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© 2003 European Association of Cardio-Thoracic Surgery
Is off-pump coronary surgery justified in EuroSCORE high-risk cases? A propensity score analysis
a Department of Cardiothoracic Surgery, The Cardiothoracic Centre, Thomas Drive, Liverpool, L14 3PE, UK
* Corresponding author. Tel.: +44-151-293-2397; fax: +44-151-220-8573 Received April 29, 2003; received in revised form July 31, 2003; accepted August 26, 2003
We aimed to quantify the effect of avoiding cardiopulmonary bypass on outcomes in high-risk patients. Of the 2079 consecutive CABG's performed by three surgeons between April 1997 and September 2002, 389 were classified as high-risk according to the European System for Cardiac Operative Risk Evaluation (EuroSCORE) risk stratification, with a score of >5. The off-pump group had 196 patients and the on-pump group had 193 patients. Multivariate logistic regression was used to assess the effect of off-pump on in-hospital outcomes, while adjusting for treatment selection bias with a propensity score. The incidence of hypertension, hypercholesterolaemia, and renal dysfunction were higher in the off-pump group. The median EuroSCORE for off-pump patients was 7 (6–8), while for the on-pump patients was 7 (6–8;  ). After adjusting for the propensity score, off-pump patients were less likely to have stroke (OR 0.17;  ), renal failure (OR 0.35;  ), blood transfusion (OR 0.12;  ), prolonged mechanical ventilation (OR 0.36;  ), and inotrope support (OR 0.35; ). Off-pump patients also had significantly shorter post-operative hospital stays. There was no significant difference between off-pump and on-pump patients in terms of in-hospital and mid-term mortality. Off-pump CABG is justified in EuroSCORE high-risk cases.
Key Words: Off-pump coronary artery bypass surgery; Morbidity; Mortality; EuroSCORE; High-risk patients; Propensity score
Off-pump CABG is increasingly shown to have better outcomes by several institutions in matched or risk adjusted cohorts, with a reduction in morbidity and hospital costs [1,2]. Deleterious effects of cardiopulmonary bypass (CPB) might be more severe in high-risk patients undergoing coronary artery bypass surgery (CABG). Several reports have examined the impact of off-pump CABG in different high-risk groupings [3–6]. Recently, Al-Ruzzeh et al. [7] and Calafiore and colleagues [8] have published reports examining the impact of off-pump CABG on patients classified as high-risk by the European System for Cardiac Operative Risk Evaluation (EuroSCORE >5) [9]. The aim of this study was to determine the effect of avoiding CPB on EuroSCORE high-risk patients in our population.
2.1. Patient population and data Two thousand and seventy-nine patients received isolated CABG at the Cardiothoracic Centre-Liverpool between 1st April 1997 and 30th September 2002. These patients represented the entire coronary revascularisation practice of three surgeons (D.M.P., W.C.D., and B.M.F.). Of these patients, 389 (18.7%) were identified as high-risk with a EuroSCORE of 6 or more [9]. Of the 389 patients in the study, 196 (50.4%) patients underwent off-pump CABG, while 193 (49.6%) patients received conventional on-pump CABG. All three surgeons have changed practice over the last 5 years, from performing almost all cases on-pump to now performing all cases off-pump. All off-pump patients were operated through a median sternotomy approach. Target coronary arteries were stabilised using the Octopus II+ tissue stabilisation system (Medtronic, Minneapolis, MN). Appropriate size intracoronary shunts (Medtronic Clearview Shunt; Medtronic) were used in all cases to maintain distal perfusion and to achieve a bloodless operative field. Visualization was aided with a humidified carbon dioxide blower (CTS Aires CO2 Blower; Cardiothoracic Systems, Cupertino, CA). Hypothermia was prevented by application of an air warming blanket to the lower half of the body. In the on-pump group, standard CPB techniques were used with a median sternotomy approach. Membrane oxygenators and roller pump heads were used for the CPB. Normothermia or mild hypothermia (34–36 °C) was maintained and myocardial protection was achieved with antegrade induction of blood cardioplegia followed by intermittent antegrade or continuous retrograde warm blood cardioplegia. All data were collected prospectively during the patient admission as part of routine clinical practice. Methods of data collection and definitions are available from www.nwheartaudit.nhs.uk. Data were collected on the variables listed in Tables 1 and 2.
In-hospital mortality was defined as death within the same hospital admission regardless of cause. All patients transferred from the base hospital to another hospital were followed up to confirm their status at discharge. Post-operative stroke was defined as a new focal neurological deficit and comatose states occurring post-operatively that persisted for >24 h after its onset and was noted before discharge. We excluded confused states, transient events and intellectual impairment from our study to avoid any subjective bias. Renal failure was defined as patients with a post-operative creatinine level greater than 200 çmol/l or patients requiring dialysis. 2.2. Patient follow-up Patient records were linked to the National Strategic Tracing Service (NSTS), which records all deaths in the United Kingdom, to establish current vital status.2.3. Statistical methods Due to non-normality of data, continuous variables are shown as median with 25th and 75th percentiles. Categorical variables are shown as a percentage. Comparisons were made with Wilcoxon rank sum tests and Chi-square tests as appropriate. Logistic regression was used to adjust in-hospital outcomes for differences in patient and disease characteristics (treatment selection bias) [10]. Deaths occurring as a function of time were described using the product limit methodology of Kaplan and Meier [11]. Cox proportional hazards analyses was used to calculate adjusted hazard ratios (HR) and to risk adjust the Kaplan–Meier survival curves [12].Treatment selection bias was controlled for by constructing a propensity score [13]. The propensity score was the probability that a patient would receive off-pump coronary surgery, and included older age, hypercholesteroleamia, severity of NYHA class, and number of bypass grafts (C statistic=0.85) [10]. Once the propensity score is constructed for each patient, there are three ways of using the score for comparisons: matching, stratification, and multivariable adjustment. Due to the small sample size available to us for this study, we have decided to use multivariable adjustment because matching would have reduced the study size even further and stratification can be difficult to interpret. The propensity score is then included along with the comparison variable (off-pump or on-pump) in a multivariable analyses of outcome producing adjusted odds ratios as shown in Table 3. The propensity score adjusts for the treatment selection bias, which is evident in Table 1, between one group versus another [13]. In all cases a P value <0.05 was considered significant. All statistical analysis was performed retrospectively with SAS for Windows Version 8.2.
The characteristics of patients based on the procedure performed are shown in Table 1. The EuroSCORE distribution of the two surgical techniques was similar. The median EuroSCORE for off-pump patients was 7 (25th and 75th percentiles: 6–8), while for the on-pump patients it was also 7 (25th and 75th percentiles: 6–8; ).
The off-pump patients on average (median) received one less graft compared to on-pump cases [three grafts (25th and 75th percentiles: 2–3) vs. four grafts (25th and 75th percentiles: 3–4);
Off-pump patients were, on average, staying 2 days less than patients who received conventional on-pump CABG. The median post-operative length of stay for off-pump patients was 8 days (25th and 75th percentiles: 7–11), while for the on-pump patients it was 10 days (25th and 75th percentiles: 8–16); Post-operative outcomes for both surgical techniques are shown in Table 2 (unadjusted) and Table 3 (adjusted for the propensity score).
Sixty-three (16.2%) deaths occurred during a total of 730.6 patient-years, with a mean follow-up of 1.9 (S.D. 1.0) years. The crude HR of mid-term mortality for off-pump patients was 0.94 (
After adjusting for the propensity score, the adjusted HR of mid-term mortality for off-pump patients was 0.90 (
In this observational study we report on our experience of performing 196 off-pump CABG in EuroSCORE high-risk cases and the impact on adverse outcomes. Comparisons were made with a EuroSCORE group of patients who underwent conventional on-pump CABGs (  ) by the same surgeons. There were some differences between off-pump and on-pump patients in patient characteristics. Off-pump patients were more likely to have hypertension, hypercholesterolaemia, and renal dysfunction, however, they also had less extensive coronary disease. Al-Ruzzeh and colleagues also showed a significantly higher proportion of off-pump patients with preoperative renal problems. Their off-pump patients were also older with a higher number having a left ventricular ejection fraction of 30% or less. However, no attempts, at risk adjustment were made in Al-Ruzzeh's study, which they point out as a limitation [7]. We performed a propensity score analysis to account for any underlying differences in case-mix between off-pump and on-pump patients. Unlike, Calafiore and co-workers [8] who performed a propensity-matched analyses, we included the propensity score as a covariate in multivariable analyses of outcome to risk adjust for treatment selection bias [13]. After risk adjustment with the propensity score, the incidence of post-operative stroke was significantly lower in the off-pump patients. Several other reports have shown significant reductions in neurological deficits when avoiding CPB in high-risk groups [3,5,6,8]. Calafiore showed that the use of CPB increased the risk of stroke by 4.6 times in patients with a EuroSCORE >5, claiming that this was due to the avoidance of aortic manipulation in the off-pump patients [8]. Previous work involving this institution has shown that off-pump CABG significantly lowers stroke rates compared to conventional on-pump CABG, irrespective of aortic manipulation [14]. We believe the mechanism for this apparent advantage is through a reduction in the embolic load caused by the bypass circuit generating microgaseous and microparticulate emboli during the procedure [1]. We have shown a significant reduction in the incidence of post-operative renal failure in high-risk off-pump cases. Using a threshold definition of more than 200 µmol/l of post-operative serum creatinine for renal failure may imply that some patients had a trivial baseline increase in serum creatinine resulting in a classification of renal failure. However, all our patients classified as post-operative renal failure, without requiring dialysis, had a baseline serum creatinine increase of 71 µmol/l or more. This is comparable with the findings of Mangano and colleagues [15] who regarded anyone with a serum creatinine increase of 62 µmol/l or more over baseline as having clinically significant renal failure. Al-Ruzzeh et al. showed a significant reduction in renal dysfunction requiring haemofiltration or haemodialysis in elderly patients [6]. The mechanisms for reducing renal complications were described in detail in Ascione's work on non-dialysis-dependent renal insufficiency and coronary surgery, where they identified CPB as an independent predictor of acute renal failure [3]. The requirement for RBC transfusion was significantly lower when avoiding CPB, which has been shown by several other publications [3,4,8]. Yokoyama et al. identified several high-risk sub-groups who benefited from less transfusion requirements when undergoing off-pump CABG. Patients with preoperative respiratory disease, renal dysfunction, poor ejection fraction, old age, and prior cardiac surgery all had significantly lower transfusion rates [4]. With a need for blood conservation and growing concerns over safety of allogeneic blood transfusion, together with the theoretical risk of transmission of variant Creutzfeldt–Jakob disease (vCJD), off-pump coronary surgery appears to be an attractive alternative to conventional coronary bypass surgery. The question of whether avoidance of CPB has a significant benefit with respect to mortality remains inconclusive. We have found no survival advantage in in-hospital mortality and mid-term mortality between off-pump and on-pump patients. Many reports have demonstrated significantly lower mortality in off-pump patients [5,7,8], however, other reports have failed to reach a statistical significant difference [3,4]. It is important to note that these reports show that off-pump CABG incurs no increased risk of mortality and exhibits a tendency towards decreased mortality compared to on-pump patients. There are some limitations, which may effect the conclusions drawn from our study. These include variables not measured in this study such as the quality of the coronary vessels, which is important in selecting the type of surgery and in determining the outcome, and selection bias resulting from the operating surgeon's decision to perform the procedure off-pump or on-pump. For this to effect our conclusions by a significant amount, the variables used in the propensity score (e.g. diabetes, age, sex) would have to be uncorrelated with the variables not measured (e.g. quality of coronary vessels or distal coronary disease), but we do not believe that this is likely. A further limitation is the fact that this is an observational study. We have used propensity score methodologies to adjust for any potential treatment selection bias. However, propensity score adjustment is no substitute for a properly designed randomised control trial. On the other hand, retrospective comparisons with propensity score adjustment are more versatile and may be more widely acceptable than randomised control trials [13].
Off-pump CABG is justified in EuroSCORE high-risk cases and can significantly reduce post-operative stroke, renal failure, blood transfusion and inotrope requirements. The length of ventilation and post-operative hospital stay can also be significantly reduced in off-pump patients. These reductions in morbidity have a significant impact on financial costs and resource utilisation in the treatment of high-risk patients.
The authors would like to thank Miss Janet Deane for ensuring completeness and maintaining the quality of data collected.
Presented at the 2003 Annual Meeting of the Society of Cardiothoracic Surgeons of Great Britain and Ireland, Edinburgh, UK, March 2003. doi:10.1016/S1569-9293(03)00193-2
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Abstract
1. Introduction
). Observed freedom from death in the off-pump patients at 30 days, 1, 2, and 3 years was 94.4%, 88.6%, 84.9% and 79.7%, respectively, compared with 94.3%, 87.5%, 83.1% and 80.5% for the on-pump patients. 
). The adjusted Kaplan–Meier survival curves are shown in 
