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J Thorac Cardiovasc Surg 2005;129:314-321
© 2005 The American Association for Thoracic Surgery
Surgery for Acquired Cardiovascular Disease |
a College of Physicians and Surgeons of Columbia University, Department of Cardiothoracic Surgery, St Luke'sRoosevelt Hospital Center, New York, NY
b Department of Hygiene and Epidemiology, University of Ioannina School of Medicine, Ioannina, Greece
c Institute for Clinical Research and Health Policy Studies, Department of Medicine, TuftsNew England Medical Center, Tufts University School of Medicine, Boston, Mass
d Department of Cardiac Surgery, University of Athens School of Medicine, Attikon Hospital Center, Athens, Greece
Received for publication February 21, 2004; revisions received May 18, 2004; accepted for publication May 24, 2004. * Address for reprints: Joseph J. DeRose, Jr, MD, Department of Cardiothoracic Surgery, St Luke'sRoosevelt Hospital Center, 1111 Amsterdam Ave, MU217, New York, NY 10025 (E-mail: jjd11{at}columbia.edu).
| Abstract |
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METHODS: We retrospectively evaluated 544 consecutive patients with ejection fraction 25% or less who underwent coronary artery bypass grafting from 1992 to 2002 at a single institution. Long-term survival data (mean follow-up 4.1 years) were obtained from the National Death Index. Multivariable Cox regression analysis was performed to construct a predictive score for long-term mortality. A split-sample approach was also used building a model on a training group (n = 360); this model was then tested on a separate validation group (n = 184).
RESULTS: From the entire database, the predictive score was calculated according to the following equation: 0.430(if past congestive heart failure) + 0.049(age in years) + 0.507(if peripheral vascular disease) + 0.580(if emergency operation) + 0.366(if chronic obstructive pulmonary disease). The 5-year survivals of the predictive score quartiles were 82.3%, 78.2%, 65.5%, and 45.5% (P < .0001). The model based on the training group had four independent predictors for long-term mortality (the same as the listed equation except for past congestive heart failure). The 5-year survival rates of the quartiles were 90.1%, 75.4%, 64.3%, and 49.2% in the training group (P < .0001) and 77.4%, 71.2%, 65.8%, and 45.5% in the validation group (P = .0001).
CONCLUSION: Coronary artery bypass grafting in patients with severe ischemic cardiomyopathy achieves satisfactory midterm and long-term survival in selected patients. This new score, which is based on long-term data from a large number of patients, may aid clinicians in selecting therapeutic interventions for patients with ischemic cardiomyopathy.
Multiple preoperative risk factors have been associated in previous studies with decreased survival benefit. These include age at the time of surgery, female sex, emergency status, end-systolic volume index, number of previous myocardial infarctions, the quality of coronary arteries, complete revascularization, myocardial viability, left ventricular end diastolic pressure, and NYHA functional class IV status.8,11,12,14-19 These various risk factors may increase mortality to a point that outweighs any potential long-term benefits of revascularization. In this study, we used long-term follow-up data of 544 consecutive patients with low EF (
25%) to create a preoperative score that can be used in predicting long-term CABG risk for death and assist in determining therapy for patients with a markedly low EF and coronary artery disease.
| Materials and methods |
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25%), who underwent isolated CABG between January 1992 and March 2002 at the St. Luke'sRoosevelt Hospital Center affiliated with Columbia University College of Physicians and Surgeons in New York. Patients who underwent concomitant valve surgery or aneurysmectomy were excluded from the study. Indications for CABG were based on standard clinical and angiographic criteria. Complete revascularization was the policy of our institution, and reliance on echocardiography, particularly transesophageal echocardiography, in addition to anatomy determined the number of anastomoses. Moving and thick segments indicating myocardial viability were all revascularized. Data were prospectively collected during admission as part of routine clinical practice and entered into the New York State adult cardiac surgery report for the variables shown in Tables 1 and 2. CHF was diagnosed by paroxysmal nocturnal dyspnea or dyspnea on exertion from heart failure or chest radiograph showing pulmonary congestion. CHF was characterized as current when diagnosed within 2 weeks before the procedure or past when diagnosed between 2 weeks to 6 months before the procedure. Peripheral vascular disease (PVD) was diagnosed by one of the following criteria: (1) a history of stroke with or without residual deficit; (2) angiographic or ultrasonographic demonstration of at least 50% narrowing in a major cerebral or carotid artery (common or internal) and history of nonembolic stroke or previous surgery for such disease (a history of bruits or transient ischemic attacks is not sufficient evidence for carotid or cerebrovascular disease); (3) angiographic demonstration of at least 50% narrowing in a major aortoiliac vessel and previous surgery for such disease, absent femoral pulses, or inability to insert a catheter or intra-aortic balloon pump because of iliac aneurysm or obstruction of the aortoiliac arteries (tortuosity of the vessel alone does not constitute aortoiliac disease as well as abdominal aortic aneurysm, history of aortobifemoral bypass, and absence of a femoral pulse on either right or left); and (4) angiographic demonstration of at least 50% narrowing in a major femoral or popliteal vessel and previous surgery for such disease, absent pedal pulses, or inability to insert a catheter or intra-aortic balloon pump because of obstruction in the femoral arteries (both dorsalis pedis and posterior tibial need to be absent in one foot; a negative popliteal pulse alone does not constitute femoral or popliteal disease as well as medications for PVD without indication of femoral or popliteal disease). Chronic obstructive pulmonary disease (COPD) was diagnosed by one of the following criteria: (1) chronic bronchodilator therapy to avoid disability from obstructive airway disease longer than 3 months, (2) a forced expiratory volume in 1 second less than 75% of the predicted value or less than 1.25 L, and (3) a room air PaO2 less than 60 mm Hg or PaCO2 greater than 50 mm Hg. Preoperative risk stratification was performed according to the EuroSCORE model.20
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Statistical methods
Numerica variables are presented as mean ± SD, whereas discrete variables were summarized by percentages. Multivariable Cox proportional hazard regression models22 were used to reveal the statistically significant risk factors that were associated with long-term mortality. Hazard ratios (HRs) and 95% confidence intervals were calculated. The model selection was done with backward stepwise method starting from all variables with P < .05 in univariate analyses. Interaction terms were considered but were not important. Continuous variables were checked for conformity with linear gradient. This multivariable model was then used to calculate a predictive score for long-term mortality after CABG.
We also used a split-sample approach. The total number of patients was divided randomly into two groups (training and validation sets). We built a multivariable model from the data of the 360 patients in the training set, and we tested the discriminating performance of the derived model on the remaining 184 patients of the validation set. The coefficients of the model derived in the full data set may be inflated because of overfitting; that is, the relative differences in prognosis between the risk quartiles may be smaller than suggested by the full data set model. This is not a problem in the split-sample approach. A measure of the potential shrinkage that needs to be applied to the coefficients of the full data set model may be obtained from the shrinkage slope b of a Cox regression that is modeling (in the validation data set) the relative hazard of death as a function of the score derived in the training data set.23 We estimated the shrinkage b in our data.
For all models tested, we generated Kaplan-Meier curves24 according to score quartiles, and the four curves were compared with the log-rank test, with adjustment for trend as appropriate. We also estimated for each model and data set the extreme quartile odds ratio (EQuOR)25; that is, the odds of death in the high-risk quartile divided by the odds of death in the low-risk quartile. The larger the EQuOR value, the better the discrimination of a model between high- and low-risk patients. All analyses were performed with SPSS version 11.0 software (SPSS, Inc, Chicago, Ill), and all P values are 2-tailed.
| Results |
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| Discussion |
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Several previous studies have examined preoperative risk factors in this population. Age is clearly a predictor of mortality risk.11,12,15 Several studies have also suggested that the severity of cardiac dysfunction, as measured by end-systolic volume index,15 number of previous myocardial infarctions,15 NYHA functional class IV status,8,11-13 and myocardial viability,6,16-18 are important determinants of death risk. The type of intervention is probably also important, and previously suggested determinants have included the quality of coronary arteries12,16 as well as emergency versus elective status,16 which was also identified as an independent predictor in our model. Female sex, diabetes, number of diseased vessels, and hypertension have been proposed in other studies14,19,26 but were not independently statistically significant in our data set, and they were also not found to be important in another large study.16
The identification of preoperative risk factors regarding the long-term mortality after CABG in patients with low EF is often hampered by limitations in the study designs, inadequate sample size or follow-up, and inconsistent definitions of low EF. Almost all the reports in the literature are retrospective investigations, and the prospective study by Shah and colleagues19 was limited to 57 patients. Moreover, these studies differed from our analysis in the preoperative factors being considered. Most of them did not analyze other extracardiac factors and comorbidities, such as COPD and PVD. Such factors were found in our analysis to be strong independent predictors for long-term mortality, and they suggest that pulmonary and vascular comorbidities may have a key impact in determining long-term outcome for these patients. Finally, there is no consensus in the literature regarding the definition of left ventricular dysfunction. We, along with others,8,26 analyzed patients with an EF of 25% or less, whereas still others have studied patients with an EF less than 30%6,13,14,16 or even less than 40%.11,15,19 On the other hand, other groups have used more strict criteria and analyzed only patients with EF less than 20%.7,9,12 These differences in definitions may affect the importance of predictive factors.
Previous studies have noted a low rate of coronary revascularization in patients with ischemic cardiomyopathy. A retrospective study of reversible defects in patients with ischemic cardiomyopathy at Mayo Clinic revealed a revascularization rate of only 13% through 5 years. This low rate of revascularization reflects at least in part the absence of optimal treatment for this complex group of patients and points to the need for a randomized clinical trial.27 It also highlights that there is some skepticism regarding proceeding with CABG for these patients because of the uncertainty about their long-term outcomes. Severe low left ventricular dysfunction is known to increase morbidity and mortality after CABG. However, recent retrospective studies have shown that CABG in patients with an EF less than 30% can have an acceptable perioperative mortality, ranging from 4% to 11%.8-10,12,14,26 Patients demonstrate improvements in both left ventricular EF and NYHA functional class after surgical revascularization in most studies,8-11 and for some patients long-term survival may be feasible after CABG. The 5- and 10-year survivals in our study were 68% ± 2% and 45% ± 4%, respectively, well in concordance with other recent studies in which 5-year survival rates have ranged from 56% to 72%.8,9,12,14,19,26 However, the patients undergoing OPCAB had a significantly lower long-term survival than those undergoing on-pump CABG. In our study this undoubtedly was related to selection bias, because the patients undergoing OPCAB had a significantly higher mean EuroSCORE. In addition, we have shown that the detrimental effect of OPCAB on midterm survival changed during the last 5 years in our institution after innovative techniques became available.28 The improvement in operative technique, the current application of robotic techniques, and the more common use of bilateral internal thoracic artery grafts may achieve even better survivals without any increase in operative mortality.5,29,30
Other than CABG, the other major option for therapy in ischemic cardiomyopathy is orthotopic heart transplantation. In determining whether a patient is a candidate for CABG or heart transplantation, a number of factors need to be considered. Heart transplantation is limited by a severe shortage of donor organs, considerable expense, moderate long-term survival, and need for continuous immunosuppression. However, it has been shown to be beneficial in patients with NYHA functional class IV symptoms refractory to medical management who have no other impairments.4 Unfortunately, transplantation remains an option limited to only a small number of patients.7 With the aging population, more patients are living well past the traditional age ceilings for acceptability onto a transplant list. The HAVOC score developed in this study may serve to guide physicians in making decisions regarding transplantation versus CABG. Thus the HAVOC score may be used to modify and clarify our previously proposed algorithm for selection of intervention in patients with ischemic cardiomyopathy (Figure 4).31
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There are several limitations in this study. First, this was a retrospective investigation. Nevertheless, the information on preoperative risk factors was collected with highly standardized methods for the New York State database. Second, a most important limitation, is that preoperative myocardial viability, which has consistently been shown to be a strong predictor of long-term mortality, was not routinely determined. However, Samady and coauthors32 have shown that lack of improvement of ventricular function is associated with a similar survival and similar improvement of angina and heart failure as seen in patients with improved ventricular function, suggesting that in patients with low EF methods for assessment of myocardial viability may underestimate the potential for symptomatic and survival benefit achieved by CABG. Importantly, our model needs to be validated externally in data sets derived from other centers and settings. The split-sample approach ensures that the derived discriminatory ability is probably not inflated, but no internal validation approach (split-sample, bootstrapping, or other) can ensure satisfactory performance on different external data sets. For external applications, the coefficients of the HAVOC model may be shrunk (eg, by 0.78) to get more accurate estimates of expected survival. Nevertheless, we generally recommend that the HAVOC score should be used for general decision making on whether a patient exceeds the threshold for CABG candidacy, rather than for estimating the exact mortality risk. Finally, the cause of death in these patients is not documented and was not necessarily cardiac related, whereas survival in relation to graft patency is unknown. However, for practical purposes prediction of overall mortality is probably more important than prediction of cardiac deaths alone. Administrative follow-up can be extremely useful, particularly when the outcomes to be examined are limited to all-cause mortality, which is less prone to error than administrative coding of medical diagnoses or procedures. However, there are still limitations of this approach, including inability to match patients and to confirm a patient's status as alive.
In conclusion, and acknowledging these limitations, this first multivariable predictive score of midterm and long-term survival from a large patient database for many years should be helpful in determining the optimal therapy for the growing group of patients with ischemic cardiomyopathy.
| References |
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