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European Heart Journal Advance Access originally published online on September 15, 2006
European Heart Journal 2006 27(20):2473-2480; doi:10.1093/eurheartj/ehl256
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© The European Society of Cardiology 2006. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Outcomes in off-pump vs. on-pump coronary artery bypass grafting stratified by pre-operative risk profile: an assessment using propensity score

Ignacio J. Ferreira-González1,2, Aida Ribera1,3, Purificación Cascant1, Gaietá Permanyer-Miralda1,* for the ARCA study group

1 Epidemiology Unit, Department of Cardiology, Vall d'Hebron Hospital, Paseo Vall d'Hebron 119-129, Barcelona 08035, Spain
2 Department of Internal Medicine, Universitat Autónoma, Barcelona, Spain
3 Department of Health and Experimental Sciences, Universitat Pompeu Fabra, Barcelona, Spain

Received 21 March 2006; revised 21 August 2006; accepted 31 August 2006; online publish-ahead-of-print 15 September 2006.

* Corresponding author. Tel: +34 932746177; fax: +34 933746063. E-mail address: gpermany{at}vhebron.net


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Appendix: EuroSCORE scale
 Acknowledgements
 References
 
Aims To assess the benefit of off-pump coronary surgery stratified by the pre-operative risk profile.

Methods and results Prospective and multicentric cohort study. All consecutive patients undergoing a first coronary bypass procedure between November 2001 and November 2003 were potentially eligible. Pre-operative EuroSCORE and in-hospital outcomes were prospectively collected using strictly standardized criteria. To ensure optimal adjustment, a propensity score was constructed using clinically relevant variables and incorporating individual centres. Of 1602 patients who underwent a first coronary bypass, EuroSCORE could be calculated in 1585: 787 were of moderate/high pre-operative risk profile (EuroSCORE>3), of which 347 underwent off-pump procedures, and 798 were of low pre-operative risk profile (EuroSCORE≤3), of which 349 underwent off-pump procedures. After risk adjusting for propensity score, off-pump patients had less major events (post-operative death, myocardial infarction, and need for reoperation). This benefit was higher in the low-risk stratum (OR ranged between 0.27 and 0.4; P=0.02–0.07) than in the high-risk stratum (OR between 0.4 and 0.7; P, not significant).

Conclusion In real-life conditions, off-pump coronary surgery may be more effective than on-pump surgery. In contrast with previous reports, our results suggest that this benefit may be higher in patients with low pre-operative risk.

Key Words: Coronary disease • Revascularization • Extracorporeal circulation • Surgery


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Appendix: EuroSCORE scale
 Acknowledgements
 References
 
The increasing popularity of coronary bypass surgery without cardiopulmonary bypass (‘off pump’) may be based on a reduced morbidity in the post-operative period.1 Although clinical trials have not demonstrated a benefit of off-pump surgery on clinical hard endpoints,1 the majority of them have been conducted in selected populations with, as a rule, low pre-operative risk.1,2 On the contrary, in observational studies of patients with a higher pre-operative risk profile, there is some evidence of a prognostic advantage on more relevant endpoints.3–8 A recent meta-analysis9 has shown consistently better results over most outcomes in observational than in randomized studies. While its authors could not rule out undetected confounding in the former, relevant factors in clinical practice that could influence their findings were not identified. In particular, the effect of pre-operative risk could not be assessed in the studies included in the meta-analysis.

In the last decade, the EuroSCORE has been validated as an effective and accurate method to pre-operatively quantify the risk of death and other serious post-operative complications.10,11 Therefore, the aim of the present study was to prospectively assess, using accurate adjustment techniques, the effectiveness of off-pump vs. on-pump coronary bypass surgery in two major pre-operative risk profile strata as assessed by the EuroSCORE in a real clinical practice setting.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Appendix: EuroSCORE scale
 Acknowledgements
 References
 
This prospective cohort study was carried out in five tertiary hospitals belonging to a single Spanish region (Barcelona, Catalonia, Spain) serving a population whose health care is funded by the social security system. Off-pump surgery was performed on the individual surgeon judgement or habit.

Patients and data abstraction
All consecutive patients who underwent a first coronary bypass surgery from November 2001 to November 2003 were included. A clinical researcher in each centre was designated to identify the patients meeting the inclusion criteria and to record the information in a specifically designed questionnaire, which included variables related to the EuroSCORE pre-operative risk profile,11 the surgical procedure, and the following post-operative in-hospital outcomes: myocardial infarction, stroke, need for reintervention, long hospital stay, post-operative haemodynamic instability, and post-opearative infection. In addition, we investigated the rate of ‘post-operative mortality’ defined as death occurring during the first 3 months after the procedure regardless of the cause. To calculate the rate of post-operative mortality, we searched the vital status for each patient at the regional database registry of deaths.

Other outcomes were defined as follows. Acute myocardial infarction (AMI): a rise of creatine kinase-MB or troponine-I levels more than two-fold above the normal level for the specific laboratory of each hospital or a new Q-wave after the operation in two or more continuous leads. Post-operative stroke: a new focal neurological deficit appearing after the surgical operation that persisted for more than 24 h. Both variables, post-operative AMI and stroke, were studied between the index intervention and the discharge. Need for reoperation: any other new surgical intervention performed during the current admission and after the index operation. Post-operative haemorrhage: occurring before discharge and requiring blood transfusion or reoperation. Post-operative haemodynamic instability: the need for using vasoactive drugs for more than 24 h or counterpulsation balloon to maintain the arterial pressure. Post-operative infection was also defined between the index intervention and discharge, and included sternal wound infection or the presence of fever of any origin persisting for more than 24 h.

The quality of the data collection was checked by a review of randomly selected clinical records (10% of the recruited sample) by the research team and by a head-to-head comparison of the list of included patients with the administrative databases.

Statistical analysis
Continuous variables are shown as median with 95% confidence interval (CI), and categorical variables are shown as percentages. Comparisons were performed with Student's t or Mann–Wittney's tests or with {chi}2 test or Fischer's exact test, when appropriate.

The pre-operative risk EuroSCORE was computed in each patient. The EuroSCORE was developed and validated10–12 in European countries to predict the risk of surgical mortality and its use in European countries has been encouraged by the European Cardiology and Cardio-Thoracic Surgery Societies (see in Appendix, the EuroSCORE variables). The resulting score can be stratified in three levels: low (0–2), moderate (3–5), and high risk (6 or more). For analysis purposes, we considered the subgroups of low–moderate risk patients, (scores below 4), and the moderate–high risk patients (scores of 4 or upper), since it has been shown that the in-hospital mortality abruptly increases above this cutoff.10,11

Control of potential confounders was attempted by constructing a propensity score13,14 for the whole population and for each stratum risk using logistic regression. The propensity score was the probability that a patient would receive off-pump coronary intervention, and was computed using an extensive, non-parsimonious, logistic regression modelling including the following covariates: age, gender, ventricular dysfunction, number of vessels affected, proximal anterior descending stenosis, left main (LM) stenosis, angina in 72 h before intervention, previous percutaneous revascularization, previous admission to the hospital for heart failure, previous stroke, urgency of the intervention, serum creatinine above 1.5 mg/dL, respiratory dysfunction, peripheral vascular disease, diabetes, and the particular centre where the patient was operated. The selection of the variables was made so as to get the best discriminating model as assessed by the C-statistics. Covariate interactions and higher-order terms for the continous variables proved unnecessary for the balance of baseline characteristics across quintiles. We judged that estimation of missing data was not required, since in 99.1% of the patients (1570) all data were complete. The resulting propensity score was then used for adjustment in two ways: by performing a stratified analysis and by performing a multivariable analysis. In the first case, the propensity score was used to define five risk strata for receiving off-pump, corresponding to the quintiles of propensity score or quintiles of the computed probability to receive off-pump surgery. Then, a common OR across the five quintiles was computed using the Maentel–Haenszel method. This method is appropriate if Wald's test is not statistically significant, indicating an acceptable homogeneity across the OR estimated through each quintile.15 If, on the contrary, Wald's test is statistically significant, a multivariable method including the variable ‘propensity score’ along with the variable type of surgery (off-pump vs. on pump), as the solely independent variables, is more adequate to control for selection bias.13 We applied both methods indicating in the Results section the nominal P-value of Wald's homogeneity test for each computed OR. Finally, we constructed several conventional logistic regression models with the study variables using the corresponding outcomes as dependent variables plus the variable, ‘type of surgery’ (off-pump vs. on-pump), as another independent variable.

In all cases, P<0.05 was considered significant. Corrections were not made for multiple comparisons. All statistical analysis was performed with SPSS 11.0 (SPSS Inc., Chicago, IL, USA).


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Appendix: EuroSCORE scale
 Acknowledgements
 References
 
Patient characteristics
Table 1 shows the baseline characteristics and the average grafts per patient in the whole population and in the respective risk strata. A total of 1602 patients were included: 706 undergoing off-pump procedures and 896 on-pump procedures. Among the former, previous coronary artery disease, neurological dysfunction in the index admission, and previous admissions to the hospital for heart-failure episodes were more frequent, whereas the prevalence of diabetes, hypercholesterolaemia, and three-vessel disease was higher in those with on-pump procedures. The information needed to compute the EuroSCORE was available in 1585 patients (99%). Nearly half of the sample size had moderate/high pre-operative risk (EuroSCORE>3; n=787), whereas the remainder of patients had low/moderate pre-operative risk (EuroSCORE≤3; n=798). The difference in the pre-operative risk profile between the off-pump vs. on-pump surgery was similar in each stratum and in the whole population. However, variables more related to an adverse prognosis were, as expected, more frequent in the high-risk stratum. There were slightly more grafts implanted per patient in the on-pump procedures.


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  		Table 1 Baseline characteristics of the patients in the whole population and in each risk stratum

 
Crude post-operative in-hospital outcomes
Off-pump surgery benefited almost all the unadjusted outcome variables, except for post-operative haemorrhage and haemodynamic instability in the moderate/high-risk stratum (Table 2). However, the magnitude of the difference did not reach statistical significance in most of them. Remarkably, the benefit was consistently higher in the low-risk stratum both in absolute and relative terms, whereas the difference in the results between both techniques was attenuated in the high-risk stratum. Need for reoperation, post-operative infection, and post-operative stay longer than 10 days were the most benefited outcomes with off-pump surgery in both the whole and the low-risk population. In the latter, the reduction in post-operative haemorrhage and post-operative myocardial infarction approached statistical significance, in spite of a smaller sample size after stratifying the study population.


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  		Table 2 Unadjusted in-hospital events with the off-pump vs. the on-pump procedures, in the whole population and in each risk stratum

 
Adjusted post-operative in-hospital outcomes
The model constructed to compute the propensity score reached a C-statistic (area under the receiver operating characteristic curve) of 0.85 (CI 0.82–0.87), 0.85 (CI 0.82–0.87), and 0.86 (CI 0.84–0.89) in the whole population, the moderate/high-risk stratum, and the low/moderate risk-stratum, respectively. Distribution of propensity score quintiles showed that each quintile in the whole population was composed by roughly 300 patients, whereas only about 150 patients were available for each quintile in the respective risk strata. Propensity scores for off-pump patients were fairly comparable with those of on-pump patients within each quintile, but at the expense of having unbalanced numbers between treatments within each quintile. Thus, the number of patients favoured on-pump in the lower and off-pump in the upper quintiles. For this reason, in the low-risk stratum, it was not possible to perform stratified analysis in three outcomes because of the few patients suffering outcomes in several quintiles.

Tables 3–5 show the risk of adverse outcomes adjusted for propensity score. Although Wald's homogeneity test reached statistical significance in nearly half the outcomes, the common inter-strata OR weighted adjusted using Mantel–Haenszel method was fairly close to the OR adjusted using multivariable analysis. The benefit of the off-pump procedure was usually higher after adjusting the OR, except in the high-risk stratum, where it barely varied (Table 4). As in the unadjusted outcomes, the low-risk stratum was the most benefited by the off-pump procedure, since most outcomes reached statistical significance (Table 5). To note, in spite of the low rate of post-operative death in the low-risk stratum (1.7% for off-pump and 2% for on-pump; Table 2), the adjusted OR for the off-pump procedure almost reached statistical significance (OR 0.34; 95% CI 0.1–1.1; P=0.07). Although the benefit of the off-pump was also present in the whole population and in the high-risk stratum, it was progressively diluted while increasing the pre-operative risk profile. Thus, ORs were fairly closer to the unit in the high-risk strata, where significance was lost throughout outcomes except for the need for reoperation and long post-operative stay, where it had borderline value (Tables 3 and 4). In Figure 1, the adjusted results for the main outcomes in the three patient subgroups are summarized.


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  		Table 3 In-hospital risk of adverse outcomes with the off-pump vs. the on-pump procedures, in the whole population, adjusted for propensity score

 

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  		Table 5 In-hospital risk of adverse outcomes with the off-pump vs. on-pump procedures, in the moderate/low-risk stratum, adjusted for propensity score

 

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  		Table 4 In-hospital risk of adverse outcomes with the off-pump vs. the on-pump procedures, in the moderate/high stratum, adjusted for propensity score

 

Figure 2561
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  		Figure 1 Adjusted results adjusted for propensity score (OR and 95% CI) for the main outcomes in the three patient subgroups.

 
As an additional analysis, we computed the ORs for post-operative mortality in each subgroup of patients using a conventional logistic regression model. We then obtained results closely similar to those previously shown: OR for in-hospital death with off-pump surgery in the whole population 0.49 (95% CI 0.27–0.89; P=0.02); in the high-risk stratum 0.54 (95% CI 0.3–1.1; P=0.1); and in the low-risk stratum 0.34 (95% CI 0.1–1.06; P=0.06).


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Appendix: EuroSCORE scale
 Acknowledgements
 References
 
The actual benefit of the off-pump surgery is still controversial, since late trials have shown a small benefit limited to the endpoints atrial fibrillation, inotrope requirements, respiratory infections, and need for blood transfusion.1 This modest advantage has been attributed to the low pre-operative risk profile of the population included in the trials.1,2 In fact, it has been speculated that the benefit of off-pump may be greater in high-risk patients and that it would be more efficient to limit future trials to the high-risk subgroups.1 On the other hand, the majority of observational studies, although fairly benefiting off-pump surgery, are retrospective and either do not include a large cohort of patients or, in those cases with large sample sizes, their information is limited by being extracted from administrative databases.5,7,16,17 Nevertheless, from the results of these observational studies, it would seem reasonable to expect a greater benefit with the off-pump in high-risk subgroups,5,18 such as the elderly7,16,17 and the diabetic patients.8 A recent meta-analysis9 has found consistently better results in observational studies than in clinical trials. Although observational studies may be more vulnerable to confusion, clinical trials may represent a highly selected population, as suggested by the number of patients in the former (293 617) and in the latter (3449). The question, then, remains open as to which are the real benefits of off-pump surgery and to what patient subgroups they apply.

This controversy encouraged us to design a study to assess the results of the off-pump surgery in a real clinical world setting, accounting for bias in the most accurately possible way in the scenario of a prospective observational study and considering results in separate surgical risk strata. This combined approach has been, to date, uncommon among observational studies and is relevant to the present status of the controversy.

In patients who undergo a first coronary bypass intervention, our study has shown a definite advantage of off-pump surgery in the low-risk subgroup patients except for post-operative haemorrhage and stroke. There was a benefit nearly significant for in-hospital mortality (OR 0.34; 95% CI 0.1–1.1 P=0.07), in spite of the relative low mortality rate of this group (1.7% for off-pump and 2% for on-pump). The benefit was smaller, but significant, in the whole population for in-hospital mortality, post-operative AMI, need for reoperation, and post-operative stay >10 days (Table 3). However, and in striking disagreement with current suggestions, such benefit was much smaller and inconclusive among the high-risk subgroup patients, except for the ‘softer’ outcomes need for reoperation and post-operative stay >10 days, which approached a statistical significance (Table 4).

Our findings have some particular features. For instance, the operative mortality rate was higher than reported in other contexts.19 Although this may be partly explained by the higher pre-operative risk profile in our patients, other series with similar pre-operative risk, as suggested by a similar average EuroSCORE, had smaller rates of unfavourable outcomes.8 In any case, the high rate of outcomes in our study could be associated with many variables. Some of them could be related to the care process (e.g. the volume of annual interventions performed per centre and per surgeon may be lower in our study than it is in other series), while other unrecognized variables, not necessarily implying a poor surgical performance, may also be involved. The comparatively high rate of such outcomes in our study, on the other hand, may have permitted clinical benefits of off-pump procedure to more easily achieve statistical significance.

A selection bias as a determinant of our results is unlikely since all the patients were consecutively and prospectively included. In addition, we were extremely cautious in standardizing the definitions of the variables and in instructing the hospital researchers to avoid a potential information bias. Finally, it is unlikely that a major unknown confounder may explain our results, mostly considering that all the usual variables in other studies and those recommended by the American College of Cardiology guidelines20 were included in our questionnaire.

The reliability of our results is supported by their internal consistency. On the one hand, virtually all outcomes were improved in off-pump procedures. On the other hand, we reached the same results using multivariable and stratified analyses, particularly for outcomes in which Wald's homogeneity test was non-significant (suggesting homogeneity, i.e. greater comparability across ORs in the five quintiles). Moreover, the finding that the low-risk subgroup was the most benefited is consistent with the fact that in this group the difference between off-pump and on-pump EuroSCORE was smaller (Table 1). Therefore, adjusting for the pre-operative risk factors, the relative benefit of low risk should be higher than in the high-risk group and in the whole population.

Two questions arise regarding the external validity of our findings: why were our results so advantageous to off-pump surgery when this benefit has not been observed in the relevant clinical trials? and why was this advantage clearly greater in the low-risk subgroup, again in contrast with rigorous trials?

The first query may also be applied to other observational studies in which a morbidity and mortality benefits of the off-pump surgery have been found.7,21–24 In fact, the discrepancy observed between the results of any clinical trial and observational studies can be partly related to factors associated to the quality of care process, such as the surgical team expertise or the post-operative care, which are heterogeneous across countries, regions, or even centres, but ultimately determine the results of the procedure in the ‘real world’. In this sense, our results are in agreement with those shown in other observational studies. They may even be more valid, since this is a prospective study specifically directed to answer a prespecified question. There may be reasons for the better outcomes in off-pump surgery found in observational studies9 that, even if not detected in ideal comparisons such as randomized trials, are relevant for clinical decision making. For instance, the surgeon preference may be a leading factor in choosing the procedure type. Therefore, it could be speculated that the more proficient and experienced surgical teams tend to incorporate new procedures more rapidly and thus prefer off-pump surgery. In this sense, we constructed an additional propensity score adding the variable ‘surgeon’ to the preceding ones. It was found that the individual surgeon was the strongest predictor of the surgical procedure that was chosen. The question then arose as to whether adjustment for the individual surgeons performing each operation would be the best approach. We decided against this, however, as each of the surgeons predominantly chose either on- or off-pump procedures for most patients, thus leaving only a small proportion of the sample as valid for this adjusted comparison. In any case, when the logistic regression analysis was performed adjusting for this propensity scores, the ORs were virtually unchanged but the effect was diluted.

The second question is more troublesome. Two factors should be taken into account to explain the better results of the off-pump surgery in the low-risk subgroup. First, it has been assumed that the pathophysiologic sequelae of cardiopulmonary bypass are greater in high-risk subsets.5 Consequently, most observational studies have been done in high-risk patients leaving out low-risk patients. However, it is not unreasonable to speculate that, if on-pump procedures are more aggressive than off-pump, the greater mortality associated with the former may be ‘diluted’ and harder to disclose the high-risk subgroups (where higher mortality rate is less procedure dependent). The second factor is related to the comparatively higher mortality rate in the low-risk subset of our study than in other studies, above all in clinical trials,1 which might arguably be a limitation to extrapolate our findings. Moreover, the fact of having included a single Health Care Spanish region is itself a limitation of the study that must be emphasized. Nevertheless, the observed mortality in the low-risk group is within the 95% CI of the expected mortality according to the EuroSCORE model. Therefore, our results do not necessarily suggest an unduly poor result. In addition, observational reports may, as any study, be influenced by publication bias and poorer outcomes, as those reflecting results in whole regions may not be communicated. Therefore, it is likely that our findings may reflect the pattern of care in many parts of the world and thus be generalizable to similar areas.

In conclusion, the present study supports the view that the greater benefits of off-pump surgery in observational than in randomized studies9 are not spurious. In real clinical practice, off-pump surgery in those patients undergoing a primary coronary bypass procedure may be associated with slightly but definitely better outcomes than on-pump surgery on a range of clinically important outcomes. This advantage may be greater than previously reported and predominantly apparent in low-risk patients. The effect of patterns of care on these results cannot be ruled out but, if present, it is likely to depend on widely prevalent practice characteristics.


   Appendix: EuroSCORE scale
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Appendix: EuroSCORE scale
Acknowledgements
 References
 

Patient-related factors
Age Per 5 years or part thereof over 60 years 1
Sex Female 1
Chronic pulmonary disease Long-term use of bronchodilators or steroids for lung disease 1
Extracardiac arteriopathy Any one or more of the following: claudication, carotid occlusion or >50% stenosis, previous or planned intervention on the abdominal aorta, limb arteries or carotids 2
Neurological dysfunction disease Severely affecting ambulation or day-to-day functioning 2
Previous cardiac surgery Requiring opening of the pericardium 3
Serum creatinine >200 µmol/L pre-operatively 2
Active endocarditis Patient still under antibiotic treatment for endocarditis at the time of surgery 3
Critical pre-operative state Any one or more of the following: ventricular tachycardia or fibrillation or aborted sudden death, pre-operative cardiac massage, pre-operative ventilation before arrival in the anaesthetic room, pre-operative inotropic support, intra-aortic balloon counterpulsation, or pre-operative acute renal failure (anuria or oliguria<10 mL/h) 3
Cardiac-related factors
Unstable angina Rest angina requiring iv nitrates until arrival in the anaesthetic room 2
LV dysfunction Moderate or LVEF 30–50% 1
Poor or LVEF <30% 3
Recent myocardial infarct <90 days 2
Pulmonary hypertension Systolic PA pressure >60 mmHg 2
Operation-related factors
Emergency Carried out on referral before the beginning of the next working day 2
Other than isolated CABG Major cardiac procedure other than or in addition to CABG 2
Surgery on thoracic aorta For disorder of ascending, arch or descending aorta 3
Post-infarct septal rupture 4


   Acknowledgements
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Appendix: EuroSCORE scale
 Acknowledgements
References
 
This study has been supported by a grant from the Agència d'Avaluació de Tecnologia i Recerca Mèdiques de Catalunya (Spain). I.F.-G. is funded by Carlos III Spanish Institute of Health Research Fellowship Award (FIS).

Conflict of interest: The authors declare that they have no conflict of interest regarding this study.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Appendix: EuroSCORE scale
 Acknowledgements
 References
 

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