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Ann Thorac Surg 2010;90:753-759. doi:10.1016/j.athoracsur.2010.04.018
© 2010 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Off-Pump Bypass Surgery and Postoperative Stroke: California Coronary Bypass Outcomes Reporting Program

Zhongmin Li, PhDa,*, Timothy Denton, MD, FACCb, Khung Keong Yeo, MBBSa, Joseph P. Parker, PhDc, Richard White, MDa, J. Nilas Young, MDa, Ezra A. Amsterdam, MDa

a University of California, Davis Medical Center, Sacramento, California
b High Desert Heart Institute, Victorville, California
c California Office of Statewide Health Planning and Development, Sacramento, California

Accepted for publication April 5, 2010.

Abbreviations and Acronyms CABG = coronary artery bypass graft surgery; CCB = conventional on-pump coronary bypass; CCORP = California CABG Outcomes Reporting Program; CI = confidence interval; CLD = chronic lung disease; CVA = cerebrovascular accident; CVD = cerebrovascular disease; MI = myocardial infarction; NYHA = New York Heart Association; OPCAB = off-pump coronary artery bypass surgery; OSHPD = California Office of Health Planning and Development; PAD = peripheral arterial disease; PDD = patient discharge data; STS = The Society of Thoracic Surgeons

* Address correspondence to Dr Li, Department of Internal Medicine, University of California, Davis Medical Center, 4150 V St, Ste 2400, Sacramento, CA 95817 (Email: zmli{at}ucdavis.edu).


   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 References
 
Background: Coronary artery bypass surgery (CABG) is associated with a significant risk of stroke. Some studies suggest that off-pump CABG (OPCAB) may reduce postoperative stroke rate. We performed this study to evaluate the relationship between postoperative stroke and OPCAB compared with conventional on-pump CABG (CCB) in a recent, large cohort of patients.

Methods: Data from the California CABG Outcomes Reporting Program were analyzed in patients who had OPCAB or CCB for isolated CABGs in 2006 to 2007. Two multivariable logistic regression models were developed for the analysis, and the "recycled predictions" method was used to compute risk-adjusted postoperative stroke rates in the two surgical groups.

Results: Of 30,426 isolated CABGs, 7,720 (23.7%) were OPCAB. The model developed in the CCB subset indicated that CCB had a lower predicted stroke risk than OPCAB, yet the observed rate of stroke was higher in the CCB subset. The model using both CCB and OPCAB patients revealed that OPCAB was associated with a reduction in postoperative stroke (adjusted odds ratio: 0.76, 95% confidence interval [CI] 0.59 to 0.98). For patients with cardiogenic shock, OPCAB was also associated with a lower risk-adjusted postoperative stroke rate compared with CCB (OPCAB: 3.06%, 95% CI 2.83% to 3.28%; CCB: 4.05%, 95% CI 3.76% to 4.33%, p < 0.001). However, the 793 (11%) OPCAB patients who were converted to CCB intraoperatively had an increased postoperative stroke rate (with conversion: 2.02%, 95% CI 1.04% to 3.00% versus without conversion: 0.96%, 95% CI 0.73% to 1.20%, p < 0.001).

Conclusions: The OPCAB was associated with a significantly lower postoperative stroke rate compared with CCB even for older and higher risk patients. However, intraoperative OPCAB to CCB conversion was associated with the highest postoperative stroke rate.


   Introduction
Top
 Abstract
 Introduction
Material and Methods
 Results
 Comment
 References
 
Off-pump coronary artery bypass surgery (OPCAB) was developed to reduce complications associated with cardiopulmonary bypass surgery [1, 2]. Stroke is one of the most serious complications of conventional on-pump coronary bypass (CCB). Several mechanisms for this complication have been proposed [3–5], with OPCAB potentially addressing cannulation, pump time, inflammation, and microembolization. Several randomized and observational trials have reported no difference or a trend toward reduced postoperative stroke in patients undergoing OPCAB compared with CCB [6–10]. However, these trials are limited by relatively small sample size or incomplete data. Our objective was to evaluate the relationship between postoperative stroke and OPCAB compared with CCB in a large, contemporary cohort of patients.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 References
 
Source
Data were obtained from the California Coronary Artery Bypass Graft (CABG) Outcomes Reporting Program (CCORP) data registry on patients who underwent CABG surgery in 2006 and 2007 from 121 reporting hospitals. Since 2003, California state law has required that all state-licensed hospitals report isolated and nonisolated CABG cases to CCORP. Isolated CABG is defined as CABG performed without other major procedures such as valve repair, carotid endarterectomy, aortic surgery, lung surgery, or ventricular surgery during the same operation. The study was approved by the Committee for the Protection of Human Subjects of the California Health and Human Services Agency on June 11, 2009 (Project No. 04-08-58).

The CCORP clinical data registry utilizes a subset of data elements collected by the Society of Thoracic Surgeons for the National Database of Cardiac Surgery. After data submission by hospitals, CCORP performs data validation procedures, including verification of complete submissions, using the statewide hospital patient discharge data), verification of deaths using the state's death registry, and an annual independent medical chart audit of randomly selected hospitals. The program produces hospital and surgeon-specific report cards based on risk-adjusted operative mortality rates. A detailed description of the CCORP data collection methods and analysis has been documented elsewhere [11–13].

Postoperative stroke is defined as a neurologic deficit of abrupt onset caused by a disturbance in cerebral blood supply (ischemic or hemorrhagic) that did not resolve within 24 hours after CABG and lasted for more than 72 hours. Postoperative stroke does not include the neurologic deficits of confusion, delirium, and (or) encephalopathic (anoxic or metabolic) events. Our analysis was based on the "intention-to-treat" principle, in which OPCAB procedures that were converted to CCB intraoperatively were categorized as OPCAB for the purpose of the study.

Statistical Analysis
Categoric variables were compared by {chi}2 analysis and continuous variables were compared by the Student's t test. Differences were considered statistically significant if the p value was less than 0.05.

To determine the impact of OPCAB on postoperative stroke while controlling for individual patient demographic and clinical characteristics, we used two different multivariable regression models and the "recycled predictions" method [14, 15] to compute and compare the mean risk-adjusted marginal effects for OPCAB versus CCB. The first logistic model was developed for all patients who underwent CCB only (ie, excluding OPCAB patients); this model was then applied to OPCAB patients to compare the risk of stroke in the CCB and OPCAB groups. The second risk model was developed on both CCB and OPCAB patients, and applied to all isolated CABGs in 2006 to 2007. However, because the raw logistic regression coefficients and odds ratios are nonlinear expressions of the impact of individual covariates on the response variables, they provide an imperfect picture of the impact of OPCAB on predicted risk for postoperative stroke. The method of recycled predictions, also referred to as "averaging the individual marginal effects" or "predictive margins," provides more easily interpreted statistics than raw logistic regression coefficients [14, 15].

Marginal effects measure the expected instantaneous change in the dependent variable (ie, postoperative stroke) as a function of a change in a certain explanatory variable (ie, surgery type: OPCAB vs CCB) while keeping all the other covariates constant. By using the recycled predictions method for the first run, we assumed all isolated CABGs were performed with CCB, and for the second run we assumed all were performed with OPCAB. With the same multivariate logistic regression model we calculated the mean predicted marginal probability of postoperative stroke for OPCAB and CCB independently, while keeping the same attributes for all other covariates fixed.

A further subgroup analysis was performed for selected risk factors based on their significance in affecting postoperative stroke as demonstrated by the regression model. Within the OPCAB group we further compared the risk-adjusted postoperative stroke rates between patients who were converted from OPCAB to CCB during the operation (converters) and patients who underwent OPCAB without intraoperative conversion (nonconverters), and applied a multivariable logistic regression model to identify patient characteristics associatied with intraoperative conversion from OPCAB to CCB. All data analyses were conducted with SAS version 9.2 (SAS Inc, Cary, NC).


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 References
 
During the two-year study period, a total of 30,426 isolated CABG procedures were performed, of which 7,220 (23.7%) were OPCAB. This is consistent with 2003 to 2005, in which OPCAB accounted for 22.9% of all isolated CABGs. Compared with CCB (Table 1), OPCAB patients were older, more likely female, and had a higher prevalence of cerebrovascular disease, peripheral arterial disease, dialysis, immunosuppresive treatment, and prior CABG, but had a lower prevalence of diabetes, hypertension, recent myocardial infarction (MI) (1 to 7 days prior to CABG), New York Heart Association class IV heart failure or angina, cardiogenic shock, and 3-vessel or greater coronary artery disease (all p < 0.01).


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  		Table 1 Isolated CABG Patient Preoperative Clinical Profile Comparison: Off-Pump (OPCAB) Versus On-Pump (CCB), California, 2006 to 2007 a
 
Using the multivariable logistic risk model developed with the CCB-only subset, the predicted risk of postoperative stroke was slightly lower for the CCB subset (CCB: 1.42%, 95% confidence interval [CI]: 1.40% to 1.44%; OPCAB: 1.46%, 95% CI: 1.43% to 1.49%, p = 0.026). However, the observed postoperative stroke rate was lower for the OPCAB group (CCB: 1.42%, 95% CI 1.27% to 1.57%; OPCAB: 1.08%, 95% CI 0.84% to 1.32%, p = 0.018). This model has a c-statistic of 0.728 and a p value of 0.5552 for the Hosmer-Lemeshow goodness-of-fit test, indicating the model has an acceptable discrimination power and good overall data calibration.

The multivariable logistic risk model with OPCAB added as a risk factor (Table 2) has a c-statistic of 0.73 and p value of 0.1263 for the Hosmer-Lemeshow goodness-of-fit test, also indicating a good discimination and data calibration. Using this risk model and controlling for demographic and clinical variables, OPCAB was associated with a significantly lower rate of postoperative stroke (adjusted odds ratio: 0.76, 95% CI 0.59 to 0.98, p < 0.001). Using the recycled predictions method, the predicted risk for postoperative stroke, after controlling for patient demographic and clinical risk factors for OPCAB, was significantly less than that of CCB (OPCAB 1.09%, 95% CI 1.08% to 1.10% vs CCB 1.43%, 95% CI 1.41% to 1.45%, p < 0.001; Table 3). Even for patients with cardiogenic shock, OPCAB was associated with lower risk-adjusted postoperative stroke rates compared with CCB (OPCAB: 3.06%, 95% CI 2.83% to 3.28%; CCB: 4.05%, 95% CI 3.76% to 4.33%, p < 0.001). The odds ratio of predicted postoperative stroke for OPCAB (0.0109)/(1 to 0.0109) divided by the predicted postoperative stroke for CCB (0.0143)/(1 to 0.0143) = 0.76, is close to the odds ratio of 0.755 from the multivariable logistic parameter estimate. This similarity supports the marginal effects of OPCAB on the outcome computed with the recycled predictions method.


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  		Table 2 Multivariate Logistic Risk Adjustment Model for Postoperative Stroke
 

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  		Table 3 Comparison of Observed and Risk-Adjusted Postoperative Stroke: OPCAB Versus CCB
 
Table 3 presents subgroup analyses for other patient characteristics, and in all cases OPCAB resulted in a significantly lower risk of postoperative stroke than CCB. Further, the protective effect of OPCAB was not evenly distributed by patient age, this effect being more pronounced in older patients compared with younger patients (Fig 1).


Figure 1
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  		Fig 1. The effect of off-pump coronary artery bypass surgery on postoperative stroke rate by age, 2006 to 2007. ({blacksquare} = conventional on-pump coronary bypass; {blacktriangleup} = off-pump coronary artery bypass surgery.)

 
The group of 793 (11%) OPCAB patients who converted intraoperatively to CCB had an increased postoperative incidence of stroke (with conversion: 2.02%, 95% CI 1.04% to 3.00% versus without conversion: 0.96%, 95% CI 0.73% to 1.20%, p = 0.041), although both subgroups had similar predicted postoperative stroke rates (with conversion: 1.16%, 95% CI 1.08% to 1.25%; without conversion: 1.11% 95% CI 1.08% to 1.13%, p = 0.242; Table 4). Additionally, those patients with MI 24 hours or less prior to surgery, left main coronary artery stenosis, and 3-vessel or greater coronary disease were more likely to have intraoperative conversion from OPCAB to CCB (adjusted odds ratio for MI within 24 hours: 1.72, 95% CI 1.15 to 2.55, p = 0.008; for left main stenosis [each additional 1% increase]: 1.01, 95% CI 1.01 to 1.02, p < 0.001; for 3-vessel or greater disease: 2.29, 95% CI 1.88 to 2.78, p < 0.001).


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  		Table 4 Observed and Mean Predicted Risk-Adjusted Postoperative Stroke Rates: Patients Undergoing Intraoperative Conversion From OPCAB to CCB
 

   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
References
 
Our study demonstrates a significantly reduced risk of postoperative stroke in a large cohort of patients undergoing OPCAB compared with CCB. Three different methods of analysis were applied in this retrospective study comprising patients of differing risk profiles. First, in a multivariable model which was developed with CCB patients only, and then used to compare that subset to OPCAB patients, the predicted risk of stroke was similar in the two groups but the observed rate of stroke was significantly lower with OPCAB. In a second model, in which OPCAB was included as a predictor variable, OPCAB was associated with a significantly lower rate of postoperative stroke. Third, the recycled predictions method confirmed the previously demonstrated lower rate of stroke in the OPCAB subset.

Previous trials [6–10, 16] that suggested a protective effect of OPCAB on postoperative complications are limited by small sample sizes, the largest of which was only 281 patients [10]. By contrast, our findings are based on a large patient population and rigorous statistical analysis which revealed that the observed rate of postoperative stroke with OPCAB was lower than the predicted rate. This finding was similar in the subgroups with cerebrovascular disease, recent stroke, or MI within 24 hours prior to surgery. In patients with cardiogenic shock, left ventricular ejection fraction less than 0.40 or with 3 or greater diseased coronary arteries, OPCAB was also associated with lower risk-adjusted postoperative stroke rates. Further, the reduced rate of stroke associated with OPCAB was greater in elderly than younger patients (Fig 1).

In contrast to the abovementioned results, we found an increased risk of stroke in patients who required intraoperative conversion from OPCAB to CCB. This finding may reflect an unanticipated requirement for urgent conversion resulting in higher operative risk, including that of stroke. Conversion to CCB occurred in 11% of OPCAB patients, accounting for a significant minority. Our data suggest that this group has high-risk features such as recent MI, left main coronary artery disease, or 3-vessel or greater disease which were associated with conversion to CCB.

This study does not provide a mechanistic explanation for the lower risk of stroke associated with OPCAB. However, cardiopulmonary bypass is associated with significant risk of neurocognitive dysfunction [17]. This may be related to atheroembolism, air embolism, or other undetermined factors. In patients with atherosclerosis, the aorta and arch vessels are likely to contain atherosclerotic plaques which may be disrupted during aortic clamping placement of cannulas during cardiopulmonary bypass, resulting in atheroemboli [4]. This adverse potential has led to the development of epiaortic ultrasound techniques to avoid cannulating the aorta where there is significant plaque burden [18–20]. In this regard, methods which reduce aortic manipulation during surgery may be beneficial.

While this study suggests that OPCAB may be associated with a lower risk of postoperative stroke, it has important limitations. First, this is a retrospective analysis with the inherent limitations of this method. However, it utilizes an extensive risk model to account for patients' hazard of postoperative stroke and to reduce the problem of factors that may be unaccounted for. Second, the CCORP database is dependent on the accuracy of information provided by the individual hospitals. However, a well-established and documented CCORP auditing system supports the validity of the data [11–13]. Third, CCORP does not collect data on the specific coronary arteries bypassed during CABG and, therefore, factors pertaining to these vessels such as site and extent of revascularization, which may influence the technical complexity of the procedure, could not be assessed. Fourth, CCORP does not collect data such as degree of aortic root calcification which may have a significant impact on the type of CABG performed and the risk of complications [21]. Finally, a study of this nature cannot account for individual and unrecorded surgical decisions and strategies in the performance of the operative procedure. Because of these limitations, this study does not provide definitive evidence that OPCAB is associated with a lower risk of postoperative stroke. In this regard, it is noted that conversion to CCB was associated with an increased risk of stroke. However, this study does provide a basis for a randomized controlled trial of the comparative risk of stroke associated with CCB and OPCAB.

In this large retrospective study, OPCAB was associated with a significantly lower postoperative stroke rate compared with CCB, even for older and higher risk patients. However, intraoperative conversion from OPCAB to CCB was associated with the highest postoperative stroke rate.


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 References
 

  1. Benetti FJ, Naselli G, Wood M, Geffner L. Direct myocardial revascularization without extracorporeal circulation. Experience in 700 patients. Chest 1991;100:312-316.[Medline]
  2. Sellke FW, DiMaio JM, Caplan LR, et al. Comparing on-pump and off-pump coronary artery bypass grafting: numerous studies but few conclusions: a scientific statement from the American Heart Association council on cardiovascular surgery and anesthesia in collaboration with the interdisciplinary working group on quality of care and outcomes research Circulation 2005;111:2858-2864.[Abstract/Free Full Text]
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  4. Kapetanakis EI, Stamou SC, Dullum MK, et al. The impact of aortic manipulation on neurologic outcomes after coronary artery bypass surgery: a risk-adjusted study Ann Thorac Surg 2004;78:1564-1571.[Abstract/Free Full Text]
  5. Peel GK, Stamou SC, Dullum MK, et al. Chronologic distribution of stroke after minimally invasive versus conventional coronary artery bypass J Am Coll Cardiol 2004;43:752-756.[Medline]
  6. Al-Ruzzeh S, Ambler G, Asimakopoulos G, et al. Off-pump coronary artery bypass (OPCAB) surgery reduces risk-stratified morbidity and mortality: a United Kingdom Multi-Center Comparative Analysis of Early Clinical Outcome Circulation 2003;108(Suppl 1):II1-II8.[Medline]
  7. Angelini GD, Taylor FC, Reeves BC, Ascione R. Early and midterm outcome after off-pump and on-pump surgery in Beating Heart Against Cardioplegic Arrest Studies (BHACAS 1 and 2): a pooled analysis of two randomised controlled trials Lancet 2002;359:1194-1199.[Medline]
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  14. Greene WH. Econometric AnalysisThird edition. Upper Saddle River, NJ: Prentice Hall; 1997. pp. 339-350.
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