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Ann Thorac Surg 2011;92:571-578. doi:10.1016/j.athoracsur.2011.03.100
© 2011 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Comparing Long-Term Survival Between Patients Undergoing Off-Pump and On-Pump Coronary Artery Bypass Graft Operations

Giovanni Filardo, PhD, MPHa,b,c,*, Paul A. Grayburn, MDd, Cody Hamilton, PhDe, Robert F. Hebeler, Jr, MDf, William B. Cooksey, RN, MBAa, Baron Hamman, MDf

a Institute for Health Care Research and Improvement, Baylor Research Institute, Dallas, Texas
b Department of Statistical Science, Southern Methodist University, Dallas, Texas
d Baylor Heart and Vascular Institute, Dallas, Texas
f Department of Cardiothoracic Surgery, Baylor University Medical Center, Dallas, Texas
c Department of Infectious Diseases, University of Louisville, Louisville, Kentucky
e Department of Global Clinical Operations, Edwards Lifesciences, Irvine, California

Accepted for publication March 11, 2011.

* Address correspondence to Dr Filardo, Institute for Health Care Research and Improvement, 8080 N Central Expy, Ste 500, Dallas, TX 76206 (Email: giovanfi{at}baylorhealth.edu).

Presented at the Forty-seventh Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Jan 31–Feb 2, 2011.


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 Discussion
 Acknowledgments
 References
 
Background: As the population of the United States and Western Europe ages, the number of patients undergoing isolated coronary artery bypass grafting (CABG) for revascularization can be expected to increase. This study investigated long-term survival in patients undergoing off-pump vs on-pump CABG.

Methods: Survival was assessed in 8081 consecutive patients who underwent isolated CABG (732 received off-pump) between January 1, 1997, and December 31, 2008. A propensity-adjusted model controlling for preoperative risk factors identified by the Society of Thoracic Surgeons and other preoperative clinical and nonclinical details was used to assess adjusted long-term mortality differences between off-pump and on-pump CABG.

Results: Ten-year unadjusted survival was 54.7% (95% confidence interval, 47.2% to 61.6%) in off-pump CABG patients and 62.3% (95% confidence interval 60.9% to 63.8%) in on-pump CABG patients. The log-rank test (p = 0.012) indicated a significantly higher risk of death in off-pump CABG patients. After adjustment, the risk of death remained significantly higher in the off-pump CABG patients (hazard ratio, 1.18; 95% confidence interval, 1.02 to 1.38). The adjusted association regarding off-pump learning curve and survival was assessed separately and was not statistically significant (p = 0.774), further validating our findings regarding off-pump CABG.

Conclusions: After controlling for preoperative severity of disease and other possible confounders, the risk of long-term mortality in patients undergoing off-pump CABG is significantly higher than in those undergoing on-pump CABG. For multivessel coronary disease, on-pump CABG might be preferable to off-pump CABG given that it may achieve a more complete and durable revascularization.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 Discussion
 Acknowledgments
 References
 

Dr Hamilton discloses that he has a financial relationship with Edwards Lifesciences.

 

Off-pump coronary artery bypass grafting (OPCAB) operations have been increasingly used in Western medicine since the early 1990s, when Benetti [1] and Buffolo [2] and their colleagues demonstrated unexpected benefits possibly associated with the avoidance of cardiopulmonary bypass [3]. Today, an estimated 15% to 20% of CABG procedures in the Western world and most CABG operations in Asia are performed without the use of cardiopulmonary bypass [3].

Multiple randomized controlled trials, observational studies, and meta-analyses have compared perioperative mortality, surgical complications, and resource use for OPCAB and on-pump CABG [3]. The randomized controlled trials, typically conducted in low-risk patients, generally found no significant differences in perioperative mortality, but did find a reduced need for transfusions and ventilation and shorter hospital stays with OPCAB [4–8]. Meta-analyses showed similar results [9–14], whereas large, observational studies reported significant reductions in perioperative mortality and morbidity [5,15–20], or similar perioperative outcomes [21–23].

Fewer studies have examined longer-term outcomes, but one randomized trial considered a composite outcome of death, repeat revascularization, or myocardial infarction within 1 year and observed a significantly higher rate in patients who received OPCAB [24]. One observational study included 2-year survival and found similar rates for off-pump and on-pump CABG, and two observational studies examined 3-year survival, with one showing similar survival in on-pump and off-pump CABG [15], and the second showing a survival benefit with on-pump CABG [22]. Only one study to date has rigorously considered 10-year survival: Puskas and colleagues [5] analyzed data from 12,812 consecutive isolated CABG patients from 1997 to 2006 to determine whether sex or type of operation affected long-term survival and observed no significant difference between off-pump and on-pump CABG, regardless of sex. To add to this currently sparse literature on long-term comparative effectiveness of off-pump and on-pump CABG, we assessed long-term survival (over 12 years) in a cohort of 8,081 consecutive patients who underwent isolated CABG at Baylor University Medical Center, Dallas, Texas, between 1997 and 2008.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 Discussion
 Acknowledgments
 References
 
This study was approved by the Baylor Research Institute Investigational Review Board, and waiver of the requirement for informed consent was obtained in May 2008.

Patient Data
The study cohort included all consecutive patients who underwent isolated CABG between January 1, 1997, and December 31, 2008. Data collected by Baylor University Medical Center and the Society of Thoracic Surgery Adult Cardiac Surgery Database [25] were considered for the study. Data abstraction and collection procedures used for the Society of Thoracic Surgery Adult Cardiac Surgery Database, which comprehends more than 80% of the cardiac surgery programs in the United States, are standardized and have been described elsewhere [25, 26].

Recognized Society of Thoracic Surgery risk factors for postcardiac adverse outcomes [27, 28], including mortality, are the key elements for this study. These included age, sex, race, body surface area (BSA), diabetes, preoperative renal failure, preoperative creatinine level, chronic lung disease, hypertension, peripheral vascular disease, cerebrovascular disease, smoking status, atrial fibrillation, congestive heart failure, previous revascularization procedure, myocardial infarction (MI) timing (hours since MI), preoperative angina, ejection fraction, left main disease, urgency of operation (status), and preoperative use of an intraaortic balloon pump (IABP).

The patient cohort was limited to individuals without previous valve operations, preoperative endocarditis, or a ventricular assist device. Also excluded were 98 patients (1.2%) with missing values for MI timing.

Exposure and Outcome Definition
Exposure was defined as a patient who received off-pump or on-pump CABG. Outcome was survival in the 8081 patients from January 1, 1997, to June 2009 was assessed by using the National Death Index data. Survival was measured in all 8081 patients as time (in days) to death or the last follow-up (June 2009) from the date of the operation.

Statistical Analysis
Differences between demographics for on-pump and off-pump patients were tested with a Wilcoxon test for continuous factors or a {chi}2 test for categorical factors. A Bonferroni adjustment was used to adjust for multiplicity.

Unadjusted survival estimates were obtained using the Kaplan-Meier method. The unadjusted difference between the survival curves for on-pump and off-pump patients was tested with a log-rank statistic. An unadjusted hazard ratio was obtained by fitting a Cox proportional hazards model with on-pump or off-pump as the sole predictor.

A propensity score approach [29, 30] was used to adjust the effect of OPCAB on long-term survival. To calculate the propensity score, we used a logistic regression model with OPCAB as the dependent variable and the factors in Table 1 as independent variables. Multiple imputation by predictive mean matching [29, 31] was used to account for missing data in this model (0.3% missing status, 5.9% missing ejection fraction, 7.6% missing creatinine, 1.5% missing BSA).


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  		Table 1 Society of Thoracic Surgeons Operative Mortality Risk Factors and Other Clinical and Nonclinical Details
 
We statistically compared balance between the off-pump and on-pump patients for the risk factors listed in Table 1 before and after adjustment by the propensity score. First, we fit a logistic regression model (with OPCAB as the outcome) for each factor unadjusted by the propensity score and then re-fit the model adjusting for the propensity score. Wald {chi}2 statistics were collected for each model. All {chi}2 statistics were statistically insignificant after propensity score adjustment, indicating good balance was achieved.

The linear predicted value (Xβ) from the model forms the propensity score. If we let yp (1) denote the propensity score and z denote whether the patient received OPCAB, then the final model was:


Formula 0005

(0005)
Here


Formula 0010

(0010)
for j = 1 to 6, where k j denotes the j th knot for the cubic spline and Formula if Formula and = 0 otherwise.

The Grambsch and Therneau test statistic [32] was used to test for proportionality of hazards in the final model. The adjusted survival curves were created using the Kalbfleisch and Prentice method [33].

Possible effect modification by age, sex, and surgeon was investigated. First, sex and an interaction between sex and the exposure (off-pump) were added to the propensity-adjusted model. The test statistic for the interaction term was then investigated to determine if sex produced any effect modification. This process was repeated for age in similar fashion (although the effect of age, which is continuous, was modeled with a cubic spline). To test for possible effect modification produced by the surgeon, the surgeon identification number (ID), and an interaction term between surgeon ID and the exposure was added to the final model. Patients of surgeons who had performed fewer than 10 off-pump procedures were grouped in a single surgeon ID to allow the surgeon effect to be estimated. Again, the interaction term was used to investigate whether there was effect modification produced by surgeon.

Besides testing for a possible effect associated with surgeon, we also tested the effect of the surgeons' learning curve for OPCAB to further validate the study findings. A generalized propensity approach was used for this purpose, and the following steps were executed:

1 a model was fit with log(count) as the outcome where count is the number of OPCAB operations performed by the surgeon before the given patient's operation (independent variables were all the factors in Table 1);
2 the exponentiated linear predictors from this model comprised the propensity scores, which were used to adjust the effect of "count" in a Cox proportional hazards model.

All analyses were performed using R 2.8.1 software.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 Discussion
 Acknowledgments
 References
 
A total of 732 patients (9.1%) received OPCAB. These patients were generally older (p < 0.001) and were more likely to have renal failure (p = 0.012) and a previous percutaneous coronary intervention (p = 0.011) compared with patients who received on-pump CABG. OPCAB patients were also more likely to have had a previous stroke, although this association was not significant at the 0.05 level after Bonferroni adjustment (p = 0.088). However, these patients generally had higher ejection fraction (p < 0.001), were less likely to have had an intraaortic balloon pump (p = 0.010), and were less likely to undergo an emergency operation (p < 0.001). See Table 1 for more details.

Total follow-up amounted to 48,165 patient-years. The unadjusted hazard ratio comparing off-pump with on-pump patients was 1.21 (95% confidence interval [CI], 1.04 to 1.41), indicating that patients receiving OPCAB were at higher risk for long-term death (p = 0.012). The unadjusted survival at 5 and 10 years was 80.8% (95% CI, 79.8% to 81.7%) and 62.3% (95% CI, 60.9% to 63.8%), respectively, for on-pump patients and 77.4% (95% CI, 74.0% to 80.5%) and 54.7% (95% CI, 47.2% to 61.6%), respectively, for off-pump patients. Figure 1 presents the Kaplan-Meier survival curves.


Figure 1
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  		Fig 1. Unadjusted Kaplan-Meier curves depict long-term survival in patients who underwent off-pump (dotted line) and on-pump (solid line) coronary artery bypass grafting at Baylor University Medical Center (Dallas, TX) between January 1997 and December 2008. Unadjusted p = 0.012 (log-rank).

 
After propensity score adjustment, the survival difference between on-pump and off-pump patients was still statistically significant (p = 0.028). The estimated adjusted hazard ratio was 1.18 (95% CI, 1.02 to 1.38), indicating that patients undergoing OPCAB were at almost 20% higher risk for long-term death compared with patients undergoing on-pump CABG. Figure 2 presents the adjusted survival curves for on-pump and off-pump patients.


Figure 2
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  		Fig 2. Adjusted proportional hazard model curves depict long-term survival in patients who underwent on-pump (solid line) and off-pump (dotted line) coronary artery bypass grafting at Baylor University Medical Center (Dallas, TX) between January 1997 and December 2008. Adjusted hazard ratio, 1.18 (95% confidence intervals, 1.02 to 1.38). Please see text for the risk factors included in the proportional hazard model.

 
The test for nonproportionality of hazards was highly insignificant; the checks regarding the balance produced by the propensity score (as described in Statistical Analysis) yielded no concerns. No effect modification produced by gender (p = 0.639) or age (p = 0.597) was detected.

Three surgeons performed 81% (267, 250, and 77 operations each) of the OPCAB procedures. The remaining operations (19%) were performed by 9 surgeons (range, 1 to 24 operations). The test for the propensity-adjusted effect of the learning curve was not significant (p = 0.774; Fig 3 ). Likewise, the test to assess the possible differential effect revealed no significant differential effect due to surgeon (p = 0.808).


Figure 3
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  		Fig 3. Five-year adjusted survival probability, estimated with a proportional hazard model, depicting long-term survival by number of procedures performed by the surgeon in patients who underwent off-pump coronary artery bypass grafting at Baylor University Medical Center (Dallas, TX) between January 1997 and December 2008. The solid line shows the five-year survival probability and the dotted lines show the 95% confidence interval. Adjusted p = 0.774. Please see the text for the risk factors included in the adjusted propensity model.

 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
Discussion
 Acknowledgments
 References
 
Our study complements the existing knowledge regarding the apparently lower adverse event rates associated with OPAB [5–23, 34] and provides novel data regarding the effectiveness of OPCAB vs on-pump CABG for long-term survival, showing higher long-term survival (over 12 years of follow-up) in patients undergoing on-pump CABG procedures. Puskas and colleagues [5], in a similarly sized cohort of patients monitored for up to 10 years, observed no significant survival difference between OPCAB and on-pump CABG.

Our data showed that, in patients undergoing isolated CABG, those receiving OPCAB were at increased long-term risk of death independent of their preoperative severity, as defined by the combination of available preoperative data. Specifically, the adjusted survival differences observed in our study could not be attributed to patient age, sex, race, BSA, diabetes, preoperative renal failure, preoperative creatinine level, chronic lung disease, hypertension, peripheral vascular disease, cerebrovascular disease, smoking status, congestive heart failure, previous revascularization procedure, preoperative atrial fibrillation, time since MI, preoperative angina, ejection fraction, left main disease, urgency of operation (status), or the preoperative use of a intraaortic balloon pump, which were all risk factors accounted for in the propensity model used for the analysis.

The risk of long-term death in patients undergoing OPCAB was significantly higher (18%) than in patients who underwent on-pump CABG, even after controlling for a comprehensive array of risk factors and confounders associated with death after CABG and other adverse outcomes. These results indicate that a patient undergoing OPCAB in this cohort had 18% higher risk of long-term death than a patient with exactly the same preoperative risk profile who underwent on-pump CABG.

In our study cohort, the propensity-adjusted effect of the learning curve on survival was not significant (p = 0.774; Fig 3), indicating that the survival differences could not be explained by the possible effect associated with the surgeons' learning curve in performing OPCAB. Likewise, survival differences could not be explained by a differential effect due to surgeon (the test for interaction was not significant, p = 0.808).

We do not know for certain why survival was reduced in patients undergoing OPCAB; however, we hypothesize that complete revascularization may have been lower in these patients, which may have contributed to reduced long-term survival. Several studies have shown lower rates of complete revascularization in OPCAB patients [24, 35, 36], substantiating our hypothesis. We also speculate that lower revascularization partly occurs because of a perception that fewer bypasses are "as good" as more bypasses if decreasing the number of bypasses makes the immediate operation safer. This paradigm also would explain the seemingly lower short-term adverse outcomes attributable to OPCAB if surgeons are less willing to risk the additional bypasses than they are in on-pump CABG. Surgeons may embrace OPCAB because of the immediate operative benefits and safety, trading off potential long-term benefits associated with complete revascularization. Most decisions regarding the number of bypasses are made on a continuing basis during the operation, within the parameter of the type of operation. Perhaps, given the technical complexity of OPCAB, decisions about on-pump vs off-pump CABG should be based on the anatomy and location of the needed revascularizations as well as the total number of revascularizations needed. The fundamental questions regarding which patients benefit the most from OPCAB and who should be selected have not been answer yet.

The ability to generalize our results may be limited because our study was conducted at a single center in Dallas, Texas. In addition, as in any observational study, causality between type of operation and death could not be definitively established, and further research is necessary to assess it.

Our findings, nonetheless, have important public health implications given the advancing age of the population [37] and the consequently increasing demand for CABG operations. Our data provide new evidence regarding the long-term effectiveness of OPCAB and pose important questions regarding how the decision off-pump vs on-pump should be made—specifically, whether it should be heavily based on the anatomy and location of the needed revascularizations and on the total number of revascularizations needed. Accordingly, future randomized trials comparing off-pump vs on-pump CABG should be balanced not only by surgeon and surgeon experience with OPCAB techniques, but also stratified according to the number and location of the needed revascularizations.

In the meantime, it may be that OPCAB should be used primarily for patients needing fewer bypasses, all easily accessible during beating-heart operations. For multivessel coronary disease, however, on-pump CABG might be preferable to percutaneous coronary intervention and an OPCAB given that it usually achieves more complete and durable revascularization.


   Discussion
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Discussion
Acknowledgments
 References
 
DR OMAR LATTOUF (Atlanta, GA): Dr Hamman, thank you for a very interesting presentation. I have a number of questions for you. The first question, what is your definition of an experienced surgeon? Second question. You have done 10 to 1 on-pump to off-pump cases. That means you have done 51 on-pump per month to 5 cases off-pump per month for your entire institution. Third question. You have 3 top performers in your study. The first performer in a 12-year period performed 267 cases off-pump. That means the top performer performed 1.79 cases per month off-pump. The second top performer did 250 cases in a 12-year period off-pump. That is 1.7 cases per month. The third top performer performed 70 cases over a 12-year period. That is half a case per month. Would that be your definition of experienced off-pump surgeons? The 9 other surgeons performed an average of one to two off-pump cases per year. Would that be defined as experienced off-pump surgeon?

In regards to patient selection, off-pump patients, on the average, were 6 months older than the on-pump, they were more likely to have renal failure and cerebrovascular disease. If you look at the adjusted risk chart and incorporate into it the 6 months' age difference between the off-pump and the on-pump, would that eliminate the survival difference between the two groups?

The fourth question: the number of diseased vessels and the number of bypassed vessels were not clearly stated; thus, the index of revascularization between one group and another was not correlated to outcome. Was the off-pump group under-revascularized as compared to the on-pump group? Thank you for an interesting paper.

DR HAMMAN: Thank you, Dr Lattouf, for those insightful questions. Your first question was my definition of an experienced surgeon. Our definition of an experienced surgeon for this study was a surgeon who performed greater than 24 cases. In fact, all of the surgeons were there for greater than half of the study duration and all of the surgeons had been practicing for greater than 2 years at the beginning of the study.

I do note the ratio of off-pump to on-pump, being 1 to 10, and that leads to the next question regarding the top performer or the "more frequent" off-pump surgeons. During this study time, there were variable rates of off-pump surgery performed as we became good at doing the operation and understanding its applicability. When you spread the experience over 12 years that is not entirely representative of the experience. Over the last 7 years of our study, the rate of off-pump surgery at this institution has been roughly 20% and the top performing surgeon has done as much as 50%.

Finally, the third question pertained to patient selection, and you pointed out the differences that I highlighted in the slides during the talk. And when you use a statistical adjustment, such as we did, it does allow for comparison of different groups using a hard end point. We intentionally didn't report the myriad of differences in our results, including target vessels, number of diseased vessels attempted or not attempted, and the number of grafts performed, because it wasn't germane to our single hard data end point.

DR JOHN D. PUSKAS (Atlanta, GA): Baron, could you describe for us your thoughts on comparing the, I believe, six factors you used to risk adjust as opposed to picking, for instance, the predictive risk of mortality score, which is available for every patient in your Society of Thoracic Surgeons (STS) database and is derived from 30 preoperative risk factors? Why not pick the PROM (predictive risk of mortality) score as a means of risk adjusting these patients rather than picking your sort of top 6 suspects?

DR HAMMAN: We used, actually, all of the demographics and all of the preoperative variables as our risk adjustments. I just highlighted those that were glaringly different in the unadjusted data.

DR JUERGEN CARL ENNKER (Lahr, Germany): Congratulations to your interesting presentation; however, I want to elaborate on the definition of experienced surgeons and by a simple question. First, what was the rate of conversion in your off-pump surgery? Second, I also want to ask you, was the level of arterial revascularizations the same in on-pump as in off-pump surgery? And third, did you measure flow in all of your anastomoses in both on-pump and off-pump surgery. Thank you so much. One comment. Complete off-pump surgery is not only used in Asia but by also a couple of institutions in Europe doing more than 90% to 95% of their bypass procedures off pump.

DR HAMMAN: Thank you. I appreciate the comments about the expertise in Germany and other places. The rate of conversion was not recorded in our database and so it is not reportable. But I will say that in subgroups of our data when we were seriously learning off-pump and training, our conversion rate remained less than 10% at all times.

With respect to arterial anastomoses, happily, at this institution, we have a 97% to 98% utilization of arterial grafts in both arms, off-pump and on-pump.

And regarding the question of flow measurements, we did measure flow in some of the vein grafts, but because our flow measurement technique varied throughout the study, they weren't really comparable. We used the SPY technique (Novadaq Technologies Inc, Mississauga, Ontario, Canada), we used MediStim Flow (MediStim ASA, Oslo, Norway), and handheld Doppler imaging.


   Acknowledgments
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Discussion
 Acknowledgments
References
 
We acknowledge the use of software from Professor Frank Harrell's Hmisc and Design libraries and thank Briget da Graca for writing and editorial assistance.

Grant support was provided by the Cardiovascular Research Review Committee in cooperation with the Baylor Heart and Vascular Institute.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Discussion
 Acknowledgments
 References
 

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