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a Emory University, Atlanta, Georgia
b University of Florida, Jacksonville, Florida
c Duke Clinical Research Institute, Durham
d Duke University School of Medicine, Durham
e Eastern Carolina University, Greenville, North Carolina
Accepted for publication June 7, 2007.
* Address correspondence to Dr Puskas, Emory University School of Medicine, 6th Floor Medical Office Tower, Emory Crawford Long Hospital, 550 Peachtree St, NE, Atlanta, GA 30308 (Email: john.puskas{at}emoryhealthcare.org).
Presented at the Forty-third Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Jan 29–31, 2007. Winner of the J. Maxwell Chamberlain Memorial Award for Adult Cardiac Surgery
| Dr Puskas discloses that he has a financial relationship with Medtronic and Guidant.
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Abstract |
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 Top Abstract IntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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Methods: The Society of Thoracic Surgeons National Cardiac Database was reviewed for risk factors and clinical outcomes of 42,477 consecutive, nonemergency, isolated, primary ONCAB or OPCAB cases performed at 63 North American centers that performed more than 100 OPCAB cases between January 1, 2004, and December 31, 2005. Odds ratios for adverse events, adjusted for 32 clinical and demographic covariates, were compared by multiple logistic regression models between women and men who had OPCAB versus ONCAB. All analyses were by intention-to-treat; 355 (2.2%) patients converted from OPCAB to ONCAB intraoperatively were included in the OPCAB group.
Results: Women (n = 11,785) and those treated with OPCAB (n = 16,245) were older and had more comorbidities than men (n = 30,662) and those treated with conventional ONCAB (n = 26,202). Overall, adjusted odds ratios for death and most major complications in both men and women were significantly lower with OPCAB than with ONCAB. Among ONCAB cases only, women had a significantly greater adjusted risk of death, prolonged ventilation, and long length of stay than men. In contrast, among OPCAB cases, women had lower risk of reexploration than men and similar risks for death, myocardial infarction, and prolonged ventilation and hospital stay.
Conclusions: OPCAB is associated with lower adjusted risk of death and major adverse events than ONCAB. OPCAB benefits both men and women and reduces the gender disparity in clinical outcomes after CABG.
In an effort to reduce morbidity and mortality attributable to cardiopulmonary bypass (CPB) [1–3], surgeons in the United States performed more than 20% of all coronary artery bypass graft (CABG) operations without the use of CPB in 2005 [4]. Several large retrospective reviews have demonstrated that coronary artery bypass grafting (CABG) performed without CPB (OPCAB) is associated with a lower risk-adjusted rate of morbidity and mortality than CABG with CPB (ONCAB) [5–7]. These analyses have been subject to legitimate criticism, however, because outcomes for patients converted intraoperatively from OPCAB to ONCAB were recorded in the ONCAB group. This is important, because patients who require emergency conversion from OPCAB to ONCAB have a high incidence of multiple negative outcomes, including death [8].
No large multicenter database has previously been able to report outcomes on an intention-to-treat basis. In 2004, however, the Society of Thoracic Surgeons (STS) National Adult Cardiac Database added a data field to record intraoperative conversions, permitting the comparison of outcomes after OPCAB and ONCAB by intention-to-treat.
Prospective, randomized trials [9–12] and meta-analysis [13] have also demonstrated improved outcomes after OPCAB compared with ONCAB. However, perhaps owing to small samples, these randomized trials have inadequate statistical power to demonstrate improvements in infrequently occurring events such as death, stroke, and myocardial infarction (MI) or to evaluate gender-specific outcomes.
Historically, comparisons of outcomes for women versus men having CABG have, with few exceptions [14, 15], shown a higher incidence of morbidity and mortality for women [16–20]. Of importance is that most of the larger retrospective comparisons of outcomes between women and men did not include large numbers of OPCAB patients, usually because these comparisons were performed before OPCAB gained wide acceptance.
The present study was designed to compare in-hospital outcomes for patients having OPCAB and ONCAB in an intention-to-treat analysis and to determine whether OPCAB might alter the gender-based disparity in morbidity and mortality commonly observed after CABG.
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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This 2-year time frame was chosen because it included the entire period during which complete data were available since the introduction within the STS database of the data field for intraoperative conversion of CABG cases from OPCAB to ONCAB. Therefore, all analyses were by intention-to-treat; patients converted intraoperatively from OPCAB to ONCAB were included in their originally intended OPCAB group.
Inclusion/Exclusion Criteria
The study excluded 2822 emergency and 2364 redo cases because so few of these are performed off-pump that selection bias may especially complicate comparisons between the treatment groups. Also excluded were 539 patients who had a planned combination of OPCAB and ONCAB or who were missing data for surgery type, as well as patients having CABG combined with valvular or other cardiac procedures. Forty patients with endocarditis and 3 patients with missing data points for gender or age were also excluded.
In an effort to include cases from institutions committed to performing CABG by both techniques and to exclude the outcomes of institutions very inexperienced in OPCAB or coronary surgery in general, we excluded patients from institutions that performed less than a total of 100 cases of each type of operation during the 2 calendar years under study. This resulted in a final sample of 42,477 consecutive patients available for analysis from 63 experienced institutions (including eight with cardiothoracic residency programs) that contribute data to the STS National Adult Cardiac Database.
Treatment
Each patient underwent a single surgical session consisting of OPCAB or ONCAB, presumably at the discretion of the attending surgeon. Because this is a multicenter, retrospective study, we were not able to gauge the uniformity of surgical technique applied to patients in either group beyond what was reported in the STS database.
Data Management
Patient medical data, prospectively entered and retrospectively reviewed, included risk factors such as demographic data, preexisting comorbidities, perioperative status, operative strategy, and clinical outcomes. This study used the data fields and definitions of the STS National Adult Cardiac Database. Data for each institution were managed by local database staff and warehoused in locked, secure facilities, and protected by computer passwords and firewalls. The STS maintains strict reporting criteria and uses an exhaustive data-checking algorithm to help ensure data quality for patient records that are uploaded to the database.
All comparisons and model terms were preplanned. Before analysis, 32 clinical and demographic risk factors were identified as possible confounders of outcome in addition to gender and surgery type. These are listed in Table 1. Of importance, diabetes was classified into three classes: insulin dependent, noninsulin dependent, and none. Chronic lung disease (COPD) was classified as severe, moderate, mild, or none. MI was classified in terms of time elapsed between MI and surgery: less than one day, 1 to 7 days, 1 to 3 weeks, more than 3 weeks, or none.
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Because this database is collected and monitored in a prospective fashion, missing data were sparse. One of the exclusion criteria mandated that gender, age, and procedure type (OPCAB or ONCAB) be recorded in the database for all study patients. In cases where other covariates were missing, the missing value was imputed with the most frequent response. Data for in-hospital death were complete for all but 8 patients; data for perioperative stroke and MI were missing in 3.2% and 3.1% of patients, respectively. All missing data were assumed to be missing at random.
Statistical Analysis
The study had two primary aims. The first was to compare outcomes after OPCAB and ONCAB by an intention-to-treat analysis, and the second was to examine whether surgery type (OPCAB versus ONCAB) alters the gender-based disparity in morbidity and mortality historically observed after CABG.
To accomplish these goals, separate logistic regression models related the primary variables of interest (gender and surgery type), adjusted for 32 preplanned covariates (Table 1), to each outcome. These outcomes included operative mortality, stroke, MI, major cardiac adverse event (MACE, a composite endpoint of mortality, stroke, and MI) any reoperation, deep sternal wound infection, atrial fibrillation, renal failure, dialysis, prolonged ventilation (> 24 hours), and length of stay (LOS; dichotomous: < 14 days, 
14 days).
Each outcome was expressed as a dichotomy so that odds ratios (ORs) could be computed for purposes of comparison. Adjusted ORs with 95% confidence intervals were computed by multiple logistic regression (MLR) models for each comparison of interest between women and men who had OPCAB versus ONCAB. For each outcome, ORs for the effect of gender and surgery type were estimated using a model that included main effects for gender and surgery type but did not include the interaction between gender and surgery type. Subsequently, the interaction between gender and surgery type was added to the model to assess the extent to which surgery type modified the effect of gender, and vice versa.
STS personnel were integral in the planning phases of this study and in performing all analyses, consistent with stated policies of the STS. The data were analyzed with SAS 8.2 software (SAS Inc, Cary, NC). All statistical comparisons were evaluated at the 
= .05 significance level. The values of p for Tables 1, 2, and 3
were computed using a 
2 test for categoric variables, the Wilcoxon rank-sum test for continuous variables compared with a dichotomous response, and the Kruskal-Wallis test for continuous variables compared with a categoric response with more than two levels.
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Results |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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Unadjusted Outcomes
Of the 16,245 OPCAB cases, 355 (2.2%) were converted during surgery from an initial OPCAB approach to ONCAB. These 355 patients had an operative mortality of 6.5% and were analyzed within the OPCAB group.
After CABG, women had significantly higher incidences of death (2.8% versus 1.4%), stroke (1.8% versus 1.1%), MI (1.5% versus 1.1%), and the combined end point of MACE (5.6% versus 3.2%) than men. Women similarly had statistically greater incidence of prolonged ventilation, renal failure, new dialysis, reoperation for bleeding, and prolonged LOS (Table 2). Men and women had a similar incidence of deep sternal wound infection. Men had statistically greater incidence of postoperative atrial fibrillation (AF) compared with women.
The OPCAB patients had significantly lower incidences of stroke (0.9% versus 1.5%), MI (0.9% versus 1.4%), and the combined end point of MACE (3.1% versus 4.4%) than the ONCAB patients. The unadjusted mortality between ONCAB and OPCAB (1.8 versus 1.6%; p = 0.17) was not significant; however, incidences of prolonged ventilation, renal failure, new dialysis, AF, and prolonged LOS were all significantly lower after OPCAB than after ONCAB.
Table 3 summarizes unadjusted comparisons of outcomes for combinations of gender and surgery type. Among men, OPCAB was associated with a significantly lower incidence of stroke (p = 0.07), MI (p < 0.001), MACE (p < 0.001), prolonged ventilation (p < 0.001), renal failure (p < 0.001), new dialysis (p = 0.02), and prolonged LOS (p < 0.001). Among women, OPCAB was associated with a significantly lower incidence of death (p < 0.001), stroke (p < 0.001), MI (p < 0.001), MACE (p < 0.001), prolonged ventilation (p < 0.001), renal failure (p < 0.001), new dialysis (p < 0.001), AF (p = 0.03), and reoperation (p < 0.001).
The unadjusted outcomes comparisons (men versus women; ONCAB versus OPCAB, etc) presented above are similar to those for the entire STS National Cardiac Database during the same time period.
Risk-Adjusted Outcomes
Figure 1
and Tables 4 and 5
show ORs for MACE and other outcomes adjusted for the 32 preselected risk factors listed in Table 1. Overall, OPCAB was associated with a significantly reduced risk-adjusted incidence of death (OR, 0.83; p = 0.03), stroke (OR, 0.65; p < 0001), MI (OR, 0.67; p < 0001), MACE (OR, 0.71; p < 0001), renal failure (OR, 0.74; p < 0.001), new dialysis (OR, 0.63; p < 0.001), deep sternal wound infection (OR, 0.67; p = 0.04), reoperation (OR, 0.86; p = 0.004), AF (OR, 0.88; p < 0.001), prolonged ventilation (OR, 0.75; p < 0.001), and prolonged LOS (OR, 0.70; p < 0.001) compared with ONCAB.
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The risk-adjusted benefit of OPCAB was slightly less dramatic for men than for women. Among men, OPCAB was associated with a statistically similar OR of 0.88 (95% confidence interval, 0.70 to 1.09; p = 0.24) for death compared with treatment with ONCAB. However, consistent with the broad benefit seen in women, among men, risk of stroke (OR, 0.58; p < 0.001), MI (OR, 0.68; p = 0.005), MACE (OR, 0.72; p < 0.001), renal failure (OR, 0.79; p = 0.005), dialysis (OR, 0.70; p = 0.01), AF (OR, 0.90; p = 0.001), prolonged ventilation (OR, 0.81; p < 0.001), and prolonged LOS (OR, 0.72; p < 0.001) were all significantly reduced after OPCAB compared with ONCAB.
Overall, female gender was associated with increased risk-adjusted incidence of death (OR, 1.39; p = 0.002), stroke (OR, 1.30; p = 0.03), MACE (OR, 1.32; p < 0.001), and prolonged ventilation (OR, 1.16; p = 0.005), a similar incidence of deep sternal wound infection, reoperation, AF, and prolonged LOS, but a lower incidence of renal failure (OR, 0.61; p < 0.001) and dialysis (0.69, p = 0.005) compared with male gender. In particular, women treated with ONCAB had risk-adjusted ORs of 1.47 for death (p = 0.001), 1.19 for stroke (p = 0.16), 1.23 for MI (p = 0.18), and 1.32 for MACE (p < 0.001) compared with men who had ONCAB. Of importance, however, with the exception of stroke (OR, 1.56; p < 0.02) and MACE (likely driven by stroke [OR, 1.30; p = 0.01]), women treated with OPCAB had major adverse outcomes statistically similar to men, with risk-adjusted ORs of 1.27 for death (p = 0.10) and 1.18 for MI (p = 0.42) compared with men who had OPCAB.
Interactions between gender and surgery type were statistically insignificant for all outcomes except prolonged ventilation (p = 0.01) using the final MLR models, adjusted for 32 preselected covariates of risk. This reflects that both men and women benefited significantly from OPCAB and that women benefited only modestly more than men.
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Comment |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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Female patients were generally sicker and older than male patients at the time of operation. Their predicted risk of mortality was therefore higher than that of their male counterparts. More important, even when adjusted for 32 preselected risk factors, women had a significantly increased early incidence of death, stroke, MI, and numerous other adverse events. Female gender was a strong, independent predictor of negative outcomes. Thus, this large contemporary data set confirms the historic gender disparity in clinical outcomes reported after CABG.
It has long been noted that women fare less well than men after cardiac surgery, and numerous explanations have been offered [22–27], usually in the form of risk factors that might be confounders of the impact of gender on outcomes. None of these has been conclusively proved. In the present analysis, the most credible explanations, namely, body habitus, prevalence of diabetes, completeness of revascularization, institutional identity, and use of internal mammary arteries, were all tested as variables within our multivariable analyses. All were found to influence adverse cardiac events independent of gender and surgical technique. Their inclusion in these multivariable logistic regression analyses did not negate the effect of gender or surgical technique on the risk-adjusted ORs for negative outcomes.
Patient height, weight, BMI, and body surface area were included as continuous variable covariates in the MLR models, thoroughly adjusting ORs for differences in body habitus. The three categories of diabetes (insulin dependent, noninsulin dependent, and none) were also included as covariates in the MLR models.
Some authors have proposed that poorer outcomes for women after CABG are caused by less complete or effective revascularization, perhaps owing to the small coronary arteries sometimes found in women. To evaluate this possible explanation, an index of completeness of revascularization (ICOR) was calculated for each patient and compared between groups. ICOR was defined as the number of distal anastomoses constructed divided by the number of diseased vessels reported on the preoperative coronary arteriogram and was included as a covariate in the final MLR models. The ICOR was lower for both men and women treated with OPCAB than for those treated with ONCAB (Table 1). The longer-term clinical implications of these differences are unknown. It is clear, however, that the improvement in early clinical outcomes for both men and women after OPCAB compared with conventional CABG on CPB is not due to more complete revascularization during OPCAB.
It is well known that the use of at least one internal mammary artery (especially when placed to the left anterior descending coronary artery) confers significant in-hospital survival benefit [28, 29]. Less frequent use of internal mammary arteries in women than in men has been reported in numerous series [22, 28, 30, 31]. Accordingly, the number of internal mammary arteries used was included in the MLR models as a covariate.
The 42,477 CABG procedures analyzed in this report were performed by 63 different centers during a 2-year period. Institutional identity was included in the MLR models as a fixed effects covariate, adjusting for a theoretically possible effect of institutional identity on surgical outcomes; the ORs for group comparisons in Tables 4 and 5 reflect this adjustment.
This study has several limitations. Its retrospective, observational nature does not permit complete accounting for all sources of bias, despite advanced statistical methodology designed to correct for treatment selection bias and potential confounders of outcomes in preplanned analyses. Moreover, the 63 institutions included in this study varied greatly in their interest in OPCAB: several infrequently performed OPCAB, whereas several used OPCAB in most of their cases. These institutions collectively have greater institutional interest and presumably greater expertise in OPCAB than the national norm, limiting generalizability of these results.
In conclusion, after adjustment for preoperative risk factors, this study found that patients treated with OPCAB have significantly reduced risk of death, stroke, MI, and numerous other adverse outcomes compared with those treated with CABG on CPB. Overall, women undergoing CABG are at increased risk of death, stroke, MI, and numerous other adverse outcomes compared with men. OPCAB is associated with improved outcomes for both men and women and reduced the gender disparity in outcomes after CABG.
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Discussion |
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Previous randomized trials comparing these strategies have shown relatively few differences in outcomes for the low-risk patients that were randomized to on- or off-pump surgery. In nonrandomized studies, usually carried out by enthusiasts for off- pump surgery, there usually has been a greater benefit shown for off-pump surgery. In this trial, there appears to be an overall advantage, although not dramatic, in terms of doing surgery off-pump, with that advantage focused in a definable group: women.
In examining the statistical methods and the assumptions that were made for this study, they all seem to me to be very reasonable. I am certainly not capable of doing a detailed analysis of the statistical methods, but the authors’ choice seems reasonable.
My questions are few. One is the issue about the number of vessels diseased. That was not included as a variable. In fact, much more complex variables relating to the extent of disease and complete revascularization were included, so those data should be available.
The second question is, John, now that you have gone through all this, do you have any greater insight into the difference in outcome between women and men that we have seen for 30 years in regard to their procedure-related risk of bypass surgery?
And finally, in doing studies that are comparing outcomes for very large groups of patients, finding that there is an average difference or that there is an overall mild benefit doesn’t help us all that much. Within a large group of patients, there are likely to be patients that benefit a great deal from one technique, some that benefit modestly, and some that may not benefit at all and, in fact, do worse. This is something that my colleague, Dr Eugene Blackstone, is very good in ferreting out. Can we define a subgroup of patients where off-pump surgery doesn’t just produce a modest advantage but an advantage so great that it should be standard of care? Have you looked at that at all or is that something for another day?
I think this is a very interesting study. Thank you very much.
DR PUSKAS: Dr Lytle, thank you for your kind remarks and for your questions. I would concur that among prospective randomized trials done to date, there have been few differences demonstrated for critical variables that occur infrequently, namely, death, stroke and MI. That is a product of mathematics and the way we do statistics. To demonstrate a difference between a 3% mortality and a 2% mortality, one requires thousands of patients. Fortunately, those critical end points occur so infrequently that the randomized trials, which enrolled between 100 and 200 patients in each limb, are simply mathematically incapable of demonstrating an improvement of as much as 50% in mortality or stroke or MI. You were right in that retrospective observational trials have shown greater differences between groups, including differences for those critical end points of death, stroke, and MI. They have been able to do so because they include a much larger sample size in each group, and those patients typically are much sicker than patients randomized in those earlier randomized controlled trials.
Addressing your specific questions, Dr Lytle, you asked about the inclusion or exclusion of the number of diseased vessels as a covariate in our multivariable logistic regression models. It was not specifically included as a covariate. We did include an index of completeness of revascularization, which was the number of grafts performed divided by the number of diseased vessels, because we were concerned that perhaps women with smaller coronary arteries were getting a less complete revascularization, and that could be the explanation for poorer outcomes. Similarly, as you and others have pointed out, women are less likely to receive an internal mammary artery graft, and that was the case in our more recent sample of the national database. However, IMA usage was included as a covariate and cannot explain the differences in outcomes for women.
There is a paper due to be presented tomorrow morning by my colleague from Emory, Dr Lattouf, which will examine the impact of the number of diseased vessels on the surgeon’s choice of OPCAB versus on-pump and on the relative benefit of OPCAB versus on-pump depending on the number of grafts needed.
You have asked, Dr Lytle, whether I have any new insights into the differences between men and women. You are right. We have been noticing this difference in outcomes after coronary bypass for 30 years, and despite the fact that I have been married for 20 years, I do not understand the differences between men and women. [Laughter.]
Quite frankly, this remains something of a mystery. The data analysis that was performed by the DCRI [Duke Clinical Research Institute] folks for us in this study examined every possible confounder that has been published in the literature, and we could find no difference except that women are different from men. You will have to go back to the My Fair Lady question, "How do we make a woman more like a man?" We can make a woman more like a man by operating on her off-pump.
Who benefits most? It is logical to assume, and it has been demonstrated numerous times, that those patients benefit most from OPCAB who would have had or could be expected to have the highest risk of morbidity or mortality from the heart-lung machine, and you all know who those are. Very elderly patients and patients with very heavily atherosclerotic ascending aortas are perhaps the highest-risk group for cardiopulmonary bypass or conventional CABG; patients with renal failure and pulmonary failure are among the next tier of high-risk groups. I have no other real insight into the difference between men and women. Thank you.
DR J. SCOTT RANKIN (Nashville, TN): John, I would like to congratulate you and your co-workers for an excellent study, one that illustrates the power of the STS database to address contemporary surgical issues. I have several questions.
One important principle of observational analysis is the assessment of all important variables affecting a comparison. Can we assume that all of the on-pump patients were comparable to the off-pump patients and that unmeasured confounding variables did not exist? There was some reason in each center that the patients were treated on-pump versus off-pump, perhaps related to coronary anatomy, and these variables were not collected.
Second, it worries me that patients were excluded from the analysis. Would it not have been better to have greater than and less than 100 cases or emergency versus elective as an independent variable in the model, and then perhaps the importance of surgical volume could have been measured? What was the result of the study if the low volume centers were not excluded? Similarly, were there differences in outcome before the adjustment? It might be good initially to show raw data to illustrate how much the statistical adjustment affected the results.
Third, over 20 tests were done here, and it is likely that one or two could have been positive by chance alone. Would you agree it might have been preferable to generate one large model, perhaps with male versus female gender and off-versus on-pump bypass as independent variables, evaluated with a formal treatment interaction and related with the logistic regression to a composite end point of all events?
Fourth, with such large sample sizes, even small differences tend to be statistically significant. Don’t you think one should always ask if the absolute difference is also clinically significant? For example, a 33% relative risk reduction of a 1.5% operative event would yield an absolute difference of 0.5%. In the context of a composite event rate of 10% to 15% over the first 3 years after coronary bypass in most of our studies, is the half of 1% difference at the beginning of much clinical significance?
Finally, the largest differences between off-versus on-pump bypass have been in the 3- to 5-year composite end points, and most of them have been in favor of the on-pump procedures. Wouldn’t you agree that late outcomes should be included in future studies?
Thank you again for this excellent work and for stimulating our thinking on this subject. Such careful consideration can only improve the care of our patients. And I also would like to thank the Society for the privilege of discussing this paper.
DR PUSKAS: Thank you. As we acknowledged, there are in fact unmeasured variables that can impact both selection bias and treatment bias. We have done the best job that we are able to do with the database available to us, and I think we all know that those data fields do not capture every aspect of the risk that every patient brings to the operating room. I would think it is fair also to state that we surgeons don’t understand every one of those risks well, and hence, our ability to account for those differences in risk is limited.
We did choose experienced centers, and we chose not to complicate our analysis with emergency cases. We felt this would make a muddy statistical task somewhat cleaner.
The notion of using a composite end point with a single very large MLR model is an interesting one. We were more interested in teasing out individual outcomes, and that is simply a different approach. To your question, is an absolute difference of .5% in a critical end point clinically relevant, I would answer that it is. I think if I could reduce by 30% the number of bereaved families I had to talk to after heart surgery, I would consider that an excellent thing.
And finally, late outcomes have not demonstrated an increased need for revascularization in OPCAB, but there is a paucity of late outcomes data for off- pump and on-pump surgery. Unfortunately, our national database does not include any long term follow-up. Thank you.
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