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Ann Thorac Surg 2006;81:542-546
© 2006 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Acute Renal Failure After Cardiac Surgery: Evaluation of the RIFLE Classification

^a Department of Anesthesia and Intensive Care Medicine, Helsinki University Central Hospital, Helsinki, Finland
^b Department of Cardiothoracic Surgery, Helsinki University Central Hospital, Helsinki, Finland

Accepted for publication July 18, 2005.

^_* Address correspondence to Dr Kuitunen, Department of Anesthesia and Intensive Care Medicine, Helsinki University Hospital, PO Box 340, Helsinki, FIN-00029 HUS, Finland (Email: anne.kuitunen{at}hus.fi).

	Abstract

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BACKGROUND: Acute renal failure increases risk of death after cardiac surgery. However, the definition of acute renal failure is not standardized. The Acute Dialysis Quality Initiative Workgroup has outlined consensus criteria. The aim of the present study was to evaluate this determination of postoperative renal impairment in cardiac surgical patients, and its association with mortality.

METHODS: The 813 consecutive patients undergoing cardiac surgery at Helsinki University Central Hospital were analyzed. According to the RIFLE classification (named by the severity of renal impairment: Risk, Injury, Failure, Loss, End-stage kidney disease) patients were divided into three levels based on either plasma creatine level or urine output.The discrimination with death within 90 days after surgery was determined.

RESULTS: According to the RIFLE criteria, 19.3% of patients had renal impairment after cardiac surgery. Patients in the more severe category, RIFLE-F (failure), had a 90-day mortality rate of 32.5% compared with 8.0% for those in RIFLE-R (risk) and 21.4% for RIFLE-I (injury) patients. The RIFLE classification discriminated 90-day mortality quite well (area under curve 0.824) compared with the change of plasma creatinine and the change of estimated gromerular filtration rate (areas under curve 0.849 and 0.829, respectively). The results of the multivariate forward stepwise logistic regression analysis found that RIFLE classification was an independent risk factor for 90-day mortality (p < 0.0001), unlike change in glomerular filtration rate and change in plasma creatinine.

CONCLUSIONS: We propose that the RIFLE classification is a valuable method to evaluate acute renal failure after cardiac surgery. The severity of RIFLE classification may be associated with increased 90-day mortality rate.

	Introduction

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Postoperative renal insufficiency remains a serious complication of cardiac surgery. It is associated with increases in mortality, morbidity, and intensive care unit (ICU) stay, particularly if there is requirement for hemodialysis [1–3]. Acute renal failure (ARF) develops in 5% to 30% of patients who undergo cardiac surgery [1, 4–10], and it is asscociated with a high risk of death (up to 80%) [1–4, 9, 11–15]. The development of ARF has been identified as the strongest risk factor for death with an odds ratio of 7.9 (95% confidence interval: 6 to 10) in cardiac surgical patients [2].

The observed incidences have been varied depending on many factors, such as the study population, exclusion criteria, and especially criteria for defining renal dysfunction. The spectrum of definitions of ARF has ranges from severe (for example, ARF requiring dialysis) [2, 3] to relatively modest observable increases in serum creatinine concentration [6, 9], or decreases in calculated glomerular filtration rate (GFR) [5]. To optimize the approach to ARF, there is an international demand to the recommendations for the criteria of ARF. The Acute Dialysis Quality Initiative Workgroup has outlined consensus criteria [16]. This classification is divided into three levels based on either plasma creatinine level or urine output. This recommendation for the definition of ARF has not been evaluated in special patient populations. Thus, in the present investigation, we tested this classification of ARF known as RIFLE (for Risk, Injury, Failure, Loss, End-stage kidney disease) postoperatively after cardiac surgical procedures.

	Material and Methods

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This is an observational study approved by the Institutional Review Board (February 25, 2004). The study is based on data collected from routine care hospital records, thus individual consent was waived. All patients undergoing cardiac surgery at the Helsinki University Central Hospital between April 15 and December 31, 2003 were eligible. There were no exclusion criteria. In all, 813 patients were included in the study.

Routine computerized data collection consisted plasma creatinine levels and urine outputs during the stay in the cardiac surgical intensive care unit (CareSuite6.3; Picis, Wakefield, Massachusetts). Our laboratory determined plasma values of creatinine instead of serum values, but the values have been evaluated to correspond each other [17]. From obtained values, the classification for ARF by the Acute Dialysis Quality Initiative Workgroup was constructed [16]. This classification has been named RIFLE according the severity of renal impairment (Table 1). According to GFR, plasma creatinine (where change from the patient's individual baseline was measured), and urine output, patients were classified into three severity categories: risk, injury, and failure. The patients with renal replacement therapy (RRT) for renal indications should be considered to have ARF regardless of their plasma creatinine level or urine output. The patients without any injury have been classified as RIFLE–0 level.

The European System for Cardiac Operative Risk Evaluation (EuroSCORE) is used in many European countries and in all cardiosurgical centers in Finland. The EuroSCORE has been originally validated for the prediction of mortality [19]. The EuroSCORE was entered into a computerized database on the day of operation.

The studied outcome variables were the 90-day mortality and the prolonged stay (5 or more days) in the ICU. Renal replacement therapy was recorded; it was either intermittent hemodialysis or continuous venovenous hemodiafiltration. Any other major morbidity was not evaluated.

Data are presented in medians with interquartile ranges. The Mann-Whitney U test or Pearson ² test were performed to evaluate differences between survivors and dead patients. The discrimination of each risk factor (RIFLE, change in plasma creatinine, change in GFR, EuroSCORE) was determined and compared by receiver operating characteristic curves for 90-day mortality. With this test, an area under the receiver operating characteristic curve of 1.0 indicates perfect discrimination, whereas an area less than 0.5 means that it is no better than chance. Areas of 0.5 to 0.7 suggest a low predictive discrimination, and values greater than 0.7 confirm the usefulness of the model as a risk predictor [20]. Stepwise multiple logistic regression analyses were performed to determine the independent effect of each factor (RIFLE and type of surgery as category variables; change in plasma creatinine, change in GFR, and EuroSCORE as continuous variables) on 90-day mortality. The SPSS 10.1.3 software (SPSS, Chicago, Illinois) was used in all analyses. For statistics, a p value less than 0.05 was considered significant.

	Results

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During the study period, 813 patients underwent cardiac surgery. Within this population, 434 patients (53.4%) underwent coronary artery bypass graft surgery with cardiopulmonary bypass, 116 patients (14.3%) underwent the surgery without cardiopulmonary bypass, 122 patients (15.0%) had valvular surgery, 58 patients (7.1%) had combined coronary artery bypass graft and valvular surgery, and 79 patients (9.7%) had other types of cardiac surgery (transplantation, thoracic aorta reconstruction, and so forth).

The postoperative proportions of 90-day mortality, prolonged stay in the ICU (5 or more days), and renal failure requiring RRT were 4.3% (number of patients, 35 of 813), 17.8% (145 of 813), and 3.2% (26 of 813), respectively. The median postoperative length of stay in ICU for the whole study population was 1 day with interquartile ranges of 1 to 3.

The preoperative EuroSCORE was 5.0 (3.0 to 7.0). The estimated preoperative GFR was 81.8 mL · min^–1 · 1.73 m^–2 (67.7 to 98.2 mL · min^–1 · 1.73 m^–2), and the lowest postoperative GFR was 85.1 mL · min^–1 · 1.73 m^–2 (64.1 to 103.9 mL · min^–1 · 1.73 m^–2). The GFR could be estimated in 796 patients preoperatively and in 805 patients postoperatively. The preoperative plasma creatinine level was 81.0 µmol/L (69.0 to 94.0 µmol/L), and the highest postoperative plasma creatinine level was 79.0 µmol/L (65.0 to 100.0 µmol/L). Table 2. shows the number of patients according to the RIFLE classification. The proportions of RRT, 90-day mortality, and prolonged ICU stay (5 or more days) in different RIFLE categories are also presented in Table 2. The RRTs in RIFLE classes 0 and R were not for renal indications. The purpose was to remove fluid overload.

The results of the multivariate forward stepwise logistic regression analysis found that RIFLE classification was an independent risk factor for 90-day mortality (Exp [B] 2.616, p < 0.001). Change in GFR and change in plasma creatinine level were not independent risk factors for 90-day mortality. The EuroSCORE was also an independent risk factor for 90-day mortality (Exp [B] 1.275, p < 0.001).

The RIFLE criteria include two clinical outcomes: loss and end-stage renal disease. Persistant ARF (loss) is defined as need for RRT for more than 4 weeks (RIFLE class L); whereas end-stage renal disease is defined by need for dialysis more than 3 months (RIFLE class E). In the present study, there were 3 patients requiring RRT more than 4 weeks, and 3 patients requiring RRT more than 3 months.

	Comment

Top Abstract Introduction Material and Methods Results Comment References

 
In this observational study, we found that ARF after cardiac surgery can be classified by the new consensus criteria. According to this classification, 80.7% of patients had no ARF, whereas 19.3% of patients had renal impairment during their stay in ICU after cardiac surgery. The 90-day mortality rate of patients with ARF was high (16.7%). The mortality rate was highest (32.5%) in the severest category of ARF classified by consensus criteria of RIFLE. Acute renal failure defined by the RIFLE classsification was also an independent risk factor for 90-day mortality.

Postoperative renal dysfunction has been confirmed as one of the most severe complications of cardiac surgery. Furthermore, RRT has markedly increased mortality [2, 3, 9]. The risk of ARF after cardiac surgery has ranged from 5% to 30%, depending on the criteria used to define this complication [1, 2, 4–10, 12, 21], whereas in 1% to 5% of patients, severe disease requiring RRT develops [1, 2]. In the present study, ARF incidence according to RIFLE classification was 19.3%. The comparison with the previous reports is difficult, as this is the first report evaluating the consensus recommendations for ARF definition in cardiac surgical patients. The mortality rate for ARF has remained high, greater than 50% in most series of patients requiring RRT, despite improvements in intensive care and RRT technology [22, 23]. In the present study, mortality was also increased with ARF. The mortality rate without ARF was 0.9%, whereas the mortality rate with ARF defined as RIFLE class F was 32.5%. Furthermore, the present definition system of ARF had a good association with mortality. That is not so surprising, because both the change in plasma creatinine level and the change in GFR have been previously identified to increase the risk of death [6]. Both of these variables are included in the RIFLE classification.

Because there has been a lack of consensus criteria to define ARF [24], the choice of definition requires discussion. In the context of cardiac surgery, subclinical renal dysfunction, as measured using various makers of renal tubular damage (1–microglobulin, glutathione transferase–, glutathione transferase–pi, N-acetyl-ß-D-glucosaminidase), has been described [25]. However, there is no evidence that perioperative increases of these markers are associated with postoperative morbidity or mortality, and thus these surrogate changes can not be used for definition of ARF after cardiac surgery [26]. Generally, plasma concentration of creatinine has been used for definition criteria of ARF. This variable has been used to predict morbidity and mortality after cardiac surgery [4–6, 9, 11–13, 27]. However, different authors have chosen different degrees of abnormality in plasma creatinine levels as cutoffs for diagnosis of ARF. Furthermore, plasma creatinine concentration will not be an accurate reflection of GFR in the nonsteady-state condition of ARF. During the evolution of dysfunction, plasma creatinine will underestimate the degree dysfunction whereas the opposite will be true as renal function recovers. Nonetheless, the degree to which plasma creatinine changes from baseline will reflect the change in glomerular filtration.

Although the kidney has numerous functions, in clinical practice the unique functions of the kidney are both the excretion of waste products of nitrogen metabolism and production of urine. Thus, for clinical research, ARF has to be defined as an abrupt and sustained decrease in glomerular filtration, urine output, or both. Once glomerular filtration has reached a steady state, it can be quantified by measuring 24-hour creatinine clearance or estimate clearance from the plasma concentrations of creatinine. Unfortunately, the accuracy of creatinine clearance is limited because as GFR falls, creatinine secretion is increased, and thus the rise in plasma creatinine is less, resulting in a potentially large overestimation of GFR. Therefore, creatinine clearance represents the upper limit of what the true GFR is under steady-state conditions. Furthermore, as patients with ARF are not in steady state, creatinine clearance will not accurately reflect GFR.

The urine output is a very rarely studied variable in publications concerning ARF after cardiac surgery, although, urine output is more sensitive to changes in renal function than biochemical markers [8]. However, it is far less specific except when severely decreased or absent; the changes in urine output often occur long before biochemical changes are apparent. The new recommendation for the definition of ARF takes account of both renal functions, the excretion of creatinine and the production of urine. The consensus criteria take also into account the patients whose baseline renal function is abnormal before the current insult. In many studies concerning ARF after cardiac surgery, these patients have been excluded from the study material.

These new recommendation for ARF have not yet been widely validated. Thus, we tested the utility of this definition in cardiac surgery, as the renal dysfunction is there still one of the major complications. Postoperative renal failure always causes costs. And despite all these efforts, the clinical outcome is poor, which was also seen in the present study: the deaths and prolonged stay in ICU were directly connected to ARF. These nonspecific clinical endpoints were also chosen in the present evaluation to validate the utility of this new classification, as ARF lacks a gold standard for diagnosis and relatively few survivors go on to end-stage renal failure. Certainly, ARF may serve as an indicator of the severity of disease or associated complications. Nevertheless, several studies have shown that ARF exerts a profound impact on prognosis independent of the severity of the condition [2, 6, 28]. Renal injury itself further increases the risk of developing severe nonrenal complications that may lead to death. Large studies have evaluated the predictive value of classification systems using plasma creatinine or estimated creatinine clearance [6, 11].

The results of the present study confirmed our hypothesis that the association between postoperative ARF defined by new consensus criteria and mortality is strong. Thus, by calculating the RIFLE level, clinicians may identify and consistently classify patients with increased risk of death due to impairment in renal function after cardiac surgery.

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