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Ann Thorac Surg 2005;79:1297-1302
© 2005 The Society of Thoracic Surgeons

Original articles: Cardiovascular

Multiple Risk Assessment of Cardiovascular Surgery in Chronic Renal Failure Patients

Bartlomiej Witczak, MDa,*, Anders Hartmann, MD, PhDa, Jan L. Svennevig, MD, PhDb

a Department of Medicine (Section of Nephrology), Rikshospitalet University Hospital, University of Oslo, Oslo, Norway
b Department of Thoracic Surgery, Rikshospitalet University Hospital, University of Oslo, Oslo, Norway

Accepted for publication September 3, 2004.

* Address reprint requests to Dr Witczak, Department of Medicine, Section of Nephrology, Rikshospitalet University Hospital, 0027 Oslo, Norway (E-mail: bartlomiej.witczak{at}rikshospitalet.no).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 References
 
BACKGROUND: Chronic renal failure is a major risk factor in cardiovascular surgery. We evaluated results of cardiovascular surgery in chronic renal failure patients (s-creatinine > 200 µmol/L or established dialysis) at our center from 1990 to 2000.

METHODS: One hundred and six chronic renal failure patients underwent cardiovascular surgery (56 coronary artery bypass operations, 25 valve replacements with or without coronary bypass, and 25 other major cardiovascular operations [8 thoracic aorta, 10 abdominal aorta, 7 other]). Matched controls were selected (n = 106) based on age, sex, year, and type of operation and occurrence of diabetes.

RESULTS: There were 88 men and 18 women and mean age was 64 ± 10 years (standard deviation). Demographics did not differ between chronic renal failure and control patients, except for hypertension (more prevalent in chronic renal failure group, p < 0.05). Intraoperative hemorrhage, perfusion and ischemia time, and reoperation did not differ between groups. Chronic renal failure patients received more transfusions of red blood cells, plasma, and platelets (p < 0.02). Ventilation support (27.6 ± 59.3 hours), intensive care unit stay (7.7 ± 8.3 days), and hospital stay (12.3 ± 10.5 days) were longer (p < 0.02). Early mortality was 16% versus 6.6% (p = 0.04) and 5-year mortality was 79% versus 39% (p < 0.05) for chronic renal failure and control patients, respectively. Independent preoperative risk factors of mortality for chronic renal patients were age greater than 70 years (relative risk = 2.32, p = 0.001), chronic obstructive pulmonary disease (relative risk = 2.59, p = 0.001), diabetes (relative risk = 1.80, p = 0.037), and dialysis (relative risk = 2.03, p = 0.005).

CONCLUSIONS: Chronic renal failure patients suffered more postoperative complications and had substantially increased short-term and long-term mortality rates. Independent preoperative mortality risk factors for chronic renal failure patients were age, chronic obstructive pulmonary disease, diabetes, and chronic dialysis.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 References
 
Since the 1970s, a series of studies have reported on early and late outcomes of cardiopulmonary bypass operations (coronary artery bypass graft [CABG] and/or valve operations) in chronic renal failure (CRF) patients [1–26]. However, most of the studies reported on less than 50 patients and only a few comprise a predialysis population [1–4]. More detailed analyses of intraoperative and postoperative complications are only performed in a small number of studies [5–10]. We have found no previous studies that have included a matched control group in their analyses. The purpose of this study was (1) to identify the intraoperative and postoperative complications of major cardiovascular (CV) surgery in CRF patients as compared with nonrenal failure matched controls and (2) to investigate short-term and long-term survival and assess independent risk factors of mortality.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 References
 
During the inclusion period, 1990 through 2000, greater than 17,500 CV operations were performed at the Department of Thoracic Surgery at our hospital. Among these, 106 were CRF patients, defined as s-creatinine greater than 200 µmol/L or established dialysis treatment (56 CABG, 25 valve replacements with or without CABG, 25 other CV operations, including combined procedures [8 thoracic aorta grafts, 10 abdominal aorta grafts, and 7 other major CV procedures]). Chronic renal failure patients were selected by searching the database containing all procedures performed at the Department of Thoracic Surgery since 1987. Appropriate control data, matched for age, sex, diabetes, and type and year of surgery were randomly selected from the same database. These factors were selected as they are possible important risk factors for mortality and were readily available from our database. The inclusion of the year of operation was meant to eliminate any differences in operation results over the ten-year period. It was, however, not possible to match the groups perfectly. There are three diabetes patients less among CRF patients than controls (25 vs 28, respectively). Also, whereas there were 25 valve replacements (19 aorta and 6 mitral) in the CRF group, there were 24 in the control group (21 aorta and 3 mitral). Consequently, there was one more patient in the "other CV operation" group among controls. Incidence of CRF in our material was approximately one third of what has been reported from a similar database by the New England Cardiovascular Disease Study Group [10]. This is likely due to the very low prevalence of CRF patients in the Norwegian population due both to a low incidence rate of uremia and a high renal transplantation rate [27]. Closing date of follow-up was December 31, 2002, resulting in an average of 3.82 ± 3.25 (0–12) years follow-up. Quality of data was ensured by carefully reviewing the regular paper records for all patients. In addition, information on death was obtained from the Norwegian Renal Registry and Public Census.

Early mortality is defined as any death within 30 days of surgery or during the hospital stay. All long-term survival rates are presented as Kaplan-Meier survival estimates. The analyses comparing continuous variables between groups are based on independent two sample t tests. The relative risk of mortality for the various preoperative, intraoperative, and postoperative risk factors are based on Cox proportional hazards analyses (univariate and multivariate step forward regression). All statistical analyses were performed using SPSS for Windows (SPSS Inc, Chicago, IL) or BMDP (BMDP Statistical Software Inc, Los Angeles, CA).


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 References
 
The patient population consisted of 83% men. Average age was 64 ± 10 years. Demographics did not differ, except for hypertension which was more prevalent among CRF patients (p < 0.05), see Table 1. Intraoperative data and complications are shown in Table 2. There were no differences between the groups. Postoperative complications are shown in Table 3. Chronic renal failure patients had more postoperative complications than controls, except for full blood transfusions and reoperation for hemorrhage.


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  		Table 1. Demographics: Comparison Between 106 Chronic Renal Failure Patients and 106 Control Patients Undergoing Major Cardiovascular Operations
 

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  		Table 2. Intraoperative Data and Complications
 

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  		Table 3. Postoperative Data
 
Both early and late mortality rates were higher for CRF patients compared with controls. One, three, and five-year survival rates were 69.9 ± 4.5, 45.2 ± 5.0, 39.3 ± 5.1% for CRF patients and 89.6 ± 3.0, 85.7 ± 3.4, 78.7 ± 4.4% for controls (Fig 1).



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  		Fig 1. Kaplan-Meier survival curves after cardiovascular surgery (CV)—chronic renal failure (CRF) versus controls (log-rank test, p < 0.001). Relative risk of mortality in CRF patients over the observation period was 4.25 (p < 0.001).

 
All preoperative risk factors of mortality included in the analyses are listed in Table 4. All mortality risk factors from the univariate analyses were included in the step forward multivariate Cox proportional hazards model. The univariate model revealed age (relative risk [RR] = 1.97, p = 0.005), chronic obstructive pulmonary disease (RR = 2.35, p = 0.002), diabetes (RR = 1.59, p = 0.083), dialysis (RR = 1.83, p = 0.012), and previous cerebrovascular event (RR = 1.66, p = 0.089) to be significant or borderline significant risk factors. In control patients age (RR = 3.10, p = 0.006), chronic obstructive pulmonary disease (RR = 3.61, p = 0.003), and diabetes (RR = 0.224, p = 0.043) turned out to be significant determinants of survival. The results of the multivariate Cox proportional hazards analysis for independent preoperative risk factors are shown in Table 5 and Figure 2. The Kaplan-Meier survival curves for the dialysis and predialysis populations are shown in Figure 3.


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  		Table 4. Preoperative Risk Factors of Mortality Included in Statistical Analyses
 

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  		Table 5. Multivariate Cox Proportional Hazards Analysis of Preoperative Risk Factors of Mortalitya
 


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  		Fig 2. Independent preoperative risk factors of mortality in chronic renal failure patients.

 


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  		Fig 3. Kaplan-Meier survival curves after cardiovascular surgery (CV)—predialysis versus dialysis (log-rank test, p = 0.011). Relative risk of mortality in dialysis patients over the observation period was 1.83 (p = 0.012).

 
Among the operative risk factors of mortality (Table 6) investigated in both CRF and control patients, only CABG operation among the CRF group turned out significant. Relative risk of mortality was 0.54 (p = 0.011) compared to the other, more complicated, CV procedures.


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  		Table 6. Operative Risk Factors of Mortality Included in Statistical Analyses
 
Among postoperative risk factors, organ failure (heart, lung, kidney [only applicable to controls], liver) and isolated respiratory failure were found to be of significance in the univariate analyses. The relative risk of death for 27 patients with organ failure was 1.60 (p = 0.076) and 1.97 (p = 0.018) for 19 patients with respiratory failure. In control patients organ failure carried an RR of 3.36 (p = 0.005, n = 16) and respiratory failure an RR of 4.40 (p = 0.017 and n = 4). The results of multivariate Cox proportional hazards analysis are presented in Table 7.


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  		Table 7. Multivariate Cox Proportional Hazards Analysis of Postoperative Risk Factors of Mortality
 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
References
 
We could not demonstrate that intraoperative data and complications are different in CRF patients compared with matched controls. Although the view that CRF patients need more transfusions has been put forward [11], clinical data are scarce. We demonstrate that CRF patients have an increased need of blood products during their hospital stay. We found that transfusions of both red cells and plasma were twice as high as for controls. The need for platelets was almost five times as high. Postoperatively, the need for ventilation support, intensive care unit stay and hospital stay were increased in CRF patients and this is in accordance with some other studies [5, 6, 8].

Early mortality in CRF patients in our material was 16%, which is in the upper range of previously reported figures (0% to 23%) [9, 4]. In a study with a similar design to ours, that included dialysis patients undergoing CABG surgery, in-hospital mortality was 12% [10]. Also, Horst and colleagues [5] reported a very similar rate of 12.5% as an average for 20 studies investigating results of cardiac operations among the end-stage renal disease population. The higher mortality rate in our study may be explained by a high proportion of high-risk and combined procedures. Only 50% of the surgical procedures were simple CABG operations. It is therefore more reasonable to compare our result with Jault and colleagues [12] reporting 16.9%. Early mortality rate was 6.6% in our control patients. This is in line with the 4.8% reported by Roques and colleagues [13].

The one, three, and five-year survival rates for CRF patients were 66%, 44%, and 39%, and for control patients 90%, 85%, and 79%, respectively. The relative risk of mortality for CRF patients over the five-year period was about fourfold, similar to the RR of 3.2 reported by Dacey and colleagues [14]. In our dialysis population, the survival rates were 56%, 26%, and 23%. Dacey and colleagues report five-year survival at 55.8% in the dialysis population undergoing CABG surgery. Other studies on CRF populations undergoing major CV surgery report five-year survival rates as low as 30% [9, 15] and as high as 80% [9, 16]. Herzog and colleagues [17] calculated late survival based on almost 7,000 patients in the US Renal Data System database in the period 1995 to 1998. These registry data show a four-year survival of 32% for CABG operations, in line with our results. However, comparison of survival rates is difficult as patient populations, operative procedures, and statistical methodologies vary. One study even excluded early mortality when calculating late mortality [9]. One, three, and five-year survival rates for our predialysis group (n = 56) was 75%, 60%, and 54%, respectively. Nakayama and colleagues [1] reported a five-year survival of 65%, which is in agreement with our results.

Cox proportional hazards analysis showed age, chronic obstructive pulmonary disease, diabetes, and dialysis to be independent preoperative risk factors of mortality among the CRF group. Relative risks were between 1.8 and 2.6. The significant risk factors were similar for the control group with the obvious exception of dialysis. Surprisingly, RR for mortality in diabetes patients was not increased in the control group. A possible explanation for this finding may be that the diabetes patients without renal failure had less generalized disease than the ones with diabetic nephropathy.

The independent preoperative risk factors of mortality vary widely in different studies. Many found a low ejection fraction to be an independent risk factor [3, 8, 13, 16–18]. We did not investigate this risk factor due to lack of data. However, the preoperative risk factors of mortality that we report as significant (age, diabetes, chronic obstructive pulmonary disease, dialysis) are also reported by Herzog and colleagues' registry data studies [15, 17], with the obvious exception of dialysis (Herzog and colleagues' study is only on dialysis population). Franga and colleagues [8] reported the only preoperative risk factor of late mortality in their series to be smoking. Since data on smoking history in our study were based on patient records and not personal interviews, we do not regard the data to be reliable. Chronic obstructive pulmonary disease may therefore better reflect the adverse impact of smoking in our series.

The CABG operation was a significant operative risk factor among the CRF group. It halved the risk of long-term mortality compared to other CV procedures (r = 0.54, p = 0.011). Data on the relative risks of long-term mortality for different CV procedures among the CRF population are scarce. Two studies support our finding; however, their results are based on perioperative mortality [5, 13] (RR = 0.3 and 0.6).

Among the postoperative risk factors, organ failure and respiratory failure, only the latter was significant in the multivariate analysis for the CRF group. We could find only one study that investigated postoperative risk factors of mortality [2], finding also respiratory failure to be significant.

It is rather difficult to compare the results from various reports. Most studies have few patients enrolled, the demographics of the different populations vary widely, and the types of CV operations may not be comparable. What is evident is that there is a preponderance of men in all studies investigated [1–26] and that most studies report five-year survival of less than 50%.

Conclusions
Chronic renal failure and control patients were comparable with regard to preoperative risk factors as well as intraoperative data. A higher incidence of postoperative complications was found in CRF patients. Higher early and late mortality rates after major CV operations were found in CRF patients compared with the control population. Age, diabetes, dialysis, and chronic obstructive pulmonary disease were identified as independent risk factors of long-term mortality.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 References
 

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