Outcome in a post-cardiac surgery population with acute renal failure requiring dialysis: does age make a difference?

Nele Van Den Noortgate1,, Veerle Mouton1, Caroline Lamot1, Guido Van Nooten2, Annemieke Dhondt3, Raymond Vanholder3, Marcel Afschrift1 and Norbert Lameire3

1 Division of Geriatric Medicine, Department of Internal Medicine, 2 Division of Cardiovascular Surgery, Department of Surgery and 3 Renal Division, Department of Internal Medicine, University Hospital Gent, Gent, Belgium



   Abstract
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Background. Acute renal failure (ARF), requiring dialysis (ARF-d), develops in 1–5% of patients undergoing cardiac surgery and is associated with higher in-hospital mortality. Age is one of the known risk factors for the development of ARF. As the ageing population is increasing, the nephrologist will be faced with a large population of elderly patients requiring dialysis following cardiac surgery. The aim of our study was to evaluate the influence of age on and the risk factors for in-hospital mortality.

Methods. Eighty-two patients with ARF following cardiac surgery and requiring dialysis between January 1997 and October 2001 were included. Two groups of patients were studied: the younger population (<70 years, 42 patients, mean age 59±10) and an elderly population (>=70 years, 40 patients, mean age 76±4). Severity of disease was evaluated using the SAPS (Simplified Acute Physiology Score), the Liano score and the SHARF (Stuivenberg Hospital Acute Renal Failure) score.

Results. Overall mortality in the population with ARF-d was 56.1%. No difference in mortality rate was found between the younger (61.9%) and elderly patient group (50.0%). The two groups were very similar in baseline and procedural characteristics with exception of body weight (P=0.02) and preoperative glomerular filtration rate (P=0.0001). No significant difference was found in the scoring systems between the old and the young (SAPS P=0.52; Liano P=0.96; SHARF T0 P=0.06; SHARF T48 P=0.15). Mortality in the elderly was significantly correlated with hypotension before starting renal replacement therapy (RRT) (P=0.002), mechanical ventilation (P=0.002), presence of multiorgan failure (MOF) (P=0.0001) and higher scores in the severity models (SAPS: P=0.01; Liano: P<0.0001 and SHARF: P<0.0001).

Conclusion. The outcome in the elderly requiring dialysis due to ARF post-cardiac surgery is comparable with the outcome in a younger population. No significant difference was found in severity of disease between the elderly and the younger. Variables predicting mortality in the elderly are the presence of MOF, mechanical ventilation and hypotension 24 h before starting RRT. These findings indicate that at the time the nephrologist is called for an elderly patient requiring dialysis due to ARF following cardiac surgery, age per se is not a reason to withhold RRT.

Keywords: acute renal failure; cardiac surgery; disease severity; elderly; haemodialysis; outcome



   Introduction
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
The elderly are the fastest growing age group of the general population. In Western Europe and the US, the number of subjects >60 years is projected to rise from 231 million in 2000 to 395 million in 2050. Because cardiovascular disease is the leading cause of morbidity and mortality in the elderly, the ageing of the population has led to an increasing number of patients with symptomatic coronary heart and valve disease in whom cardiac surgery is needed. A serious complication of cardiac surgery is acute renal failure (ARF). ARF requiring dialysis (ARF-d) occurs in 1–5% of patients following cardiac surgery and is associated with high morbidity and mortality [17]. The influence of preoperative and intraoperative factors on the development of ARF-d is well known from previous studies [211]. One of the risk factors for developing ARF following cardiac surgery is age [3,6,911]. Because of this evolution, the nephrologist will, in the near future, be faced more frequently with the problem of dialysis requiring ARF following cardiac surgery in the elderly. The policy in our hospital until now is to start renal replacement therapy (RRT) in every patient following cardiac surgery developing ARF with a need for RRT.

The aim of this study was to evaluate the question of whether the age of the patient should influence this policy. Therefore, we compared the outcome in the elderly with those in younger patients requiring dialysis following cardiac surgery. Concomitantly, we tried to evaluate whether the risk factors for in-hospital mortality and the underlying severity of disease were comparable in both age groups.



   Subjects and methods
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
This study is a retrospective chart review of patients developing post-cardiac surgery ARF, requiring dialysis, at the University Hospital Gent. Between January 1997 and October 2001, 3736 adult patients (20 years or older) of which 1497 patients 70 years or older (40%), underwent cardiac surgery. Of these populations 85 patients or 2.3% developed ARF, requiring dialysis. Three patients with ARF but in whom RRT had already been started before surgery were excluded. The mean age of the 82 patients was 67.5±11.5 years. For this analysis, the population was divided into two groups according to age: the younger (<70 years, 42 patients, mean age 59±10 years) and the elderly (>=70 years, 40 patients, mean age 76±4 years). In-hospital mortality was recorded and based on known risk factors [111], the following baseline characteristics were evaluated: age, gender, body weight, height, medical history, medication at time of surgery, preoperative serum creatinine and glomerular filtration rate estimated by Cockcroft–Gault [12], type of cardiac surgery, preoperative use of intra-aortic balloon pump (IABP), mechanical ventilation, need of inotropics and nitroglycerin. In the medical history of the patients, previously known renal impairment, the presence of diabetes (defined as treatment with oral antidiabetics or insulin), peripheral vascular disease (defined as previous vascular procedure, a history of claudication or presence of femoral bruits), previous cerebrovascular insult, arterial hypertension (defined as taking antihypertensive drugs or systolic or diastolic blood pressure >140/90 mmHg at the moment of hospitalization), chronic obstructive pulmonary disease, recent (<2 weeks) or previous myocardial infarction and dyslipidaemia were noted. The following perioperative and postoperative variables were studied: the use of and duration of extra-corporeal circulation and cross clamping, the number of distal anastomoses, the insertion of IABP, the presence of intraoperative hypotension (defined as a systolic blood pressure <90 mmHg) and the use of inotropic drugs. Characteristics of RRT such as the type of RRT and the onset and duration of RRT were recorded. Also, the presence of hypotension, mechanical ventilation, cardiac arrest, oliguria, defined as a diuresis <400 cc over 24 h, and serum creatinine at onset of RRT were noted.

To evaluate the severity of illness associated with ARF, scores to predict mortality in Intensive Care Unit (ICU) patients were retrospectively calculated using the worst values obtained 24 h before starting RRT. The first method, of use in ICU patients, is the Simplified Acute Physiology Score II (SAPS II) [13]. Le Gall et al. [13] preferred this method to the APACHE II (Acute Physiology and Health Evaluation) method to avoid a systematic bias. Indeed missing values may induce a bias if they are reported as normal values in the APACHE II method. Secondly, more specific models to predict mortality in patients with ARF such as the Liano score [14] and the SHARF (Stuivenberg Hospital Acute Renal Failure) score [15] were applied as well. In a retrospective study comparing various scoring methods, Douma et al. [16] found that the Liano model was the most accurate predictor of a fatal outcome. The SHARF score was recently developed as a bedside tool and compares favourably with other published scores [15]. The SHARF score was evaluated 24 h before and 48 h after starting RRT. Because age is included as predictive factor in all these scores, the scores were also calculated without age.

First, a search was performed for differences in baseline characteristics, peri- and postoperative variables, scores of predictive models and dialysis characteristics between the younger and the elderly. They are presented as means with standard deviations for continuous variables and as percentage for dichotomous variables. All statistical comparisons are made across age category (younger and elderly) using the Student's t-test, the Fisher's exact test or {chi}2 test where appropriate. Significance was defined as P<0.05.

Secondly, the presence of a difference between the models including age or not was evaluated. Receiver operating curve analysis and area under the curve (AUC) were used to compare the accuracy in predicting mortality, comparing the same score calculated with and without age. If the AUC is significantly different, a difference in the models must be present.

Thirdly, the mortality and the variables correlated with mortality were evaluated in the whole patient group and separately in the elderly group. Univariate analysis was performed using the Student's t-test, the Fisher's exact test or {chi}2 test where appropriate.

Statistical analysis was performed using the Medcalc package (Medcalc, Mariakerke, Belgium) and SPSS version 10.



   Results
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Baseline and procedural characteristics
The patient characteristics and clinical data (pre-, peri- and postoperative and 24 h before starting RRT) are shown in Table 1Go. The number of isolated coronary artery bypass grafting (CABG) procedures was comparable with the younger (69%) and the elderly (45%), although there was a significantly higher number of distal anastomoses in the elderly (3.4±0.9) than in the younger patients (2.8±0.9) (P=0.02). Other surgical procedures such as valve replacement and combined surgical interventions were also comparable between the younger and the elderly. Elective surgery was performed in 45% in the young and in 40% in the elderly (P=0.81). No significant difference was found between the young and the elderly in medical history and in the preoperative use of cardiac medication. The cause of ARF was similar in both age groups. The most frequent reason of ARF was acute tubular necrosis (ATN) due to low cardiac output (53.7% in the younger and 58.3% in the elderly; P=0.85) or septic shock (24% in the younger and 8% in the elderly; P=0.12). Other reasons of ARF were underlying chronic renal insufficiency due to nephroangiosclerosis and diabetic nephropathy, contrast-induced nephropathy and cholesterol embolization. No differences in applied type of RRT between the different age groups were found. In the younger age group 54.8% underwent CVVH (continuous venovenous haemofiltration), 23.8% underwent CVVHD (continuous venovenous haemodialysis) and 21.4% haemodialysis. A similar distribution was found in the older age group (65, 10 and 25%, respectively). The most frequent indications for RRT were oliguria (65%), acidosis (8%), hyperkalaemia (12%) and rhabdomyolysis (5%) in both age groups. The moment of initiating RRT was the same in the elderly (day 2.42±1.10) as in the young patients (day 2.73±2.57) (P=0.24). RRT was performed during 5.07±4.36 days in the elderly and 5.69±1.16 days in the young (P=0.61). Five patients younger than 70 years and none of those older than 70 years, remained dialysis dependent after cardiac surgery.


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Table 1.  Patients demographic and clinical data

 
There was no statistically significant difference between the age groups in the values of the different predictive models. However, comparing the scores between the non-survivors in both age groups, a significantly lower score in the elderly was found for the SAPS II. In contrast, a significantly higher score in the elderly was found for the SHARF (Table 2Go).


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Table 2.  Values of the different scoring systems used to evaluate severity of disease

 
To evaluate whether some scoring systems without age are more useful to predict mortality in our study population, the AUC for the different methods with and without age were compared (Table 3Go). Only for the SHARF T0 score was a significant difference in favour of the models without age obtained (P=0.04). Therefore, we decided to use the scoring systems as validated with age.


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Table 3.  Evaluating the difference in the methods used to predict mortality calculated with or without age using the AUC (with 95% confidence interval)

 
The overall in-hospital mortality (n=3736) was 3.7%. Mortality rate was significantly higher in the elderly patients (5.1%) than in the young (2.7%) (P=0.004). A mortality rate of 56% was found in the population with ARF-d. The outcome was comparable in both age group with a mortality rate of 61.9% in the younger and 50% in the elderly. The surgical procedure was not correlated with significant differences in mortality rate between both age groups (mortality rate for CABG and valve replacement of 41.4 and 75% in the younger versus 55.6 and 50% in the elderly). Compared with urgent and emergency surgery, elective surgery was correlated with lower mortality (P=0.02). No significant difference in mortality was found between the younger (45%) or older (40%) elective surgical patients. Patients undergoing CVVH had a significantly higher mortality (65.3%) than patients treated with intermittent haemodialysis (31.6%) (P=0.04). This difference was not present in the elderly population (mortality 57.7 and 40% for CVVH and haemodialysis, respectively). No difference between survivors and non-survivors in both age groups was found in the day of appearance of ARF and the peak serum creatinine (2.73±2.57 day and 3.76±1.58 mg/dl in the young versus 2.42±1.10 day and 3.74±1.05 mg/dl in the elderly) at onset of RRT or duration of RRT (5.69±1.16 day in the young versus 5.07±4.36 day in the elderly).

In univariate analysis, the variables significantly correlating with in-hospital mortality are shown in Table 4Go. In contrast with the overall population, only hypotension before RRT, the need for mechanical ventilation, the presence of multiorgan failure (MOF) and the score of the predictive models were significantly correlated with mortality in the elderly. The studied baseline preoperative and postoperative variables not correlating with in-hospital mortality are not mentioned in the table.


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Table 4.  Predictors of in-hospital mortality

 



   Discussion
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Our study found that age does not influence the outcome of the dialysed ARF patient post-cardiac surgery, suggesting that age may not be a reason for the nephrologist to withhold the elderly patient from necessary RRT in post-cardiac surgery ARF.

First, to avoid bias by changes in surgical techniques or postoperative care, this study was limited to a maximal observation period of 5 years. This contributed to maintain the homogenous care pattern; as a drawback, this limited the number of patients enrolled in this study.

Secondly, a major objection to this conclusion can be the retrospective character of this study, inducing a possible bias. As the policy in our department is to dialyse every patient requiring RRT after cardiac surgery, the selection bias cannot occur at the moment of taking the patient in RRT. However, a selection bias made by the surgeon or cardiologist before the surgical intervention cannot be excluded. As figures regarding overall mortality (3.7%) and the development of ARF in the cardiac surgery department (2.2%) are comparable with those observed in the literature [17,17,18], our population can be considered as a representative sample for the actual policies in most Western cardiac surgery departments. Therefore, we think that our results can be useful for every nephrologist consulting an elderly patient with ARF in cardiac surgery departments.

It might be surprising that there are no differences in outcome between the elderly and the younger patients. The relatively beneficial outcome in the elderly might, however, be the consequence of a difference in severity of underlying disease. The ageing kidney by itself, due to structural and functional alterations, is less able to cope with rapid haemodynamic changes and electrolyte balance [19]. Hence, it can be accepted that ARF develops more easily in the older patients whereas comorbid factors that are at the origin of the ARF, might be less preponderant and/or severe than in the younger population. After all, comorbid conditions probably more frequently lead to a fatal outcome than renal failure per se, for which RRT can be offered.

To evaluate our hypothesis, we scored severity of illness at the development of ARF by three different models namely the SAPS II, the Liano and the SHARF score. We expected the score to be higher in the younger population. This was however only the case for the SAPS II score (with or without age as correcting factor) and not for the Liano score or the SHARF score. One way to explain this difference is the fact that in these scoring systems, the same factors are not always weighted. For example, in the SAPS II, a high value is attributed to a low Glasgow coma score. The factor of consciousness is less important in the Liano score and completely absent in the SHARF score. In the younger population, a higher percentage of the population developed ARF after cardiac arrest than in the elderly population. Because of the cardiac arrest, lower Glasgow coma scores could be expected and hence younger people had significantly higher SAPS II scores. So far it was not possible in our study to prove conclusively that the younger and older group are equivalent. Further prospective studies on larger numbers of patients will be necessary to evaluate whether there is a difference in severity of illness between younger and older patients developing post-cardiac surgery ARF-d.

In whatever the underlying condition, the comparable mortality rate between the younger and the elderly population suggests that age is not a reason to withhold the elderly cardiac surgical patient with ARF from RRT. The absence of a relationship between age and poor outcome in the elderly developing ARF in the ICU was already confirmed by us [19] and by other authors [20].

The most predictive factors for in-hospital mortality in our study population were non-elective surgery, duration of extra-corporeal circulation during surgery, and the presence of mechanical ventilation, hypotension before RRT, IABP, MOF and cardiac arrest. Of these variables, only the presence of MOF, mechanical ventilation and hypotension pre-RRT were predictive in the elderly patients. Only a few other studies considered independent predictors of mortality in ARF-d. Chertow et al. [10] mentioned low cardiac output requiring inotropic support, prolonged mechanical ventilation, cardiac arrest and stroke or coma. Liopart found only postoperative events (sepsis) to be related with higher mortality in ARF-d [8]. Osterman et al. [4] reported a poor outcome in patients with ARF and cardiovascular failure and in patients with three or more failing organ systems before haemofiltration.

In conclusion, the outcome in the elderly requiring dialysis due to post-cardiac surgery ARF is comparable with that in a younger population. No significant difference was found in severity of disease between the elderly and the younger. Variables predicting mortality in the elderly are the presence of MOF, mechanical ventilation and hypotension 24 h before starting RRT. These findings indicate that at the time the nephrologist is called for an elderly patient requiring dialysis due to ARF folowing cardiac surgery, age per se is not a reason to withhold RRT. We therefore believe that once the cardiologist and cardiac surgeon have decided to perform surgery in a given patient, the nephrologist should not interfere with this decision, once post-surgery ARF develops.

Conflict of interest statement. None declared.



   Notes
 
Correspondence and offprint requests to: Nele Van Den Noortgate, Division of Geriatric Medicine, University Hospital Gent, De Pintelaan 185, B-9000 Gent, Belgium. Email: nele.vandennoortgate{at}rug.ac.be Back



   References
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 

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Received for publication: 4. 7.02
Accepted in revised form: 5.12.02





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