Author + information
- Received August 13, 2012
- Revision received September 26, 2012
- Accepted September 30, 2012
- Published online February 12, 2013.
- Matthias Heringlake, MD⁎,⁎ (, )
- Efstratios I. Charitos, MD†,
- Nicola Gatz⁎,
- Jan-Hendrik Käbler⁎,
- Anna Beilharz⁎,
- Daniel Holz⁎,
- Julika Schön, MD⁎,
- Hauke Paarmann, MD⁎,
- Michael Petersen, MD† and
- Thorsten Hanke, MD†
- ↵⁎Reprint requests and correspondence:
Dr. Matthias Heringlake, Department of Anesthesiology, University of Lübeck, Ratzeburger Allee 160, D-23538 Lübeck, Germany
Objectives This study sought to determine the usefulness of plasma growth differentiation factor 15 (GDF-15) for risk stratification in patients undergoing cardiac surgery in comparison with the additive European System of Cardiac Operative Risk Evaluation (EuroSCORE), N-terminal pro–B-type natriuretic peptide (NTproBNP), and high-sensitive troponin T (hsTNT).
Background GDF-15 is emerging as a humoral marker for risk stratification in cardiovascular disease. No data are available if this marker may also be used for risk stratification in cardiac surgery.
Methods In total, 1,458 consecutive patients were prospectively studied. Pre-operative plasma GDF-15, NTproBNP, hsTNT, clinical outcomes, and 30-day and 1-year mortality were recorded. GDF-15 was determined with a pre-commercial electrochemiluminescence immunoassay.
Results Median additive EuroSCORE (addES) was 5 (interquartile range: 3 to 8); 30-day and 1-year mortality were 3.4% and 7.6%, respectively. Median GDF-15 levels were 1.04 ng/ml (95% confidence interval [CI]: 1.0 to 1.07 ng/ml) in 30-day survivors and 2.62 ng/ml (95% CI: 1.88 to 3.88) in 30-day nonsurvivors (p < 0.0001). C-statistics showed that the area under the curve of a combined model of GDF-15 and addES for 30-day mortality was significantly greater (0.85 vs. 0.81; p = 0.0091) than of the addES alone. For the EuroSCORE categories (0 to 2, 3 to 5, 6 to 10, >10) the presence of GDF-15 ≥1.8 ng/ml resulted in a significant 41.4% (95% CI: 19.2 to 63.7%; p < 0.001) net reclassification improvement and an integrated discrimination improvement of 0.038 (95% CI: 0.022 to 0.0547; p < 0.0001) compared to the model including only the addES, whereas the presence of NTproBNP (cutoff ≥2,000 pg/ml) or hsTNT (cutoff 14 pg/ml) did not result in significant reclassification.
Conclusions The pre-operative plasma GDF-15 level is an independent predictor of post-operative mortality and morbidity in cardiac surgery patients, can further stratify beyond established risk scores and cardiovascular markers, and thus adds important additional information for risk stratification in these patients. (The Usefulness of Growth Differentiation Factor 15 [GDF-15] for Risk Stratification in Cardiac Surgery; NCT01166360).
Growth differentiation factor (GDF)-15, formerly entitled macrophage-inhibitory cytokine, is a member of the transforming growth factor family (1). Low quantities of GDF-15 are expressed in most tissues, including myocardium, lung, kidney, brain, liver, and the intestine. Increased expression of this cytokine can be induced by myocardial stretch, volume overload, and experimental cardiomyopathy as well as oxidative stress, inflammatory cytokines, and ischemia/reperfusion (2), suggesting that the plasma levels of this cytokine are not only related to myocardial dysfunction but also to circulatory stress.
Recent data revealed that the plasma concentrations of this peptide may be used for predicting short- and long-term mortality in patients with coronary artery disease (3), myocardial infarction (4), and chronic heart failure (5,6). Interestingly, in some of these studies, GDF-15's predictive capacity was additive or even superior to conventional risk markers such as N-terminal pro–B-type natriuretic peptide (NTproBNP) (6) or high-sensitive troponin T (hsTNT) (7).
The present study was designed to determine the usefulness of GDF-15 for risk stratification in patients undergoing cardiac surgery and if plasma levels of this cytokine add prognostic information to conventional risk stratification strategies such as the additive European System of Cardiac Operative Risk Evaluation (EuroSCORE) (addES) and markers of cardiovascular dysfunction like NTproBNP or hsTNT.
The study was designed as a prospective observational cohort study and is part of a project analyzing the prognostic relevance of pre-operative cerebral oxygen saturation and markers of cardiopulmonary dysfunction with respect to clinical outcomes in patients undergoing cardiac surgery (8).
The primary objective of the present analysis was to determine the relation between pre-operative plasma GDF-15 levels and 30-day mortality in comparison with the additive EuroSCORE as well as to investigate if GDF-15 levels can further improve risk stratification of patients undergoing cardiac surgery beyond that obtained by this established risk score.
The secondary objective was to determine the usefulness of GDF-15 for predicting 1-year mortality and major complications as well as to determine the additive information of GDF-15 in comparison with NTproBNP (10) and hsTNT (7).
Following approval by the local ethical committee (Ethikkommission der Universität zu Lübeck, Lübeck, Germany) all patients scheduled for cardiac surgery at the University of Lübeck from January 1, 2008, to December 1, 2008, and from April 1, 2009, to December 31, 2009, were screened for participation in this prospective, observational trial. The exclusion criterion was age <18 years. Written informed consent was obtained from all elective and urgent patients as well as emergency patients capable to communicate. In case of sedated and/or intubated patients scheduled for emergency surgery consent was obtained from the next of kin and reconfirmed after recovery.
It was our primary intention to analyze the association between pre-operative GDF-15 levels and clinical outcomes in a “derivation” cohort of patients undergoing cardiac surgery in 2009 and to confirm our findings in a “validation” cohort of patients. The latter should be accomplished by using stored plasma samples of a cohort of patients studied in 2008 that has been described recently (8). Unfortunately, and for unresolved reasons, a relevant number of plasma specimen were lost during transport and/or analysis. Consequently, plasma samples were only available from 499 of 796 patients in the derivation cohort (Fig. 1). Because the low event rate (30-day mortality: 3.4%) limits statistical reliability, especially of the logistic regression models (9), we chose to pool the data from both cohorts, analyze them as 1 group, and use the present cohort for validation of cutoff levels derived from previous studies in patients with non–ST-segment elevation acute coronary syndrome (6,7) to determine the usefulness of GDF-15 for risk stratification in the specific setting of cardiac surgery.
In total, 2,026 patients were screened during the study period. Five patients refused to participate in the study, and in 76 patients surgery was cancelled. Complete data sets including GDF-15 levels were available from 1,458 patients (Fig. 1). One-year follow-up was 99.6% (n = 1,452 patients).
Determination of GDF-15, NTproBNP, and hsTNT
Arterial blood samples for determination of GDF-15, NTproBNP, and hsTNT were obtained immediately before induction of anesthesia. Plasma was separated and stored at –80° C for further analysis. Analyses were performed by electrochemiluminescence immunoassays on Elecsys 2010 analyzers (Roche Diagnostics, Mannheim, Germany).
GDF-15 was determined in plasma with a pre-commercial assay based on the electrochemiluminesence immunoassay principle on Elecsys (Roche Diagnostics, Mannheim, Germany) (10). The assay has a sandwich format with 2 monoclonal antibodies used for capture and detection of serum or plasma GDF-15. The intra- and interassay coefficient of variation for GDF-15 levels between 0.1 and 16.0 ng/ml varied between 0.7% and 2.2%. This new assay is highly correlated with a previously described immunoradiometric assay for GDF-15 (11).
NTproBNP was determined as described recently (8,12) utilizing an electrochemiluminescence immunoassay (Elecsys proBNP sandwich immunoassay; Roche Diagnostics) on Elecsys 2010. The lower detection limit of the assay was 5 pg/ml. The interassay coefficient of variation was 9.3% at 130 pg/ml and 14.4% at 3,890 pg/ml.
hsTNT was determined by the electrochemiluminescence method as described previously (electrochemiluminesence immunoassay; Elecsys 2010 analyzer, Roche Diagnostics) (13). The lower detection limit of this assay was 3 pg/ml. The interassay coefficient of variation was 5.4% at 28 pg/ml and 7.1% at 2,350 pg/ml.
Standard risk factors and clinical outcomes
Pre-operative left ventricular ejection fraction (LVEF) was graded as severely reduced (LVEF <30%), moderately reduced (LVEF 30 to 49%), or normal (LVEF >50%). The addES (14) was calculated from the prospectively sampled cardiac surgery database and analyzed as a continuous variable. The variables of the addES are presented in Online Table 1. Estimated glomerular filtration rate was calculated from pre-operative plasma creatinine by the abbreviated Modifications of Diet in Renal Disease equation (15).
Clinical outcomes (30-day mortality, major complications, duration of treatment in the high-dependency unit) were derived from the prospectively sampled cardiac surgery database. All-cause 1-year mortality was determined from the hospital database, by contacting the patient's primary physician and/or the patient or his or her next of kin.
Anesthesia, surgery, and cardiopulmonary bypass management
Anesthesia and cardiopulmonary bypass management were performed as described recently (8). With the exception of patients undergoing off-pump coronary artery bypass grafting (n = 21) and transapical aortic valve replacement (n = 11) in 2009, surgery was performed with cardiopulmonary bypass in moderate hypothermia using antegrade blood cardioplegia. Forty-six patients underwent deep hypothermic circulatory arrest with or without selective perfusion. A description of the surgical procedures is given in Figure 1.
Data were analyzed by MedCalc 12.3.0 statistical software package (MedCalc Software, Mariakerke, Belgium) and R version 2.14.2 (R Development Core Team, Vienna, Austria).
Data are presented as mean ± SD if normally distributed or otherwise as median and 25%/75% quartiles or 95% confidence interval (CI) of the median. Statistical significance was assessed at the 5% level (p < 0.05 is statistically significant). Correlation analyses were performed by Spearman's rank correlation test.
To allow better comparisons with the addES, mortality was primarily calculated as 30-day mortality. One-year mortality was used for construction of Kaplan-Meier survival curves and Cox proportional hazards regression analyses.
Post-operative morbidity was defined as a combined endpoint of: 1) more than 1 point in the Major Adverse Events and Complications Score according to Schön et al. (16) (need of renal replacement therapy, reintubation, stroke (Rankin disability score >1 ), low cardiac output syndrome); and/or 2) need for high-dependency unit (intensive care unit plus intermediate care) treatment of ≥10 days.
Comparisons between groups for univariate predictors of outcome were performed by a 2-sided chi-square test for categorical variables and Mann-Whitney test or Kruskal-Wallis test for continuous variables, where appropriate.
Area under the curve (AUC) as well as cutoff values for mortality and morbidity (i.e., the values with the maximal sum of sensitivity and specificity) were derived from receiver-operating characteristic (ROC) curve analyses (C-statistics). Differences between AUCs were calculated according to DeLong and coworkers (18).
To determine the predictive capacity of GDF-15, NTproBNP, and hsTNT in addition to the addES and duration of surgery as a measure of the severity of the surgical insult, multiple logistic regression and Cox proportional hazards models were employed. The addES was forced in all models and backward elimination of the variables GDF-15, NTproBNP, hsTNT, and duration of surgery was employed for final model selection. Survival analysis with the Cox proportional hazards methods were used to identify which factors have a significant influence on 30-day and 1-year survival. In all models addES and duration of surgery were entered as continuous variables. GDF-15, NTproBNP, and hsTNT were analyzed separately both as continuous as well as dichotomized variables (cutoffs: GDF-15 <1.8 or ≥1.8 ng/ml, NTproBNP <2,000 pg/ml or ≥2000 pg/ml, hsTNT<14 pg/ml or ≥14 pg/ml) according to established cutoff values (19,20). Additionally, analyses according to surgical priority (elective vs. urgent or emergency cases) were performed.
Net reclassification improvement and integrated discrimination improvement were calculated as described recently (21).
Demographics, surgical course, and general outcomes
Demographics, cardiovascular risk factors, risk scores including different versions of EuroSCORE, estimated glomerular filtration rate, and pre-operative plasma levels of GDF-15, NTproBNP, and hsTNT are presented in Table 1. Thirty-day mortality, 1-year mortality, and post-operative morbidity (defined as more than 2 major complications and/or a high-dependency unit stay of ≥10 days) were 3.4%, 7.6%, and 14.4%, respectively. Univariate analyses of patients with post-operative mortality and morbidity revealed that patients with an adverse outcome had a higher addES and a more pronounced cardiovascular risk profile (Table 1).
Factors influencing GDF-15
GDF-15 was correlated with various demographic and physiological variables representative of cardiovascular risk including NTproBNP and hsTNT concentrations (see Online Table 2). Analysis of GDF-15 in different addES mortality risk categories revealed significantly different and increasing GDF-15 levels in each EuroSCORE category (Fig. 2).
ROC curve analyses
ROC curve analysis of GDF-15 and 30-day mortality in the total cohort revealed an AUC of 0.83 (95% CI: 0.81 to 0.85; p < 0.0001) and a cutoff value of >1.63 ng/ml (sensitivity 76.5%, specificity 78.0%). The respective results for the combined morbidity endpoint (Major Adverse Events and Complications Score ≥2 and/or high-dependency unit time ≥10 days) revealed an AUC of 0.8 (95% CI: 0.78 to 0.82; p < 0.0001) in the total cohort. The cutoff value was GDF-15 ≥1.66 ng/ml with a sensitivity of 62.9% and a specificity of 83.6%.
An analysis restricted to elective patients revealed a cutoff level of 1.76 ng/ml for 30-day mortality (AUC: 0.78; 95% CI: 0.76 to 0.81; sensitivity 72.0, specificity 83.6; p < 0.0001).
The ROC curve analysis for 30-day mortality and duration of surgery and the respective cutoff value was an AUC of 0.71 (95% CI: 0.69 to 0.74; p < 0.0001; cutoff value >284 min).
ROC curve analyses revealed no significant differences between the AUC of the addES and GDF-15 levels for 30-day mortality (AUC: 0.81; 95% CI: 0.79 to 0.83; p = 0.515) and for the combined morbidity endpoint (AUC: 0.78; 95% CI: 0.75 to 0.80; p = 0.256).
The AUCs of combined multiple regression models of the addES and GDF-15, hsTNT, and NTproBNP are given in Table 2. The AUC of the combined model with addES and GDF-15 was significantly higher than of the addES alone. The combined model of addES and hsTNT was superior to the addES alone but had a significantly lower AUC than the combined addES and GDF-15 model. It is of note that multiple regression failed to identify NTproBNP as an independent risk factors/predictor of 30-day mortality.
GDF-15, NTproBNP, hsTNT in survivors and nonsurvivors
Comparative analysis of GDF-15 levels in survivors and nonsurvivors showed significantly higher GDF-15 levels in 30-day nonsurvivors (Table 1). Employing a cutoff level of 1.8 ng/ml revealed a markedly increased 1-year mortality rate in the total cohort, after stratification for different EuroSCORE risk categories, and cutoff levels of 2000 pg/ml for NTproBNP and 14 pg/ml for hsTNT (Table 3).
Employing the respective cutoff level for NTproBNP in the different addES risk categories revealed a significant difference only in patients with an addES from 6 to 10 (30-day mortality of 8.2% in 294 patients with NTproBNP <2,000 pg/ml vs. 16.1% in 124 patients with a NTproBNP ≥2000 pg/ml; difference: 7.9%; 95% CI: 0.91 to 16.2%; p = 0.0244). Comparably, stratification according to the respective hsTNT cutoff showed a significant difference only in patients with an addES from 6 to 10 (30-day mortality of 4.6% in 172 patients with hsTNT <14 pg/ml vs. 14.6% in 246 patients with a hsTNT ≥14 pg/ml; difference: 10%; 95% CI: 4.0 to 15.7%; p = 0.0019).
Logistic regression analysis
Logistic regression analysis for 30-day and 1-year mortality (Table 4) including addES, GDF-15, NTproBNP, hsTNT, and duration of surgery revealed that only the addES and pre-operative GDF-15 were consistent independent predictors of mortality in the total cohort and in elective patients.
Cox proportional hazard regression
Cox proportional hazards regression including addES, GDF-15, NTproBNP, hsTNT, and duration of surgery as continuous variables revealed that pre-operative GDF-15 levels are significant factors influencing 1-year mortality. NTproBNP either in elective as well as in urgent or emergency cases was not entered in these models. For elective patients only the addES and GDF-15 were predictors of mortality (Online Table 3).
Kaplan-Meier survival analyses
The effects of high pre-operative GDF-15 levels (cutoff level <1.8 ng/ml or ≥1.8 ng/ml) on post-operative mortality analyzed by Kaplan-Meier statistics are displayed in Figure 3. In patients with high NTproBNP (≥2,000 pg/ml) or high hsTNT levels (>14 pg/ml), the presence of high GDF-15 levels (≥1.8 ng/ml) were associated with significantly worse survival rate.
Net reclassification improvement and integrated discrimination improvement analyses
The addition of GDF-15 (categorized in 2 groups according to a cutoff level of ≥1.8 ng/ml) resulted in a 41.4% net reclassification improvement (95% CI: 19.2 to 63.7%; p < 0.0001); for the model predicted mortality risk categories: 0 to 2%, 2 to 5%, 5 to 10%, and >10%) and an integrated discrimination improvement of 0.038 (95% CI: 0.022 to 0.0547; p<0.0001) compared to a model only including the addES (Table 5;Online Table 4).
In line with recent observations in patients with heart failure and coronary artery disease (1–7,11) the findings of the present study show that pre-operative plasma GDF-15 levels are closely related to relevant measures of cardiopulmonary function, post-operative morbidity, and short- and long-term mortality in patients undergoing cardiac surgery.
In addition to clinical judgment, pre-operative risk assessment in cardiac surgery is typically performed by risk scores. The most frequently used scoring system in European cardiac surgery is the EuroSCORE. It is available in a classical additive (14) and 2 logistic versions, with the latest version, the logistic EuroSCORE II, published very recently (22).
In contrast to the setting of cardiac surgery, risk stratification in medical patients with heart failure and coronary artery disease is typically accomplished by determination of humoral markers that are reflective of the severity of cardiopulmonary dysfunction (i.e., natriuretic peptides [19) and myocardial integrity (troponins [13,20]). Interestingly, these variables may also be used for risk stratification in cardiac surgery patients; sometimes adding additional information if performed “on top” (e.g., in addition to a conventional risk scores such as the EuroSCORE ).
The cytokine GDF-15 differs from the humoral markers presently used for risk stratification in cardiac patients in so far that it is not only increased by cardiac dysfunction (i.e., volume overload and cardiac distension) but also by “circulatory stress” (1,2). Thus, the plasma levels of this peptide may be not only representative of cardiac dysfunction but also reflective of the circulatory disturbances associated with severe cardiovascular disease and/or heart failure (i.e., inflammation and systemic hypoperfusion). In line with these assumptions it has been shown that GDF-15 had a high predictive capacity for a combined morbidity and mortality endpoint in patients with heart failure (5) and that patients presenting with acute myocardial infarction and with high plasma GDF-15 levels benefited more from an aggressive interventional approach than patients with low levels of this cytokine (6).
The findings of the present study extend the prognostic relevance of increased plasma levels GDF-15 also to patients undergoing cardiac surgery. The 30-day mortality cutoff level derived from the present study for elective patients (≥1.78 ng/ml) is highly comparable to the 1-year mortality cutoff derived from medical patients with an acute coronary syndrome (>1.8 ng/ml) (6,7); a finding supporting the use of this “medical” cutoff also in the present comparative analyses in “surgical” patients.
GDF-15 significantly improved the predictive capacity of the addES for predicting 30-day mortality. Interestingly, the discriminatory capability of GDF-15 remained significant even in the presence of other well-established risk factors. As highlighted in Table 3 and Figure 3, patients with GDF-15 levels higher than 1.8 ng/ml always had a much higher mortality than those with GDF-15 levels below this cutoff, not only in the different EuroSCORE risk categories, but also after stratification for high plasma concentrations of NTproBNP or hsTNT. This finding has important clinical implications, as NTproBNP and hsTNT added only limited prognostic information on the mortality risk: NTproBNP and hsTNT were only helpful to stratify patients with an addES between 6 and 10. In contrast to other studies showing that NTproBNP adds prognostic capabilities in addition to the EuroSCORE in cardiac surgical patients (23), this hormone was no independent risk factor/predictor for mortality in this study: neither in a combined multiple regression model of the additive EuroSCORE and NTproBNP nor in the logistic regression and the Cox regression analyses for 30-day and 1-year mortality. One explanation for this observation may be the fact that the sample size of the present study is much larger than in previous studies analyzing the prognostic capabilities of NTproBNP. This, however, is speculative.
Comparably, as highlighted by the Cox regression analysis, hsTNT and duration of surgery were only predictors of mortality in the total cohort, including patients scheduled for surgery as urgent or emergency cases, but not in elective patients. Taken together these findings clearly show that the determination of GDF-15 adds important prognostic information in addition to conventional risk stratification tools.
First, despite very recently a new version of the EuroSCORE, the logistic EuroSCORE II, has been presented, we used the addES as a comparator for “conventional” risk stratification as primarily planned and described in the study protocol. We recognize that the logistic EuroSCORE II has been suggested to have a better discriminatory power than the older versions. However, this score has been developed to estimate hospital mortality at the base hospital and thus is difficult to be applied in an institution such as ours in which a relevant number of patients is transferred rather early after surgery to other departments or other hospitals.
Second, to avoid overfitting of the logistic regression and the Cox proportional hazards regression analyses, not all variables with significant differences in the univariate analyses were integrated in the model. Thus, it cannot be completely ruled out that a relevant physiological variable influencing the plasma concentration of GDF-15 might have been missed in this analysis.
Third, besides taking into account surgical priority, no further meaningful subgroup analyses of specific patient groups and indications could be performed due to the low number of events. It is rather plausible that the relationship among GDF-15, hsTNT, and NTproBNP in specific subsets of patients and indications (i.e., emergency revascularization for myocardial infarction vs. elective aortic valve replacement) may differ from the “mixed” relationship observed in the total cohort of patients. This is an important limitation and needs to be specifically addressed in future studies.
The present analysis of a large population shows that the pre-operative plasma level of GDF-15 is an independent predictor of post-operative mortality and morbidity in cardiac surgery patients, can further stratify beyond established risk scores such as the EuroSCORE and other cardiovascular risk markers such as NTproBNP or hsTNT, and thus adds important additional information for risk stratification in these patients.
Hormone analyses were kindly performed by Roche Diagnostics (Mannheim, Germany). Additionally, the authors thank the members of the cardiac anesthesia unit, the staff of the intensive care unit 15i, the Cardiac Surgery documentation service, and the personnel of the Department of Clinical Chemistry for their help in data and blood sample acquisition.
For expanded tables, please see the online version of this article.
All authors have reported that they have no relationships relevant to the contents of this paper to disclose. Dr. Heringlake and Dr. Charitos contributed equally to this manuscript.
The work was presented in part at the Annual Meeting of the German Association for Thoracic, Vascular, and Cardiac Surgery (DGTHG) from February 14 to 17, 2010, in Stuttgart, Germany, and at the Annual Meeting of the European Association of Cardiothoracic Anesthesiologists (EACTA) from May 31 to June 4, 2011, in Vienna, Austria.
- Abbreviations and Acronyms
- additive EuroSCORE
- area under the curve
- growth differentiation factor 15
- high-sensitive troponin T
- left ventricular ejection fraction
- N-terminal pro–B-type natriuretic peptide
- receiver-operating characteristic
- Received August 13, 2012.
- Revision received September 26, 2012.
- Accepted September 30, 2012.
- American College of Cardiology Foundation
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