19 декабря 2001 00:00 |
Low Ejection Fraction Is Not a Contraindication to Off-Pump
INTRODUCTION
Patients with coronary artery disease (CAD) and severe left
ventricular dysfunction (LVD) have a bleak prognosis despite recent
advances in medical therapy, with an estimated two-year survival
rate of only 30% [Isom 1975, Cosgrove 1984]. Although cardiac
transplantation is an effective treatment for end stage
cardiomyopathy, limited donor availability and high mortality among
patients on the waiting list make transplantation an increasingly
limited option.
Myocardial revascularization prevents further ischemic injury to
functional myocardium, restores function to hibernating myocardium,
and has been shown to improve survival in patients with moderate to
severe LVD [Isom 1975, Faulkner 1977]. Three large control trials of
coronary artery bypass grafting (CABG) versus medical management
have shown that patients with three vessel CAD and mild to moderate
LVD (35%−50%) have better survival with CABG [Faulkner 1977, Tyras
1984, CASS 1983]. Cohort studies [Bounous 1988] of patients with
LVEF of < 35% have shown that it is exactly this population of
patients who will benefit most from revascularization, particularly
if they have symptoms of angina.
It is difficult to predict the actual operative mortality in
patients with severe left ventricular dysfunction who undergo CABG.
When only patients with LVEF, less than 35% are considered. Results
of CABG are confusing because mortality rates range from 1.6% to
almost 40% [Bounous 1988]. Christakis et al. at the University of
Toronto identified contemporary risk factors for isolated CABG
[Christakis 1998]. As expected, operative mortality rates varied
with LVEF: patients with LVEF higher than 40% had a lower operative
mortality rate (2.3%) than patients with LVEF between 20%−40% (4.8%)
or patients with LVEF lower than 20% (9.8% p < 0.001). Traditional
risk factors[~]urgency of surgery, female sex, reoperation, left
main coronary artery stenosis, and age-predicted operative mortality
in patients with LVEF higher than 40%. In patients with LVEF between
20%−40%, operative risk was predicted not only by these risk
factors, but also by myocardial protection. However, the only
multivariate predictor of operative mortality for patients with LVEF
less than 20%, was urgency of operation [Christakis 1998].
A significant predictor of operative mortality in patients with LVEF
between 20%−40% was myocardial protection, probably because these
patients are more likely to have extensive areas of jeopardized
myocardium [Guyton 1987]. Perioperative mortality rates in excellent
centers appear to range from 3%−10% in carefully selected patients
with severe LVD, provided that meticulous attention is given to
preoperative myocardial protection [Elefteriades 1997].
Despite the success of CABG with CPB, the deleterious effects of CPB
are well documented. Many centers, including ours, have shown that
beating heart CABG can be performed safely with results similar to
CABG with CPB [Buffalo 1996, Pfister 1992, Calafiore 1997,
Subramaniam 1997]. Today advances in technology allow for a near
bloodless operative field and near motionless target area to ease
the technical difficulty of performing a beating heart anastamosis.
The aim of the present study was to analyze the potential beneficial
role that CABG without CPB (i.e., off CPB) may have in reducing
morbidity and improving outcomes in patients with LVEF < 40%. This
was accomplished by comparing the outcomes of matched patients in
whom revascularization was performed with and without CPB.
MATERIALS AND METHODS
We performed a retrospective, non-randomized, non-blinded study on
903 patients between 1995−1999 with ejection fractions (EF) below
40%. Seven hundred two of these patients underwent conventional CPB
CABG and the remaining 201 had off pump CABG's. In the time frame of
this study, it is important to keep in mind some technical
milestones that included the use of stabilizers as of 1997 and the
left internal mammary artery (LIMA) stitch in January of 1999
[Bergsland 1999]. Median sternotomy was the exposure of choice in
most instances for complete myocardial revascularization in both
groups. Revascularization of the marginal branches of the circumflex
were not considered contraindications to «off pump» coronary
grafting.
Technical considerations included use of the single suture («LIMA»
stitch) technique in the oblique sinus of the posterior pericardium
used to obtain exposure [Bergsland 1999], and mechanical
stabilization with an epicardial foot-plate to reduce motion.
Ischemic preconditioning especially of LAD vessels was routine for
3−5 minutes and this was the first vessel grafted during off pump
cases. Intracoronary shunts were used to facilitate flow during
anastamosis, the details of which have been previously described
[Rivetti 1998]. The CO2 blower/saline aerosolizer [Bergsland 1998]
was used to maintain a bloodless field of vision. All off pump
grafts were interrogated by use of the transit time flow meter.
All relevant information which included demographic data,
preoperative risk factors and comorbid conditions, angiographic data
with severity and distribution of significant coronary artery
disease, morbidity and mortality rates were recorded. The severity
of angina was categorized according to the Canadian Cardiovascular
Society (CCS) classification. The LVEF was determined in all cases
by left ventriculography during coronary angiography. The type of
operative priority was defined as emergent when severity and
distribution of coronary pathology in combination with hemodynamic
instability mandated immediate intervention. The management of some
of these patients had included vasopressors, intra-aortic balloon
counterpulsation, and cardiopulmonary resuscitation. Patients in
whom surgical intervention was promptly undertaken in the face of
ongoing ischemia, failed angioplasty, or as a result of unfavorable
anatomy (i.e., left main disease), were referred to as urgent.
The data collected from both study groups was statistically analyzed
and compared. Statistical analysis was conducted using Epi Info,
version 6. Continuous variables were contrasted using the Student's
t test. The Fisher exact test was used when the expected value of a
cell was less than 5. Differences between variables were considered
significant when the p value was less than 0.05.
RESULTS
The risk adjusted mortality for the off- pump CABG group was not
statistically significant (1.5 % vs. 2.2 %, p = NS). A higher
proportion of the off CPB group were «re-do» cases compared to the
CPB group (34.3% vs. 9.7%, p < 0.005) and the incidence of calcified
aorta was higher in the off pump group (8.0% vs. 3.6%, p = 0.016).
Postoperative complications (stroke, transmural MI, deep sternal
wound infection, bleeding, renal and respiratory failure) were
identical in both groups. The overall lack of complications was
88.6% vs. 81.8% (p = 0.024) for the off CPB and CPB groups
respectively. There were no differences in age, sex, or
elective/urgent status. Preoperative risk factors (stroke,
hypertension, previous MI, diabetes, and CHF) were identical in both
groups.
DISCUSSION
Revascularization of ischemic myocardium in patients with severely
impaired left ventricular (LV) function remains a surgical
challenge. In the past, perioperative mortality after CABG in
patients with poor LV function has been reported to be between
10%−37% [Kaul 1996] but more recent reports indicate a much lower
mortality (2.3%−5%) attributed to advances in myocardial management
and surgical technique.
In the New York State Cardiac Surgery Reporting System, the risk of
perioperative myocardial infarction was 1.5%, the risk of stroke was
1.6% and the risk of all major nonfatal complications was 9.2%.
These data suggest that CABG can be safely performed in patients
with severe LVD with low operative mortality and minimal morbidity
[CASS 1983, Mochtar 1985, Milano 1993].
As noted in the results, there was a significantly higher percentage
of patients in the off pump group who were re-do's with a higher
percentage having calcified aortas. Other preop risk factors that
were higher in the off pump group included a higher pre-op stroke
and renal failure incidence. When followed and calculated, the risk
adjusted mortality rate was 1.5% in the off pump group as compared
to the 2.2 % in the CPB group but was not of statistical
significance. While not statistically significant, the stroke rates
(new neurological deficit) in the off pump group was 2.5% and 2.7%
in the CPB group. There was also a statistically significant higher
freedom from overall complications (88.6% off pump and 81.8% CPB).
There was no significant difference in the grafts per patient,
distribution of diseased vessels, male-female ratios, length of
stay, or specific post operative complications (stroke, GI bleeding,
sepsis, wound infection, cardiac and respiratory compromise).
The concept of chronic myocardial ischemia has broadened to include
the metabolic responses of stunned and hibernating myocardium, both
of which have been identified as independent predictors of
cardiovascular complications or death [Jones 1978, Pryor 1987].
Hibernation, defined as cessation of contraction without loss of
viability, is caused by sustained or repetitive hypoperfusion and
can be metabolically expressed by an increased uptake of
18fluorodeoxyglucose (FDG+) in myocardial segments with reduced
blood flow (flow-metabolism mismatch) on positron emission
tomography (PET). The most widely available and accepted physiologic
test for determining the presence of ischemic myocardium is
myocardial perfusion scintigraphy, such as Thallium 201 imaging with
poststress, redistribution, and rest injection imaging. The absence
of angina in patients with advanced symptoms of heart failure does
not exclude the presence of extensive areas of myocardial viability
[Pryor 1987]. Despite improvements in surgical technique and
perioperative care, LVEF remains an important predictor of operative
mortality [Hung 1980]. However it has been shown that patients at
highest risk are also the ones who derive the greatest benefit from
CABG. The reported benefits of beating heart or off-pump CABG
include: shorter post operative hospital stays, shorter time with
ventilatory support, less blood loss and need for transfusions, less
likelihood of low output syndrome, reduced systemic inflammatory
response, fewer postoperative arrhythmia and neurologic
complications, and potential cost savings [Moshkovitz 1997,
Bergsland 1998].
In terms of the technical aspects of bypass, Winkel et al. [Winkel
1997] state that the operation should be expedient, avoiding
prolonged aortic cross clamping. The demographics and presentation
of more patients today include the fact that patients are older, are
more frequently re-do operations, have more associated
viability and myocardial tolerance to any further insult is likely
to be smaller. Off pump complete myocardial revascularization may
prove to be that improvement in surgical technique necessary to
overcome the problems plagued by the impaired, dysfunctional
ventricle [Bergsland 1998, Moshkovitz 1997].
While off-pump CABG surgery is proving, at the very least, as
effective as CPB CABG in hemodynamically stable patients with EF
greater than 40%, we have shown here its safe and efficacious use in
patients with ejection fractions below 40%. The question of «how low
is too low» in the evaluation of ejection fraction as a risk factor
[Bolling 1999] may soon be made irrelevant in the decision to offer
revascularization via off-pump CABG to patients.
REVIEW AND COMMENTARY
1. Editorial Board Member MB134 writes:
The authors state that the complications are the same in both
studied groups, but does not elaborate. What were the complication
rates? There should at least be a table of data showing the
complication rates in both groups with their corresponding p-values.
There was no comment on whether conversion to CPB was more common
with low EF than normal EF. I would like to know this.
It would be helpful if the authors can examine their database to
find if the usage of the IABP was more or less common with OPCAB
compared with on CPB. This would support the claim for lessened
disturbance of the remaining, viable myocardium by OPCAB as compared
with standard CPB and cardioplegia.
Authors' Response by Reginald Abraham, MD:
The reviewer raised the question as to what the complications were
(that were compared in the study), and suggested a table of data
showing the complications (stroke, transmural MI, bleeding, renal
and respiratory failure) and rates in both groups and their
corresponding p-values. I have included such a table as an appendix
below.
The table also addresses this reviewer's other query regarding IABP
use: 6.9% CPB and 1.7% off pump (p=0.045). IABP use is 18% in the
off pump group postoperatively.
Regarding conversion to CPB in low EF vs. normal EF, in the overall
Buffalo experience, there was no greater rate of conversion based on
EF. As can be expected, an increased CPB conversion rate was noted
in the early off pump experiences in many institutional series.
2. Editorial Board Member NC124 writes:
The authors explained that in some cases, hemodynamic support was
needed either with drugs or IABP. My question is if they considered
or utilized «elective» pre-op IABP or off-pump cases as part of the
management of these low EF-high risk patients?
Authors' Response by Reginald Abraham, MD:
Use of IABP was not considered «routine» in all low EF patients.
Despite strong evidence in the efficacy of IABP use pre-op in this
setting, the judgment of the individual surgeon mandated its use in
each low EF case (numbers shown in table below).
3. Editorial Board Member GX21 writes:
The last sentence of the Introduction says that this was a
comparison of «matched patients», but the Materials and Methods
section does not mention the matching nor how it was done.
The Results section mentions «risk adjusted mortality» in the first
sentence. It is not stated how this risk adjustment was done.
Authors' Response by Reginald Abraham, MD:
The matching of these 381 retrospective patients was done on the
basis of number of grafts, gender distribution, EF operative status
the risk adjusted mortality, it was calculated by NYS Database
standards.
AUTHOR/ARTICLE INFORMATION
VIEW APPENDIX
Submitted October 6, 2000; accepted October 9, 2000.
Address correspondence to: Hratch L. Karamanoukian, M.D. Kaleida
York 14203, Phone: (716) 859−1080, Fax: (716) 859−4687, Email:
lisbon5@yahoo.com
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