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Ann Thorac Surg 2002;73:1196-1203
© 2002 The Society of Thoracic Surgeons
a Cardiopulmonary Research Science and Technology Institute, Dallas, Texas, USA
b Statistics and Computer Center, MedStar Research Institute, Washington, DC, USA
c Washington Hospital Center, Washington, DC, USA
* Address reprint requests to Dr Magee, 7777 Forest Lane, Suite A323, Dallas, TX, USA 75230
e-mail: mmagee{at}csant.com
Presented at the Thirty-seventh Annual Meeting of The Society of Thoracic Surgeons, New Orleans, LA, Jan 29–31, 2001.
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Abstract |
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Methods. A review of two large databases employed by multiple surgeons in the hospitals of two institutions identified 8,758 multivessel CABG procedures performed from January 1998 through July 2000. In all, 8,449 procedures were included in a multivariate logistic regression analysis to determine the relative impact of CPB on mortality independent of known risk factors for mortality. Procedures were also divided into two treatment groups based on the use of CPB: 6,466 had CABG with CPB (CABG-CPB), 1,983 had CABG without CPB (OPCABG). Disparities between groups were identified by univariate analysis of 17 preoperative risk factors and treatment groups were compared by Parsonnet’s risk stratification model. Finally, computer-matched groups based on propensity score for institution selection for OPCABG were combined and analyzed by a logistic regression model predicting risk for mortality.
Results. CABG-CPB was associated with increased mortality compared with OPCABG by univariate analysis, 3.5% versus 1.8%, despite a lower predicted risk in the CABG-CPB group. CPB was associated with increased mortality by multiple logistic regression analysis with an odds ratio of 1.79 (95% confidence interval = 1.24 to 2.67). An increased risk of mortality associated with CPB was also determined by logistic regression analysis of the combined computer-matched groups based on OPCABG-selection propensity scores with an odds ratio of 1.9 (95% confidence interval = 1.2 to 3.1).
Conclusions. Elimination of CPB improves early survival in multivessel CABG patients. Rigorous attempts to statistically account for selection bias maintained a clear association between CPB and increased mortality. Larger multiinstitutional studies are needed to confirm these findings and determine the most appropriate application of OPCABG.
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Introduction |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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Patients and methods |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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Patient data were collected and analyzed according to The Society of Thoracic Surgeons (STS) National Cardiac Surgery Database guidelines and definitions (http://www.ctsnet.org/doc/4314). Preoperative data were obtained relative to diabetes, chronic pulmonary disease (COPD), prior cerebrovascular event (CVA), peripheral vascular disease (PVD), obesity, hypertension, hypercholesterolemia, family history of CAD, renal failure, renal failure on dialysis, current smoking, prior myocardial infarction, preoperative IABP use, preoperative cardiogenic shock, left main coronary disease, age, gender, left ventricular function, and prior CABG. The Parsonnet risk stratification model [8] was used to broadly define differences between treatment groups that might influence selection and patient outcomes. Perioperative data collected or calculated and compared between groups included number of bypass grafts performed, blood product use, reoperation for bleeding, perioperative myocardial infarction (MI), prolonged ventilation, renal failure, neurologic dysfunction, surgery to discharge length of hospital stay, and in-hospital operative mortality.
Statistical analysis
Patients were grouped and compared according to surgical treatment, OPCABG versus CABG-CPB. Preoperative patient characteristics and individual risk factors, intraoperative course, and operative outcomes including mortality were compared by univariate and multivariate analyses using SAS (SAS Institute, Cary, NC) or S-PLUS (Insightful Corporation, Seattle, WA) statistical software. Data are reported as a percentage or as a mean ± standard deviation or for variables not normally distributed the median, minimum and maximum are reported. Univariate comparisons were computed using the Mantel-Haenszel 
2 test of general association unless otherwise stated. Fisher’s exact test was used in comparisons of small cell counts. Ordinal categorical data were compared using the Cochran-Armitage test for trends. Comparisons of continuous data were done using the Wilcoxon rank test. All tests are two-sided. All p values less than or equal to 0.05 were considered significant.
Multivariate logistic regression analysis was used to determine the significant independent predictors of mortality which are reported as odds ratios with 95% confidence intervals (CI). The maximum likelihood approach was used to compute the estimates and residual analysis and the Hosmer and Lemeshow goodness-of-fit statistic were used to evaluate the model fit. Additionally, a logistic regression comparing preoperative risk factors between the CABG-CPB and OPCABG groups for each institution was used to create OPCABG-selection propensity scores. OPCABG patients were then computer matched to CABG-CPB patients in a 1:2 ratio within the same institution by propensity score. A logistic regression model predicting mortality was then computed using the method of generalized estimating equations. This method takes into consideration the correlation between matched sets when computing estimates.
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Results |
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To better define differences in risk between the two disparate treatment groups, Parsonnet’s predicted risk of mortality was calculated for each patient and the means and medians compared. The OPCABG group had a significantly higher predicted risk for mortality than the CABG-CPB group (Table 2).
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Comment |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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The issue of complete revascularization could not be determined in either treatment group given that the number of diseased vessels or the preoperative assessment of number of bypasses needed was not available for comparison with the number of bypasses completed. The decreased median number of bypass grafts in the OPCABG group may simply be part of the selection process as shown in the analysis and corrected for in the computer-matched model.
The database employed in this study does not characterize coronary anatomy, therefore it could not be determined whether a disproportionate number of what might be considered difficult revascularizations, eg, diffuse, small, calcified vessel disease, are represented in one treatment group versus the other. This characteristic is not incorporated into the risk stratification model either; therefore it is difficult to determine the impact if any this may have on outcome. It is not our impression, however, that this is a significant negative selection factor in patients considered to otherwise be appropriate and preferred candidates for OPCAB based on other more compelling risk factors.
Every effort was made within the limitations of a retrospective analysis to minimize selection bias. The OPCABG and CABG-CPB groups were clearly disparate with respect to risk factors and the differences were not consistent at the two contributing institutions. These findings are expected given that the majority of surgeons currently do not broadly apply OPCABG techniques in their practice. The OPCABG patients represent a selected group and this can only be completely eliminated by a prospective randomized trial; however, meaningful comparisons can be made and selection bias minimized by the statistical models used. The varied differences in risk factors noted in the univariate analysis can be distilled down to mean or median predicted risk scores comparable between groups through risk stratification models. The OPCABG group has a significantly higher predicted risk for mortality than the CABG-CPB group. This is not surprising, given the fact that many surgeons select patients for OPCABG based on medical comorbidities considered to increase the risks of CPB. The selection process is complex, however, and is not likely explained by this difference in risk alone. Other contributions of the selection process to mortality differences were considered. The multivariate logistic regression analysis for risk factors associated with selection for OPCABG helps to define and quantify the selection process for each institution. The institution-specific propensity score for each OPCABG patient derived from this analysis was used to computer-match with an equivalent patient in the CABG-CPB group. The statistical analysis of the computer-matched group, taking into account the correlation between matched sets, found that the CABG-CPB patients are almost two times (1.9) more likely to die than the OPCABG patients. The standard multivariate logistic regression analysis also showed an independent risk for mortality associated with CPB of 1.79 in tandem with a number of other known risk factors associated with mortality. Finally, the univariate analysis also found a statistically significant increased observed mortality in the CABG-CPB group, 3.1 versus 1.8, as well as increased morbidity including increased reoperation for bleeding, patients receiving blood transfusions, prolonged ventilation, and renal failure. It is interesting that the causes of death were similar in both groups, particularly the neurologic and pulmonary related mortalities, with the exception of a higher proportion of cardiac deaths occurring in the CABG-CPB group and a higher proportion of vascular deaths in the OPCABG group.
The consistent findings of increased early mortality associated with CPB in multiple, increasingly rigorous, analyses are even more compelling given the size of this study population derived from multiple surgeons in two large institutions. Only a large, multi-institutional prospective study would more convincingly show that elimination of cardiopulmonary bypass improves early survival in multivessel CABG patients. The need for such a study to confirm these findings and determine the most appropriate application of OPCABG is likely but debatable.
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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Discussion |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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Our results at Lenox Hill also echo the results that have been found at the Washington Heart Center and also in Dallas. I would like to present these results very briefly, because we looked at an unselected group of patients, so we did not have any intuitional bias or surgeon bias, evaluating 430 consecutive OPCAB patients, comparing those with 453 conventional patients that were done 2 years previously before OPCAB surgery became a part of our institution. As in the previous presented presentation, the OPCAB group had a higher preoperative risk profile. The mean number of grafts were slightly higher in the conventional group, at 3.4 compared with 3.2 in the OPCAB group. In this total intention to treat OPCAB group, we had a 2.6% conversion rate with one death.
Overall complication rate, sternal infection, reoperation for bleeding, transfusion requirements, and operative mortality and risk-adjusted New York State operative mortality were significantly less in the OPCAB group. Length of stay was also, as stated in the previous group, approximately 1 day less in the OPCAB group. Perioperative MI was not significant or different between the two groups, and interestingly, perioperative stroke rate—again, these are unselected patients—was approaching significance with a 1.4% stroke rate in the OPCAB group compared with 3% in the conventional group (p < 0.07). Multiregression analysis also, as in the previous discussion, conventional coronary bypass grafting was an independent risk factor for operative mortality.
As more and more studies become available demonstrating the potential benefits of OPCAB surgery, I would like to ask the authors if they did evaluate their data based on surgeon-specific preference? Of course, experience with this procedure may affect the results. Secondly, there has been a lot of talk for randomization, and I would like to ask the author, who is an OPCAB surgeon, whether he could unbiasedly randomize his patients to on-pump? And the third question is how "beating heart" surgery has affected his patients’ awareness and referral practice at his institution, and how that could potentially affect the implementation of a randomized trial?
DR VAUGHN A. STARNES (Los Angeles, CA): I want to follow up on one of Dr. Connolly’s points. I think we do owe a great debt of gratitude to these two institutions, which have given us large numbers and actually very excellent results. I would like to pose to them a null hypothesis, that the surgeon makes no difference in this data selection, and could they tell me if that is true. I mean, they have a very large group of surgeons at each institution, and yet we see in a prospective randomized trial 23% of their patients were done off-pump, so that means 70-some percent were done on. Is that a surgeon-selection issue, patient-selection issue, if they could be so kind to help me with that question.
DR RALPH J. DAMIANO (St. Louis, MO): I would like to congratulate Dr. Magee and all the co-authors for their pioneering work in the field and an absolutely beautiful study with very sophisticated statistical analysis. One of the weaknesses of this study is that there were different surgeons in each group. This is a weakness of many of the studies that were presented in this session. Obviously you wonder, would the OPCAB surgeons get a lower mortality even if they did an on-pump comparison? I was wondering if you have looked back at the pre-1995 "on-pump" mortality of the two groups of surgeons to see if there was a difference in their operative mortality. If their mortality was the same with "on-pump" procedures, before the era of off-pump surgery, it would make a more convincing argument that the mortality difference in this study was due to eliminating cardiopulmonary bypass and not due to the different technical abilities of the two different groups of surgeons.
DR PAUL KURLANSKY (Miami Beach, FL): In view of the previous excellent presentation and the findings of Dr. Grover with the STS database, it was very interesting that the incidence of stroke rate was not significantly different in this study. I was just wondering if perhaps you could comment on the reason for that difference.
I would obviously like to echo the congratulations of the other previous discussants on an excellent study. Thank you.
DR MAGEE: I would like to thank the discussants for their kind comments and insightful questions. I will attempt to briefly answer each of them.
Individual, surgeon-specific impact was not examined in this particular study. We believe that this may indeed be important and have just begun to examine this effect.
To reiterate and clarify the surgeon composition in this particular study, the CRSTI research group at Dallas consists of 22 surgeons and the Washington Hospital Center 5 surgeons. As I mentioned, individual surgeon experience with off-pump surgery varies diversely and represents a wide degree of adoption, particularly in Dallas. Our concern about surgeon-specific influence on patient selection and outcome was one of the reasons we made a concerted effort to correct for selection bias in this particular study. The argument that off-pump surgeons represent a unique group and on-pump surgeons another, each with unique qualities, is an interesting one and difficult to address and indeed was not addressed specifically in this study. The corollary that off-pump surgeons represent a skilled group that might achieve technically superior results in all patients, on-pump or off-pump is intriguing, difficult to assess, and politically-charged. This certainly was not addressed in this study. Certainly most surgeons will perform procedures better, and with improved outcomes, as experience increases with a given procedure. Other than through a randomized trial, these surgeon-specific issues will be difficult to address.
In terms of using data prior to 1995 there are several reasons why we chose not to use that for comparison purposes. We wanted to limit any impact that a learning curve might have on our results. Also, prior to 1995 the off-pump surgery patients included a large number of single-vessel bypasses with alternative incision approaches, introducing a variability that we wanted to avoid. Looking at a group of surgeons experience in on-pump surgery prior to 1995 and comparing that to the same surgeon’s current experience with off-pump surgery addresses surgeon-specific technical impact but comparing two different time periods represents change in the surgeon’s experience and change in the patient populations compared.
In response to the question about a randomized trial, as I mentioned this would address some of these surgeon specific issues. Speaking from experience, referral patterns and patient preferences have impacted on our ability to implement a randomized trial. I would be willing to participate in such a study, although I know from our experience that enrollment would be difficult.
In terms of the stroke rate being similar in our two study groups, I can’t offer any specific explanations. I can tell you that in our smaller, single-institution study population we examined in the past, there were significant differences in the stroke rate not seen in this larger population of patients. Although the data do not reflect it, a partial occlusion clamp was likely used in the majority of the study patients and this may have impacted on our findings.
I would again like to thank the discussants for their questions and kind comments and to the Society for the opportunity to present our results.
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