the collagen network continues.
During the first 4 days after infarction in rats, there is a progressive decrease in the
number of normally birefringent collagen fibers, and by 4 days there is a significant
reduction in the number of collagen struts that laterally connect myocytes (86). As
the necrotic phase concludes, deposition of new ECM components begins, forming
a scaffold for the deposition of new collagen. Fibronectin (87, 88), laminin (89),
and collagen type IV (89) all appear at 34 days in the healing rat infarct, ap-
proximately the same time that mRNA for type III (first) and I (slightly later)
procollagens is first detected (90).
Changes in Mechanical Properties During the Necrotic Phase
Two changes in mechanics are apparent in the necrotic infarct. First, circumferen-
tial and longitudinal stiffness increase under multiaxial loading, whereas uniaxial
tests show no change in stiffness, suggesting increased mechanical coupling be-
tween the two directions. Second, unstressed segment length increases, at least in
the circumferential direction, whereas end-diastolic length does not. The net effect
is an increase of segment lengths below end-diastolic pressure but a decrease in
segment lengths at higher pressures.Theroux et al. tracked the distance between pairs of circumferentially oriented
sonomicrometers implanted in the subendocardium over 4 weeks following exper-
imental infarction in dogs (63). Circumferential segmental shortening remained
approximately zero in the infarct throughout the first week (0% at 1 day, 1.9
0.1% at 1 week). EDP and segment length were unchanged from preinfarction
control at 1 day and 1 week. However, the slope of the diastolic pressure-length
relationship during filling was increased more than fivefold at 1 day, 2 days, and
1 week. Hood reported a similarly dramatic increase in the slope of the diastolic
pressure-circumferential segment length relation and an increase in unstressedlength in 5-day-old canine infarcts both in vivo and in isolated arrested hearts (91),
whereas Lima et al. found reduced systolic principal strains in 1-week-old ovine
infarcts using MRI tagging (92). Because mild thinning of the infarcted region
typically occurs over the first week, stresses in the infarcts were likely similar to
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
15/32
INFARCT STRUCTURE AND MECHANICS 237
or slightly greater than in control regions at a given diastolic pressure. Therefore,
these studies imply increased stiffness and unstressed length in healing myocardial
infarcts throughout the necrotic phase.
By contrast, uniaxial tests of strips of healing infarct tissue have consistentlyindicated that infarct material properties do not change during the necrotic phase of
healing. Laird et al. studied uniaxial strips cut from the midwall of infarcted rabbit
hearts along the original myofiber direction (nearly circumferential) and found no
change in stiffness during the 10 days following infarction (24). In a more detailed
study at a single time point, Przyklenk et al. tested longitudinally oriented strips
cut from several transmural layers of normal canine myocardium and 24-h-old
infarcts. They found no differences between normal and necrotic myocardium in
stiffness, tensile strength, or strain at rupture (93).
Only a single report of biaxial mechanical testing of healing infarct tissue iscurrently available, and the results agree better with in vivo studies. Gupta and
coworkers measured circumferential and longitudinal stresses at 15% equibiaxial
stretch in healing anterior ovine infarcts during each of the phases of infarct healing
outlined in this review (29). At 1 week, although collagen content had increased by
less than twofold, longitudinal stress at 15% equibiaxial stretch reached its peak
value for the entire time course studied, roughly six times control values (Figure 2).
Circumferential stress at 15% equibiaxial stretch was also increased at 1 week to
more than eight times its control value, although it did not peak until 2 weeks.
Although the use of only a single test protocol limits the interpretation of their datasomewhat, their equibiaxial stretch data, like the in situ pressure-length curves,
suggest a several-fold increase in infarct stiffness at 1 week, before the bulk of
new collagen deposition occurs.
Determinants of Infarct Mechanics During the Necrotic Phase
Unfortunately, very little direct information is available regarding the determinants
of mechanical properties during the necrotic phase. Most evidence is either correla-
tive, relating pathologic observations to functional ones, or circumstantial, derivedfrom the outcome of various experimental interventions. In addition, most of the
evidence relates to the prevention or aggravation of infarct expansion. Although
the degree of infarct expansion likely depends on infarct material properties, the
process is not sufficiently well understood to draw conclusions about specific prop-
erties, such as infarct stiffness or tensile strength, from data on expansion. In spite
of these difficulties, the evidence reviewed below strongly suggests that interstitial
edema is responsible for reported increases in infarct stiffness during the necrotic
phase, whereas infarct expansion is the most likely basis for the reported increase
in unstressed dimensions of the necrotic infarct.
MATRIX AND MYOFIBRILLAR NECROSIS Thetwoprimarystructuralproteinsinpas-
sive myocardium, titin and collagen, both undergo degradation during the necrotic
phase. Necrotic myocytes lose their striations within hours (42), reflecting damage
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
16/32
238 HOLMES BORG COVELL
to the major myofibrillar proteins that compose the sarcomere, including titin.
Progressive damage to collagen is also seen in the first days following infarction.
Although the impact of this damage on infarct material properties has not been
studied directly, the degree of damage correlates with the degree of infarct expan-sion (86), and selective MMP inhibition limits infarct expansion (94). If titin or
collagen normally bear some tension in the stress-free state, their degradation could
produce the increase in unstressed segment length reported in necrotic infarcts, but
would not explain the reported increase in infarct stiffness.
INTERSTITIAL EDEMA Several lines of evidence support the idea that interstitial
edema increases myocardial stiffness. First, studies of iatrogenic edema asso-
ciated with cardioplegia have shown that experimentally induced global edema
decreases ventricular compliance, but have not consistently found changes in un-stressed chamber volume (95, 96). Second, studies of the role of edema in postis-
chemic reperfusion injury have shown that interstitial edema increases stiffness
in the ischemic region. For example, reperfusion following experimental global
ischemia increased ventricular water content and diastolic pressure at a fixed vol-
ume, whereas reperfusion with a hypertonic solution returned water content to
normal and diastolic pressure toward normal (97).
Although these studies demonstrate that edema could increase stiffness in
necrotic myocardium, the evidence that edema actually does this in necrotic in-
farcts is more circumstantial. A recent MRI study by Gerber et al. found thatreperfused experimental infarcts with high levels of microvascular obstruction
(MO) showed less systolic stretching at 48 h postinfarction than infarcts with
low levels of MO (98). Another interesting finding was that the high-MO infarcts
appeared to be not only stiffer but also more isotropic than low-MO infarcts. How-
ever, although high-MO infarcts would likely have more intramyocardial hem-
orrhage and edema than low-MO infarcts, the degree of infarct edema was not
directly verified in this study. Other studies have indicated that infarct water con-
tent is significantly increased several days after infarction, even in the absence of
reperfusion (99).The final line of evidence that edema is an important determinant of mechani-
cal properties in the necrotic infarct is that a variety of pharmacologic agents that
reduce edema and inflammation, including high-dose steroids (100, 101), ibupro-
fen (102), and indomethacin (103, 104), also aggravate infarct expansion in the
first days following experimental infarction. One of the best of these studies, by
Mannisi et al., showed that water content was significantly increased in the infarct
region at 24 h in rats, steroids prevented this water increase, and steroids did not
change infarct size or the prevalence of expansion but did increase the extent of
infarct expansion when it occurred (101). Although the relationship between in-farct material properties and infarct expansion is not well understood, these studies
suggest that edema reinforces the necrotic infarct against expansion by increasing
stiffness and/or tensile strength, and antiinflammatory agents promote expansion
by reducing edema.
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
17/32
-
7/31/2019 Structure And Mechanics of Healing Mi
18/32
240 HOLMES BORG COVELL
to increase rapidly in the healing infarct [approximately 7 days after infarction
in the human (41) and 5 days after infarction in the rat (40) (Figure 4)], and
ending when collagen accumulation slows and mechanical properties decouple
from collagen content. This occurs at approximately 3 weeks in large animalmodels (29), presumably earlier in the rat and later in humans (Figure 4).
Structural Changes During the Fibrotic Phase
Collagen content increases steadily from 1 to 6 weeks after experimental infarction
in dogs (112, 114) and sheep (29). Qualitative observations at autopsy indicate a
similar time course for human myocardial infarction (41, 42). In rats, the collagen
content begins rising on day 4 or 5 (40) and continues to increase for at least
3 weeks (40, 115). The healing infarct contains a mixture of collagen types I, III,
and other minor subtypes (115), and Whittaker et al. have suggested that an initialmesh of type III collagen forms the scaffold for subsequent deposition of large,
highly aligned type I collagen fibers (116). By 3 weeks after infarction in pig, the
scar is dominated by large type I collagen fibers highly aligned with one another
in each transmural layer (117). The mean orientation of the collagen fibers varies
with depth below the epicardium in a pattern similar to that for normal muscle
fibers except that the transmural range of mean angles is smaller (30). The net
result of this pattern is that the majority of large collagen fibers in the scar are
oriented within 30 of the circumferential direction (118). A similar pattern of
collagen fiber alignment has been reported 2 weeks after infarction in rat (119)and at 6 weeks in dog (116).
Changes in Mechanical Properties During the Fibrotic Phase
Only a few studies have evaluated mechanics during this phase of healing. The
available evidence suggests that during this phase infarct stiffness peaks and the
healing infarct acquires a distinctive anisotropy. Theroux reported that segment
lengths changed only approximately 2% over the cardiac cycle at 1, 2, and 3 weeks
after infarction in dogs, suggesting high stiffness in the healing infarcts (63). Theslope of the passive pressure-segment length relationship confirmed elevated stiff-
ness, varying from six to nine times control values depending on EDP (63). Gibbons
et al. found that the circumferential extent of abnormal wall motion peaked 48 h
after infarction in the dog and then decreased over the next 6 weeks (120). When
we studied the three-dimensional mechanics of healing porcine infarcts, we also
found that systolic strains were not different from zero at 1 week, consistent with
elevated stiffness (121). However, although circumferential stretching remained
minimal at 3 weeks, significant passive longitudinal shortening and radial thick-
ening returned, suggesting developing mechanical anisotropy in the healing scar.Connelly and Lerman both reported that uniaxial tensile strength of excised
strips of 1-week-old rabbit myocardial scar tissue was roughly double that for
control myocardium, but did not report stiffness values at this time point (113, 122).
Gupta et al. performed equibiaxial mechanical tests of excised ovine scar tissue and
found that stress at 15% equibiaxial stretch peaked at 1 week in the longitudinal
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
19/32
INFARCT STRUCTURE AND MECHANICS 241
direction at a value 6 times control and at 2 weeks in the circumferential direction
at a value 16 times control (Figure 2) (29). Although longitudinal stresses at 15%
equibiaxial stretch remained roughly twice circumferential stresses at all time
points for noninfarcted myocardium, the healing scar switched from stiffer inthe longitudinal direction through the first week to stiffer in the circumferential
direction beyond the second week (29). We found similar anisotropy in 3-week-
old porcine infarcts, which displayed little circumferential stretch in the healing
scar during passive inflation of isolated arrested hearts over a physiologic range of
cavity pressures, but roughly 50% greater longitudinal stretch at any pressure as
remote noninfarcted myocardium (Figure 5) (123). By contrast, Omens et al. found
a greater reduction in longitudinal than in circumferential epicardial strains in the
scar during passive inflation of isolated arrested hearts 2 weeks after infarction in
rat (119). They also found that collagen fibers in the scar straightened more rapidlywith pressure but were not straighter in the unloaded state than collagen fibers in
normal myocardium.
Figure 5 Anisotropy in 3-week-old porcine scar with large collagen fibers oriented
predominantly in the circumferential direction. Lines show transmural pattern of strains
as isolated arrested heart is inflated from a cavity pressure of 5 mm Hg (lowest linein each panel, with symbols) in 5-mm Hg increments to 25 mm Hg (highest line, with
symbols). Circumferential strains are much smaller at all depths and pressures in the
scar (upper right panel ) compared to remote noninfarcted myocardium (upper left),
whereas longitudinal strains are similar in the scar (lower right) and muscle (lower
left) (123).
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
20/32
242 HOLMES BORG COVELL
Determinants of Infarct Mechanics During the Fibrotic Phase
In this review, we define the fibrotic phase as the phase of healing dominated by
new collagen deposition. During this phase, both the amount of collagen and the
three-dimensional structure of the collagen fibers are important determinants of
infarct mechanics. Other matrix components may also be important, but there is
not yet enough information to assess their role relative to collagen.
COLLAGEN CONTENT Because infarct stiffness and collagen content increase in
parallel during the fibrotic phase, it seems obvious that collagen content is one
primary determinant of the mechanical properties of the healing infarct during
this phase. However, the effects of alterations in collagen content and subtype
ratios on scar mechanics have not been systematically studied. Lerman found that
passive stiffness of the rabbit LV correlated with hydroxyproline content over the
first week after infarction (113), as would be expected if hydroxyproline content
correlates with stiffness of the healing infarct (21, 26).
THREE-DIMENSIONAL COLLAGEN STRUCTURE The findingthat myocardial infarcts
are highly anisotropic during the fibrotic phase of healing (29, 30) implicates the
highly aligned large collagen fiber structure as the second primary determinant
of infarct properties during this phase. The predominance of large collagen fibers
oriented in the circumferential direction (116, 117) is consistent with reports thatmyocardialscar is stiffer in thecircumferential direction in most animal models (29,
30). However, more work is needed, particularly in the development of structural
constitutive models for myocardial scar tissue (118).
Ventricular Function During the Fibrotic Phase
The two mechanisms by which infarcts in the fibrotic phase of healing may depress
LV function are impaired filling owing to elevated chamber stiffness (mechanism 3)
and impaired systolic function of adjacent noninfarcted myocardium owing to
coupling with the infarct (mechanism 5). Janz (26) and Bogen (21) both predictedthat the primary adverse effect of a very stiff infarct would be impaired filling
owing to decreased LV compliance. Janz also suggested that diastolic stretch of
adjacent noninfarcted myocardium would be limited during filling by tethering to
the very stiff infarct, reducing systolic function via the Frank-Starling mechanism
(26). We have proposed that tethering of adjacent noninfarcted myocardium to a
stiff isotropic infarct would directly retard both systolic shortening parallel to the
infarct border and radial thickening (30). The anisotropy we observed in 3-week-
old porcine scars oriented longitudinally on the LV appears to minimize this effect:
high circumferential stiffness prevents stretching of the infarct perpendicular toits border, whereas low longitudinal and radial stiffness allow the scar to deform
compatibly with adjacent myocardium in these directions (30). Evidence in support
of this hypothesis includes the fact that longitudinal shortening and wall thickening
in the healing infarct disappear at 1 week in this animal model (when the infarct is
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
21/32
INFARCT STRUCTURE AND MECHANICS 243
stiff and isotropic) then reappear at 3 weeks (once infarct anisotropy is established),
despite the absence of viable myocardium. Another consistent observation is that,
in the study by Gerber et al. discussed above, high-MO infarcts, which appeared
to be stiffer and more isotropic, reduced wall thickening in adjacent noninfarctedmyocardium much more than low-MO infarcts (98).
THE REMODELING PHASE
As healing continues, the mechanical properties of the infarct decouple from colla-
gen content. Collagen content may continue to rise for several weeks while infarct
stiffness drops (29), suggesting that other factors now dominate the mechanics.
We term this phase the remodeling phase, and although its onset can be defined,the healing scar tissue is a dynamic, biologically active tissue that may never reach
a stable, mature configuration (healed as opposed to healing) that could be taken
to mark the end of remodeling (124).
Structural Remodeling of Myocardial Scar Tissue
Remodeling of the myocardial scar occurs during this phase at both the gross
and microscopic levels. On the gross level, the dominant effect is shrinkage of
the scar to occupy a reduced percentage of the LV wall. In canine models, direct
topographic measurements indicated a 40% shrinkage of the infarct over 6 weeks
(114), whereas condensation of microspheres indicated 30% to more than 70%
shrinkage (99, 125), depending on infarct size and location. At the microscopic
level, the rise in collagen content slows but cross-linking continues to increase.
After a tenfold increase in the first 4 weeks, Jugdutt found that hydroxyproline
increased only an additional 20% from week 4 to week 6 in dogs (114). Vivaldi
founda50%increaseincollagencontentbetween2and4weeksintheratcompared
to a doubling of cross-link concentration over the same period (115). Data from
McCormick et al. at 13 weeks in the rat showed the same collagen content and
another 50% increase in cross-linking compared to Vivaldis 4-week data (126).
Qualitative changes in collagen have also beenreported. Whittaker found continued
increases in molecular organization as assessed by optical retardation for at least
6 weeks in a canine model (116).
Changes in Mechanical Properties During theRemodeling Phase
There is some disagreement in the literature regarding changes in scar mechanical
properties during the remodeling phase. Although Parmley found that strips offibrous human aneurysms tested uniaxially months to years after infarction were
many times stiffer than muscular aneurysms from patients who died days after
infarction (23), Connelly reported only moderately (twofold) increased stiffness in
samples from 3-week-old rabbit scar tissue compared to noninfarcted myocardium
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
22/32
244 HOLMES BORG COVELL
(122). Scar anisotropy may largely explain these differences; in 6-week-old ovine
scar stretched equibiaxially by 15%, Gupta et al. reported longitudinal stresses
identical to those in control myocardium, whereas circumferential stresses were
tenfold greater in the scar (Figure 2) (29).
Determinants of Infarct Mechanics During theRemodeling Phase
During the early part of the remodeling phase, infarct stiffness decreases while
collagen content continues to increase, indicating that collagen content and fiber
structure are no longer the only important determinants of the mechanical prop-
erties of the healing infarct. Structural changes during this phase of healing have
received much less attention, but one factor that does appear to correlate with
infarct mechanics late in healing is the degree of cross-linking.
COLLAGEN CROSS-LINKING Connelly et al. compared uniaxial mechanics of strips
of rabbit scar tissue 3 weeks after infarction in rabbits, with or without reperfusion.
Late reperfusion (3 h after infarction) did not change scar collagen content or
stiffness, but it did reduce cross-link density and tensile strength, suggesting that
cross-linking can influence the mechanics of healing scar tissue. Similar findings
have been reported in healing rabbit ligament, where reduced crosslink density is
associated with reduced failure strength despite normal collagen concentrations(127). More work is needed to determine the effect of cross-linking on multiaxial
mechanics of healing myocardial scar.
Ventricular Function During the Remodeling Phase
In many patients and experimental models, LV function improves as healing
reaches the later stages. Clinical studies show improved hemodynamics and par-
tially normalized LV compliance and EDP 46 weeks after infarction (60, 128),
with few additional functional changes over the remainder of the first year (128,
129). All of the mechanisms for depression of function discussed at the beginningof this review except infarct rupture are involved to some extent in this late improve-
ment in LV function. Scar stiffness remains higher than that of passive or acutely
ischemic myocardium, limiting energy loss owing to systolic stretching (mech-
anism 2), and anisotropy appears to limit local tethering effects (mechanism 5).
Scar shrinkage acts like infarct expansion in reverse, reducing the volume of the
scar and infarct-associated cavity dilation (mechanism 4): wall motion abnormali-
ties partially resolve (120, 125, 130), and the reduction in wall motion abnormality
correlates closely with scar contraction (125). To the extent that LV dysfunction
remains, it primarily reflects limitation of diastolic function owing to reduced di-astolic compliance (mechanism 3, Figure 1). For example, Weisse found normal
hemodynamics with mildly depressed ventricular function curves at 34 and 6
8 weeks following infarction in dogs. The depressed ventricular function curves
were due entirely to a stiffer end-diastolic pressure-volume relationship (EDPVR),
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
23/32
INFARCT STRUCTURE AND MECHANICS 245
resolving when stroke work was plotted as a function of end-diastolic circumfer-
ence rather than pressure (131).
However, there are some exceptions to this generally improving course. When
very large infarcts are present, cavity dilation dominates other effects, such asscar shrinkage, leading to increased wall stresses and progressively depressed
function (mechanism 6) (132, 133). If an aneurysm forms, the severely altered local
geometry increases stresses (134) and depresses function (134136) in the adjacent
myocardium, creating a functional border zone analogous to that discussed for
acute ischemia.
SUMMARY AND CONCLUSIONS
Based on temporal correlation of reported changes in structure and mechanics
of healing myocardial infarcts, we have defined four phases of infarct healing
and hypothesized the following: (a) Mechanical properties during acute ischemia
(the first few hours) are essentially the normal constitutive properties of passive
myocardium, (b) mechanical properties during the necrotic phase (the first 5
7 days depending on animal model) are dominated by edema, (c) mechanical
properties during the fibrotic phase (up to 24 weeks) arise from the large collagen
fiber structure, and (d) mechanical properties during the remodeling phase (the
remainder of the healing process) are determined primarily by collagen cross-linking. We intend these hypotheses to stimulate further, mechanistic research
on the mechanics of healing myocardial infarcts. Certainly, this review suggests
many areas where more data are needed: Quantitative structural studies of the three-
dimensional organization of important matrix components and determination of
constitutive relations for scar tissue at multiple time points during healing would
head our list.
However, the mechanics of healing infarct tissue, like those of heart tissue in
general, depend both on constitutive properties and on loading conditions, which
in turn are determined by hemodynamics, ventricular and local geometry, andcoupling to adjacent myocardium. The individual studies reviewed here typically
provide complementary, incomplete subsets of information about infarct mechan-
ics. Studies using ventriculography and echocardiography provide information on
global ventricular function and shape, plus more limited information on regional
deformation in the infarct. Studies using implanted sonomicrometers or radiopaque
markers provide more regional detail, with the advantages that infarct mechanics
can still be related to overall ventricular function and that the deformation of an
infarcted segment can be tracked not only throughout the cardiac cycle but also
throughout longer-term remodeling; the primary disadvantage is that stresses can-not be measured directly but must be estimated from hemodynamic and geometric
data by modeling (118). Finally, excision and mechanical testing of tissue provides
the most direct characterization of tissue material properties, with the caveat that
excision itself may alter those properties.
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
24/32
246 HOLMES BORG COVELL
Remarkably few studies have tried to integrate these different experimental
methods to obtain a complete picture of infarct mechanics even at a single time
in a single animal model. The primary consequence of this lack of integration is
that the wealth of information on changes in infarct deformation patterns over thecourseof postinfarction healing is difficult to interpret. In future studies, much more
attention needs to be paid to differentiating changes in material properties (shifts
of the stress-strain curve) from changes in loading (shifts along the stress-strain
curve) because completely different therapeutic approaches may be appropriate
to address these two different bases for altered mechanics. Multiaxial testing of
infarcts at the various stages of healing is needed, but should include careful
registration of these data to the in vivo working range. The other type of integration
that is largely missing from the literature is direct integration of structural and
mechanical data. No study we reviewed reported collagen content, cross-linking,and fiber structure along with mechanics of a healing infarct, and none of the
studies directly altered tissue composition to test hypotheses about the structural
basis for observed mechanical properties.
In summary, although much is known about changes in ventricular function, re-
gional deformation, and tissue composition during the course of infarct healing, the
underlying mechanics of the simplest case, permanent coronary occlusion without
reperfusion, are still not sufficiently understood to predict the impact of proposed
interventions or to specify the design requirements for a tissue-engineered re-
placement. Integrative studies combining material testing, quantitative structuralanalysis, and in vivo functional studies are needed, as are structural constitutive
models. By allowing prediction of the changes in mechanics and function that will
follow from proposed changes to healing infarct structure, these new studies would
allow rational design of bioengineering therapies to improve long-term survival
and quality of life for patients who suffer myocardial infarction.
ACKNOWLEDGMENTS
This work was supported by National Institutes of Health grant HL-075639(J.W.H.). The authors wish to acknowledge Dr. Kevin Costa and the students
of the Cardiac Biomechanics Group at Columbia University for comments on the
developing manuscript.
The Annual Review of Biomedical Engineering is online at
http://biomed.annualreviews.org
LITERATURE CITED
1. American Heart Association. 2003.Heart
Disease and Stroke Statistics2004 Up-
date. Dallas, TX: Am. Heart Assoc.
2. Topol EJ. 2003. Current status and future
prospects for acute myocardial infarction
therapy. Circulation 108:III613
3. Armstrong PW, Collen D, Antman E.
2003. Fibrinolysis for acute myocardial
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
25/32
INFARCT STRUCTURE AND MECHANICS 247
infarction: the future is here and now. Cir-
culation 107:253337
4. Udelson JE. 2004. Ventricular remodel-
ing in heart failure and the effect of beta-
blockade. Am. J. Cardiol. 93:43B48B
5. Sharpe N. 2004. Cardiac remodeling in
coronary artery disease. Am. J. Cardiol.
93:17B20B
6. Sutton MG, Sharpe N. 2000. Left ventric-
ular remodeling after myocardial infarc-
tion: pathophysiology and therapy. Circu-
lation 101:298188
7. Forrester JS, Price MJ, Makkar RR. 2003.
Stem cell repair of infarcted myocardium:an overview for clinicians. Circulation
108:113945
8. Thompson RB, Emani SM, Davis BH, van
den Bos EJ, Morimoto Y, et al. 2003.
Comparison of intracardiac cell trans-
plantation: autologous skeletal myoblasts
versus bone marrow cells. Circulation
108(Suppl. 1):II26471
9. Ghostine S, Carrion C, Souza LC, Richard
P, Bruneval P, et al. 2002. Long-term ef-ficacy of myoblast transplantation on re-
gional structure and function after my-
ocardial infarction. Circulation 106:I131
36
10. Jain M, DerSimonian H, Brenner DA,
Ngoy S, Teller P, et al. 2001. Cell therapy
attenuates deleterious ventricular remod-
eling and improves cardiac performance
after myocardial infarction. Circulation
103:19202711. Zimmermann WH, Melnychenko I, Es-
chenhagen T. 2004. Engineered heart tis-
sue for regeneration of diseased hearts.
Biomaterials 25:163947
12. Matsubayashi K, Fedak PW, Mickle DA,
Weisel RD, Ozawa T, Li RK. 2003.
Improved left ventricular aneurysm re-
pair with bioengineered vascular smooth
muscle grafts. Circulation 108(Suppl. 1):
II2192513. Roberts R, DeMello V, Sobel BE. 1976.
Deleterious effects of methylprednisolone
in patients with myocardial infarction.
Circulation 53:I-2045
14. Birnbaum Y, Chamoun AJ, Anzuini A,
Lick SD, Ahmad M, Uretsky BF. 2003.
Ventricular free wall rupture following
acute myocardial infarction. Coron Artery
Dis. 14:46370
15. Wehrens XH, Doevendans PA. 2004.
Cardiac rupture complicating myocar-
dial infarction. Int. J. Cardiol. 95:285
92
16. Bogen DK, McMahon TA. 1979. Do car-
diac aneurysms blow out? Biophys. J. 27:
30116
17. Radhakrishnan S, Ghista DN, Jayaraman
G. 1980. Mechanical analysis of the de-velopment of left ventricular aneurysms.
J. Biomech. 13:103139
18. Radhakrishnan S, Ghista DN, Jayaraman
G. 1986. Mechanics of left ventricular
aneurysm. J. Biomed. Eng. 8:923
19. Page DL, Caulfield JB, Kastor JA, De-
Sanctis RW, Sanders CA. 1971. Myocar-
dial changes associated with cardiogenic
shock. N. Engl. J. Med. 285:13337
20. Alonso DR, Scheidt S, Post M, Kil-lip T. 1973. Pathophysiology of cardio-
genic shock. Quantification of myocardial
necrosis, clinical, pathologic and elec-
trocardiographic correlations. Circulation
48:58896
21. Bogen DK, Rabinowitz SA, Needleman
A, McMahon TA, Abelmann WH. 1980.
An analysis of the mechanical disadvan-
tage of myocardial infarction in the canine
left ventricle. Circ. Res. 47:7284122. Swan HJ, Forrester JS, Diamond G, Chat-
terjee K, Parmley WW. 1972. Hemo-
dynamic spectrum of myocardial infarc-
tion and cardiogenic shock. A conceptual
model. Circulation 45:1097110
23. Parmley WW, Chuck L, Kivowitz C, Mat-
loff JM, Swan HJ. 1973. In vitro length-
tension relations of human ventricular
aneurysms. Relation of stiffness to me-
chanical disadvantage Am. J. Cardiol. 32:88994
24. Laird JD, Vellekoop HP. 1977. Time
course of passive elasticity of myocardial
tissue following experimental infarction
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
26/32
248 HOLMES BORG COVELL
in rabbits and its relation to mechanical
dysfunction. Circ. Res. 41:71521
25. Smith M, Russell RO, Feild BJ, Rack-
ley CE. 1974. Left ventricular compliance
and abnormally contracting segments in
postmyocardial infarction patients. Chest
65:36878
26. Janz RF, Waldron RJ. 1978. Predicted ef-
fect of chronic apical aneurysms on the
passive stiffness of the human left ventri-
cle. Circ. Res. 42:25563
27. Weisman HF, Healy B. Myocardial in-
farct expansion, infarct extension, and
reinfarction: pathophysiologic concepts.Prog. Cardiovasc. Dis. 1987:73110
28. Bogaert J, Bosmans H, Maes A, Suetens
P, Marchal G, Rademakers FE. 2000. Re-
mote myocardial dysfunction after acute
anterior myocardial infarction: impact of
left ventricular shape on regional func-
tion: a magnetic resonance myocardial
tagging study. J. Am. Coll. Cardiol. 35:
152534
29. Gupta KB, Ratcliffe MB, Fallert MA, Ed-munds LH Jr, Bogen DK. 1994. Changes
in passive mechanical stiffness of myocar-
dial tissue with aneurysm formation. Cir-
culation 89:231526
30. Holmes JW, Nunez JA, Covell JW. 1997.
Functional implications of myocardial
scar structure. Am. J. Physiol. Heart Circ.
Physiol. 272:H212330
31. Gallagher KP, Gerren RA, Stirling MC,
Choy M, Dysko RC, et al. 1986. The dis-tribution of functional impairment across
the lateral border of acutely ischemic my-
ocardium. Circ. Res. 58:57083
32. Pfeffer MA, Braunwald E. 1990. Ventric-
ular remodeling after myocardial infarc-
tion. Experimental observations and clin-
ical implications. Circulation 81:1161
72
33. Holmes JW. 2004. Candidate mechani-
cal stimuli for hypertrophy during vol-ume overload. J. Appl. Physiol. 97:1453
60
34. Maroko PR, Kjekshus JK, Sobel BE,
Watanabe T, Covell JW, et al. 1971.
Factors influencing infarct size following
experimental coronary artery occlusions.
Circulation 43:6782
35. Harken AH, Simson MB, Haselgrove J,
Wetstein L, Harden WR, Barlow CH.
1981. Early ischemia after complete coro-
nary ligation in the rabbit, dog, pig, and
monkey.Am. J. Physiol. Heart Circ. Phys-
iol. 241:H20210
36. Connelly C, Vogel WM, Hernandez YM,
Apstein CS. 1982. Movement of necrotic
wavefront after coronary artery occlusion
in the rabbit. Am. J. Physiol. Heart Circ.
Physiol. 243:H6829037. Miura T, Yellon DM, Hearse DJ, Downey
JM. 1987. Determinants of infarct size
during permanent occlusion of a coronary
artery in the closed chest dog.J. Am. Coll.
Cardiol. 9:64754
38. Pirzada FA, Ekong EA, Vokonas PS, Ap-
stein CS, Hood WB. 1976. Experimen-
tal myocardial infarction. XIII. Sequential
changes in left ventricular pressure-length
relationships in the acute phase. Circula-tion 53:97075
39. Vokonas PS, Pirzada FA, Hood WB.
1976. Experimental myocardial infarc-
tion. XII. Dynamic changes in segmen-
tal mechanical behavior of infarcted and
non-infarcted myocardium. Am. J. Car-
diol. 37:85359
40. Fishbein MC, Maclean D, Maroko PR.
1978. Experimental myocardial infarc-
tion in the rat: qualitative and quantitativechanges during pathologic evolution. Am.
J. Pathol. 90:5770
41. Fishbein MC, Maclean D, Maroko PR.
1978. The histopathologic evolution of
myocardial infarction. Chest 73:843
49
42. Mallory KG, White PD, Salcedo-Salgar
J. 1939. The speed of healing of myocar-
dial infarction: a study of the pathologic
anatomy in seventy-two cases. Am. HeartJ. 18:64771
43. Caulfield JB, Borg TK. 1979. The col-
lagen network of the heart. Lab Invest.
40:36472
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
27/32
INFARCT STRUCTURE AND MECHANICS 249
44. Borg TK, Caulfield JB. 1981. The colla-
gen matrix of the heart. Fed. Proc. 40:
203741
45. Weber KT. 1989. Cardiac interstitium in
health and disease: the fibrillar collagen
network. J. Am. Coll. Cardiol. 13:1637
53
46. Legrice IJ, Hunter PJ, Smaill BH. 1997.
Laminar structure of the heart: a mathe-
matical model.Am. J. Physiol. Heart Circ.
Physiol. 272:H246676
47. Goldsmith EC, Borg TK. 2002. The dy-
namic interaction of the extracellular ma-
trix in cardiac remodeling. J. Card. Fail.8:S31418
48. Goldsmith EC, Carver W, McFadden A,
Goldsmith JG, Price RL, et al. 2003. Inte-
grin shedding as a mechanism of cellular
adaptation during cardiac growth. Am. J.
Physiol. Heart Circ. Physiol. 284:H2227
34
49. Jugdutt BI. 2003. Ventricular remodeling
after infarction and the extracellular colla-
gen matrix: when is enough enough? Cir-culation 108:1395403
50. Whittaker P. 1998. Collagen organization
in wound healing after myocardial injury.
Basic Res. Cardiol. 93(Suppl. 3):2325
51. Sato S, Ashraf M, Millard RW, Fujiwara
H, Schwartz A. 1983. Connective tissue
changes in early ischemia of porcine my-
ocardium: an ultrastructural study.J. Mol.
Cell. Cardiol. 15:26175
52. Caulfield JB, Tao SB, Nachtigal M. 1985.Ventricular collagen matrix and alter-
ations. Adv. Myocardiol. 5:25769
53. Takahashi S, Barry AC, Factor SM. 1990.
Collagen degradation in ischaemic rat
hearts. Biochem. J. 265:23341
54. Etoh T, Joffs C, Deschamps AM, Davis
J, Dowdy K, et al. 2001. Myocardial and
interstitialmatrix metalloproteinase activ-
ity after acute myocardial infarction in
pigs. Am. J. Physiol. Heart Circ. Physiol.281:H98794
55. Tennant R, Wiggers CJ. 1935. The effect
of coronary occlusion on myocardial con-
traction. Am. J. Physiol. 112:35136
56. Tyberg JV, Forrester JS, Wyatt HL, Gold-
ner SJ, Parmley WW, Swan HJC. 1974.
An analysis of segmental ischemic dys-
function using the pressure-length loop.
Circulation 49:74854
57. Villarreal FJ, Lew WYW, Waldman LK,
Covell JW. 1991. Transmural myocardial
deformation in the ischemic canine left
ventricle. Circ. Res. 68:36881
58. Forrester JS, Diamond G, Parmley WW,
Swan HJC. 1972. Early increase in left
ventricular compliance after myocardial
infarction. J. Clin. Invest. 51:598603
59. Diamond G, Forrester JS. 1972. Effect ofcoronary artery disease and acute myocar-
dial infarction on left ventricular compli-
ance in man. Circulation 45:1119
60. Bleifeld W, Mathey D, Hanrath P. 1974.
Acute myocardial infarction. VI. Left ven-
tricular wall stiffness in the acute phase
and in the convalescent phase.Eur. J. Car-
diol. 2:19198
61. Theroux P, Franklin D, Ross J Jr,
Kemper WS. 1974. Regional myocar-dial function during acute coronary artery
occlusion and its modification by phar-
macologic agents in the dog. Circ. Res.
35:896908
62. Akaishi M, Weintraub WS, Schneider
RM, Klein LW, Agarwal JB, Helfant
RH. 1986. Analysis of systolic bulging.
Mechanical characteristics of acutely is-
chemic myocardium in the conscious dog.
Circ. Res. 58:2091763. Theroux P, Ross J Jr, Franklin D, Covell
JW, Bloor CM, Sasayama S. 1977. Re-
gional myocardial function and dimen-
sions early and late after myocardial in-
farction in the unanesthetized dog. Circ.
Res. 40:15865
64. Gillam LD, Franklin TD, Foale RA,
Wiske PS, Guyer DE, et al. 1986. The nat-
ural history of regional wall motion in the
acutely infarcted canine ventricle. J. Am.Coll. Cardiol. 7:132534
65. Kass DA, Maughan WL, Ciuffo A,
Graves W, Healy B, Weisfeldt ML. 1988.
Disproportionate epicardial dilation after
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
28/32
250 HOLMES BORG COVELL
transmural infarction of the canine left
ventricle: acute and chronic differences.
J. Am. Coll. Cardiol. 11:17785
66. Kirton RS, Taberner AJ, Young AA,
Nielsen PMF, Loiselle DS. 2004. Strain
softening is not present during axial ex-
tensions of rat intact right ventricular tra-
beculae in the presence or absence of 2,3-
butanedione monoxime. Am. J. Physiol.
Heart Circ. Physiol. 286:H70815
67. Emery JL, Omens JH, McCulloch AD.
1997. Biaxial mechanics of the passively
overstretched left ventricle. Am. J. Phys-
iol. Heart Circ. Physiol. 272:H229930568. Emery JL, Omens JH, McCulloch AD.
1997. Strain softening in the rat left ven-
tricular myocardium. J. Biomech. Eng.
119:612
69. Summerour SR, Emery JL, Fazeli B,
Omens JH, McCulloch AD. 1998. Resid-
ual strain in ischemic ventricular my-
ocardium. J. Biomech. Eng. 120:71014
70. Paulus WJ, Grossman W, Serizawa T,
Bourdillon PD, Pasipoularides A, MirskyI. 1985. Different effects of two types of
ischemia on myocardial systolic and dias-
tolic function. Am. J. Physiol. Heart Circ.
Physiol. 248:H71928
71. Granzier HL, Irving TC. 1995. Passive
tension in cardiac muscle: contribution
of collagen, titin, microtubules, and inter-
mediate filaments. Biophys. J. 68:1027
44
72. MacKenna DA, Omens JH, McCullochAD, Covell JW. 1994. Contribution of
collagen matrix to passive left ventricu-
lar mechanics in isolated rat hearts.Am. J.
Physiol. Heart Circ. Physiol. 266:H1007
18
73. Zhao MJ, Zhang H, Robinson TF, Factor
SM, Sonnenblick EH, Eng C. 1987. Pro-
found structural alterations of the extra-
cellular collagen matrix in postischemic
dysfunctional (stunned) but viable my-ocardium. J. Am. Coll. Cardiol. 10:1322
34
74. Whittaker P, Boughner DR, Kloner RA,
Przyklenk K. 1991. Stunned myocardium
and myocardial collagen damage: differ-
ential effects of single and repeated occlu-
sions. Am. Heart J. 121:43441
75. Allaart CP, Sipkema P, Westerhof N.
1995. Effect of perfusion pressure on dias-
tolic stress-strain relations of isolated rat
papillary muscle. Am. J. Physiol. Heart
Circ. Physiol. 268:H94554
76. May-Newman K, Omens JH, Pavelec RS,
McCulloch AD. 1994. Three-dimensional
transmural mechanical interaction be-
tween the coronary vasculature and pas-
sive myocardium in the dog. Circ. Res.
74:11667877. Sunagawa K, Maughan WL, Sagawa K.
1983. Effect of regional ischemia on
the left ventricular end-systolic pressure-
volume relationship of isolated canine
hearts. Circ. Res. 52:17078
78. Seals AA, Pratt CM, Mahmarian JJ,
Tadros S, Kleiman N, et al. 1988. Relation
of left ventricular dilation during acute
myocardial infarction to systolic perfor-
mance, diastolic dysfunction, infarct sizeand location. Am. J. Cardiol. 61:224
29
79. Bovendeerd PH, Arts T, Delhaas T,
Huyghe JM, van Campen DH, Reneman
RS. 1996. Regional wall mechanics in the
ischemic left ventricle: numerical model-
ing and dog experiments. Am. J. Physiol.
Heart Circ. Physiol. 270:H398410
80. Schuster EH, Bulkley BH. 1979. Expan-
sion of transmural myocardial infarction:a pathophysiologic factor in cardiac rup-
ture. Circulation 60:153238
81. Mazhari R, Omens JH, Covell JW, Mc-
Culloch AD. 2000. Structural basis of re-
gional dysfunction in acutely ischemic
myocardium. Cardiovasc. Res. 47:284
93
82. Mazhari R, McCulloch AD. 2000. In-
tegrative models for understanding the
structural basis of regional mechanicaldysfunction in ischemic myocardium.
Ann. Biomed. Eng. 28:97990
83. Jackson BM, Gorman JH 3rd, Salgo IS,
Moainie SL, Plappert T, et al. 2003.
y
y
g
p
y
-
7/31/2019 Structure And Mechanics of Healing Mi
29/32
INFARCT STRUCTURE AND MECHANICS 251
Border zone geometry increases wall
stress after myocardial infarction: contrast
echocardiographic assessment. Am. J.
Physiol. Heart Circ. Physiol. 284:H475
79
84. Cleutjens JP, Kandala JC, Guarda E, Gun-
taka RV, Weber KT. 1995. Regulation
of collagen degradation in the rat my-
ocardium after infarction. J. Mol. Cell.
Cardiol. 27:128192
85. Carlyle WC,Jacobson AW, Judd DL, Tian
B, Chu C, et al. 1997. Delayed reperfu-
sion alters matrix metalloproteinase ac-
tivity and fibronectin mRNA expressionin the infarct zone of the ligated rat heart.
J. Mol. Cell. Cardiol. 29:245163
86. Whittaker P, Boughner DR, Kloner RA.
1991. Role of collagen in acute myocar-
dial infarct expansion. Circulation 84:
212334
87. Knowlton AA, Connelly CM, Romo GM,
Mamuya W, Apstein CS, Brecher P. 1992.
Rapid expression of fibronectin in the rab-
bit heart after myocardial infarction withand without reperfusion. J. Clin. Invest.
89:106068
88. Ulrich MM, Janssen AM, Daemen MJ,
Rappaport L, Samuel JL, et al. 1997.
Increased expression of fibronectin iso-
forms after myocardial infarction in rats.
J. Mol. Cell. Cardiol. 29:253343
89. Morishita N, Kusachi S, Yamasaki S,
Kondo J, Tsuji T. 1996. Sequential
changes in laminin and type IV collagen inthe infarct zoneimmunohistochemical
study in rat myocardial infarction. Jpn.
Circ. J. 60:10814
90. Cleutjens JP, Verluyten MJ, Smiths JF,
Daemen MJ. 1995. Collagen remodeling
after myocardial infarction in the rat heart.
Am. J. Pathol. 147:32538
91. Hood WB, Bianco JA, Kumar R, Whiting
RB. 1970. Experimental myocardial in-
farction. IV. Reduction of left ventricularcompliance in the healing phase. J. Clin.
Invest. 49:131623
92. Lima J, Ferrari V, Reichek N, Kramer