Interactive Cardiovascular and Thoracic Surgery 3:562-565(2004)
© 2004 European Association of Cardio-Thoracic Surgery
Institutional report - Cardiac general |
Application of European system for cardiac operative risk evaluation (EuroSCORE) in coronary artery bypass surgery for Taiwanese
Chien-Chang Chen,
Chung-Chi Wang,
Shih-Rong Hsieh,
Hong-Wen Tsai,
Hao-Ji Wei and
Yen Chang*
Division of Cardiovascular Surgery, Cardiovascular Center, Taichung Veterans General Hospital, Taichung, Taiwan, ROC
* Corresponding author. Tel.: +886-4-2359-2525x5063; fax: +886-4-2374-1323. (E-mail: ychang{at}mail.vghtc.gov.tw).
Received April 27, 2004;
received in revised form June 14, 2004;
accepted June 16, 2004
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Abstract
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The applicability for Taiwanese cardiac surgery is unclear. The preliminary goal of our study was to evaluate the validity of EuroSCORE in coronary artery bypass surgery (CABG). From January 1999 to January 2004, 801 consecutive adult patients who received primary or re-operative off- and on-pump CABG in our institute were collected. Both simple additive and logistic scores were calculated. Patients were categorized into low-risk group (simple additive score 0–2), medium-risk group (simple additive score 3–5), and high-risk group (simple additive score 6 plus). Mean age was 68.0±10.1 years. Patients aged 75 or more were 27.6%. Male-to-female ratio was 3.8:1. The mean simple additive and logistic scores of all patients were 5.0±3.5 and 8.0±11.9, respectively. The observed overall average mortality rate was 10.6%. There were 18.6% of patients in low-risk group, 40.0% in medium-risk group, and 41.4% in high-risk group. The mortality rate was 3.3% in low-risk group, 5.3% in medium-risk group, 19.0% in high-risk group. The area under the curve (c-index) was 0.75 for the simple additive score and 0.74 for the logistic score. Our results suggest that despite demographic differences, our study demonstrates preliminarily that EuroSCORE is valid in CABG for Taiwanese.
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1. Introduction
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The European System for Cardiac Operative Risk Evaluation (EuroSCORE) was developed between 1995 and 1999 to provide an evaluation model for in-hospital or 30-day mortality in cardiac surgery [1,2]. Despite demographic differences, it has been widely accepted in Europe, North America [3], and Japan [4]. Both simple additive and logistic scores accurately predict early surgical mortality. However, prior to the current study, the applicability of EuroSCORE for Taiwanese cardiac surgery was unclear. The preliminary goal of our study is to evaluate the validity of EuroSCORE in the domain of coronary artery bypass surgery (CABG).
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2. Patients and methods
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From January 1999 to January 2004, 801 consecutive adult patients who received primary or re-operative CABG in our institute were collected. These included both off- and on-pump CABG patients and those who underwent combined procedures. Without exclusion criteria, total arterialized CABG using left internal mammary artery and radial artery was routine. The exclusion criteria were positive Allen's test, emergency, age over 80 years, injury of internal mammary artery during harvest, poor respiratory function which was not in favor of entering pleural space, uremic patients with A–V fistula for hemodialysis.
Throughout the study we strictly conformed to the definitions of risk factors described in the original publication by Nashef et al. Mortality was defined as death from any cause within the same hospital admission of operation. Each patient's simple additive and logistic scores were calculated according to the formula provided on the website www.euroscore.org. Patients were divided into three risk groups: low-risk group (simple additive score 0–2), medium-risk group (simple additive score 3–5), and high-risk group (simple additive score 6 plus).
2.1. Statistic analysis
Numerical variables were presented as mean±standard deviation. A P value of less than 0.05 was considered statistically significant. The predictive values of the EuroSCORE items were tested by Yates' correction  2-test. The odds ratios were calculated. Analysis of variance (ANOVA) test was used for comparison of both simple additive and logistic scores among different years. Changes in the proportion of the three risk levels from year 2000 to 2004 were examined using 2-test. Receiver operating characteristic (ROC) curves were used to assess the discriminatory ability of EuroSCORE. To be discriminatory, the ROC curve of a risk model should have an area of more than 0.5.
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3. Results
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Mean age was 68.0±10.1 (31–93) years. Patients aged 75 or over were 27.6%. Male-to-female ratio was 3.8:1. Diabetes mellitus was present in 20.5% of patients. There were 46.0% of patients who had history of hypertension. The mean numbers of significant coronary artery stenosis were 2.6 per patient. The mean preoperative NYHA functional class was 2.5 and the mean angina class was 2. Twenty-one percent of patients had significant left main coronary artery stenosis (>50%). The mean preoperative creatinine level was 1.6µmol/l. Patients receiving long-term hemodialysis were 1.4%.
The distribution of the risk factors among the study patients is listed in Table 1. The patients who received procedures other than isolated CABG occupied 19.2%. The average numbers of anastomosis were 3.3 per CABG operation. There were 72.4% of patients who received total arterialized CABG. The mean cross-clamping and cardiopulmonary bypass time was 115 and 150min, respectively. The mean postoperative NYHA functional class improved to 1.9. The average ICU and hospital stays were 4.2 and 10.8 days, respectively.
The mean simple additive score of all patients was 5.0±3.5. The mean logistic score was 8.0±11.9. The observed mortality rate was 10.6%. Calculation of odds ratios showed that the predictive value of risk factors such as age, emergency, EF<30%, critical preoperative status, other than isolated CABG, poor renal function, neurological dysfunction were statistically significant with regard to mortality. However, the predictive values of other risk factors were not significant (Table 2). There were 149 patients (18.6%) in low-risk group, 320 patients (40.0%) in medium-risk group, and 332 patients (41.4%) in high-risk group. The mortality rate was 3.3% in low-risk group, 5.3% in medium-risk group, 19.0% in high-risk group. The discriminatory ability of EuroSCORE on the prediction of mortality was assessed using receiver operating curve (ROC) curves. The area under the curve (c-index) was 0.75 for the simple additive score and 0.74 for the logistic score (Fig. 1A and B). The accuracy rate was 89.6% for the simple additive score and 89.8% for the logistic score. When the study patients were grouped by the year of operation, the observed mortality rate was approximately 10% in each year while the average scores increased with time (Table 3). This may be attributed to the gradually increasing percentage of high-risk patients from 33.7% in year 2000 to 51.1% in January 2004.
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Fig. 1 (A) ROC curve of simple additive score. The area under curve is 0.746. (B) ROC curve of logistic score. The area under curve is 0.744.
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4. Discussion
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The concept of risk stratification has been increasingly emphasized in cardiac surgery [5]. Although risk-scoring systems do not allow for decisions to be made for individual patient, they provide a tool for measuring the quality of care in cardiac institutes. In addition, the surgical results of different institutes or countries can be compared on the objective basis of the patients' risk profile. A universal risk stratification system that worked well regardless of demographic differences was needed. Several scoring systems had been used for evaluation of cardiac surgical patients before the advent of EuroSCORE [4–6]. Of these, the Parsonnet system was most notably used although it has been criticized for including subjective variables [7]. The major advantage of EuroSCORE is the clearly objective definition for each risk factor. Furthermore, the simple additive score allows the surgeon to estimate easily the surgical risk of the patient at bedside. The EuroSCORE has been validated in Europe [6,8,9], North America [3], Turkey [10], and Japan [4]. Therefore, we were interested in evaluating the applicability of EuroSCORE for Taiwanese cardiac surgery.
There are two different measures that can be used to evaluate a predictive model: calibration and discrimination. Of the two, discrimination is generally regarded as more practical because model adjustments can be made to overcome poor calibration. The discriminatory power of a model is assessed by ROC curve analysis [11]. A model is considered to have discriminatory ability when the area under the curve (c-index) is more than 0.5. A c-index value more than 0.7 indicates good discrimination [12]. The c-indexes of the European and North American studies were 0.76 and 0.77, respectively. In this study, the c-indexes were 0.74 and 0.75 for simple additive and logistic scores, respectively, which demonstrates that the EuroSCORE was valid in our hospital.
The epidemiologic differences were reflected in the prevalence of risk factors (Table 1), which was consistent with the findings between the European and North American studies [3]. The mean age of our patients was 68 years, which was higher than those of the other studies. Patients aged 75 or over were 27.6% and they were much more than those in the European (10%) and North American (19.1%) studies [1,3]. They had at least four points in calculating scores. The elder patients usually had other risk factors. Therefore, the higher average additive and logistic scores in our institute could be attributed to this portion of elder patients. The proportion of female patients was lower than that in the European and North American studies (20.8 versus 27.8 and 30.9%). The prevalence rates of renal insufficiency (11%) and poor left ventricular function (13.4%) were higher than those in the studies conducted in Europe and North America. However, percentage of patients who had undergone previous cardiac surgery was significantly lower in our study compared to those reported in European and North American studies (VGHTC: 1.2%, Europe: 7.3%, North America: 11.7%). This may be explained by differences in the general medical environment and the habits of general population in seeking medical resources in these parts of the world. The proportion of extracardiac arteriopathy in Europe and North America was 11.3 and 19.0%, respectively, compared with 2.9% in the current study. This figure may have been underestimated in our study as we did not routinely measure the ankle-brachial index or perform Doppler on carotid arteries in our patients.
From Table 3, it can be seen that the risk of our patients significantly increased with time while the actual mortality rate was maintained at around 10%. The advantage of EuroSCORE is that it can accurately reveal improvements in surgical results and quality of care. The average age of coronary artery bypass patients is increasing in Taiwan, which is largely due to the ageing population, and this is resulting in increased comorbidity. It follows that the risk of CABG patients is higher now than it has been in the past. Furthermore, our hospital as a medical center is dealing with increasing numbers of high-risk patients while more and more lower risk patients are being managed in the district hospitals. Therefore, increase in proportion of high-risk patients with time as described above is not surprising.
4.1. Limitations of study
This retrospective study was limited in completely retrieving patients' medical histories, especially in extracardiac arteriopathy and neurological dysfunction. As such, patients' risks were underestimated to some extent although we believe this was restricted to a minor proportion of patients. Theoretically, the predictive mortality rates would have been closer to the observed ones if such errors had been taken into consideration.
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5. Conclusion
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EuroSCORE is simple and easy to use. It is useful in evaluation and comparison of surgical results and medical care. Despite differences in demographic data, risk and surgical characteristics, our study demonstrates preliminarily that EuroSCORE is valid in CABG for Taiwanese.
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Acknowledgements
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We would like to thank Ms Nien for her assistance with the statistical analysis. The research was carried out at Division of Cardiovascular Surgery, Cardiovascular Center, Taichung Veterans General Hospital, Taiwan.
doi:10.1016/j.icvts.2004.06.006
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References
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Abstract
1. Introduction