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Ann Thorac Surg 2000;70:84-90
© 2000 The Society of Thoracic Surgeons
a Division of Cardiovascular Surgery, Sunnybrook Health Sciences Center, University of Toronto, Toronto, Ontario, Canada
Address reprint requests to Dr Fremes, Division of Cardiovascular Surgery, Sunnybrook and Women’s College Health Sciences Center, 2075 Bayview Ave, Toronto, ON, M4N 3M5, Canada
e-mail: stephen.fremes{at}swchsc.on.ca
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Abstract |
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Methods. Prospectively collected data concerning 4,839 CABG operations was divided into three time cohorts (1990 to 1992, 1993 to 1995, 1996 to 1998) and analyzed by univariate and multivariate techniques.
Results. Mean age and female gender frequency increased in the later time cohorts (60.7 ± 9.0 to 63.4 ± 9.9 years and 16.5% to 21.4%, respectively). The following comorbidities were more prevalent in the later time cohorts: diabetes (26.7% versus 18.6%), renal failure (8.5% versus 2.2%), peripheral vascular disease (20.7% versus 11.0%), previous cerebrovascular accident (6.7% versus 5.0%), urgent procedures (41.5% versus 26.9%), unstable angina (47.8% versus 31.7%), urgent CABG following myocardial infarction (17.1% versus 7.3%), previous percutaneous transluminal coronary angioplasty (8.0% versus 4.5%), ejection fraction less than 35% (20.5% versus 10.4%), (all p < 0.05). Procedurally, increased utilization of the left internal mammary artery, multiple arterial conduits, and warm blood cardioplegia occurred in the later cohorts (91.2%, 22.2%, and 80.4% versus 78.7%, 3.4%, and 38.0%, respectively). The mortality rate was 2.0% and the M + M rate was 15.6% in all 4,839 patients.
The mortality and M + M for the three cohorts were 1.6%, 2.0%, and 2.3% and 18.4%, 17.2% and 12.5%, respectively. The risk-adjusted mortality and M + M decreased from 2.4% and 15.9%, respectively, in 1990 to 1992 to 1.8% and 8.4% in 1996 to 1998 (p < 0.001). The difference in adjusted event rates was minimized when the surgical factors were entered into the model.
Conclusions. Over time, there has been a trend toward operating on older patients with more comorbidities. Though hospital mortality has been stable, risk-adjusted M + M has been in a constant decline. This decline was associated with an increased use of left internal mammary artery grafts, multiple arterial conduits, and warm blood cardioplegia during the later years of the study.
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Introduction |
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During the 1980s, Christakis [1] and others [2, 3] demonstrated that perioperative mortality decreased or remained stable despite an increasing incidence of high-risk patients. During the 1990s, there was a small increase in postoperative mortality rates in most centers in the United States [4] though the perioperative risk factors of patients proceeding to surgery had increased substantially. Recent studies from New York [5, 6], northern New England [7], Massachusetts [8], and Canada [9–12] have reported substantial and consistent trends of decreasing adjusted mortality (more than 40% decrease during the last years). Studies that focused on elderly CABG patients have also shown that operative mortality has decreased significantly during the last decade [4].
Though evidence for reduction in operative mortality during the last decade is abundant, the reasons for this trend were not studied, primarily because most of the previously mentioned databases studied included only a small number of parameters on each patient.
This study was designed to define the contemporary risk factors for isolated CABG in our practice and to describe the trends in surgery during a 9-year period. Unlike the previous studies [2–12], we used an extensive database with more than 250 parameters on each patient. We thus tried to determine if protective factors such as utilization of the internal mammary artery graft, multiple arterial grafts, and warm bypass and warm cardioplegia could explain these trends.
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Material and methods |
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Anesthetic and operative techniques
Low dose fentanyl citrate (10 to 15 µg/kg) midazolam (2 to 3 mg) and isoflurane (0.5% to 2%) were used for induction and maintenance of anesthesia. Since 1993, propofol (100 to 150 µg · kg-1 · min-1) has been added for the same purpose. Standard median sternotomy and aorta-right atrial cannulation were performed for cardiopulmonary bypass. Patients were either cooled to 28°C (in previous years of the study) or remained normothermic (32° to 37°C) (88% of operations from 1996 to 1998). Revascularization was performed during single aortic cross-clamp and cardioplegic arrest in most instances. Blood cardioplegic solution was delivered in a 4:1 ratio before 1996 and an 8:1 ratio since then. Cold cardioplegia (10°C) was utilized in the earlier years of the study, while warm or tepid (33°C) cardioplegia was used more frequently in later years (80% of operations from 1996 to 1998). Cardioplegia was delivered either antegrade via the aortic root and completed vein grafts or retrograde via the coronary sinus (more common in later years). After cardioplegic induction, additional doses of 300 to 500 mL were administered after completion of each distal and proximal anastomoses. The left internal mammary artery (LIMA) has been used to bypass the left anterior descending artery with increasing frequency over the years (75% in 1990 to 1991 versus 91% in 1996 to 1998). Utilization of more than one arterial graft has also increased in frequency with each time period (6%, 16% and 27% respectively).
Statistical analysis
Clinical, operative, and outcome data were collected prospectively in a computerized database, for 4,839 consecutive patients undergoing isolated CABG between January 1, 1990 and August 30, 1998 at the Sunnybrook and Women’s College Health Sciences Center. For statistical analysis, data were divided into three time cohorts: 1990 to 1992, 1993 to 1995, and 1996 to 1998.
The outcomes of interest were hospital mortality or perioperative complications: postoperative myocardial infarction [MI], low output syndrome, intraaortic balloon pump [IABP] insertion, and cerebrovascular accident [CVA].
Preoperative risk factors were evaluated and modelled individually, rather than as a Charlson comorbidity index score [11], because the weighting of the original Charlson indexes does not accurately reflect the prognostic impact for CABG of each constituent comorbidity.
Data were collected and managed in dBASE IV datasets. The SAS for PC software [12] was used for statistical analyses.
Clinical and angiographic features were analyzed by descriptive statistical methods. Continuous variables are summarized as means ± SD and categorical variables as absolute frequencies or proportions. Continuous variables were compared by analysis of variance for the three time cohorts, and categorical variables were compared by Fisher’s exact test or Chi-square. Stepwise multiple logistic regression analysis using the maximum likelihood estimates was used to determine independent predictors of operative mortality and early nonfatal complications. Model discrimination was evaluated by the area under the receiver operating characteristic curve [13, 14], and the Hosmer–Lemeshow goodness-of-fit statistic [15] was used to examine the fit of the model.
We used the logistic model to calculate predicted probability of in-hospital death for each patient who underwent CABG:
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To determine whether any of the modifications in surgical management that have occurred in the past decade could explain the trends in mortality and M + M, certain prespecified factors (utilization of internal mammary artery, multiple arterial conduits, warm cardioplegia, or cardiopulmonary bypass [16, 17]) were entered into the multiple logistic regression analysis by increasing the p value needed to enter the model to 0.15. We searched for those variables that when included into the statistical model, neutralized the difference in adjusted M + M between the different time cohorts.
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Results |
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LIMA, multiple arterial grafts, and continuous warm blood cardioplegia with warm cardiopulmonary bypass were used more frequently in 1996 to 1998 than in 1990 to 1992.
Effects on morbidity and mortality
Hospital mortality for the entire study population was 2.0% (Table 1). Mortality rates for the three time cohorts were 1.6%, 2.0%, and 2.3%, respectively (p = NS). The rate of M + M (MI, low output syndrome, IABP insertion, CVA or death) for the entire population was 15.6%. The observed rates of M + M decreased from 18.4% in 1990 to 1992% to 12.5% in 1996 to 1998 (p = 0.001). There was a significant decrease in postoperative mechanical ventilation time and intensive care unit stay reflecting a "fast track" approach. The total hospitalization time did not change significantly, although the patients were older and sicker in later time cohorts.
Predicted and adjusted morbidity and mortality rates
Stepwise logistic regression analysis revealed the following independent predictors for perioperative mortality for the entire study population (Table 2): Left ventricular grade, urgent-emergent operation, redo CABG, age more than 70, PVD (including carotid artery disease), and chronic renal failure. Table 3 shows the same analysis for the prespecified composite endpoint of perioperative M + M.
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Comment |
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TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
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Recent studies from New York [5, 6], northern New England [7], and Massachusetts [8] report trends of decreasing mortality after CABG surgery. In New York, in-hospital mortality decreased from 3.5% to 2.8% between 1989 and 1992, with a 41% decrease in adjusted mortality [5]. Similarly, in northern New England (Maine, Vermont, and New Hampshire), mortality decreased from 4.5% to 3.6% between 1987 and 1993 [7]. A report from Massachusetts revealed a decrease in unadjusted mortality from 4.7% to 3.3% between 1990 and 1994 and a decrease of 42% in adjusted mortality for the same time period (from 5.7% to 3.0%) [8].
Two recent studies from Canada have shown similar results. The first study evaluated temporal trends in case-mix and short-term outcomes of patients who underwent CABG in Ontario between 1981 and 1995. A relative decline in the risk-adjusted death rate of 52% between 1986 and 1995 was observed [9]. The second study evaluated Canada-wide outcome trends for CABG between 1992 to 1995. The adjusted mortality rate decreased by 17% during that period (similar to the decline of 18% reported by Peterson for CABG performed in US Medicare beneficiaries). Individual provinces, such as Manitoba, have reported decreases in adjusted death rates of more than 40%, resembling the results in New York, New England, and Massachusetts [12]. None of the abovementioned studies explained the causes for this trend.
In the current study, incremental risk factors for operative M + M were identified in 4,839 patients undergoing isolated CABG surgery in our institute between 1990 and 1998. Urgent surgery, left ventricular ejection fraction, previous CABG surgery, age, chronic renal failure, and PVD were independent predictors of operative mortality. The independent predictors of operative M + M were urgent surgery, left ventricular ejection fraction, age, previous CABG surgery, female gender, and chronic renal failure.
The mortality rate of our entire study population was low (2.0%). Our findings confirm that there has been a time-related increase in the severity of the preoperative risk profile of these patients. Despite this increase in high-risk patients, our risk-adjusted hospital mortality and M + M have decreased considerably (28.0% reduction in adjusted mortality and 45.6% reduction in adjusted M + M over the study period).
In 1996 the Department of Veteran Affairs Cardiac Surgery Consultants Committee published a review of the quality of cardiac surgical treatment at the 43 Veterans Affairs cardiac surgical centers [18]. It did so by reviewing both the unadjusted and risk-adjusted operative mortality data and the incidence of perioperative complications. The committee divided the operating centers according to two parameters: the operative mortality rates and the trends in outcome over time. Our data reflect the best combination of results: a low operative mortality with a trend toward reduction of the adjusted death rates over the years.
Using an extensive database, we showed that the decreased adjusted M + M in the later time periods can be partially explained by the more frequent usage of LIMA grafts, multiple arterial grafts, and increased utilization of warm bypass and cardioplegia (by including these parameters into the multiple logistic regression analysis, we accounted for most of the difference in adjusted mortality and M + M between the time cohorts).
One of the key limitations of this study is its retrospective nature. Although the temporal reductions in risk-adjusted outcomes were associated with a greater usage of arterial conduits and warm blood cardioplegia, such an association could be spurious or an epiphenomena, rather than truly explanatory. It must also be mentioned that the introduction of these parameters into the statistical model for calculating risk factors for M + M only partially eliminated the protective effect of the later time cohort. In support of a causative role for these changes, we have previously shown that warm as opposed to cold blood cardioplegia was associated with reduction in low output syndrome (6.1% versus 9.3%, p = 0.001), enzymatic MI (12.3% versus 17.3%, p < 0.001), and mortality (1.4% versus 2.5%, p = 0.12) [19]. An association between internal mammary artery usage and decreased perioperative mortality had been recognized previously [20]. Although selection bias cannot be excluded, subtle differences in early patency between arterial conduits and saphenous grafts could be important.
In 1991, the American College of Cardiology and the American Heart Association established guidelines and indications for CABG, describing conditions for which the operation is indicated on the basis of a demonstrated advantage over medical treatment in terms of longevity, relief of symptoms, or both [21]. These guidelines were based mainly on data from the Veterans Administration, European, and CASS studies [22–24], which reflected surgical results of the 1970s. When we compared the risk factors for operative mortality in our model with that of the CASS study [21] we could see that variables such as left main disease and angina class have lost their predictive power for mortality. This finding has also been confirmed by other investigators [18]. New risk factors for operative mortality such as PVD, chronic renal failure, and chronic obstructive pulmonary disease [18] have emerged.
The current study revealed that surgical practice is constantly changing. Advances in technology and anesthesia, new philosophies about intensive care unit care, improvement in medical management as well as methods of myocardial protection, and use of internal mammary artery grafts and multiple arterial conduits may have all contributed to a reduced risk-adjusted mortality rate in the past decade [9]. We therefore believe that earlier indications for CABG should be revised and expanded criteria for CABG referral should be considered.
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