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Ann Thorac Surg 2009;87:731-736. doi:10.1016/j.athoracsur.2008.11.055
© 2009 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Impact of Preoperative Renal Dysfunction on In-hospital Mortality After Solitary Valve and Combined Valve and Coronary Procedures

Claudius Diez, MDa,*, Peter Mohr, MDb, Oliver Kuss, PhDc, Bernd Osten, MDb, Rolf-Edgar Silber, MDd, Hans-Stefan Hofmann, MDe

a Department of Cardiothoracic Surgery, University Regensburg, Regensburg, Germany
e Department of Thoracic Surgery, University Regensburg, Regensburg, Germany
b Department of Internal Medicine II, Martin Luther University Halle-Wittenberg, Germany
d Department of Cardiothoracic Surgery, Martin Luther University Halle-Wittenberg, Germany
c Institute of Medical Epidemiology, Biostatistics, and Informatics, Halle (Saale), Germany

Accepted for publication November 17, 2008.

* Address correspondence to Dr Diez, University Hospital Regensburg, Department of Cardiothoracic Surgery, Franz-Josef-Strauß-Allee 11, Regensburg, D-93053, Germany (Email: claudius.diez{at}t-online.de).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
Background: Limited information exists on the influence of preoperative renal dysfunction on in-hospital mortality after valve and combined valve and coronary procedures. The impact of preoperative renal dysfunction on patient outcome was investigated.

Methods: This was a retrospective observational study of 916 patients who underwent solitary valve or combined procedures. Primary outcome was in-hospital mortality. Preoperative estimated glomerular filtration rate (eGFR) was calculated with the abbreviated Modification of Diet in Renal Disease formula.

Results: Independent predictors of death were prolonged stay in the intensive care unit (odds ratio [OR], 1.03; 95% confidence interval [CI], 1.01 to 1.05), preoperative atrial fibrillation (OR, 1.61; 95% CI, 1.02 to 2.54), chronic obstructive pulmonary disease (OR, 2.2; 95% CI, 1.06 to 4.55), and prolonged operation time (OR, 1.01; 95% CI, 1.00 to 1.01). Each unit of the eGFR (mL/min/1.73m2) above average exerted a renoprotective effect (OR, 0.97; 95% CI, 0.96 to 0.98). The final regression model showed no lack of fit (Hosmer-Lemeshow test, p = 0.38) and a good discrimination performance in a receiver operating characteristic analysis (area under the curve, 0.84; 95% CI, 0.80 to 0.88). The lower the preoperative eGFR rate, the longer the postoperative stay at the intensive care unit.

Conclusions: Renal dysfunction is an important independent predictor of in-hospital mortality in adult patients after valve and combined valve and coronary procedures.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
Preoperative renal dysfunction, defined as reduced estimated glomerular filtration rate (eGFR), is one of the most important risk factors for perioperative morbidity and mortality after cardiac operations. Several studies addressed this issue in patients with coronary artery bypass grafting (CABG) [1–3], but only a few studies examined effects of preoperative impaired renal function in patients with valve and combined procedures [4–6]. Furthermore, some reports indicated that proper risk stratification for valve and combined operations heavily depends on the scoring system that is used to calculate perioperative mortality [7–9].

In Germany, mortality rates in 2006 after isolated valve operations and concomitant coronary revascularization procedures ranged from 4.2% for single-valve operations to 17% for aortic and mitral valve replacement with CABG [10]. Isolated aortic valve replacement was associated with a mortality rate of 3.9%, whereas isolated mitral valve replacement had a mortality rate of 5.3%. Combination procedures of CABG with aortic valve replacement and CABG plus mitral valve replacements had mortality rates of 7.1% and 13%, respectively.

In this study, we evaluated the impact of preoperative renal dysfunction on in-hospital mortality after isolated valve and combined valve operations among our high-risk patients.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 Acknowledgments
 References
 
Patients and Study Design
The initial study sample comprised 994 patients who underwent isolated valve operations or combined valve and coronary artery procedures between January 1999 and June 2003 at the University Hospital Halle. Institutional Review Board approval was obtained in March 2003. The requirement of individual patient consent was waived because of the study's retrospective design and the data collection from routine care.

We excluded 78 patients because 17 patients were aged younger than 20 years, 18 patients had terminal renal failure and dialysis, 32 had insufficient documentation, and 11 were without preoperative creatinine measurement. The final study sample comprised 916 patients. Patients who died during the index hospitalization were defined as nonsurvivors.

Data Collection and Variables
We retrospectively reviewed patient medical records and collected the data in an Excel spreadsheet (Microsoft Corp, Redmond, WA). The primary outcome variable was in-hospital mortality, defined as death during the index hospitalization. A secondary outcome variable was the postoperative length of stay at the intensive care unit (ICU).

Preoperative risk factors such as gender, age, diabetes mellitus, and chronic obstructive pulmonary disease were defined as in the European System for Cardiac Operative Risk Evaluation (EuroSCORE).

Preoperative serum creatinine (SCr) was measured in µmol/L at the day of hospital admission and then converted to mg/dL (1 mg/dL = 88.4 µmol/L). The eGFR was calculated with the abbreviated Modification of Diet in Renal Disease (MDRD) formula and expressed in mL/min/1.73m2:


Formula

For further evaluation and based on the current clinical practice guidelines for chronic kidney disease (CKD), patients were grouped into CKD classes according to their eGFR [11].

Surgical Technique
All operations were performed with cardiopulmonary bypass (CPB) that was instituted by cannulation of the ascending aorta and right atrium or bicaval cannulation. A standard circuit with a hollow-fiber membrane oxygenator and a roller pump was used. The body temperature was kept at 37°C in 85% of the patients. Otherwise, mild hypothermia (32°C) was induced. Myocardial protection was achieved by antegrade crystalloid (45%) or warm blood cardioplegia. No statistical significant difference was noted in the outcomes of those patients with hypothermia and blood cardioplegia. All operations were performed by senior cardiothoracic surgeons. Cardiac anesthesia was performed according to the institution's guidelines.

Statistics
Data were analyzed with the statistical software SPSS 15.0 (SPSS Inc, Chicago, IL) and SAS 9.1 (SAS Institute, Cary, NC). Differences between normally distributed continuous data were analyzed with the t test and presented as mean differences with the 95% confidence interval (CI). The Mann-Whitney U test was used for nonnormally distributed data. The CIs for the difference of two medians were calculated as proposed by Bonett and Price in 2002 [12]. Overall differences between more than two groups were analyzed with analysis of variance (ANOVA) or, where appropriate, with the Kruskal-Wallis test. The Fisher exact test was used for categoric variables in a 2 x 2 table.

Logistic regression analysis helped to examine the relationship between potential risk factors and in-hospital mortality. A conditional forward method was used with inclusion of clinically relevant variables with p < 0.25 from the univariate analysis as indicated in Table 1. Variables were retained when p < 0.05. In a second analysis, a stepwise forward method (Wald) was used to calculate the odds ratios using the different chronic kidney disease classes. The logistic EuroSCORE was calculated with the downloadable Excel spreadsheet (www.euroscore.org) and expressed with the 95% CI as proposed by Kuss and Börgermann [13].


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  		Table 1 Characteristics of Study Sample Categorized by Primary Outcome
 
SigmaPlot 10.0 software (Systat Software GmbH, Erkrath, Germany) was used for calculating the area under the curve (AUC) for the receiver operating characteristics (ROC) analysis and for calculating the differences between ROC curves. The CIs for these differences were determined by bootstrap [14]. All figures were created with SigmaPlot 10.0. A value of p < 0.05 was considered statistically significant. Data are shown as the mean and standard deviation (SD) or, where appropriate and indicated, as the median and interquartile range (IQR).


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 Acknowledgments
 References
 
Preoperative Data
Of 916 patients, 504 underwent a solitary valve procedure and 412 had combined valve and coronary operations. Table 1 summarizes patient characteristics categorized by primary outcome after operation and odds ratios associated with poor outcome postoperatively. The median age difference between survivors (66 years) and nonsurvivors (71 years) was 5 years (95% CI, 3.90 to 6.10, p < 0.0001). Nonsurvivors had a slightly lower body mass index of 25.1 vs 26.5 kg/m2, with a median BMI difference of 1.4 kg/m2 (95% CI, 0.49 to 2.25 kg/m2; p = 0.009).

Among several other variables, a critical preoperative state, operations on the thoracic aorta, New York Heart Association (NYHA) functional class IV, previous cardiac operation, age older than 65 years, and preoperative fibrillation were strongly associated with poor postoperative outcome. Female gender was also associated with poor outcome (Table 1).

Operative and Perioperative Data
Table 2 summarizes operative procedures within each group. Almost 55% of all operations were solitary valve procedures, and 412 (45%) were combined valve and coronary operations. Perioperative data categorized by outcome are outlined in Table 3. Nonsurvivors had a significantly longer operation time, including longer CPB and aortic clamping times, a longer ICU stay, lower preoperative eGFR, and a higher risk of perioperative death as calculated by the logistic EuroSCORE.


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  		Table 2 Operative Procedures Among the Study Sample of 916 Patients
 

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  		Table 3 Perioperative Data Among the Study Sample of 916 Patients
 
Independent Risk Factors for In-Hospital Mortality
The multivariate regression model was used to identify independent predictors for "nonsurvivor" outcome status after operation (Table 4). We identified four independent predictors of in-hospital mortality. An eGFR above average had a protective effect. Each unit of eGFR (mL/min/1.72m2) reduced the odds for death by 3%. Each additional day at the ICU increased it by 3%. The length of operation time had a significant influence: every additional minute above the average increased the odds of death by 1.1%. Including aortic cross-clamping time or CPB time instead of operation time as a variable in the regression model did not lead to a higher odds ratio than the operation time itself. Interestingly, other significant variables from the univariate analysis, such as NYHA class IV or age older than 65 years, were found not to be included in our final model.


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  		Table 4 Logistic Regression Model to Predict the Outcome Status "Nonsurvivor" After Operation
 
The Hosmer-Lemeshow test was used to evaluate the goodness of fit, and the generated model showed a useful goodness of fit (final model: {chi}2 = 8.60, p = 0.38). To evaluate the model's discrimination performance between patients who survived and died, a ROC analysis was performed with the probabilities of death from the logistic regression analysis. The AUC was 0.84 (95% CI, 0.80 to 0.88; p < 0.0001), which translated into good discrimination of the final model (Fig 1).


Figure 1
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  		Fig 1. Receiver operator characteristic curve for death as predicted by the final logistic regression model. The area under the curve (AUC; black line) is 0.84, which translates into good discrimination of the final logistic regression model.

 
The primary reasons why patients did not survive the operation were severe low cardiac output in 34, postoperative myocardial infarction in 9, multisystem organ failure in 58, sepsis in 14, severe stroke in 4, severe pulmonary artery embolism in 3, and unknown cause in 3.

Preoperative eGFR and Postoperative ICU Stay
Patients with severely impaired renal function (<30 mL/min/1.73m2) experienced the longest ICU stay (Table 5). Patients with an eGFR of 60 mL/min/1.73m2 or higher had a median ICU stay of 3 days; however, there was a considerable interindividual variation.


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  		Table 5 Estimated Preoperative Glomerular Filtration Rate and Postoperative Intensive Care Unit Stay
 
Preoperative eGFR and Outcome
Impaired preoperative renal function has been associated with increased mortality and risk of acute renal failure after cardiac procedures. Thus, we examined the association between the preoperative eGFR and primary postoperative outcome. The data are summarized in Table 6. The lower the eGFR values, the higher the actual and predicted mortalities.


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  		Table 6 In-hospital Mortality, Preoperative Estimated Glomerular Filtration Rate, and Postoperative Dialysis
 
Because it is clinically important to detect patients with preoperative renal dysfunction, we assigned patients into three eGFR groups: (1) less than 30 mL/min/1.73m2, (2) 30 to 59 mL/min/1.73m2, and (3) 60 mL/min/1.73m2 or higher. For each group, we calculated odds ratios for in-hospital mortality in a logistic regression analysis, in which the group 60 mL/min/1.73m2 or higher represented the reference group with a normal eGFR. Our use of the MDRD formula resulted in a threefold increase (95% CI, 2.00 to 4.51) in risk of death among the 30 to 59 mL/min/1.73m2 eGFR group and an eightfold increase (95% CI, 3.60 to 18.0) for the less than 30 mL/min/1.73m2 eGFR group compared with the reference group.


   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
Acknowledgments
 References
 
CKD is a worldwide major public health problem. In the United States, about 8% of the population has CKD, defined by an eGFR of less than 60 mL/min/1.73m2 [15]. CKD is explained by a higher prevalence of diabetes, hypertension, and higher body mass index. In addition, CKD remains an unrecognized condition in 80% to 90% of cases, and its presence is one of the most potent risk factors for cardiovascular disease [16]. Unfortunately, large population-based studies on renal dysfunction have not yet been conducted in Germany.

Estimating the GFR instead of simply reporting the serum creatinine concentration has become the most widely used method for evaluating renal function today. The generation of creatinine is determined primarily by muscle mass and dietary intake, which accounts for the variations in the level of serum creatinine observed among different age, geographic, ethnic, and racial groups. Extrarenal elimination of creatinine may be increased at low levels of GFR. For these reasons, the relationship between the levels of serum creatinine and GFR varies substantially among persons and over time [17]. Several studies have shown that serum creatinine is an inadequate screening test for renal failure in elderly patients [18, 19].

This study adds to the evidence that preoperative renal dysfunction has an adverse effect on survival and the postoperative ICU stay of patients undergoing valve and combined valve and coronary operations. Although a number of studies have shown that preoperative renal dysfunction is a predictor of adverse outcome in patients undergoing CABG [20, 21], this study extends this finding to patients with solitary valve and combined valve and coronary procedures.

Renal Dysfunction and In-Hospital Mortality
A persistent reduction in the eGFR to less than 60 mL/min/1.73m2 is defined as CKD [11], and despite an ongoing debate on a potential misclassification, large cohort studies showed that an eGFR of less than 60 mL/min/1.73m2 is associated with an increased risk of adverse outcomes of CKD [22]. Grouping patients into several CKD classes according to their estimated GFR provides clinicians with a straightforward classification of renal impairment. Our data show that the lower the eGFR, the higher the mortality and postoperative ICU stay.

Our results are in line with two recently published studies [6, 23] that demonstrated a strong association between preoperative renal impairment and early and late mortality. In addition, the results of our logistic regression model clearly indicated a renoprotective effect of an eGFR above average.

Because patients in our study with an eGFR of less than 30 mL/min/1.73m2 had a mortality rate of up to 44%, one might question whether patients should undergo preoperative dialysis to reduce postoperative mortality. To our knowledge, no large study has examined the effect of a single preoperative dialysis in nondialysis-dependent patients on postoperative outcome; thus, one can only speculate about the effects. Although preoperative dialysis might lower increased serum creatinine levels to a normal range, an eGFR of less than 30 mL/min/1.73m2 usually reflects a long-term renal insufficiency and its sequelae on many organs. We doubt that dialysis would ameliorate the outcome.

It is not clear how preoperative renal impairment contributes to postoperative mortality. Recent evidence, however, suggests that renal and cardiovascular conditions have been closely linked. An impaired preoperative eGFR may simply be a marker of a more advanced cardiovascular disease with increased levels of inflammatory mediators and hypercoagulability, endothelial dysfunction, arterial stiffness or calcification, and left ventricular hypertrophy [24–28]. Another hypothesis considers renal dysfunction secondary to cardiac dysfunction. A decline in renal perfusion and activation of compensatory mechanisms such as the renin-angiotensin-aldosterone-system occurs in patients with a reduced cardiac output.

Implications
Our data suggest that cardiovascular surgeons should be very aware of subtle changes in the preoperative GFR as an independent risk factor for patients undergoing valve and combined procedures. The association between renal dysfunction and an increased incidence of postoperative complications such as prolonged ICU and hospital stay requires improved resource planning by those responsible for health care provision. A better understanding of mechanisms underlying progressive renal dysfunction and improved renal protection strategies during the operative period may ameliorate both in-hospital and late survival after cardiovascular operations.

Strengths and Limitations
This was a large contemporary German single-center analysis of preoperative renal dysfunction in patients undergoing valve and combined procedures. Data provided were directly collected from medical records, and the database is 98% complete for all analyzed fields. The database was revalidated by another coauthor before analysis. These procedures should have reduced errors, but did not eliminate them completely.

This study has limitations. First, conclusions from a retrospective observational study are necessarily limited in their application. Second, we cannot provide long-term survival data because it is difficult to retrieve postoperative survival data in Germany. Third, observational bias, particularly for outcomes defined by clinical interventions, which are dependent on various treatment thresholds used by different clinicians, are a common error found in database derived studies. Finally, our study is purely exploratory, especially by using automatic selection procedures, which are known to generally overestimate the influences of prognostic factors [29]. As such, we would like to see replications of our analyses in independent samples.


   Acknowledgments
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
References
 
We would like to thank the secretaries of the Department of Cardiothoracic Surgery at the University hospital Halle and all members of the Medical Record Archive for collecting and providing the files.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 

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