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Department of Thoracic and Cardiovascular Surgery, West-German Heart Center, University of Duisburg-Essen, Essen, Germany
Accepted for publication April 14, 2009.
* Address correspondence to Dr Wendt, Department of Thoracic and Cardiovascular Surgery, West-German Heart Center Essen, University Hospital Essen, Hufelandstraße 55, Essen, 45122, Germany (Email: daniel.wendt{at}uk-essen.de).
Presented at the Forty-fifth Annual Meeting of The Society of Thoracic Surgeons, San Francisco, CA, Jan 26–28, 2009.
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Abstract |
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Methods: Six hundred and fifty-two patients underwent isolated AVR from January 1999 through June 2007. Emergency and redo operations were included; acute endocarditis was excluded. Evaluation was performed by logistic regression analysis. Data collection was prospective.
Results: The mean logistic EuroSCORE of all patients was 8.5 ± 7.9%, the mean STS score was 4.4 ± 3.9%, and the mean logistic Parsonnet score was 9.8 ± 8.5%. In-hospital mortality was 2.5% (n = 16). Freedom from all-cause death was 93.4% at 1 year, 90.2% at 2 years, and 75.8% at 5 years, respectively. A total of 182 patients had a logistic EuroSCORE greater than 10. For the group of patients with a EuroSCORE between 10% and 20% (n = 130) the mean EuroSCORE was 13.9 ± 2.8% and the STS score was 6.5 ± 3.8%. Observed mortality was 4.6% in this group. For the 52 patients with a logistic EuroSCORE of at least 20 (mean 28.5 ± 10.3%, STS score 10.1 ± 7.3%) the observed mortality was 3.9% (n = 2). By stepwise logistic regression, none of the EuroSCORE variables could be identified as an independent predictor in the "high- risk" group.
Conclusions: The logistic EuroSCORE was primarily created to allow patient grouping for the total spectrum of cardiac surgery. In patients undergoing isolated AVR, the EuroSCORE highly overestimates mortality, whereas the STS score seems to be actually more suitable in assessing perioperative mortality for these patients.
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Introduction |
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TopAbstractIntroductionPatients and MethodsResultsCommentDiscussionReferences |
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Various standardized risk scoring algorithms exist for the preoperative stratification of patients according to their expected surgical risk. These risk scores are based on patients' preoperative cardiac and noncardiac status. Traditionally, risk scores were used to estimate perioperative mortality in surgical patients' cohorts. Recently, the same scores have been used to identify "high-risk" subgroups of patients in order to offer an alternative treatment option even for the individual patient. With increasing comorbidities such as advanced age, poor left ventricular function, chronic obstructive pulmonary disease, or renal dysfunction, some of these high-risk patients were deemed "too sick" for surgery and more than 30% remain untreated [4]. Therefore, transcatheter aortic valve implantation techniques (TAVI) have evolved as an endovascular or minimally invasive alternative to offer treatment options for patients whose surgical risk is reported as too high [5–7].
Widely used scores for the evaluation of procedural risk in cardiac surgery are the European system for cardiac operative risk evaluation (EuroSCORE) and the Society of Thoracic Surgeons predicted risk of mortality (STS-PROM), but even risk calculation using the older Parsonnet score is performed [8–10]. Recently, the STS risk algorithm was reported to be the most sensitive score in defining the risk of patients undergoing isolated AVR [11].
Despite limitations in actual scoring systems an accurate and "AVR specific" risk scoring tool, particularly for high-risk patients, is still missing. This study aimed to analyze the predictive value of the following: (1) the EuroSCORE, (2) the STS score, and (3) the Parsonnet risk scoring systems in high-risk patients undergoing isolated AVR, with regard to early outcomes. A long-term follow-up was also performed.
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Patients and Methods |
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Risk Calculation and Definition of Risk Groups
The EuroSCORE calculator available online (http://www.euroscore.org) was used for both additive (AES) and logistic EuroSCORE (LES) calculations. The EuroSCORE considers 17 variables [10]. All study patients had an isolated AVR and therefore fulfilled the item "operation other than isolated coronary artery bypass grafting," whereas "endocarditis" was excluded. The STS-PROM mortality risk calculation for aortic valve procedures was performed by the online available STS score calculator (http://66.89.112.110/STSWebRiskCalc261/de.aspx). The additive (APS) and logistic Parsonnet (LPS) score research calculator available online was used (http://www.sfar.org/scores2/parsonnet2.html) [9]. All scores were calculated for each of the 652 patients. All variables and items were calculated and considered according to their exact definition of each score.
The calculated scores were used to evaluate early mortality. According to the definition of the scores, 30-day in hospital mortality was evaluated. Receiver operating curves were calculated for all scores. Based on the EuroSCORE risk calculation, patients were divided into high-risk (logistic EuroSCORE between 10% and 20%) and excessive risk (logistic EuroSCORE > 20%) groups.
Receiver Operating Characteristic (ROC) Curve Analysis
Receiver operator curves were generated for all risk scoring systems. Due to the fact that the "estimates" describing the loading of each item within each score were not available for the proprietary STS score, only absolute values of all scores have been considered for the ROC analysis. Sensitivity and specificity of expected versus observed mortality were summarized by receiver operator curves and the area under the resulting curve (AUC). A decreasing value of this statistic from 1.0 toward 0.5 indicates decreasing distinctiveness or discrimination between patients living and dead. Results were given as AUC accompanied by 95% confidence intervals.
Statistics
Descriptive statistics are summarized for categoric variables as frequencies (%) and compared between groups using the Pearson 
2 exact test. Continuous variables were reported as mean ± standard deviation and were compared between groups using the Student t test. Observed and expected number of events for the groups were compared using the Pearson 2 or Fisher exact tests as appropriate. A p value less than 0.05 was considered to indicate statistical significance. Survival curves were generated with the Kaplan-Meier method [12]. All statistical analyses were performed using SAS version 9.1 (SAS Inc, Cary, NC).
Statement of Responsibility
The authors had full access to the data and take full responsibility for their integrity. All authors have read and agreed to the manuscript as written.
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Results |
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Preoperative risk assessment of the complete cohort revealed an additive and logistic EuroSCORE of 6.45 ± 2.6% and 8.46 ± 7.9%, a STS score of 4.4 ± 3.94%, and a mean additive and logistic Parsonnet score of 17.8 ± 5.2 and 9.9 ± 8.54. Thirty-day in-hospital mortality for the entire group was 2.5% (16 of 652 patients). Patients with a LES less than10 had an observed mortality of 1.7% (8 of 470). A total of 130 patients could be identified as having a logistic EuroSCORE greater than 10% and less than 20%. Within this group, estimated mortality ranged from 6.5% for the STS-PROM calculation to 8.9% for the AES and 13.9% for the LES, to 20.7% for the LPS and 24.2% for the APS. Thirty-day in hospital mortality for this group was 4.6% (6 of 130 patients). Within the patients at excessive risk as defined by an LES greater than 20% (n = 52 patients), the predicted mortality ranged from 10.1% as calculated by the STS-PROM algorithm to 11.4% for the AES and 18% for the LPS, to 25.8% for the APS and 28.5% for the LES calculations. Observed 30-day in hospital mortality was 3.9% in this group (2 of 52 patients). The expected mortality by numbers within the excessive risk group (logistic EuroSCORE > 20) was 6.0 by the AES (p = 0.08), 14.8 by the LES (p < 0.0001), 5.3 by the STS-PROM (p = 0.13), 13.4 by the APS (p = 0.0003), and 9.4 by the LPS (p = 0.0077), whereas only 2 patients died (3.9%). Furthermore, the observed-expected ratios for perioperative mortality in this group were 0.33, 0.14, 0.38, 0.15, and 0.21 as predicted by the AES, LES, STS, APS, and LPS, respectively. Preoperative risk calculations by each scoring system are shown in Table 1.
All risk scores overestimated the risk for mortality. However, the STS-PROM algorithm showed the most accurate prediction for mortality, especially in the group presenting a LES greater than 20, whereas the logistic EuroSCORE revealed the highest overestimation of mortality (mean LES of 28.5%). The observed to expected ratios for perioperative mortality for each group calculated by the different algorithms are shown in Table 2 and Figure 1.
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Survival
Freedom from all-cause death in all patients was 93.4% at 1 year, 90.2% at 2 years, and 75.8% at 5 years, respectively. In the high-risk group survival was 86.1% at 1 year, 80.8% at 2 years, and 54.8% at 5 years, whereas in the excessive risk group survival was 90.1% at 1 year, 84.5% at 2 years, and 53.5% at 5 years. Kaplan-Meier survival curves are given in Figures 2 and 3.
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Comment |
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TopAbstractIntroductionPatients and MethodsResultsCommentDiscussionReferences |
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The increased application of TAVI at our institution has raised the awareness for evaluating outcomes of conventional AVR patients because risk models are currently used, and potentially misused, to create a subgroup of patients at an assumed high or highest risk for conventional AVR. The aim of the present study was therefore to evaluate the predictive value of different risk algorithms in patients undergoing isolated AVR with regard to their outcomes and critically discuss the validity of the scores used.
Endocarditis was excluded in our present study. This exclusion was based on mainly two reasons: On the one hand, aortic valve replacement based on endocarditis represents a completely different issue compared with aortic valve replacement based on calcific stenosis and therefore results in different outcomes. On the other hand, endocarditis represents an exclusion criterion in transcatheter valve implantation techniques. Therefore, in order to get equal conditions when talking about surgical AVR and transcatheter techniques, endocarditis was excluded.
In the present study the mean additive and logistic EuroSCORE in very high-risk patients with a LES greater than 20 (n = 52) were 11.4% and 28.5%, respectively. The STS score for this group was calculated by 10.1%. Observed mortality, however, was 4.6% (6 of 130) in the group presenting a LES between 10% and 20%, and 3.9% (2 of 52) in the excessive risk group (LES > 20).
In order to control for any bias, and above all to allow discrimination between the used risk scoring systems, receiver operator curves were calculated for each score. However, due to the proprietary design of the STS score, only the calculated scores without the "estimates" for each item have been incorporated into the analysis. The STS score showed a moderately higher AUC compared with the logistic and additive EuroSCORE (71.8 vs 69.9), whereas the EuroSCORE nearly reached a good overall predictive value. The lowest c-values were calculated for the additive Parsonnet score (66.6). Both the EuroSCORE and the STS score can be used to accurately predict mortality in patients undergoing isolated AVR; however, the EuroSCORE and Parsonnet score highly overestimate the operative risk of AVR.
Other groups have reported excellent and highly convincing results in patients at high-risk for surgery [13–16]. Our overall 30-day in-hospital mortality in patients at high-risk was only 2.5% (despite the presence of peripheral vascular disease in 11.8% of the patients and many patients presenting an impaired left ventricular ejection fraction, which are known as independent predictors of hospital mortality [13]). In addition, mortality within the patients with a LES of at least 20% was only observed by 3.9%, although almost 27% of the patients in this group had previous cardiac surgery.
Risk overestimation by the EuroSCORE has been described earlier [13, 16–18]. The STS score was shown to accurately predict mortality in patients undergoing transcatheter aortic valve implantation who had a LES greater than 30% and a STS score greater than 15% [5]. The EuroSCORE was created primarily to allow patient grouping for the total spectrum of cardiac surgery; however, in the EuroSCORE cohort the part of patients undergoing AVR was represented by 17% of the whole cohort [17]. Furthermore, the EuroSCORE is still based on a 1995 mortality across all of cardiac surgery and has yet not been updated or recalibrated. The present study confirms recent reports and analyses that the logistic EuroSCORE clearly overestimates the risk of mortality, especially in patients at very high risk. The logistic EuroSCORE estimated nearly 15 deaths within the excessive risk group compared with an estimated mortality of 5 deaths as calculated by the STS score, whereas only 2 patients died. The excessive risk group had a calculated additive EuroSCORE of 11.4 compared with an observed mortality of 2 out of 52 patients (3.9%) (p = 0.08). These findings can be matched with results from Brown and colleagues [17], who observed a mortality of 7.4% in high-risk patients undergoing isolated AVR compared with an estimated mortality of 12.6% (p = 0.109) by the use of the additive EuroSCORE.
Thus, using the currently available algorithms, risk overestimation for surgical AVR is common [8, 10, 19]. None of the available scores seem adequate to justify transcatheter aortic valve implantation because of a suggested high risk. With this information in mind transcatheter aortic valve implantation studies in transcatheter patients with a mean logistic EuroSCORE of 11% or even as high as 27% may appear in a different light [6, 20]. However, especially considering our own policy for the indication of transcatheter aortic valve implantation, many of the patients carry risk factors that are not depicted by the current risk scoring systems. In our hands, patients undergoing conventional AVR and patients undergoing transcatheter aortic valve implantation are completely separate subgroups. To establish an indication for TAVI, factors like frailty, multiple heart valve disease, end stage liver disease, etc, may have to be considered. However, not only verifiable model results will serve as the sole determinant of patient risk but many other objective and subjective tests evaluating patients' mobility, frailty, quality of life, housing support, or social integration must be taken into account when evaluating the individual risk. Furthermore, the so-called "eyeball" test of the experienced surgeon will be useful in the preoperative evaluation of patients' risk.
The present study thus shows that most current scores systematically overestimate the risk of patients undergoing conventional AVR, even for an individual patient. The current scores, therefore, do not seem to be adequate to identify patients not suitable for conventional AVR because of a high risk. The algorithms need updating for an apparent improvement in surgical results, especially in patients at higher risk. The explanation for the systematic overestimation of scores is, on the one hand, the lack of a more specific AVR score as is true for the EuroSCORE and the Parsonnet score. As shown earlier by our group, the application of the STS score in patients undergoing transcatheter aortic valve implantation seems to adequately predict mortality in these patients but may still not be used to establish an indication for transcatheter aortic valve implantation [5].
Furthermore, the data used for risk estimation require constant recalculation, which is achieved recently only by the STS score, whereas the EuroSCORE and the older Parsonnet score have not as yet been updated or recalibrated. As well, the current clinical practice and all of the present most modern therapeutic options should be implemented in actual scoring systems. The presented data clearly demonstrate that the EuroSCORE (above all the logistic EuroSCORE) risk calculation overestimates mortality in patients undergoing isolated aortic valve implantation and mainly in those patients at high risk. Out of the available scoring systems the STS score, as to be explained from the evaluation modalities, represents the most accurate score although still an overestimation of risk is observed.
Limitations
The present study was performed at a single tertiary care medical center. Hence, the generalizability of our findings may not extend to all centers worldwide. The cutoff point for determining excessive risk was set arbitrarily at a logistic EuroSCORE greater than 20, resulting in only a small number of patients (n = 52). Furthermore, one can assume, according to the European experience, that nearly one third of patients (primarily patients at high or highest risk) were not referred to surgery [4].
Conclusion
In patients undergoing isolated AVR, scores evaluated in the present study overestimate the risk of perioperative mortality. The highest over prediction could be observed by the logistic EuroSCORE algorithm, whereas the STS score seems to be more suitable in calculating operative mortality in patients undergoing isolated AVR. By using these scores the definition of a high-risk patient population suitable for TAVI does not seem to be justified. The results of conventional AVR are excellent, even in high-risk patients. Nevertheless, an indication for transcatheter aortic valve implantation exists, which must, however, be better defined by considering population specific indications.
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Discussion |
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TopAbstractIntroductionPatients and MethodsResultsCommentDiscussionReferences |
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The other comment I want to point out is that "The Society for Cardiothoracic Surgery in Great Britain and Ireland," who is aware of the issue with the overestimation of EuroSCORE, have modified the risk scoring system so as to more accurately predict the actual mortality. They have responded to the overprediction of logistic EuroSCORE by undertaking a complex recalibration whereby they have looked at the comparisons between the observed mortality and that predicted in each operative group to derive a series of recalibration coefficients. These were then applied to the analyses of national data for hospitals and surgeons. This was first done for publication in 2007 based on data from 2002 to 2005. This is an important area because as health care practitioners our actual mortality is actually compared with the predicted mortality in the U.K., and that is published on the Internet for the public to view (http://heartsurgery.healthcarecommission.org.uk/). So once again, well done on your study.
DR WENDT: Thanks for your very supportive comments, and indeed the EuroSCORE, as you mentioned, is nearly 15 years old, and those over 19,000 patients who were examined in the original EuroSCORE study included only 3,200 patients who received an aortic valve replacement, so only 17% out of this cohort, as compared to over 30,000 patients within the STS score. Therefore, I totally agree with your comments. And, furthermore, I think this is one of the most lacking points of the EuroSCORE. It is a very old score, and a precise risk score for aortic valve replacement should be timely recalculated, perhaps every year. So the STS score was recalculated and the latest version came up last year and I think it is a more specific tool for risk calculation in aortic valve replacement. Therefore, at our institution we started with a staged approach towards transcatheter aortic valve implantation, and we performed our first transcatheter aortic valve implantation only on very high-risk patients, which is reflected by our poster concerning transcatheter aortic valve implantation in high-risk patients at this meeting. Our mean STS score of transcatheter aortic valve placement is about 20%, reflecting a really high-risk patient population.
DR ALESSANDRO PAROLARI (Milano, Italy): I agree with the part of your conclusion that says that all these methods overestimate the risk, but I totally disagree with your statement that one method is better than another one from the data you present here. I am very concerned from this kind of information because the events reported are not so high. Usually, to compare performance of risk score you have to use ROC [receiver operating characteristic] curves comparison that can evaluate the discrimination of the different scores, but in this case, due to the low frequency of the events you cannot do that. So, I would be very concerned about stating that one method is better than another one.
DR WENDT: That is right, but we mentioned this in our limitations. It is a single-center study and only 652 patients could be identified and only 52 patients were at highest risk with a logistic EuroSCORE above 20, but the other groups, like Dr Grossi presented 730 and Dr Brown presented 1,177 patients as well. Therefore, I think we need some big and large randomized trials to get more information about that.
DR PAROLARI: The problem is not in the total number of patients studied but it is in the number of events, very few, that limit this kind of analysis.
DR WENDT: Yes, that is right. For this reason we need a large volume study in order to increase the number of events and afterwards perform the ROC analysis. Unfortunately, the ROC analysis of the STS score cannot be performed to date because of its proprietary design and not published estimates.
DR MARK WAYNE BURLINGAME (Lancaster, PA): I have tried to use the STS risk stratifier to separate these patients for those that I am going to send for transcatheter aortic valve replacement. The specific deficiency in that database is the lack of inclusion of pulmonary hypertension and RV [right ventricular] function, and I wondered if the EuroSCORE includes that, and certainly shouldn't this be included on both scores?
DR WENDT: That's right, but the older version of the STS score before the beginning of 2007 included pulmonary hypertension as well. And I totally agree with your comment that pulmonary hypertension and right heart function play a major role in transcatheter aortic valve implantation.
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