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Ann Thorac Surg 2007;83:2111-2117
© 2007 The Society of Thoracic Surgeons

Original Articles: Cardiovascular

Can the EuroSCORE Predict the Early and Mid-Term Mortality After Off-Pump Coronary Artery Bypass Grafting?

Young-Nam Youn, MDa, Young-Lan Kwak, MDb, Kyung-Jong Yoo, MDa,*

a Department of Thoracic and Cardiovascular Surgery, Yonsei Cardiovascular Center, Yonsei University College of Medicine, Seoul, Korea
b Department of Anesthesiology and Pain Medicine, Yonsei Cardiovascular Center, Yonsei University College of Medicine, Seoul, Korea

Accepted for publication February 16, 2007.

* Address correspondence to Dr Yoo, Division of Cardiovascular Surgery, Room 618-1, Yonsei Cardiovascular Center, Yonsei University College of Medicine, 134 Shinchondong, Seodaemun-ku, Seoul, 120-752, Korea (Email: kjy{at}yumc.yonsei.ac.kr).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 References
 
Background: This study evaluated the role of the European System for Cardiac Operative Risk Evaluation (EuroSCORE) in the prediction of early-term and mid-term mortality in patients undergoing isolated off-pump coronary artery bypass grafting (OPCAB).

Methods: From January 2002 to August 2006, 757 consecutive patients underwent isolated OPCAB. The patients’ operative risks were calculated according to the standard and logistic EuroSCORE models. The cohort was classified into four subgroups according to both EuroSCORE scales. To evaluate the predictability, the expected mortality was compared with the observed mortality. The receiver operating characteristic curves were plotted and calibration was assessed. Mean follow-up was 32.8 ± 13.9 months.

Results: Ten (1.3%) in-hospital deaths occurred. The predicted total numbers of deaths by the EuroSCORE models were 34.2 (4.5%) for the standard EuroSCORE and 37.8 (5.0%) for the logistic EuroSCORE. The expected mortality rates were significantly higher than the observed mortality rates in all subgroups, except one. The area under curve (AUC) in in-hospital mortality was 0.72 for the standard EuroSCORE and 0.71 for the logistic EuroSCORE, but the tests of calibration for both EuroSCORE models were significant. Mid-term mortality was 3.6%. The AUC curve in mid-term mortality was 0.71 for the standard or logistic EuroSCORE. The calibration in both EuroSCORE models for mid-term mortality was nonsignificant, indicating good calibration.

Conclusions: Both EuroSCORE models overestimated the in-hospital mortality; however, both models showed good predictability for mid-term mortality. The EuroSCORE could be helpful in planning resource allocation and tailoring follow-up for patients undergoing isolated OPCAB.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 References
 
Risk stratification plays an important role in cardiac surgical practice worldwide. Many models are used to assess early mortality and morbidity, including the Parsonnet System, the Cleveland Clinic Coronary Scoring System, the United Kingdom Society of Cardiothoracic Surgeons Risk Score, and the European System for Cardiac Operative Risk Evaluation (EuroSCORE) [1–4]. The predictive value of these models is useful in deciding indications for surgery, estimating the need for resources, obtaining proper informed consent, and monitoring the quality of surgeons and institutions.

The EuroSCORE was developed between 1995 and 1999 to provide a simple, additive risk model of perioperative mortality [4, 5]. The system was constructed from data obtained from more than 19,000 consecutive adult patients undergoing open heart surgery in 128 centers in eight European countries. The EuroSCORE has gained wide acceptance in Europe [6] and has also been validated in North America [7] and Japan [8]. It has been reported to predict not only operative mortality but also long-term mortality of cardiac surgery patients [9]. The EuroSCORE model has been reported to be an especially good risk predictor of coronary artery bypass grafting (CABG) patients, even though the percentage of the enrolled patients with CABG was only 64% in the original EuroSCORE study [10–12].

Off-pump coronary artery bypass grafting (OPCAB) has achieved worldwide acceptance. The numbers of OPCAB procedures are increasing with recent improvements in retractor-stabilizer systems and techniques of exposure of all surfaces of the heart [13]. Likewise, multivessel OPCAB is currently feasible for many patients. Its use continues to be explored with case–matched reports, retrospective studies, and recently, prospective randomized trials [14]. The aim of this study was to evaluate the validity and predictability of the EuroSCORE in the short-term and mid-term outcomes of OPCAB and, if applicable, to determine the adjusted hazard ratio (HR) for various EuroSCORE groups.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 References
 
Patients and Strategy of Procedure
From January 2002 through August 2006, 769 consecutive patients underwent isolated myocardial revascularization by a single surgeon (KJY) at Severance Hospital, Yonsei University College of Medicine. Excluded were 12 patients who underwent conventional CABG with cardiopulmonary bypass as planned preoperatively. The remaining 757 patients with isolated OPCAB were enrolled in this study, which was approved by the Institutional Review Board of Medical College of Yonsei University. Individual patient consent was waived.

OPCAB in patients with multivessel coronary artery disease was first performed at our institute in 2000. Since 2002, after a 2-year with OPCAB, it has been our strategy of myocardial revascularization. In this period, we have tried to perform OPCAB for all patients with multivessel disease referred from cardiologists and other hospitals. The operations for the excluded 12 patients using cardiopulmonary bypass were the only ones planned to be performed in 2002, and from 2003 all referred patients underwent OPCAB initially. Five on-pump conversions occurred in the study period because of severe ventricular dysfunction (n = 1) and frequent ventricular arrhythmia (n = 4) during coronary stabilization. The five conversion cases were included in this study on the basis of intention to treat.

Operative Technique
The operation was performed through a full sternotomy incision, with harvesting of the left internal mammary artery in the skeletonized fashion and simultaneous harvesting of the radial artery by using a Harmonic scalpel (Ethicon Endosurgery Inc, Cincinnati, OH). The right internal mammary artery, saphenous vein, or right gastroepiploic artery were also harvested if necessary. A calcium-channel blocker (diltiazem) was infused intravenously during surgery to prevent spasm of the radial artery and continued until oral diltiazem was administered. Heparin was given at a dose of 1 mg/kg to achieve a target activated clotting time of at least 350 seconds before ligation of the distal internal mammary artery.

Stabilization of the target arteries was accomplished using an Octopus tissue stabilizer (Medtronic, Minneapolis, MN). When hemodynamic instability was expected, a Starfish heart positioner (Medtronic) was used during anastomoses of arteries that were placed on the lateral and posterior wall of the heart. In most instances, anastomosis of the left internal mammary artery to the left anterior descending artery was performed first using intracoronary shunts. A proximal silicone elastomer snare was used to anastomose other coronary arteries. A misted carbon dioxide blower and irrigation with warm saline were used to remove blood from the sites of arteriotomy.

Data Collection
Preoperative, intraoperative, and postoperative data were collected prospectively from the registry database, medical notes, and charts. Detailed data were collected on the index admission when cardiac surgery was performed, and the risk stratification was performed according to the standard and logistic EuroSCORE models. The factors used by the EuroSCORE formula, the definitions, and the scores for both the standard and logistic EuroSCORE model are referenced to the report of the EuroSCORE study group [5]. Complete revascularization was defined as placement of at least one graft to each of the three major vascular regions that included stenosis of more than 50% of diameter (if one existed) [15, 16]. Other variables were defined according to the Society of Thoracic Surgeons Adult Cardiac Database Definition of Terms Version 2.52.1 (http://www.ctsnet.org/file/rptDataSpecifications252_1_ForVendorsPGS.pdf).

Other data collected included the body mass index, the number of arteries diseased, prior percutaneous coronary intervention, Canadian Cardiovascular Society (CCS) angina class, smoking history, the presence of diabetes mellitus or hypertension, and the New York Heart Association (NYHA) functional class. Procedural data were also collected on the number and types of graft used, the number of distal anastomoses, and the completeness of revascularization. Postoperative data collected were in-hospital mortality, postoperative length of stay, and major complications after surgery, including stroke, transmural myocardial infarction, low cardiac output syndrome, intraaortic balloon pump support, deep sternal wound infection, cerebrovascular accident, reexploration for bleeding, sepsis, gastrointestinal complications, renal failure, and respiratory failure.

Risk stratification was performed according to the standard and logistic EuroSCORE models. To analyze the efficacy of the standard EuroSCORE in OPCAB procedures, we classified the cohort into four subgroups by score: 0 to 2 (n = 201), 3 to 5 (n = 309), 6 to 8 (n = 178), and higher than 9 (n = 64). To analyze the efficacy of the logistic EuroSCORE, we also classified the cohort into four subgroups: 0 to 2.00 (n = 233), 2.01 to 5.00 (n = 307), 5.01 to 10.00 (n = 135), and higher than 10.01 (n = 77). The subgroups were divided according to the large studies on the relationship between the EuroSCORE and CABG procedures [10, 17]. The standard EuroSCORE system consists of three risk groups: low risk (0 to 2), with an expected mortality of less than 2%; medium risk (3 to 5), with an expected mortality of less than 5%; and high risk (≥6), with an expected mortality of more than 10% [5].

Follow-Up
Mid-term follow-up was obtained for 95.3% of the patients by an outpatient clinic visit, telephone interview, or mailed questionnaire. In 32 patients (4.7%) lost to follow-up, mortality data was obtained from the National Health Insurance Corporation. The mean follow-up time was 32.8 ± 13.9 months (range, 4 to 61 months), and total follow-up time was 1932.1 patient-years.

Statistical Analysis
Continuous variables were expressed as mean ± standard deviation, and discrete variables were summarized by percentages. Univariate analyses were performed by the Student t test, Fisher exact test, {chi}2 test, or one-way analysis of variance.

Receiver operating characteristic curves were plotted to assess the discriminatory ability of the standard and logistic EuroSCORE models. The area under the curve (AUC) was calculated as an index for how well the EuroSCORE could discriminate patients who lived and those who died after OPCAB as determined at the follow-up. The AUC may be interpreted as the probability that a patient who died had a higher risk score than a patient who survived; thus, the AUC is the percentage of randomly drawn pairs for which this is true. The discriminatory power of the model is considered "excellent" if the AUC exceeds 0.80, "very good" if it exceeds 0.75, and "good" if it exceeds 0.70 [18].

To evaluate the accuracy of the EuroSCORE models, calibration was assessed by using the Hosmer-Lemeshow goodness-of-fit statistic [19]. For this statistic, the predicted risks of individual patients were ordered by rank and divided into 10 groups of about equal size by their predicted probability. Within each group of estimated risk, the numbers of predicted deaths were accumulated against the number of observed deaths, a value of p > 0.05 indicates acceptable calibration of the model.

We generated Kaplan-Meier curves and used the long-rank test (with adjustment for trend) for comparisons. A Cox proportional hazards regression model was conducted to identify the independent predictors for mid-term mortality and to calculate the hazard ratios (HRs) and 95% confidence intervals (CIs) of the various groups while adjusting for preoperative and intraoperative factors other than those included in the EuroSCORE formula, such as a body mass index exceeding 30 kg/m2, three-vessel disease, a CCS angina class of 3 or more, the presence of hypertension or diabetes mellitus, a NYHA functional class of III or more, a history of smoking, previous percutaneous coronary intervention, the number of arterial grafts, total arterial grafting, the completeness of revascularization, and other major complications. The analysis was first done with a backward stepwise method, and variables with a value of p < 0.05 were retained as independent predictors and confirmed using a forward stepwise selection. Values of p ≤ 0.05 indicated a significant difference; all p values were two-tailed. All analyses were performed using SPSS 12.0 software (SPSS Inc, Chicago, IL).


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 References
 
The mean age of the patients was 61.9 ± 8.8 years (range, 34 to 88 years), and 540 were men (71.3%) and 217 were women (28.7%). Patient and disease characteristics according to the factors used by the EuroSCORE formula are summarized in Tables 1 and 2. Go As the risk stratification grew, there was an increase in the mean age as well as the percentage of patients with factors listed in the EuroSCORE model. Because of the nature of the OPCAB procedure, no participating patients required a concurrent surgery (ie, thoracic aorta surgery or repair of a postinfarction septal rupture).


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  		Table 1 Patient and Disease Characteristics According to the Factors Used by the European System for Cardiac Operative Risk Evaluation (EuroSCORE) Formula (Standard EuroSCORE Groups)
 

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  		Table 2 Patient and Disease Characteristics According to the Factors Used by the European System for Cardiac Operative Risk Evaluation (EuroSCORE) Formula (Logistic EuroSCORE Groups)
 
The in-hospital mortality rate was 1.3% (n = 10). The mean standard EuroSCORE was 4.5 ± 2 .8 (range, 0 to 20), and the mean logistic EuroSCORE was 5.0 ± 6.7 (range, 0.88 to 85.53). The observed and predicted numbers of deaths according to the EuroSCORE models are summarized in Table 3. Both EuroSCOREs considerably overestimated risk across all the risk subgroups, except the first subgroup of the standard EuroSCORE: the predicted total number of deaths of 34.2 in the standard EuroSCORE and 37.8 in the logistic EuroSCORE were far in excess of the observed total of 10. The tests of calibration were significant (p = 0.03 in the standard EuroSCORE; p = 0.04 in the logistic EuroSCORE), suggesting that both EuroSCORE models are not well calibrated.


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  		Table 3 Operative Outcomes and Mid-Term Mortality
 
The AUC for in-hospital mortality was 0.72 in the standard EuroSCORE (95% CI, 0.57 to 0.87; p = 0.02), and the AUC was 0.71 in the logistic EuroSCORE (95% CI, 0.55 to 0.87; p = 0.02; Fig 1).


Figure 1
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  		Fig 1. Receiver operating characteristic (ROC) (diagonal line) curve of in-hospital mortality. The area under curve (AUC) of the standard EuroSCORE (solid line) is 0.72; the AUC of the logistic EuroSCORE (dashed line) is 0.71. EuroSCORE = European System for Cardiac Operative Risk Evaluation.

 
Total arterial grafting was performed in 510 patients (67.4%), and complete revascularization in 670 (88.5%). The mean number of distal anastomoses was 3.1 ± 0.8. Lengths of stay among the subgroups of the standard or logistic EuroSCORE study were significantly different (p < 0.001 in both groups). The incidence of major complications was 9.6% (n = 73). As the risk stratification grew, the incidence of major complications tended to increase in both EuroSCORE groups. There was no significant difference in the number of anastomoses among the subgroups (standard EuroSCORE groups, p = 0.17; logistic EuroSCORE groups, p = 0.09).

During the follow-up period, 27 patients (3.6%) died. The tests of calibration for mid-term mortality were nonsignificant in the standard EuroSCORE (p = 0.47) and in the logistic EuroSCORE (p = 0.60). The AUC were 0.71 (95% CI, 0.58 to 0.82; p = 0.02) for both the standard and logistic EuroSCORE in the analysis of mid-term mortality. The overall 58-month actuarial survival, as calculated by the Kaplan-Meier method, was 92.7% ± 1.4%. In the analysis of the standard EuroSCORE, the 58-month actuarial survival rates calculated by the Kaplan-Meier method were 97.8% ± 1.1% in the 0 to 2 group, 95.4% ± 1.4% in the 3 to 5 group, 87.1% ± 4.2% in the 6 to 8 group, and 83.2% ± 5.5% in the 9 or higher group, with significant difference (p < 0.001; Fig 2). In analysis of the logistic EuroSCORE, the 58-month actuarial survivals calculated by the Kaplan-Meier method were 97.3% ± 1.2% in the 0 to 2 group, 95.4% ± 1.4% in the 2.01 to 5 group, 88.3% ± 4.2% in the 5.01 to 10 group, and 85.3% ± 4.6% in the more than 10 group (p < 0.001; Fig 3).


Figure 2
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  		Fig 2. Actuarial survival in the study of the standard European System for Cardiac Operative Risk Evaluation (EuroSCORE). Solid line, 0–2; small dashed line, 3–5; large dashed line, 6–8; dash-dot line, ≥ 9.

 

Figure 3
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  		Fig 3. Actuarial survival in the study of the logistic European System for Cardiac Operative Risk Evaluation (EuroSCORE). Solid line, 0–2; small dashed line, 2.01–5; large dashed line, 5.01–10; dash-dot line, >10.

 
The univariate Cox regression hazard model determined that either the standard EuroSCORE (HR, 1.31; 95% CI, 1.19 to 1.43; p < 0.001) or logistic EuroSCORE (HR, 1.06; 95% CI, 1.04 to 1.08; p < 0.001) was a predictor for mid-term deaths. The multivariate Cox regression hazard model revealed that the independent factors predicting mid-term mortality included a CCS score of 3 or higher (HR, 2.74; 95% CI, 1.16 to 6.48), incomplete revascularization (HR, 2.68; 95% CI, 1.18 to 6.08), major surgical complications (HR, 3.53; 95% CI, 1.52 to 8.19), and the standard or logistic EuroSCORE. The HRs with the reference of each first subgroup of EuroSCORE models are listed in Table 4.


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  		Table 4 Adjusted Hazard Ratio for Mid-Term Mortality
 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
References
 
Scoring systems have become important tools in the performance analysis of surgical units and for medical quality control. They may also be useful in cost-benefit analyses. The risk assessment of perioperative mortality has become well established by risk score systems such as the EuroSCORE. The EuroSCORE system, which was designed primarily to estimate perioperative mortality, is based on patient-related factors, cardiac-related factors, and operation-related factors. It was constructed from pooled information on cardiac patients from several European countries. After the logistic model became available, it was found to be a better risk predictor, especially in high-risk patients [20].

Estimating the risk for CABG has some advantages. It helps to determine the indications for CABG versus cardiac transplantation in patients with ischemic cardiomyopathy, the application of cardiopulmonary bypass, to obtain proper informed consent, to monitor the quality of surgeons and institutions, and to identify patients at higher risk for long-term mortality in need of more frequent follow-up.

Only a few reports regard the utility of the EuroSCORE system when applied to the OPCAB procedure. Al-Ruzzeh and colleagues [17] suggested from a multicenter study that the AUC of the EuroSCORE was 0.75 (95% CI, 0.64 to 0.85) in 1907 OPCAB procedures with 1.3% in-hospital mortality and that the EuroSCORE had discriminatory power of in-hospital OPCAB mortality [17]. But several facts might bias their study results. First, there was no mention about the selection criteria of OPCAB technique, and there is a possibility that OPCAB was performed only in the low-risk or high-risk patients. Therefore, the discriminatory power of EuroSCORE might not be certain in their study. Second, their study started from 1998 when the OPCAB procedure was not popular. The technical problems of the procedure cannot be disregarded and might have affected the in-hospital mortality.

Regardless of their preoperative condition, nearly all patients in our study who needed myocardial revascularization underwent OPCAB by a single well-experienced surgeon, and it might decrease the bias for the evaluation of the performance of the EuroSCORE.

In our study, the EuroSCORE overestimated the in-hospital mortality in the OPCAB procedure, even though this scoring system had relatively good discriminatory power. We assume that overestimation of the EuroSCORE might be attributed to several factors. First, the EuroSCORE model was developed on patients undergoing only, or mainly, on-pump cardiac procedures. The use of cardiopulmonary bypass has been found to be an independent risk factor for in-hospital mortality [21]; therefore, the risk of mortality calculated by the EuroSCORE might be overestimated in the OPCAB patients. There has been a debate about operative outcomes between OPCAB and on-pump CABG, however, and a prospective and comparative study will be needed to clarify that fact.

Second, among the variables of the EuroSCORE, some operation-related factors (other than CABG, thoracic aorta surgery, and repair of a postinfarction septal rupture) are irrelevant to the isolated OPCAB procedure. The risk-stratification systems, including EuroSCORE, are developed by calculating the logistic regression ß-coefficients of all risk variables chosen. But, these three variables are always unselected during risk calculation in the OPCAB procedure, which may cause an error in calculating the risk of each patient.

Third, the recent improvements in retractor-stabilizer systems and techniques of exposure of all surfaces of the heart have allowed us to operate more safely, and we have been able to perform OPCAB even in patients with high risks, including old age, decreased left ventricular function, ventricular arrhythmia, and chronic renal failure. The development of technology may result in a lower on-pump conversion rate and postoperative morbidity, which can affect early mortality.

The EuroSCORE is known to have the ability to predict long-term mortality after CABG, valvular surgery, or combined CABG and valvular surgery [9, 10, 22]. The standard or logistic EuroSCORE was a strong predictor for long-term survival. In this study, the EuroSCORE models in the analysis of mid-term mortality showed an AUC of 0.71 and qualified as an applicable model, because an AUC greater than 0.70 is usually considered to be associated with a good predictive value. Also, Hosmer-Lemeshow tests revealed acceptable calibration indicating good accuracy. Both EuroSCORE models were independent predictors of mid-term mortality. The EuroSCORE model can be useful in identifying patients at high-risk for mid-term deaths. The score can be used to initiate more intensive follow-up and treatment and it may improve long-term outcomes.

This study has several limitations. First, the cases were a result of a single surgeon’s experience. Second, the 12 excluded patients who were preoperatively scheduled to undergo CABG using cardiopulmonary bypass may have influenced the results. However, the reanalysis with the patients between 2003 and 2006 revealed no difference with the results of this study. Third, only hospital survivors were considered in the present study, thereby excluding patients who died perioperatively and for whom the score was actually developed. Nevertheless, the results showed that the EuroSCORE might be useful in predicting the patients’ mid-term outcomes. Finally, to evaluate the role of the EuroSCORE in the OPCAB procedure, a comparison study using other scoring systems will be needed.

In conclusion, the standard and logistic EuroSCORE models overestimate in-hospital mortality in the OPCAB procedure, although they have relatively good discriminative power for in-hospital mortality. We suggest that the specific risk-stratification system for the OPCAB procedure should be developed to predict the risk more accurately. For mid-term mortality, the EuroSCORE models have good predictability and can be used to tailor follow-up and plan resource allocation for patients who have undergone the OPCAB procedure.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 References
 

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