(Circulation. 1999;100:1298-1304.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
Failure to Improve Left Ventricular Function After Coronary Revascularization for Ischemic Cardiomyopathy Is Not Associated With Worse Outcome
From The Department of Internal Medicine, Section of Cardiovascular Medicine (H.S., B.G.A., C.A.M., F.J.Th.W), and The Department of Surgery, Section of Cardiothoracic Surgery (J.A.E.), Yale University School of Medicine, and The Center for Outcomes Research and Evaluation (J.A.M.), Yale-New Haven Hospital, New Haven, Conn.
Correspondence to Frans J.Th. Wackers, MD, Yale University School of Medicine, Cardiovascular Nuclear Imaging Laboratory, 333 Cedar ST TE-2 PO Box 208042, New Haven, CT 06520-8042. E-mail frans.wackers{at}diagrad.med.yale.edu
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Abstract |
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Background—Preoperative identification of viable myocardium in patients with ischemic cardiomyopathy is considered important because CABG can result in recovery of left ventricular (LV) function. However, the hypothesis that lack of improvement of LV function after CABG is associated with poorer patient outcome is untested.
Methods and Results—Outcome was compared in patients with
ischemic LV dysfunction (LVEF 
0.30) with and without
improvement in LVEF after CABG. Of 135 consecutive patients, 128 (95%)
survived CABG and 104 (77%) had pre- and post-CABG LVEF assessment. Of
these 104 patients, 68 (65%) had >0.05 increase in LVEF (group A) and
36 (35%) had no significant change, or 0.05 decrease in LVEF (group
B) compared with pre-CABG LVEF. No significant differences existed in
age, gender, comorbidities, baseline symptoms, baseline LVEF, or
intraoperative variables between groups A and B. Group A increased
LVEF from 0.24±0.05 to 0.39±0.1 (P<0.005). In Group
B, LVEF did not change significantly postoperatively, 0.24±0.05 to
0.23±0.06 (P=NS). Postoperative improvement in angina
and heart failure scores were similar between the 2 groups. Survival
free of cardiac death was similar for both groups (93% in group A and
94% in group B, P=NS) at a mean follow-up of 32±23
months.
Conclusions—Lack of improvement of global LVEF after CABG is not associated with poorer outcome compared with that of patients with improved LVEF, presumably because effective revascularization of ischemic myocardium, even without improvement in ventricular function, protects against future infarction and death.
Key Words: ischemia • cardiomyopathy • revascularization • survival
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Introduction |
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Patients with ischemic cardiomyopathy have a poor prognosis when treated medically.1 2 3 4 Although perioperative risk is increased in such patients, surgical revascularization has been shown to improve left ventricular (LV) function5 6 7 8 and prognosis.9 10 11 12 In recent years, attention has focused on establishing criteria for myocardial viability in an attempt to identify those who might benefit most from surgical revascularization.
Toward this end, several imaging modalities have been used to differentiate viable myocardium from scar, presuming that patients with evidence for substantial myocardial viability improve LV function after CABG.5 6 7 8 13 Indeed, postoperative improvement of regional and global ventricular function has been considered the bench mark against which the preoperative methods for identification of myocardial viability are measured. The underlying assumption of this strategy is that postoperative improvement of LV function is necessary for good outcome in patients with severe coronary artery disease and LV dysfunction undergoing CABG surgery. Although LV ejection fraction (EF) is known to be an excellent indicator of prognosis after myocardial infarction,14 it is not clear whether lack of improvement of LVEF post-CABG portends worse outcome.
We postulated that among patients with extensive coronary artery disease and LV dysfunction, a subgroup exists in whom coronary revascularization is beneficial, even though overall LVEF is not improved. Accordingly, we tested the hypothesis that survival of patients with ischemic cardiomyopathy post-CABG is independent of post-CABG improvement in LV function.
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Methods |
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Patients
Between March 1986 and May 1995, 135 consecutive patients with LVEF
0.30 (mean LVEF 0.24±5, range 0.10 to 0.30) underwent CABG by a
single surgeon. Patients who had concomitant valve surgery and/or
aneurysmectomy were excluded from the present
analysis. These 135 patients represent 13.2% of the
1023 patients who had CABG by the same surgeon during the same time
period. The patients included 113 males and 22 females, with a mean age
of 66.5 years (range 42 to 87 years). Eighty-four percent had prior
myocardial infarction, documented either by clinical history or Q waves
on the resting ECG; 63% had angina, 61% had heart failure, and 26%
had severe ventricular arrhythmias, for which they
had simultaneous placement of an implantable cardiac
defibrillator (ICD). A general report on the effect of CABG on
postoperative survival and LVEF in these patients has been published
previously.15
Seven (5%) of the 135 patients died during the
perioperative period. Twenty-four (19%) of the 128
CABG survivors did not undergo postoperative assessment of LVEF. The
study population, therefore, consisted of 104 patients (81%) who
survived the perioperative period and had both pre- and
postoperative assessment of LVEF (Figure 1
). The patients were divided into 4
groups: Group A: 68 patients (65%) who had >0.05 LVEF units increase
of postoperative LVEF, and Group B: 36 patients (35%) who had either a
0.05 LVEF units increase or a decrease in postoperative LVEF compared
with preoperative LVEF. Twenty-four patients who survived CABG but did
not undergo postoperative assessment of LVEF comprised Group C, and the
7 patients who died in the perioperative period are
referred to as Group D.
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Operative Technique
All patients underwent CABG by a single surgeon (J.A.E.) using
standard operative techniques. Myocardial preservation was performed by
means of systemic and topical hypothermia and cold crystalloid
cardioplegia administered into the ascending aorta before each graft.
An average of 2.7 grafts (range 1 to 5) were placed per patient. The
internal mammary artery was used in 76% of patients. Intra-aortic
balloon pumps were placed in 69 patients before surgery,
prophylactically in 55 patients (80%) and emergently in 14
patients (20%). ICDs were placed in 20 patients.
LVEF Assessment
Preoperative LVEF was assessed by contrast ventriculography (CV)
in 65 patients and by equilibrium radionuclide
angiocardiography16 (ERNA) in 39 patients. Postoperative
LVEF was assessed by ERNA in 102 patients and by CV in only 2
patients.
ERNA was performed by modified in vivo labeling of patients' own red blood cells with technetium-99 m pertechnetate. Images were acquired in 3 planar views (left anterior oblique, anterior, and lateral views) using a gamma camera equipped with a general, all purpose, parallel hole collimator interfaced with a dedicated computer. LVEF was determined using a previously validated automated computer software package.17
Contrast ventriculography was performed in the RAO projection. LVEF was determined by Simpson's method, after manual or automated drawing of contours around the left ventricle in diastole and systole.
A regression equation (radionuclide LVEF=0.86xCV LVEF+2.90), developed for the TIMI trial,18 was used to relate CV LVEFs to ERNA-derived LVEF. This formula changes the absolute value of the LVEF between the 2 methods by <0.02 points.
Classification of Symptoms
Chart review and telephone conversations with patients were used
to assess angina and heart failure classification. The Canadian
Cardiovascular Society Functional Classification was
used to stratify the degree of angina. Heart failure status was
assessed using the New York Heart Association Functional
Classification. The investigators acquiring the data were blinded to
the values of LVEF after revascularization.
Follow-Up
Follow-up was obtained from office charts, hospital records,
and by interviews with primary physicians and/or the patients and was
complete in all 104 patients. Mean follow-up was 32±23 months (median,
32 months). Cardiac death was defined as unwitnessed sudden death,
death within 1 hour of new symptoms of cardiac ischemia, or
death due to heart failure. Noncardiac death was defined as death due
to all other causes (stroke, renal failure, diabetes, or cancer).
Results of electrophysiology interrogation were available in 15 of 20
patients with ICD placement during the follow-up period.
Statistical Analysis
Continuous data are expressed as mean±1 SD and as median when
appropriate. Categorical data are expressed as frequencies. Discrete
variables were compared in each ejection fraction group by
2 analysis and continuous
variables were compared using Student's t test. The
paired Student's t test was used to compare continuous
variables before and after surgery. P<0.05 was
considered statistically significant. Event-free survival was evaluated
by Kaplan Meier product-limit method. Differences between survival
curves were compared using the generalized Wilcoxon test.
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Results |
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Baseline Characteristics
No significant differences were present in preoperative characteristics and comorbidity between patients in groups A and B (Table 1
). Mean age in group A was
66±9 years and in group B, 66±10 years (P=NS). Males
comprised 83% of patients in group A and 84% of patients in group B
(P=NS). The 2 groups were similar with respect to in
preoperative angina score, heart failure score, and mean preoperative
LVEF. The prevalence of prior myocardial infarction was similar in both
groups.
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Characteristics of patients in groups C and D are included for comparison. Six of 7 patients who died perioperatively (group D) were in cardiogenic shock and required emergent intra-aortic balloon pump placement and revascularization. Because of their extreme high-risk presentation and almost inevitable death without immediate revascularization, we feel justified in excluding them from our analysis. Obviously, postoperative LVEF could not be obtained in this subgroup.
Intraoperative Variables
Intraoperative variables are shown in Table 2. Group A had a trend toward greater use
of left internal mammary grafts (83% versus 69%, P=0.09)
and longer cross clamp times (50±15 versus 44±13, P=0.10),
but neither variable achieved statistical significance. More ICDs
were placed in group B than in Group A (25% versus 16%,
P=0.41), however, this did not achieve statistical
significance. The use of intra-aortic balloon pumps, pump times, and
numbers of grafts placed were similar in both groups. No intra-aortic
balloon pumps were placed after surgery.
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Postoperative LVEF
In 104 patients, the mean time interval between CABG and
postoperative assessment of LVEF was 16±33 days (median 7 days, range
3 to 214 days). The postoperative time interval for LVEF assessment was
not significantly different between group A,18±37 days, and B, 12±24
days (P=0.38). Sixty-two (91%) patients in group A
and 35 (97%) in group B had postoperative LVEF assessed within 6 weeks
of CABG (P=0.45).
By study design, mean LVEF improved in group A from 0.24±0.05 before
surgery to 0.39±0.10 after CABG and did not change in group B
(0.24±0.05 preoperatively to 0.23±0.06 postoperatively, Figure 2).
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Postoperative Symptoms
Heart failure and angina class improved in both groups. Angina
class improved from 2.6±1.2 to 1.2±0.3 in group A and from 2.6±1.2
to 1.2±0.5 in group B, (Figure 3). Heart
failure class improved from 2.3±1.0 to 1.5±0.7 in group A and from
2.4±1.0 to 1.5±0.7 in group B (Figure 4). Of 15 patients who underwent
postoperative interrogation of the ICD devices, 4 of 8 patients in
group A and 5 of 7 patients in group B had recorded discharges
(P=NS).
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Postoperative Survival
Figure 5 shows the actuarial
survival curves for patients free of cardiac death in groups A and B.
Mean duration of follow-up was similar in both groups, 30±21 months in
group A and 36±27 months in group B, P=NS. There was no
significant difference in survival free of cardiac death between the 2
groups: 93% in Group A and 94% in Group B, P=NS. Figure 6 compares the actuarial survival curves
for patients free of cardiac death in Groups A, B (the study
population), and C (patients who survived surgery but did not have
postoperative assessment of LVEF). There were no significant
differences in survival between the 3 groups.
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Survival free of cardiac death in a subgroup of 39 patients who underwent both pre- and postoperative ERNA for assessment of LVEF revealed no differences between those who improved LVEF and those who failed to do so after surgery.
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Discussion |
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Patients in whom LV dysfunction is predominantly due to hibernating or stunned myocardium have been shown to improve function and survival following surgical revascularization.5 19 20 21 22 However, many patients with ischemic cardiomyopathy have a mixture of scar and viable myocardium and may therefore exhibit a variable degree of improvement in ventricular function after revascularization. Whether such patients, who do not completely recover ventricular function early after revascularization, enjoy benefit from CABG surgery similar to those who improve ventricular function had thus far not been established.
Our study shows that patients with ischemic cardiomyopathy who fail to improve LVEF early after surgical revascularization nevertheless have similar symptomatic benefit and survival compared with patients who recover ventricular function early after surgery.
Relationship Between Myocardial Viability and Improvement of
Ventricular Function
Current modalities used to assess myocardial viability are based
on the premise that dysfunctional viable myocardium will
recover function after revascularization. A
threshold of radiotracer uptake (eg, >50% of normal uptake) is often
adopted to simplify myocardial viability into a binary concept: either
viable or nonviable. This threshold, which corresponds to the
likelihood that regional LV function will recover after
revascularization, has become synonymous with
myocardial viability. Myocardium which does not meet this
threshold value will usually not recover function after
revascularization and is therefore assumed to be
nonviable scar which will not benefit from
revascularization.
Our study demonstrates that patients who do not recover LV function early after revascularization and who therefore might have been considered to have nonviable myocardium by the conventional definition, derive symptomatic and survival benefit similar to patients who do recover LV function after CABG.
This finding supports the concept that myocardial viability should be considered a continuum, from full-thickness viability without any scar to full-thickness scarring without viable cells. Indeed, we believe that group B represents a subgroup of patients who possess an intermediate degree of viability as exemplified by an improvement in outcome independent of recovery in ventricular function.
Several other studies lend further support to the concept myocardial
viability being a continuum. Zimmerman et al23
demonstrated a linear relationship between Thallium-201 redistribution
defect size and the histopathologic extent of myocardial fibrosis.
Several investigators6 24 have demonstrated a relationship
between continuous measures of myocardial radiopharmaceutical uptake
and the probability of functional recovery among dysfunctional
myocardial segments. Baumgartner et al,25 using
quantitative histological characterization of
myocardium from explanted hearts, demonstrated an excellent
correlation between the amount of PET metabolic evidence
for viability, the degree of resting Tl-201 uptake, and the presence of
25% viable myocytes on histopathology.
Furthermore, 2 recent studies suggest that the degree and timing of recovery of LV function after revascularization may be a continuum, depending on the underlying pathophysiologic substrate, ie, preoperative ratio of viable-to-scarred myocardium. Elsasser et al26 showed a direct correlation between the degree of recovery of LV function after revascularization and the severity of histopathologic changes in hibernating myocardium in biopsies obtained during CABG surgery. Shivalkar et al27 demonstrated that both the degree and timing of recovery in regional LV function were related to the extent of transmural myocardial fibrosis. Patients with only small amounts of viable myocardium had markedly delayed (6-month) improvement in regional LV function after CABG, compared with patients with more extensive amounts of viable myocardium.
Implications for Noninvasive Assessment of Myocardial
Viability
Postoperative recovery of ventricular function has
been used as the bench mark against which modalities for assessment of
myocardial viability are evaluated. Comparative studies have suggested
that resting Thallium-201 SPECT imaging provided similar sensitivity
but less specificity than Dobutamine
echocardiography and PET imaging in predicting
myocardial viability.28 29 Our study is the first to
provide support for the concept that there may be a degree of
myocardial viability that, although not capable of generating an early
improvement in resting contractile function, is sufficient to elicit
favorable outcomes in symptoms and survival. This implies that
postoperative recovery of ventricular function may not be
the most appropriate variable against which to validate the
preoperative assessment of myocardial viability. Although recovery of
wall motion obviously indicates the presence of viable
myocardium, lack of improvement in function does not
exclude the presence a lesser amounts of viable myocardium.
Further support for the concept that revascularization provides clinical benefits beyond improvement of LV function is found in observations by other investigators. Patients with only modest amounts of preoperative myocardial viability on PET imaging, nevertheless improved functional status significantly after revascularization.30 Furthermore, Ragosta et al5 observed that whereas only 19% of myocardial segments with severely reduced preoperative Thallium-201 uptake recovered systolic function after surgical revascularization, 58% showed improvement in Thallium-201 uptake after surgery.
Several recent studies31 32 33 have investigated the role of preoperative viability assessment in determining clinical outcome after revascularization. Pagley et al,31 in a retrospective study, showed that patients with a larger myocardial viability index, determined by Thallium-201 imaging, had improved short- and long-term outcomes compared with patients with lower viability index. However, even the group with a lower viability index had a 2-year survival rate of 75% after CABG. This survival rate is substantially better than the reported 30% to 50% survival for similar patients treated medically,1 3 suggesting substantial benefit from revascularization. Meluzin et al32 arrived at similar conclusions on the basis of the results of preoperative dobutamine 2D echocardiography. Haas et al33 reported in a retrospective study that patients with LV dysfunction who had preoperative PET viability assessment had better perioperative and 1-year survival than patients who had no viability assessment. Although mean LVEF improved modestly in both groups and was only significant in the patients with preoperative PET viability assessment, in both groups there were individual patients who failed to have improvement of LVEF. Furthermore, it is conceivable that results of preoperative PET imaging introduced an unintentional bias in the referral of patients for surgery.
Our findings and those of other investigators suggest that the focus on predicting recovery of ventricular function after surgical revascularization is at least partially misdirected and may underestimate the full beneficial effects of myocardial revascularization in the wide spectrum of patients with ischemic cardiomyopathy.
Potential Mechanisms of Benefit of
Revascularization Without Recovery of
Ventricular Function
Several pathophysiological mechanisms may be
responsible for the clinical benefits of CABG observed despite recovery
in LV function. First, restoration of blood flow to ischemic
myocardium adjacent to endocardial scar relieves resting
ischemia, enhances the reparative process of the myocyte
contractile machinery,34 and may protect against future
infarction. Second, the revascularized myocardial bed may limit infarct
expansion and ventricular dilation by providing a
scaffolding which supports the surrounding necrotic
myocardium and reduces myocardial
compliance.34 These mechanisms may also improve
diastolic function and even reduce dynamic mitral
regurgitation, culminating in further
symptomatic improvement. Finally, by reducing LV remodeling
and the ischemic burden, revascularization
of ischemic myocardium bordering endocardial scar
may reduce the incidence of ventricular
arrhythmias.
Study Limitations
The most important limitation of this study is its retrospective
design. Retrospectively, it is not possible to retrieve the number of
patients with severe ischemic
cardiomyopathy who were not considered to be
appropriate candidates for surgery. The importance of this bias in the
selection of patients remains unclear.
Another limitation is the use of 2 different modalities for the preoperative assessment of LVEF. However, using a previously established regression equation to convert CV LVEF to ERNA LVEF resulted in a maximal difference of only 0.02 between the modalities. Furthermore, a subgroup analysis in 39 patients who underwent both pre- and postoperative ERNA assessment of LVEF confirmed no difference in survival free of cardiac death between patients who did and did not improve LVEF after CABG.
Regional LVEF was not measured. It is conceivable that some patients had improvement in regional function, without an increase in global LVEF. In >90% of our patients, the postoperative LVEF was assessed within 6 weeks of surgery. It is possible that if the interval had been longer, some patients might have been categorized differently.
All surgery was performed by a single surgeon. This may potentially limit the general applicability of our findings. On the other hand, this could be considered a favorable aspect of the study because it eliminates additional variables which relate to differences in surgeons' expertise.
Despite similarity in demographics, comorbidity, preoperative symptoms, and intraoperative variables of groups A and B, minor differences did exist. These included greater use of left internal mammary conduits and greater numbers of diabetics in patients in group A. As stated, these differences did not achieve statistical significance.
A most important limitation of this study is that preoperative myocardial viability studies were not routinely performed. Retrospectively, we identified only 23 (22%) of 104 patients who had resting myocardial perfusion imaging to assess myocardial viability. This small cohort did not warrant meaningful subgroup analysis.
Although not statistically different, more patients in group B received ICD devices than in the group A. When adjusted for ICD implantation, no significant differences existed in survival free of cardiac death between the 2 groups. Furthermore, electrophysiology follow-up did not reveal any difference in ICD discharges between the 2 groups (4 of 8 patients in group A and 5 of 7 patients in group B, P=NS).
Although the median duration of follow-up was substantial (32 months), it is conceivable that with a longer follow-up, differences in survival would become apparent between groups A and B. However, for a cohort of patients as critically ill as those included in our analysis, a survival benefit over a 32-month period is of substantial clinical importance.
Clinical Implications
The selection of patients with severe ischemic
cardiomyopathy for surgical
revascularization is complex and involves the
consideration of many clinical variables. In addition to assessment
of myocardial viability, appropriate target vessels and comorbidity are
of importance. The implication of the present study is not that
assessment of myocardial viability is superfluous. Noninvasive
preoperative assessment of myocardial viability allows for risk
stratification and identification of optimal candidates for
revascularization, in whom the greatest benefit can
be expected.31 32 33 However, we believe that a subgroup of
patients with intermediate amounts of myocardial viability may be
denied the benefits of myocardial revascularization
and improved survival, if conventional threshold criteria for
preoperative viability assessment are too rigidly applied.
Conclusion
Our study demonstrates that in patients with severe
coronary artery disease and depressed LV function who survive
CABG surgery, lack of improvement of global ventricular
function is associated with a similar survival and similar improvement
of angina and heart failure as for patients who improve global
ventricular function. These findings suggest that in
patients with LV dysfunction, methods for assessment of myocardial
viability that focus on predicting improvement of
ventricular function after
revascularization may underestimate the potential
for symptomatic and survival benefit achieved by CABG
surgery. Randomized, prospective studies comparing optimal medical
therapy with surgical revascularization are
warranted in patients with LV dysfunction and intermediate amounts of
myocardial viability.
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Acknowledgments |
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The authors would like to acknowledge the invaluable assistance of Gerrit Fleige; Anke C. Pohl, MD; and Jude Clancy, MD.
Received February 23, 1999; revision received June 15, 1999; accepted June 22, 1999.
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