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Ann Thorac Surg 2009;87:1079-1084. doi:10.1016/j.athoracsur.2009.01.065
© 2009 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Impact of Renal Dysfunction on Long-Term Survival After Isolated Coronary Artery Bypass Surgery

Ye Lin, MDa,*, Zhe Zheng, MDa,*, Yan Li, MDa,*, Xin Yuan, MDa, Jianfeng Hou, MDa, Shiju Zhang, MDa, Hongguang Fan, MDa, Yang Wang, MDb, Wei Li, MDb, Shengshou Hu, MDb,*

a Department of Cardiovascular Surgery and Research Center for Cardiovascular Regenerative Medicine, Cardiovascular Institute and Fuwai Hospital, Chinese Academy of Medical Science and Peking Union Medical College, Beijing, China
b Division of Biometrices National Center for Cardiovascular Disease, Cardiovascular Institute and Fuwai Hospital, Chinese Academy of Medical Science and Peking Union Medical College, Beijing, China

Accepted for publication January 27, 2009.

* Address correspondence to Dr Hu, Cardiovascular Institute and Fuwai Hospital, Peking Union Medical College and Chinese Academy of Medical Science, 167A Beilishi Rd, Xi Chen District, Beijing, 100037, Peoples Republic of China (Email: shengshouhu{at}yahoo.com).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 Footnotes
 Acknowledgments
 References
 
Background: Preoperative renal dysfunction has been an important predictor for adverse cardiovascular events after coronary artery bypass grafting (CABG). In the past, serum creatinine was widely used to assess renal function. Until recently, estimated glomerular filtration rate (eGFR) was recommended in evaluating renal function. The Cockcroft-Gault formula and the Modification of Diet in Renal Disease (MDRD) equation are two widely used formulas in clinical practice. Which method best predicts long-term outcome after CABG is still unknown. This study compared the predictive effectiveness of the Cockcroft-Gault formula, the MDRD equation, and serum creatinine level for in-hospital and long-term mortality.

Methods: We retrospectively reviewed data collected from 5559 patients who underwent isolated CABG at Fuwai Hospital from January 1999 to December 2005. The main outcomes were in-hospital and long-term mortality. Receiver operating characteristic (ROC) curves and Cox analysis were used for the comparison.

Results: Mean follow-up was 56.5 ± 24.6 months. ROC curve analysis showed that the Cockcroft-Gault formula had the greatest accuracy for predicting in-hospital mortality (area under the curve, 0.755; p < 0.001). Multivariate analysis confirmed that the eGFR based on the Cockcroft-Gault formula was an independent predictor of in-hospital (odds ratio, 4.51, p < 0.001) and long-term (hazard ratio, 1.54; p = 0.003) mortality. Both formulas were better than the serum creatinine level.

Conclusions: Both formulas could provide a better measure of risk assessment than serum creatinine for in-hospital and long-term mortality. The Cockcroft-Gault formula was better than the MDRD equation for predicting in-hospital mortality.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 Footnotes
 Acknowledgments
 References
 
Coronary artery bypass grafting (CABG) is a successful surgical treatment for prolongation of life in selected patients with coronary artery disease [1]. Preoperative renal dysfunction has been an important predictor of adverse cardiovascular events after CABG [2–7]. Most previous studies have used the serum creatinine (SCr) concentration as an assessment of renal dysfunction. However, SCr concentration is affected by many factors other than glomerular filtration rate (GFR), such as age, sex, and body size. GFR is probably the best overall index of renal function [8].

The gold standard for determining GFR is to measure the clearance of exogenous substances such as inulin, chromium-51 ethylenediamine tetraacetic acid, technetium-99m labelled diethylenetriamine penta-acetate, and iodine-125 labeled iothalamate. However, the techniques used in determining GFR are time-consuming, labor-intensive, and expensive, which make them unpractical for routine monitoring. For this reason, the estimated glomerular filtration rate (eGFR) was recommended in evaluating renal function [8]. The formula that has been used most commonly is the Cockcroft-Gault formula [9]. More recently, the National Kidney Foundation of American recommended using the Modification of Diet in Renal Disease (MDRD) equation to estimate GFR [8, 10]. Even so, the method that best predicts in-hospital and long-term outcome after CABG is still unknown. This study retrospectively reviewed 5559 patients who underwent isolated CABG with an average follow-up of 4.7 years to compare the predictive effectiveness of the Cockcroft-Gault formula, the MDRD equation, and SCr concentration for in-hospital and long-term mortality after isolated CABG.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 Footnotes
 Acknowledgments
 References
 
Patients
The study included 5559 patients who underwent isolated CABG between January 1999 and December 2005. A prespecified case report form was designed for data collection. Definitions of preoperative characteristics were consistent with those of Society of Thoracic Surgeons (STS) database. Data were 100% complete for critical preoperative risk factors of interest as well as for each major postoperative hospital outcome.

Estimation of Renal Function
No patient in this study was receiving dialysis before CABG. The last single SCr levels (within 72 hours before operation) in µmol/l were obtained for all patients. Estimated GFR was calculated with the Cockcroft-Gault formula and the MDRD equation. The Cockcroft-Gault formula was adjusted with body surface area (BSA).

Renal dysfunction was defined according to the guideline from the National Kidney Foundation [8]. Normal renal function was defined as eGFR of 90 mL/min/1.73 m2 or more. Mild, moderate, and severe renal dysfunction were defined as eGFR of 60 to 90, 30 to 60, and less than 30 mL/min/1.73 m2, respectively.

Because the Cockcroft-Gault formula had the greatest accuracy for predicting in-hospital mortality in this study, the four major study groups were divided by the Cockcroft-Gault formula (Tables 1 and 2). Go Zakeri and colleagues [11] used preoperative SCr of 130 µmol/L or higher in their article to define mild to moderate renal dysfunction and stated that mild renal dysfunction was an important predictor of outcome in terms of in-hospital death. For this reason, we used SCr 130 µmol/L or higher to compare with eGFR of less than 60 mL/min/1.73 m2 for predicting in-hospital and long-term mortality.


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  		Table 1 Clinical Characteristics in Terms of Preoperative Renal Function
 

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  		Table 2 Clinical Outcomes in Terms of Preoperative Renal Function
 
The Cockcroft-Gault formula [9] is as follows:


Formula

The MDRD equation [8, 10, 12] is as follows:


Formula

where eGFR is the estimated glomerular filtration rate in mL/min/1.73 m2, the SCr concentration is in mg/dL, age is in years, and weight is given in kg.

Patient Follow-Up and Outcomes
According to institutional follow-up protocol, patients discharged alive were required to visit our outpatient clinic 6 months after CABG and then once every year. Patients were followed up through two cross-sectional investigations in 2007 and 2008, during which 92% of all patients were contacted by telephone and 5% by mail. The information of 0.6% patients was retrieved from their medical records. Follow-up was completed for 5485 patients (97.6%). The mean follow-up was 56.47 ± 24.64 months. The main events analyzed in this study were in-hospital mortality, long-term mortality, and major morbidity, including hemofiltration, postoperative coma, postoperative intraaortic balloon pump use, stroke, pneumonia, and atrial fibrillation.

Statistical Analysis
Values were expressed as means ± standard deviation or percentages, as appropriate. Comparisons between means were performed using the t test, whereas differences in categoric variables were assessed using the Fisher exact test or the {chi}2 test. The sensitivity and specificity of both formulas were assessed from nonparametric receiver operating characteristic (ROC) curves generated by plotting sensitivity vs 1– specificity, giving the ideal test a sensitivity = 1 and specificity = 1. Areas under the curve (AUC) were calculated and compared according to the procedure of Hanley and McNeil [13]. Survival curves of the four groups were generated using the Kaplan-Meier method, and group differences were assessed by the log-rank test. Multivariate logistic regression and Cox proportional hazard models were created with the use of perioperative variables to identify independent predictors of in-hospital and long-term mortality.

We developed three models to compare the Cockcroft-Gault formula, the MDRD equation, and SCr for predicting mortality. Renal dysfunction was defined as eGFR of less than 60 mL/min/1.73 m2 by the Cockcroft-Gault formula, eGFR of less than 60 mL/min/1.73 m2 by the MDRD equation, and SCr of 130 µmol/L or more. Other candidate variables included age older than 65 years, female gender, chronic obstructive pulmonary disease (COPD), history of stroke, hypertension, hyperlipidemia, diabetes, low ejection fraction, previous myocardial infarction, emergency operation, left main disease, triple-vessel disease, and off-pump technique. Only variables with a value of p < 0.25 at univariate analysis were included in the multivariate Cox regression model. All statistical tests were 2-tailed, and a significant level of 0.05 was used throughout. Odds ratios (OR) and hazard ratios (HR) with 95% confidence intervals (CI) were calculated. Statistical analyses were performed using SPSS 13.0 software (SPSS Inc, Chicago, IL).


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 Footnotes
 Acknowledgments
 References
 
The study included 5559 consecutive patients who underwent isolated CABG. These patients were divided by the Cockcroft-Gault formula. There were 21 patients (0.38%) with severe renal dysfunction (eGFR < 30 mL/min/1.73 m2), 1031 (18.5%) with moderate renal dysfunction (eGFR 30 to 59 mL/min/1.73 m2), 2765 (49.7%) with mild renal dysfunction (eGFR 60 to 89 mL/min/1.73 m2), and 1742 (31.3%) with normal renal function (eGFR ≥ 90 mL/min/1.73 m2). Perioperative characteristics for patients in the four main study groups are listed in Table 1. Patients with poorer renal function were older, were more often female, and had a higher prevalence of hypertension.

ROC Curve Comparisons for Predicting Mortality
To the MDRD equation, the Cockcroft-Gault formula, and SCr for predicting mortality, the ROC curves were constructed using all eGFR and SCr measures. The AUC were calculated and compared (Table 3). The Cockcroft-Gault formula had the greatest accuracy for predicting in-hospital mortality (AUC, 0.755; 95% CI, 0.695 to 0.815; p < 0.001). ROC curve analysis showed a similar AUC for eGFR and SCr in predicting long-term mortality (AUC, 0.618 for Cockcroft-Gault formula, p < 0.001; AUC, 0.608 for MDRD equation, p < 0.001; AUC, 0.609 for SCr, p < 0.001).


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  		Table 3 Comparison of Area Under Receiver Operating Curve for Modification of Diet in Renal Disease Equation, Cockcroft-Gault Formula and Serum Creatinine
 
In-Hospital Outcomes
There were 74 in-hospital all-cause deaths (1.33%). The patients with moderate to severe renal dysfunction (eGFR < 60 mL/min/1.73 m2) had a higher rate of postoperative complications (Table 2). The in-hospital mortality rates increased with increasing preoperative SCr levels and decreasing eGFR (Fig 1). Three multivariate logistic regression analysis models were created to identify independent predictors of in-hospital mortality. Renal dysfunction was defined as eGFR of less than 60 mL/min/1.73 m2 by the Cockcroft-Gault formula, eGFR of less than 60 mL/min/1.73 m2 by the MDRD equation, and SCr of 130 µmol/L or more. The Cockcroft-Gault formula had the maximum OR of 4.51 for predicting in-hospital mortality; the ORs were 3.43 for the MDRD equation and 2.86 for SCr (p < 0.001 Table 4).


Figure 1
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  		Fig 1. (A) In-hospital mortality based on increasing estimated glomerular filtration rate (eGFR) comparing the Cockcroft-Gault formula (solid line) and the Modification of Diet in Renal Disease (MDRD, dashed line). (B) In-hospital mortality based on decreasing serum creatinine. (C) Long-term mortality based on increasing eGFR comparing the Cockcroft-Gault formula (solid line) and the MDRD, dashed line). (D) Long-term mortality based on decreasing serum creatinine.

 

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  		Table 4 Multivariate Analysis of the Risk of Mortality a
 
Survival Analysis
There were 248 (4.52%) late all-cause deaths during follow-up. Figure 2 shows the Kaplan-Meier survival analyses according to the three renal assessment methods. The log-rank test was used to describe the differences among four groups. The groups with moderate to severe renal dysfunction had higher long-term mortality rates. Both formulas were better than SCr for predicting long-term mortality.


Figure 2
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  		Fig 2. Kaplan-Meier estimated survival according to estimated glomerular filtration rate (eGFR; mL/min/1.73 m2) calculated by the (top panel) Cockcroft-Gault formula, (middle panel) the Modification of Diet in Renal Disease (MDRD), and (bottom panel) and serum creatinine (µmol/L).

 
Cox Multivariate Analysis
We used Cox proportional hazard models to examine the relationship between renal dysfunction and other known preoperative risk factors for predicting long-term mortality. We also created three models by using the three renal assessment methods. Table 4 presents HRs of the three methods for predicting long-term mortality. There were no significant differences between the two formulas, and both formulas were more accurate in predicting long-term mortality than SCr (HR, 1.60 for the MDRD equation, p = 0.002; 1.54 for the Cockcroft-Gault formula, p = 0.003; 1.40 for SCr, p = 0.108). Long-term mortality rates increased with decreasing eGFR. In contrast, the relationship between SCr and long-term mortality rates was nonlinear (Fig 1).


   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
Footnotes
 Acknowledgments
 References
 
Principle Findings
We compared the predictive effectiveness of the Cockcroft-Gault formula, the MDRD equation, and SCr for in-hospital and long-term mortality in patients after isolated CABG at our institution. Both the Cockcroft-Gault formula and the MDRD equation could provide a better measure of risk assessment than SCr for in-hospital and long-term mortality. The Cockcroft-Gault formula was better than the MDRD equation for predicting in-hospital mortality. There was no significant difference between the MDRD equation and the Cockcroft-Gault formula for predicting long-term mortality.

Comparison of MDRD Equation, Cockcroft-Gault Formula, and SCr
A number of renal function tests are available, and each focuses on a unique aspect of kidney function. SCr is the most widely used assessment of renal function; however, it can be affected by many factors, such as muscle mass, dietary intake, changes in tubular secretion, and interference by other substances in the serum. As a result, SCr concentration is suboptimal for estimating renal function. Later on, eGFR was recommended in evaluating renal function [8]. The Cockcroft-Gault formula is probably one of the most widely used prediction equations [9]. The MDRD equation was recently recommended by the National Kidney Foundation [8, 10]. Its advantage over the Cockcroft-Gault formula was documented in some [14] but not all recent reports [15, 16]. Neither formula was developed for patients with cardiac disease. Moreover, they differ in variables and coefficients. The best method to predict in-hospital and long-term outcome after CABG is still unknown.

In this study, the Cockcroft-Gault formula was found to be better than the MDRD equation in terms of predictive power for in-hospital mortality. The fact that the former can incorporate more variables, such as BSA and weight, serves as the underlying reason. We assume that these factors indirectly reflect body mass index (BMI), which is generally identified as a risk factor for in-hospital mortality after CABG [17, 18]. He and colleagues [19] reported that a BMI of lower than 18.5 was an independent risk factor of death in the Chinese population. The Cockcroft-Gault formulas are adjusted using weight and BSA and therefore are more powerful for such prediction purposes.

Previous studies showed that the bias and precision of the MDRD equation were poorer and tended to underestimate true GFR in patients with levels of renal function nearer to normal [20, 21]. In our cohort, renal function in most patients ranged from normal to moderate. This may be another reason explaining the superiority of the Cockcroft-Gault formula over the MDRD equation for predicting in-hospital mortality.

Although the two formulas varied in their accuracy for predicting in-hospital mortality, we found a similar trend of decreased mortality rates with increased eGFR by both the Cockcroft-Gault formula and the MDRD equation (Fig 1). There was also no statistical difference between the two formulas for predicting in-hospital mortality using ROC curves and multivariate logistic regression analysis (p < 0.001), so both the Cockcroft-Gault formula and the MDRD equation could be used for predicting in-hospital and long-term mortality after CABG.

Previous studies showed an increase in all-cause and cardiovascular mortality in patients with renal dysfunction [4–7]. Our study results are similar to those reported. The recent analysis had indicated mild renal dysfunction as a major risk factor for cardiovascular complications and death [7, 11, 22]. Our study confirmed and extended these findings with long-term follow-up results.

Limitations
This study has several limitations. This study was performed in a single center. The GFR was estimated by using the Cockcroft-Gault formula and the MDRD equation. The formula provides an acceptable estimate of GFR but it is not the gold standard for determining GFR. Also, because renal function measurements (eGFR) were based on a single preoperative SCr value, which might fluctuate, particularly in patients with unstable hemodynamics and various medical therapies, we were unable to examine the effect of a rising or falling creatinine trajectory. Renal function in most patients in our cohort was normal to moderate. This might also affect our findings.

Conclusions
This study showed that an eGFR of less than 60 mL/min/1.73 m2 was an independent predictor of in-hospital and long-term mortality in Chinese patients after CABG. The Cockcroft-Gault formula was better than the MDRD equation for predicting in-hospital mortality, and both formulas were superior to SCr for predicting in-hospital and long-term mortality.


   Acknowledgments
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Footnotes
 Acknowledgments
References
 
Funding for this study was supported by the Key Project in the National Science and Technology Pillar Program during the Eleventh Five-Year Plan Period (2006BAI01A09) and the Key Project of Beijing Municipal Science and Technology Commission (D0906004040391).


   Footnotes
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Footnotes
Acknowledgments
 References
 
* The first three authors contributed equally to this work. Back


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Footnotes
 Acknowledgments
 References
 

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  8. Levey AS, Coresh J, Balk E, Kausz AT, et al. National Kidney Foundation National Kidney Foundation practice guidelines for chronic kidney disease: evaluation, classification, and stratification Ann Intern Med 2003;139:137-147.[Abstract/Free Full Text]
  9. Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine Nephron 1976;16:31-41.[Medline]
  10. National Kidney Foundation K/DOQI Clinical Practice Guidelines for Chronic Kidney Disease: evaluation, classification, and stratification Am J Kidney Dis 2002;39(suppl):S1-S266.[Medline]
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  12. Levey AS, Bosch JP, Lewis JB, Greene T, Rodgers N, Roth D. A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group. Ann Intern Med 1999;130:461-470.[Abstract/Free Full Text]
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  15. Vervoort G, Willems HL, Wetzels JF. Assessment of glomerular filtration rate in healthy subjects and normal albuminuric diabetic patients: validity of a new (MDRD) prediction equation Nephrol Dial Transplant 2002;17:1909-1913.[Abstract/Free Full Text]
  16. Lamb EJ, Webb MC, Simpson DE, Coakley AJ, Newman DJ, O'Riordan SE. Estimation of glomerular filtration rate in older patients with chronic renal insufficiency: is the Modification of Diet in Renal Disease formula an improvement? J Am Geriatr Soc 2003;51:1012-1017.[Medline]
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