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Eur J Cardiothorac Surg 2006;29:441-446
© 2006 Elsevier Science NL
a Department of Cardiothoracic Surgery, St Vincent's Hospital Melbourne, Melbourne, Australia
b Baker Heart Research Institute and CCRE Therapeutics, Monash University, Melbourne, Australia
c The Alfred Hospital, Melbourne, Australia
d The Geelong Hospital, Melbourne, Australia
e Royal Melbourne Hospital, Melbourne, Australia
f The Austin Hospital, Melbourne, Australia
g Monash Medical Centre, Melbourne, Australia
Received 2 November 2005; received in revised form 28 December 2005; accepted 29 December 2005.
* Corresponding author. Address: Department of Surgery, Geelong Hospital, Geelong, Vic. 3220, Australia. Tel.: +61 40201 5001; fax: +61 35226 7019. (Email: yapch{at}svhm.org.au).
| Abstract |
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Key Words: Cardiac surgery Risk score Mortality EuroSCORE
| 1. Introduction |
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Risk scoring systems are most applicable when the preoperative patient characteristics and treatment profiles are comparable with those on which the system was originated. For this reason any risk scoring system can only be used reliably when its validity has been tested in the local patient population [16]. A previously performed validation study of 2106 patients from two Australian cardiac surgical units [17] showed that the standard and logistic EuroSCORE models over predicted mortality. However, there was doubt as to whether the study was representative of cardiac surgery in Australia considering the small number of patients and institutions involved. Hence, in this study we aimed to validate the EuroSCORE model on a larger Australian cardiac surgical population by assessing the model within the Australian Society of Cardiac and Thoracic Surgeons (ASCTS) Cardiac Surgery Database.
| 2. Methods |
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The development [2,19] and validation [4,6] of the EuroSCORE risk model has been previously described. The additive and logistic EuroSCORE models were applied to all patients undergoing cardiac surgery between 1st July 2001 and 4th July 2005 within the ASCTS database who have complete data. The database was searched for the information regarding relevant patient demographics, prevalence of risk factors, operative variables and mortality outcomes. Definitions of six risk factors in our database were not identical to the EuroSCORE definitions. However, a close approximation was achieved, comparable to that used in the Society of Thoracic Surgeons validation study [3]. Table 1 lists the risk factor definitions for comparison. The EuroSCORE standard additive algorithm [6] and logistic regression equation [19] were applied to determine the predicted additive (standard) and logistic EuroSCORE for the overall patient population (n = 8331), isolated coronary artery bypass graft (CABG) population (n = 5592) and risk tertiles. The outcome measured was operative mortality defined as death within 30 days from operation or later than 30 days if still in hospital.
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Performance of the models was assessed by comparing the observed and predicted mortality figures with 95% confidence intervals. Model discrimination (statistical accuracy) and calibration (statistical precision) were analysed by determining the area under the receiver operating characteristic (ROC) curve [20] and HosmerLemeshow goodness-of-fit statistic [21], respectively. The HosmerLemeshow chi-square statistic measures the differences between expected and observed outcomes over deciles of risk. A well-calibrated model gives corresponding P-value greater than 0.05. Applicability of the models was further assessed by comparing the observed and expected mortality for risk tertiles. The thresholds were set to obtain approximately similar sized risk groups in the additive model.
| 3. Results |
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| 4. Discussion |
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We have shown, in this study of 8331 patients from six Australian cardiac surgical units, that the EuroSCORE models do not accurately predict outcomes in this group of patients. This is true of a mixed group of adult cardiac surgical patients as well as those undergoing isolated CABG. Both models over predicted outcomes, the logistic model more so than the additive model, at all risk tertiles. However, it is important to note that the discriminatory ability of the models were very good, with area under the ROC curve values of 0.820.83.
We obtained an identical definition match in 11 of the 17 EuroSCORE risk factors. A good approximation was achieved in the remaining six risk factors. The definition match used in this study was comparable to that used in the large North American (STS) validation study [3]. On this point the definition of recent myocardial infarction (within 21 days in our study vs <90 days) and moderate LV dysfunction (3045% in our study vs 3050%) if leading to a bias would underestimate mortality risk.
The reasons for the study results are unclear and likely to be multi-factorial. The epidemiology of ischaemic heart disease and co-morbid conditions in Australia may be different to that of the European population. This could be due to real differences in the prevalence of disease states. It is also likely to be artifactual due to the impact of good access to health care and the management of these conditions. For instance, the higher prevalence of neurological disease, extracardiac arteriopathy and chronic pulmonary disease may simply reflect better access to health care in a heavily government-subsidised healthcare system, and early and aggressive investigation and management of these diseases. Furthermore, with the now widespread use of serum troponin as a marker of myocyte necrosis, many more patients are being identified as having non-ST elevation myocardial infarction. Hence, the risk factor of recent myocardial infarction does not distinguish between those with high creatinine kinase and troponin rise with its associated worse prognosis following cardiac surgery, from the many patients with mild troponin rises which carry little, if any, added operative risk. The provision of high-quality medical care with greater resource allocation leading to better outcomes, relative to the European validation population, may also contribute to the poor performance of the model. Lastly, it is important to note that the EuroSCORE was developed on data from patients in 1995 and may not reflect current cardiac surgical practice both in Australia and in Europe. For instance, on both continents patients are older and fitter due to decreased prevalence of smoking, increased attention placed on healthy lifestyles and improved medical treatment of cardiovascular diseases. Hence, the significant weight placed on increasing age in the EuroSCORE model may no longer be appropriate. There is a need for a European multi-centre revalidation of the EuroSCORE to establish its validity in today's cardiac surgical practice. Only then can we know whether the failure of the model in our study is likely to be due to factors unique to Australia or factors common to cardiac surgery worldwide.
The ASCTS cardiac surgical database has been collecting data on patients undergoing cardiac surgery in the state of Victoria, Australia, since 2001 [18]. The hospital databases are subjected to external audit and quality assurance measures. We believe the data accuracy to be high. The EuroSCORE model has been used as the risk-adjustment model in the interinstitutional comparisons of results within the ASCTS database. However, this may not be appropriate given our findings that the EuroSCORE over predicts outcomes. A local model from the ASCTS dataset has recently been developed [22], which will allow more accurate risk-adjusted comparisons to be performed between participating institutions. However, risk-adjusted comparisons of Australian results on the international level will remain problematic given the performance of the EuroSCORE models.
In conclusion, we could not validate the use of the additive or logistic EuroSCORE models on this multi-institution cohort of Australian adult cardiac surgical patients. The EuroSCORE models do not accurately predict outcomes in Australia. Thus, at present the EuroSCORE models should be used with caution for risk-adjustment or risk-prediction in Australia.
| Footnotes |
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Presented at Australasian Society of Cardiac and Thoracic Society Annual Scientific Meeting, Noosa Heads, Queensland, Australia, 28th30th October 2005.
1 On behalf of the ASCTS database surgeons. ![]()
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