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Interact CardioVasc Thorac Surg 2008;7:1101-1106. doi:10.1510/icvts.2008.181438
© 2008 European Association of Cardio-Thoracic Surgery
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ESCVS article - Cardiac general

Retrospective cross-validation of simplified predictive index for renal replacement therapy after cardiac surgery{star}

Piotr Knapik*, Piotr Rozentryt, Pawel Nadziakiewicz, Lech Polonski and Marian Zembala

Silesian Centre for Heart Diseases, ul. Szpitalna 2, 41-800, Zabrze, Poland

Received 11 April 2008; received in revised form 10 July 2008; accepted 14 July 2008

{star} Presented at the 57th International Congress of the European Society for Cardiovascular Surgery, Barcelona, Spain, April 24–27, 2008.

Corresponding author. Tel./fax: +48-32-2732731.

E-mail address: pknapik{at}slam.katowice.pl (P. Knapik).


   Abstract
Top
 Abstract
1. Introduction
 2. Materials and methods
 3. Statistics
 4. Results
 5. Discussion
 Acknowledgements
 References
 
Objectives: Acute kidney impairment requiring renal replacement therapy is an infrequent but dangerous complication of cardiac surgery. Its development is associated with high mortality and morbidity. A recently published simple risk stratification engine has been developed and validated in the USA and Canada, but its discriminatory power has never been tested in Europe. We aimed to cross-validate the newly developed risk stratification algorithm in a group of patients operated on in a single centre in Poland. Methods: From electronic database we selected 1421 patients fulfilling identical inclusion and exclusion criteria as in derivation cohort in Canada. In each patient eligible for analysis we calculated simplified renal index and assessed its predictive power for the need of renal replacement therapy. Results: After surgery 33 (2.3%) patients developed acute kidney impairment and subsequently underwent renal replacement therapy. The simplified renal index predicted risk of postoperative renal replacement therapy in our group. Patients with low values of simplified renal index (0–1), medium (2–3) and high values (4 and more) were found to have increasingly higher risk for renal replacement therapy of 1.1% (95% CI: 0.5–2.1%), 3.2% (95% CI: 1.9–5%) and 12.5% (95% CI: 5.2–24.1%), respectively. The area under the ROC curve of simplified renal index as predictor of renal replacement therapy in our centre was 0.73 (95% CI: 0.62–0.81) and did not differ significantly from the values obtained in the original paper. Conclusion: The new risk stratification algorithm is effective in discrimination of patients at high risk for development of acute kidney impairment with the need of renal replacement therapy.

Key Words: Acute kidney impairment; Cardiac surgery; Renal replacement; Risk stratification; Validation


   1. Introduction
Top
 Abstract
 1. Introduction
2. Materials and methods
 3. Statistics
 4. Results
 5. Discussion
 Acknowledgements
 References
 
Open heart operations are the most common cardiac surgical procedures. More than 200 patients per 100,000 inhabitants undergo these procedures in the United States every year [1]. The situation in Central Europe may vary, but generally a rapid increase in the number of cardiac surgical procedures has been observed. In Poland for example, there are currently 20% more open heart operations when compared to the year 2002 [2].

Preoperative identification of risk factors for potential adverse outcomes is an important component of care. One of the most serious postoperative complications is acute kidney insufficiency. The incidence of such complication following cardiopulmonary bypass is high and depends both on the diagnostic criteria and the population studied. According to a recent review, acute kidney impairment may be recognized in up to 30% of patients [3]. Postoperative acute kidney impairment is invariably associated with prolonged intensive care unit stay, higher resource utilization, as well as increased morbidity and mortality [4–7].

According to the Acute Kidney Injury Network, the need for renal replacement therapy (RRT) is considered acute kidney impairment stage three [8] and occurs in approximately 1% of patients after cardiac surgery; however, the prognosis in this subgroup is particularly poor with mortality exceeding 50% [4–7, 9].

There are many identified risk factors for the development of acute kidney impairment. The most important factors are: female gender, advanced age, preexisting renal dysfunction, impaired cardiac function, urgent operations and various comorbidities, such as diabetes, peripheral vascular disease, or chronic obstructive pulmonary disease [3]. An elaboration and validation of a simple, easy to use index that aggregates preoperatively available information is of particular importance, because the number of patients with various comorbidities will be increasing worldwide. Identification of patients with high risk to develop acute kidney impairment can improve clinical management preoperatively, optimize resource utilization and possibly improve outcome.

There are only few validated scoring systems for the prediction of the need for renal replacement therapy (RRT) [10–13], and all but one [14] have been derived and validated in the United States or Canada. The most recently published scoring system for prediction of RRT offers some advantages over previous algorithms [13].

The aim of the present study was to validate this new algorithm in European patients operated on at the Silesian Centre for Heart Diseases (SCCS) in Zabrze, Poland.


   2. Materials and methods
Top
 Abstract
 1. Introduction
 2. Materials and methods
3. Statistics
 4. Results
 5. Discussion
 Acknowledgements
 References
 
The study was performed in a tertiary care, university hospital. In our electronic database we identified patients fulfilling inclusion and exclusion criteria identical to those applied in an original paper creating a derivation cohort [13]. Included were consecutive patients who underwent cardiac surgery with the use of cardiopulmonary bypass between June 2005 and November 2006. Exclusion criteria included severe preoperative renal dysfunction with the need for preoperative dialysis or creatinine concentration >3.4 mg/dl (300 µmol/l), infrequent surgical procedures such as operations with deep hypothermic circulatory arrest, heart or lung transplantations and/or ventricular assist device insertions.

Using our database we identified 1421 patients eligible for analysis. Accuracy of the database exceeded 95% when compared with 100 randomly selected medical records. Serum creatinine concentration was measured within 30 days to one day before surgery. If multiple measurements were present, the value closest to the date of surgery was used. Glomerular filtration rate (GFR) was calculated in all patients on the basis of the Cockcroft-Gault equation [15]. If multiple measurements of serum creatinine were present, the value closest to the date of surgery was used. Ejection fraction was measured within 30 days before surgery and closest result to this date was taken. All other preoperative variables were measured one day before surgery. Patients were stratified as having satisfactory renal function (GFR >60 ml/min/1.73 m2), mildly impaired (GFR between 31 and 60 ml/min/1.73 m2), and severely reduced renal function (GFR ≤30 ml/min/1.73 m2).

Original algorithm used categories of urgent and emergent indications for surgery as important predictors of RRT risk, however, strict definition applied was not cited. In our cohort we used standard ACC/AHA definitions to assign patients to the proper category. Urgent category indicated that patients were required to stay in the hospital but could be planned and operated on within a normal schedule. Emergent operations were those with ongoing refractory cardiac compromise, unresponsive to other forms of therapy except for cardiac surgery [16]. All patients who did not fulfil these definitions were considered to be elective. The list of demographic, preoperative and intraoperative variables and their comparison to the derivation cohort from reference paper is shown in Table 1.


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  		Table 1 Demographic and clinical characteristics of derivation (Canadian) and validation (SCHD) cohorts

 
As described in a referent paper [13], simplified renal index was constructed by summation of points (weights) assigned to eight clinical and laboratory variables (Table 2). The increasing values of simplified renal index reflect increasing risk for the development of postoperative RRT.


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  		Table 2 Components of simplified renal index [13]*

 
After collection of all the preoperative variables, simplified renal index value was calculated for each patient. The primary postoperative outcome was the need for postoperative RRT. In every case the decision to start RRT was undertaken by the cardiac anaesthetist and/or cardiac surgeon directly taking care of the patient. The indications for RRT were standard and included metabolic abnormalities (hyperkalemia or acidosis), prolonged anuria and/or fluid overload. No particular threshold was applied for any abnormality, rather they were interpreted in overall clinical context.


   3. Statistics
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Statistics
4. Results
 5. Discussion
 Acknowledgements
 References
 
Statistical analysis included comparison of the demographic data, operative data and the comorbidities between patients with or without RRT within our cohort. Discrimination of the simplified renal score was estimated assessing the area under the receiver operator characteristic (ROC) curve. The area under the ROC curve was calculated for our cohort and then compared to the values obtained in the reference paper.

Depending on the distribution, numerical data are shown by either mean and standard deviation or median values and their range and then compared with the use of Mann–Whitney test. Binary data are shown as the number and percentage and compared with the use of {chi}2-test. P<0.05 was considered significant.


   4. Results
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Statistics
 4. Results
5. Discussion
 Acknowledgements
 References
 
Despite identical inclusion/exclusion criteria our cohort differed markedly from the Canadian derivation group. Our patients were younger, with lower body weight and a higher percentage of female patients. Their kidney function as reflected by glomerular filtration rate was better although creatinine concentration was similar to the Canadian cohort. Impaired kidney function was less prevalent in our group, but such conditions as arterial hypertension and poor heart contractility were seen more frequently. Complex surgical procedures and elective surgery were more prevalent in our cohort (Table 1).

The mean value of simplified renal index was lower in our cohort than in the Canadian derivation group, so the percentage of patients falling into higher simplified renal index categories was also lower in our patients (Table 3).


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  		Table 3 Comparison of the preoperative SRI distribution, mean values of simplified renal index and RRT need in derivation and validation cohorts

 
RRT was commenced in 33 (2.3%) of our patients and these patients met the criteria of a primary outcome. This number was significantly higher in comparison to the Canadian derivation cohort (Table 3).

The clinical, laboratory and surgical characteristics of patients who needed RRT in our and the derivation group also differed markedly. Our patients had better preoperative kidney reserve, but more compromised heart function. Chronic obstructive pulmonary disease was much more prevalent in our group and these patients were more frequently subjected to complex surgical procedures. In contrast to the Canadian RRT patients, elective surgery was predominant. Twenty-five patients (76%) from our RRT group died during hospital treatment, which contrasted markedly with the mortality in the derivation group (76% vs. 47%, P=0.003) (Table 4).


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  		Table 4 Clinical, laboratory and surgical characteristics and in-hospital mortality of RRT patients – Canadian derivation cohort and SCHD group

 
The simplified renal index predicted risk of postoperative RRT in our group. Patients with low values of simplified renal index (0–1), medium (2–3) and high values (4 and more) were found to have increasingly higher risk for RRT of 1.1% (95% CI: 0.5–2.1%), 3.2% (95% CI: 1.9–5%) and 12.5% (95% CI: 5.2–24.1%), respectively (Fig. 1).


Figure 1
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  		Fig. 1. Proportions of patients requiring RRT in relation to SRI values of 0–1 points, 2–3 points and 4–5 points in the SCHD cohort. Circles indicate percentages of patients requiring RRT. Error bars are 95% confidence intervals. There were no patients with the SRI higher than 5 in the entire SCHD cohort. Horizontal axis shows number and percentage of patients with a particular category of SRI.

 
The simplified renal index also retained reasonably good discriminatory power in a statistical analysis. The area under the ROC curve was 0.73 (95% CI: 0.62–0.81) and this value was not different from the value described for the Canadian derivation cohort (P=0.05, Table 5).


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  		Table 5 Area under receiver operating characteristics curve – comparison of the SCHD cohort (Zabrze, Poland) and the derivation and validation cohorts 13

 

   5. Discussion
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Statistics
 4. Results
 5. Discussion
Acknowledgements
 References
 
Postoperative acute kidney impairment remains an important complication of cardiac surgery. It is associated with high mortality and morbidity, particularly in patients requiring RRT [5, 10]. The need for RRT constitutes the strongest risk factor for death with an odds ratio of 7.9 (95% CI: 6–10) in cardiac surgical patients [8]. The incidence of patients requiring RRT may vary depending on many factors, such as study population, surgical procedure as well as pre- and postoperative care. Identification of these patients preoperatively would result in more optimal resource allocation and potentially improve outcome. In high-risk patients undergoing cardiac surgery some studies have shown benefits from either non-pharmacological and pharmacological prophylaxis [17] or application of less kidney-jeopardizing surgical procedures [18, 19]. Early identification of a patient prone to acute kidney impairment development is a crucial task for the anaesthesiologist, cardiac surgeon and the team taking care of the patient postoperatively.

The clinical tool described recently [13] is simple, aggregating various preoperative characteristics of patients but was validated only in the USA and Canada. Cross-validation of this risk-stratification algorithm is mandatory before it can be accepted in clinical practice in Europe, as the characteristics of patients in this region differ markedly from those in the USA [20]. This validation is important because in European patients validation has been undertaken only for one, elaborate therefore less convenient, clinical algorithm [14].

The primary finding of our study is the confirmation of a reasonable discriminating performance of the new clinical algorithm in a population from Central Europe. The discriminating power of the algorithm as demonstrated by area under ROC curve was lower, but this difference was not significant. We would be surprised, however, to have very similar area under the ROC curve, taking into account our smaller sample size and a completely different population.

Simplified renal index performed well in our group despite substantial differences between derivation and our cohort. Our patients were younger, lighter and with a larger proportion of females. Use of the Cockroft-Gault formula for glomerular filtration rate estimation in such a group may overestimate true filtration, because body mass in women contains less muscle, resulting in smaller creatinine production, hence lower serum creatinine for given glomerular filtration rate [21].

Clinical syndrome of heart failure was not recorded either in derivation or in our study. However, low heart contractility, as reflected by reduced ejection fraction, which may be taken as crude surrogate of heart failure, was more prevalent in our cohort. The Cockroft-Gault formula used by Canadians and by our validation study was recently shown to deviate significantly from true filtration, particularly in patients with heart failure [22]. Higher percentage of heart failure patients in our group might cause relative overestimation of glomerular filtration rate in this cohort and produce artificial difference between the derivation and validation group.

In all risk stratification methods published so far, worse preoperative kidney function was more tightly linked to RRT than poor heart function [10–12]. This is also true for the new clinical algorithm [13]. As a result, simplified renal index value is driven more by reduced kidney reserve than by poor heart function [13]. Our patients had better estimated kidney reserve but were more likely to have compromised heart contractility. This might be reflected by lower simplified renal index values observed in our patients.

Despite lower mean simplified renal index value in our entire population (1.4 vs. 1.5, P=0.001), frequency of RRT in our cohort was significantly higher than in the Canadian derivation group (2.3% vs. 1.3%, P=0.002). Importantly, significantly higher also was in-hospital mortality (76% vs. 47%, P=0.003). This finding may point out the role of poor heart function for RRT development and particularly for risk of mortality in patients with likely overestimated glomerular filtration rate.

The association of heart failure or reduced perfusion with the risk of RRT need and mortality was demonstrated in many studies. In patients with GFR >60 ml/min·1.73 m2, presence of heart failure was shown as the most powerful predictor of the development of severe acute kidney impairment development in the postoperative period as defined by drop of glomerular filtration rate to the values below 30 ml/min·1.73 m2 [23]. In another study from Brazil, patients with heart failure NYHA class >2 before surgery were more likely to rise their plasma creatinine above 2.0 mg/dl or 50% above baseline, than those having preoperative creatinine >1.2 mg/dl [24].

Our patients suffered more frequently from various comorbidities. Among them, most important were arterial hypertension, cerebrovascular and peripheral atherosclerotic disease and chronic obstructive pulmonary disease. History of hypertension did not add risk in the Canadian algorithm, but in all other clinical algorithms hypertension was shown to be associated with worse kidney outcome [10–12] and higher mortality. Higher prevalence of hypertension in our group would account for more frequent need for RRT and raised mortality.

Urgent scheduling for surgery is an important risk factor of RRT. In our study we adhere to AHA/ACC definition of these clinical situations [16], in the Canadian study definition of urgent scheduling for surgery was not provided. It is worth noticing that in the Canadian cohort urgent qualification for surgery was more prevalent not only to our group but also in comparison to international registries [25]. This suggests that urgent scheduling might be more liberal in Canada, which could artificially increase value of simplified renal index. As a consequence, higher simplified renal index might be given to patients with lower risk resulting in lower rate of RRT and lower mortality.

Patients who developed acute kidney impairment and were subjected to RRT differed markedly between our and the Canadian group. Canadians had significantly higher simplified renal index but survived better. This finding might again point to an important role of co-morbidities such as chronic obstructive pulmonary disease and poor heart contractility as risk factors not only for RRT, but also for mortality in patients undergoing RRT.

There are only few risk stratification algorithms developed so far, but unfortunately all of them have been derived and validated in the USA or Canada [10–13]. The only exception validating clinical algorithm developed by Chertow et al. [10], comes from Norway and shows reasonably good performance with an area under the ROC curve of 0.71 for RRT [14]. In our study we have validated most recent, up until now not tested, less elaborate, though more practical algorithm developed in Canada. We feel that this particular algorhitm is worth considering as a global model for the prediction of the RRT among cardiac surgical patents due to its simplicity. In our opinion, we should invest in one model, that one which is: simple, developed on a basis of a large derivation cohort and performing reasonably well in various populations. We feel that such model already exists and it needs to be supported by testing it in the different parts of world with the aim to improve it further.

After receiving the results from at least a few different populations, databases from different studies should be probably combined into one large database and a statistician should be involved with the aim to improve the existing algorithm. We declare, that our rough, baseline data are available for that purpose.

We have shown reasonable discrimination power of this new risk stratification engine with area under ROC curve of 0.73, which is very close to data produced in Norway regarding an older stratification algorithm. Our cross-validation study documents its satisfactory performance in a Polish population, and we conclude that it could be used safely in the populations with similar clinical characteristics.


   Acknowledgements
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Statistics
 4. Results
 5. Discussion
 Acknowledgements
References
 
We wish to thank Mrs Jolanta Ciesla and Mr Daniel Ciesla for their help in preparing the manuscript.


   References
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Statistics
 4. Results
 5. Discussion
 Acknowledgements
 References
 

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