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Ann Thorac Surg 2005;79:538-543
© 2005 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Waiting Time and Mortality After Elective Coronary Artery Bypass Grafting

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

Accepted for publication July 12, 2004.

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

	Abstract

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 
BACKGROUND: Limited resources for coronary artery bypass grafting (CABG) results in waiting times, prioritization between patients, and to mortality among the patients on the waiting list. Waiting time is an independent predictor for mortality on the waiting list, but it is not clear if the waiting time also influences outcome after CABG.

METHODS: The study population was 5453 consecutive CABG patients who were prioritized at acceptance into three groups: imperative (CABG intended within 2 weeks), urgent (within 12 weeks), and routine (within 6 months). Postoperative mortality was compared between patients operated on within or after the intended waiting time in their respective groups. A multivariate Poisson regression model was used to further determine the effect of waiting time on postoperative mortality. Mean follow up was 24 ± 15 months.

RESULTS: Median waiting time was 55 days. Fifty-five percent of the patients were operated on within the intended waiting time. Postoperative mortality during follow-up was higher in patients operated on after the intended time (8.0 vs 6.2%, p = 0.014), but after correction for age, gender, operative risk, and angina symptoms, waiting time was not an independent predictor for postoperative death (risk ratio, 0.98 per waiting month; 95% confidence interval, 0.97 to 1.00; p = 0.44).

CONCLUSIONS: The results suggest that mortality after CABG is not influenced by prolonged waiting time. The result does not exclude subgroups of patients that might benefit from a shorter waiting time.

	Introduction

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Coronary artery bypass grafting (CABG) has emerged into becoming one of the more common elective surgical procedures, but several countries have reported a surgical capacity unable to meet the demand [1–5]. This shortage of surgical and financial resources results in waiting times before surgery and to prioritization among patients.

Waiting lists for CABG have been thoroughly investigated regarding morbidity and mortality [1–4], risk factors [2, 3, 6], quality of life [7], and patient experiences [8, 9]. Waiting time is an independent risk factor for mortality among the patients on the waiting list [2, 3], but the pivotal question of whether waiting time also influences outcome after the operation has been scarcely addressed.

Theoretically, prolonged waiting time before the CABG may worsen general condition and an already impaired cardiac function, which subsequently may influence outcome. Delayed revascularization in CABG patients with ischemic left ventricular impairment has been shown to result in declined cardiac function and a reduced likelihood of contractile improvement [10, 11], and similar findings have also previously been reported in aortic valve replacement patients [12]. These investigations, performed in small patient groups, indicate that a prolonged waiting time may negatively affect postoperative outcome, but larger studies have not supported this hypothesis.

Accordingly, in the present investigation we assessed the influence of waiting time on mortality after CABG in a large group of consecutive patients by calculating mortality incidences in patients with or without prolonged waiting time. In addition, we used a Poisson regression model to perform a multivariate analysis of the impact of waiting time, and finally, we estimated how a reduction in waiting time would influence calculated mortality in CABG patients within 24 months from acceptance at triage.

	Patients and Methods

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Patients
During the study inclusion period (January 1995 to June 1999), 6168 patients were accepted for isolated CABG or combined CABG and valve surgery at Sahlgrenska University Hospital and the Scandinavian Heart Center. The two centers have a joint waiting list. Excluded from the study were 183 patients (3%) who underwent acute surgery within 24 hours after admission to the hospital. Of the remaining 5985 patients, 121 (2%) were withdrawn from the waiting list and subsequently from the study for various reasons, such as patients declining surgery, change to angioplasty, or malignant disease. In addition, 77 patients (1.3%) died while waiting for CABG and 334 patients were excluded because they were still on the waiting list at the end of the inclusion period. The study population therefore consisted of 5453 elective patients operated on for isolated CABG (n = 4878, 89%) or combined CABG and valve surgery (n = 575, 11%). The mean age was 66 ± 9 years, and 78% were men.

When the estimated effect of a waiting time reduction on overall mortality in CABG patients (within 24 months after acceptance at triage) was calculated, the patients that died while waiting and patients that remained on the waiting list at the end of the inclusion period were also included. These calculations were thus performed on 5864 patients. The characteristics of this patient cohort have previously been reported in detail [2].

Preoperative data were registered prospectively in a database (CorBase, Journalia AB, Kungälv, Sweden). Mean follow-up time after surgery was 24 ± 15 months and did not differ between the three priority groups or between patients operated on within or after the intended waiting time. 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 at triage to operation. Early mortality was defined as 30-day mortality. Total postoperative mortality was all mortality from the operation until the end of the follow-up period (including 30-day mortality). Overall mortality was deaths from acceptance at triage to the end of the study period.

Significant stenosis was defined as a 50% reduction in the vessel diameter as measured by angiography. Unstable angina pectoris was defined as a patient who required hospitalization because of angina symptoms at the time of acceptance. The left ventricular ejection fraction (LVEF) was measured with transthoracic echocardiography in most patients and with a left ventricular injection during coronary angiography for the remaining patients.

The severity of congestive heart failure symptoms was classified according to the New York Heart Association (NYHA) [13], and the Canadian Cardiovascular Society (CCS) score [14] was used to classify the severity of angina symptoms. The Cleveland Risk Score was used for perioperative mortality and morbidity risk stratification [15].

Triage
All the patients were accepted and given priority at a triage as previously described [2]. The patients were prioritized into three groups:

1 Imperative (n = 2216, 41%), surgery planned within two weeks,

2 Urgent (n = 1902, 35%): surgery planned within 12 weeks, and

3 Routine (n = 1335, 24%), surgery within 6 months.

Statistical Analysis
The data are generally presented as the mean and standard deviation. For waiting times, the median and interquartile range is given. A p value of less than 0.05 was considered significant. Patients operated on within and after the intended waiting time were compared with the Student's t test (continuous variables) or ² test (categorical variables). Data from the Swedish Civil Registry was used to calculate background mortality in an age, gender, and calendar time-matched general Swedish population.

The impact of waiting time was first investigated by comparing early and total postoperative mortality between patients operated on within and after the intended waiting time (² test) overall and within the respective priority group. Cumulative survival was calculated according to Kaplan-Meier, followed by the log-rank test.

A multivariate Poisson regression model was then used to further investigate the effect of waiting time on total postoperative mortality [16]. The variables were age, gender, Cleveland risk score, unstable angina, and waiting time. Waiting time was entered in the model as a continuous variable with pooled data from the three priority groups. In addition, Poisson regression models were used to estimate the effect of a waiting time reduction on overall mortality (within 24 months after acceptance at triage). Poisson regression was used because of the possibility to calculate absolute mortality risk figures and to estimate effects of hypothetical changes in the included factors, such as waiting time.

	Results

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 
Baseline Variables
Baseline characteristics at acceptance for imperative, urgent, and routine patients are given in Tables 1, 2, and 3. Patients operated on within the intended waiting time in their respective groups were younger, had unstable angina and isolated coronary disease more often, and a lower Cleveland risk score.

Mortality
EARLY MORTALITY
Of the 5453 operated on patients, 121 (2.2%) died within 30 days. Thirty-day mortality was higher in the imperative group (3.0%) than in the urgent (1.9%) or in the routine groups (1.3%) (p = 0.028 and p < 0.001, respectively).

Thirty-day mortality did not differ significantly between patients operated on within or after the intended waiting time (all patients 2.1% vs 2.4%, p = 0.47). When calculated priority group-wise, 30-day mortality did not differ significantly between patients operated on within or after the intended time: imperative, 3.2% vs 2.9% (p = 0.70); urgent, 1.5% vs 2.4% (p = 0.17); and routine 1.6% vs 0.3% (p = 0.09).

In the subgroup of patients with a markedly impaired LVEF of 40% or less (n = 780), 30-day mortality did not differ between patients operated on within or after the intended waiting time in their respective groups (all patients, 3.9% vs 4.8%; p = 0.56).

TOTAL POSTOPERATIVE MORTALITY
Total long-term mortality (from CABG to end of follow-up) was 7.0% (382 patients). Crude long-term mortality was lower in the patients operated on within the intended waiting time than in the patients operated on later (all patients, 6.2% vs 8.0%; p = 0.014). This is also demonstrated in Fig 1, where the cumulative survival (Kaplan-Meier) for patients operated on within or after the intended waiting time is illustrated (p = 0.003 between groups). In Fig 2, cumulative waiting list mortality, all postoperative mortality, and mortality for an age, gender and calendar time-matched general Swedish population is given.

View larger version (16K): [in this window] [in a new window]   Fig 1. Cumulative survival (Kaplan-Meier) in patients operated on within (dashed line, n = 3014) the intended waiting time and patients operated on later (solid line, n = 2438). There was a significant difference between the two study groups (p = 0.003).

View larger version (20K): [in this window] [in a new window]   Fig 2. Cumulative survival (Kaplan-Meier) in patients on the waiting list (dashed line) and in coronary artery bypass grafting patients (solid line). Survival for an age, gender and calendar time-matched general Swedish population is also depicted in the figure (dotted line).

In the subgroup of patients with a LVEF of 40% or less, total long-term mortality was similar between patients operated on within or after the intended waiting time in their respective groups (15.7% vs 16.1%, p = 0.83).

Multivariate Analysis
When waiting time was analyzed in a Poisson multivariate regression model together with age, gender, perioperative risk, unstable angina, and all postoperative death as an outcome variable, waiting time was not an independent predictor of postoperative death (risk ratio [RR], 0.98 per waiting month; 95% confidence interval [CI], 0.97 to 1.00; p = 0.44). In this risk model, age (RR, 1.02 per year increase; 95% CI, 1.01 to 1.03; p < 0.001) and Cleveland risk score (RR, 1.37 per step in the scale; 95% CI, 1.32 to 1.42; p < 0.001) were independent risk factors for postoperative death.

Calculated Effect Waiting Time Reduction
The calculated effect of a waiting time reduction on overall mortality (within 2 years after triage) is illustrated in Table 4. If no patient had to wait for surgery more than 1 week, the relative probability of death within 2 years after acceptance is estimated to be reduced by 15% compared with the present handling. The absolute reduction is 1.2%, from 8.0% to 6.8%.

	Comment

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

Two large studies demonstrated that waiting time was one of several independent predictors for death on the waiting list for CABG [2, 3], but it is not clear if waiting time also is a risk factor for mortality after CABG. In patients with impaired left ventricular function and hibernating myocardium, prolonged waiting time has been shown to reduce the likelihood of a functional recovery and increase the risk for postoperative complications [10, 11, 17].

Two larger cohort studies have investigated if waiting time influences postoperative outcome. Carrier and colleagues investigated hospital outcome in 568 patients undergoing urgent or elective open heart surgery and found more complications in the urgent group but no relation between waiting time and complication rate in the routine group [18]. Ray and colleagues found in a larger study (n = 2102) that prolonged waiting time was not associated with adverse hospital outcome in patients undergoing miscellaneous cardiac surgery [19]. Mortality after the immediate postoperative period was not assessed in any of the previous studies.

In our present study of 5453 CABG patients followed for up to 5 years, we found that unadjusted total postoperative mortality was in fact higher in patients with prolonged waiting time (Fig 1). However, after correction for differences in baseline variables, waiting time was not an independent predictor of death after CABG; therefore, the results suggest that prolonged waiting before CABG is not associated with a higher postoperative mortality. Our findings are thus in accordance with the previous investigations.

In both previous reports on the subject, mortality was higher in patients with higher priority and shorter waiting times [18, 19]. The authors speculated that this could be because high-risk patients received higher priority. We found the same pattern with a higher perioperative risk and higher mortality in the imperative group of patients than in the urgent and routine patients (Tables 1, 2, and 3). However, within the respective priority group we found no evidence for an increased perioperative risk in patients operated on within the intended time compared with patients operated on after the intended time. On the contrary, patients operated on within the intended time had lower preoperative risk according to the Cleveland risk score, which includes factors such as age, ejection fraction, concomitant aortic and mitral surgery, and renal function [15]. Thus, the higher mortality in the patients operated on after the intended time can be explained by a higher preoperative risk, which also became evident in the multivariate analysis where Cleveland risk score—but not waiting time—was a significant predictor of postoperative mortality.

We cannot explain why patients with a high perioperative risk waited longer (within each priority group) during the study period, but the marked correlation between perioperative risk and waiting list mortality that we recently demonstrated [2] has made us attentive to the problem. Patients with high risk for perioperative complications are now receiving a higher priority at triage.

Patients with impaired left ventricular function may represent a subgroup of patients that could benefit from shorter waiting times. As already mentioned, there is evidence from smaller studies that patients with an impaired ventricular function have a reduced likelihood of contractile improvement postoperatively [10, 11]. When we analyzed our data in the subgroup of patients with impaired LVEF of 40% or less, there was no difference between patients operated on within or after the intended waiting time in early and late postoperative mortality. This suggests that the demonstrated reduced efficacy of CABG in this patient population cannot be directly translated to an increased postoperative mortality. However, in patients with impaired left ventricular function, the declined improvement after prolonged waiting [10, 11] and the consistent observations of low LVEF as a independent predictor of death on the waiting list [1–3, 10] emphasize the importance of prompt revascularization without delay in this subgroup of patients.

With Poisson regression models, hypothetic alterations in the included variables can be made and the subsequent effect on outcome variables can be estimated [16]. In the present study, this method was used to estimate the effect of a waiting time reduction on calculated probability of death within 24 months after acceptance. We found that a reduction in waiting time substantially influenced the calculated probability of death, with a 15% relative reduction if waiting time was reduced to a maximum of 1 week for all patients (Table 4). This finding does not conflict with the lack of effect of waiting time on postoperative mortality in the present study as waiting time is a significant risk factor for waiting list mortality [2]. Thus, the reduced calculated overall mortality is due to a decrease in waiting list mortality.

So even if this present study gives no evidence that shorter waiting time influences postoperative mortality, it is essential to keep the waiting time short to reduce the risk while waiting and thus, the total risk for patients accepted for elective CABG. The risk reduction by a shorter waiting time was more pronounced in the group of patients allocated as imperative (Table 5). This indicates that a shortening of the waiting time in this group of patients is more effective to minimize mortality than reductions in urgent and routine patients.

A worthy goal would of course be to operate on all the patients immediately after acceptance and thus avoid a waiting list completely. Both from the patient's perspective, with a markedly impaired quality of life during the waiting period [7], and from the hospital's point of view, this would be desirable. However, according to the many reports from different centers, it appears to be difficult to achieve this goal in countries with public-funded healthcare [1–5].

The study has some important limitations. The first is its retrospective nature. The second is the prioritization system with divergent waiting times, which in itself causes bias. At the triage, known risk factors for death while waiting are considered and patients carrying these risk factors are given a higher priority. Thus, waiting time is not randomly divided among the patients on the waiting list. Further limitations are the lack of exact predefined priority rules, the risk for duplication when a composite risk score is entered into the risk model, and changes in patient priority after acceptance. These limitations need to be considered when the study is interpreted.

In summary, this study could not demonstrate that waiting time has an impact on postoperative mortality after CABG and further, that a shortening of waiting time reduces calculated mortality in CABG patients by decreasing deaths on the waiting list.

	Acknowledgments

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 
The Gothenburg Medical Association, Göteborgs och Bohusläns County Research and Development Funds and Capio AB supported the study.

	References

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 

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