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Ann Thorac Surg 2004;77:769-774
© 2004 The Society of Thoracic Surgeons

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Original article: cardiovascular

Mortality on the waiting list for coronary artery bypass grafting: incidence and risk factors

^a Department of Cardiothoracic Surgery, Sahlgrenska University Hospital, Gothenburg, Sweden

Accepted for publication May 2, 2003.

^* Address reprint requests to Dr Jeppsson, Department of Cardiothoracic Surgery, Sahlgrenska University Hospital, 413 45 Gothenburg, Sweden
e-mail: anders.jeppsson{at}vgregion.se

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
BACKGROUND: Insufficient capacity for coronary artery bypass grafting results in waiting times before operation, prioritization of patients and, ultimately, death on the waiting list. We aimed to calculate waiting list mortality and to identify risk factors for death on the waiting list.

METHODS: The study included 5,864 consecutive patients accepted for elective coronary artery bypass grafting (78% male; mean age, 66 ± 9 years). The patients were categorized at acceptance into three priority groups: imperative (39%), urgent (36%), or routine (25%). Waiting list mortality was calculated and compared between groups, and risk factors were identified by Poisson regression.

RESULTS: Median waiting time for the whole population was 55 days. Seventy-seven patients (1.3%) died, corresponding to a mortality rate of 5.8 deaths per 100 patient-years. The mortality rate per 100 patient-years was highest for those in the imperative group, 15.1 deaths, compared with 5.3 deaths in the urgent group and 3.2 in the routine group (p < 0.001). Independent risk factors were male sex (p = 0.032), Cleveland Clinic risk score (p = 0.005), impaired left ventricular ejection fraction (p = 0.007), unstable angina pectoris (p = 0.001), concomitant aortic valve disease (p = 0.002), priority group (p < 0.001), and time after acceptance (p = 0.019). The mortality risk increased with time after acceptance by 11% a month.

CONCLUSIONS: Long waiting lists for coronary artery bypass grafting are associated with considerable mortality. The risk of death increases significantly with waiting time. Sex, unstable angina, perioperative risk, impaired left ventricular function, and concomitant aortic valve disease are independent risk factors and should be considered at triage.

	Introduction

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Coronary artery bypass grafting (CABG) has emerged as one of the more common major surgical procedures. In 1995, 583,000 CABG operations were performed worldwide [1]. Despite the large number of revascularization procedures, the surgical capacity in many countries is unable to meet the demand [2–8]. This shortage of surgical or financial resources or both results in waiting times before operation, prioritization of patients, and, ultimately, death among patients on the waiting list. Subsequently, waiting for CABG has been reported to result in an increased risk of death [2, 8, 9]. To keep waiting list mortality at a minimum, patients are allocated to different priority groups at acceptance. This triage process is, or should be, based on knowledge of the factors that influence waiting list mortality. However, only a few studies have investigated risk factors for waiting list complications in patients needing CABG [9–11]. Consequently, the first aim of this study was to calculate mortality on the waiting list (from acceptance) in a large group of patients awaiting CABG in western Sweden. We also undertook to compare mortality between different priority groups, and, finally, to identify factors that could predict death on the waiting list.

	Material and methods

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Patients
Sahlgrenska University Hospital and the Scandinavian Heart Center provide cardiac surgical care for 1.6 million citizens in western Sweden. About 1,100 isolated CABG and 100 combined CABG and valve operations are performed annually, corresponding to an operation rate of 75 procedures per 100,000 citizens. The two centers have a joint waiting list, and the same surgeons operate at both facilities.

During the study period (January 1995 to June 1999), 6,168 patients were accepted for isolated CABG or combined CABG and valve operations at the two centers. Of these, 183 patients (3%) underwent an emergency surgical procedure (within 24 hours after acceptance), and therefore were never included on the waiting list; they were excluded from this study. The remaining 5,985 were put on the waiting list. Of these patients, 121 (2%) were withdrawn from the list and subsequently from the study for various reasons; such as refusal to undergo operation, change in operation to angioplasty, or discovery of malignant disease.

The final study population consisted of 5,864 patients accepted for elective isolated CABG (n = 5,211) or elective combined CABG and valve operation (n = 653). The mean age was 66 ± 9 years (± the standard deviation), and 78% were male. Preoperative data (collected at triage) were registered prospectively in a database (CorBase; Journalia AB, Kungälv, Sweden). It is acknowledged that some of the factors included may have changed during the waiting period, but only triage data were used in the risk factor analysis. Deaths from all causes were reported. The causes of death for patients who died on the waiting list were collected from the Cause of Death Register kept by the National Board of Health and Welfare in Sweden (Socialstyrelsen).

The study was approved by the Research Ethics Committee of the Medical Faculty, University of Göteborg.

Definitions
Waiting time was defined as the time from acceptance to operation or death. Cardiac catheterization was performed within 1 week before triage in patients in stable condition and within the last 24 hours in patients with unstable angina. At the end of the study period, 334 patients were still on the waiting list. For these patients, the waiting time was defined as the time from acceptance to the end of the inclusion period (June 30, 1999). Data for these patients were censored in the risk factor analysis.

Major stenosis was defined as a 50% reduction in vessel diameter as measured by angiography. Left ventricular ejection fraction (EF) was assessed by transthoracic echocardiography in the majority of patients and in the others, with a left ventricular injection during coronary angiography. The severity of symptoms of cardiac failure was classified according to the New York Heart Association [12], and the severity of anginal symptoms was classified using the Canadian Cardiovascular Society score [13].

The Cleveland Clinic risk score was used for perioperative mortality and morbidity risk stratification [14]. In short, the preoperative risk factors are entered into a scoring system with one to six points for each factor. The factors include emergency procedure, impaired renal function, severe left ventricular dysfunction, reoperation, operative mitral insufficiency, increasing age, previous vascular surgical procedure, chronic obstructive pulmonary disease, anemia, operative aortic stenosis, body weight lower than 65 kg, diabetes mellitus, and cerebrovascular disease [14].

Triage
Each of the 5,864 patients in the study was accepted and assigned to a priority group at triage with the treating cardiologist, a senior cardiothoracic surgeon, and an interventional cardiologist. The decisions were based mainly on the severity of symptoms, the extent of coronary artery disease, and left ventricular function, and the final decision on acceptance and priority status (imperative, urgent, or routine) was up to the senior cardiothoracic surgeon. The priority groups were as follows: imperative (n = 2,301), operation planned within 2 weeks; urgent (n = 2,127), operation planned within 12 weeks; and routine (n = 1,436). If patient priority was changed during the study period, the final priority was used in the analyses. (This explains why some patients with a high priority had a long waiting time carried over from their period in a lower-priority group.) Patient characteristics for the different priority groups are given in Table 1.

	Results

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Patient characteristics
Preoperative characteristics for all patients and for the three priority groups are shown in Table 1. Patients with the highest priority (imperative) were more often female, were older, and had a lower left ventricular EF and a higher Cleveland Clinic risk score and serum creatinine level. They also were more likely to have left main stenosis and unstable angina pectoris and to be in a higher Canadian Cardiovascular Society and New York Heart Association class.

Waiting time
The median waiting time for the whole population was 55 days (interquartile range, 21 to 107 days). The median waiting time was 18 days (interquartile range, 7 to 40 days) for patients in the imperative group, 79 days (interquartile range, 47 to 129 days) for patients in the urgent group, and 100 days (interquartile range, 66 to 175 days) for patients in the routine group. Only 45% of patients in the imperative group and 53% in the urgent group were operated on within the planned waiting time.

Mortality
During the study period, 77 patients died while waiting for CABG, corresponding to an overall mortality of 1.3%, an incidence of 5.8 deaths per 100 patient-years. There was no difference in absolute mortality between groups. Twenty-eight patients (1.2%) died in the imperative group, 32 patients (1.5%) died in the urgent group, and 17 patients (1.2%) died in the routine group. When mortality was related to waiting time, however, the mortality rate was significantly higher in the imperative group (p < 0.001) (Fig 1). If the planned waiting times in the imperative group and the urgent group had been adhered to, 31 more patients would have survived the waiting period. The risk of death for a patient waiting for operation increased by 11% a month (Fig 2). After 3 months, the risk was 37% higher than at acceptance.

View larger version (8K): [in this window] [in a new window]   Fig 1. Mortality rate by priority group. Mortality in the imperative group was three times higher than in the urgent group and five times higher than in the routine group (p < 0.001).

View larger version (8K): [in this window] [in a new window]   Fig 2. Risk ratio for mortality in relation to time since acceptance. The risk increases 11% each month (p = 0.02).

Risk factor analysis
The following variables emerged as risk factors for death in the univariate analysis: a higher Cleveland Clinic risk score, concomitant aortic valve disease that required surgical treatment, left main stem stenosis, impaired left ventricular EF, diabetes mellitus, male sex, priority group, unstable angina pectoris, preoperative serum creatinine level, preoperative blood hemoglobin level, atrial fibrillation, and time after acceptance (Table 2). In the multivariate analysis, the following variables emerged as independent risk factors for death: male sex, Cleveland Clinic risk score, impaired left ventricular EF, unstable angina pectoris, priority group, concomitant aortic valve disease that required surgical treatment, and time after acceptance (Table 3).

View larger version (11K): [in this window] [in a new window]   Fig 3. Mortality rate on the waiting list related to Cleveland Clinic risk score. The risk of death increases 18% for every step in the score (p < 0.01).

	Comment

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The waiting list mortality rate in the present study was 1.3%. This is comparable to the rates of 0.4% to 4.0% reported in previous waiting-list studies [2–6, 8–10, 16]. However, it is difficult to compare mortality between different reports because waiting times and patient selection vary substantially. To improve comparisons, the mortality rate was calculated and found to be 5.8 deaths per 100 patient-years, which also is similar to the mortality rates (four to eight deaths per 100 patient-years) in earlier studies [3, 6, 9, 11]. Waiting list mortality appears to exceed mortality in patients with stable angina who are not on the waiting list (2.4 deaths per 100 patient-years) [17]. Additional evidence of the risk of death while waiting for CABG can be found in Figure 2, where it is shown that the relative risk of death rises with time since acceptance.

At first sight, waiting list mortality appears to be homogeneously distributed among the imperative, urgent, and routine groups. However, when the incidence was calculated, it was substantially higher in the imperative group (see Fig 1). This suggests that a reduction in waiting times in the imperative group would more effectively reduce total mortality than reductions in the urgent and routine groups. This could be achieved using two different approaches. The first is to reduce waiting times for all patients. The second is to devise a more refined prioritization system in which fewer patients are assigned to the imperative group, thereby reducing the waiting times for the other patients in that group. If this could be achieved without increasing the risk for the rest of the patients, total waiting-list mortality could be reduced. To do this, knowledge of the factors that predict death on the waiting list is necessary.

The severity of anginal symptoms, extensive coronary artery disease, including left main stenosis, and impaired left ventricular function are established risk factors for death on the waiting list [18, 19]. These factors are used to allocate patients to different priority groups, although attempts have been made to refine the prioritization process [5, 20, 21]. However, it is difficult to perform risk factor analyses for patients on the waiting list, as the prioritization process and subsequent differences in waiting times cause bias. For example, in the present investigation, left main stenosis was not predictive of death on the waiting list in the multivariate analysis. This should not necessarily be interpreted as meaning that this factor is of less importance. Instead, it indicates that sufficient consideration was taken of left main stenosis at triage, which is apparent from Table 1, which shows that substantially more patients with this variable were allocated to the imperative group.

On the other hand, left ventricular function, measured as EF, emerged as an independent risk factor (a 10-unit lower EF was associated with a 28% increase in risk of death, [see Table 3]), and therefore we believe that the risk of this factor was underestimated at triage. This is also illustrated in Table 1, where left ventricular EF varied minimally between the imperative, urgent, and routine groups. Impaired left ventricular function was also reported to be a risk factor in the only previous large-scale multivariate risk factor analysis of waiting list mortality [10].

An interesting finding in the present study is the close relationship between preoperative and perioperative risk (see Fig 3). It suggests that The Cleveland Clinic risk score could add valuable information to the prioritization process. Whether this is true for other risk stratification systems remains to be seen. It has been demonstrated that patients with unstable angina pectoris benefit from early revascularization [22]. This conclusion is strongly supported by the present investigation in which patients with unstable angina had an incidence of death on the waiting list that was more than six times higher than that of patients with stable angina (see Table 2).

The results of our study may have ethical and social implications in a society with limited medical resources. How should patients requiring CABG be prioritized in comparison with other patients on the waiting list, for example, patients with chronic pain waiting for orthopedic operations or patients with more acute diseases? Are the local and central authorities acting responsibly when they limit access to certain treatments because of cost considerations? And who should make the priority decisions, doctors or laymen? The present study supports a high priority for patients requiring CABG, as the risk of death while waiting is considerable and increases with time. In addition, the CABG operation itself reduces the risk of death in a large proportion of the patients [18].

One topic for further investigation is the importance of waiting time for perioperative and postoperative mortality. It has been suggested that a long waiting time makes it more likely that the patient will be in poorer condition at operation than if he or she is operated on immediately, thereby possibly increasing postoperative morbidity and mortality [23]. On the other hand, some patients may benefit from a modest waiting time. For example, there is evidence that the results are better if there is a time interval between acute myocardial infarction and CABG [24], and a short waiting time may also have psychological advantages.

This study has some important limitations. It is a retrospective study. As already mentioned, the prioritization system itself with divergent waiting times in the different groups causes bias. Further limitations are the lack of exact, predefined priority rules and the risk of duplication when a composite risk score is entered into the risk model. These limitations need to be considered when the study is interpreted.

In summary, mortality is considerable among patients waiting for CABG and rises as the waiting time increases. A better identification system for patients at risk is desirable. A system that gives higher priority to the presence of unstable angina, male sex, impaired left ventricular function, concomitant aortic valve operation, and high operative risk may contribute to reduced waiting list mortality.

	Acknowledgments

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The Gothenburg Medical Association and Göteborgs och Bohuslän County Research and Development Funds supported this study.

	References

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