doi: 10.1161/01.CIR.0000138193.51635.6f
(Circulation. 2004;110:II-27 – II-35.)
© 2004 American Heart Association, Inc.
Surgery for Coronary Artery Disease |
Single Versus Multiple Internal Mammary Artery Grafting for Coronary Artery Bypass
15-Year Follow-Up of a Clinical Practice Trial
From the Duke University Medical Center (W.R.B., D.D.G.), Durham, NC; Hahnemann University (A.S.W.), Philadelphia, Pa; the Duke Clinical Research Institute (R.H.T., L.K.S.), Durham, NC; Vanderbilt University (F.E.H., J.S.R.), Nashville, Tenn; and the Centennial Medical Center (J.S.R.), Nashville, Tenn.
Correspondence to J. Scott Rankin, MD, 2400 Patterson Street, Suite 103, Nashville, TN 37203. E-mail jsrankinmd{at}cs.com
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Abstract |
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Background— The long-term clinical advantages of using routine multiple internal mammary artery (IMA) grafts for coronary artery bypass (CAB) are not clear. This study was designed to test the hypothesis that multiple IMA grafts would provide better 15-year outcomes when compared with single IMA and vein grafts.
Methods and Results— Between 1984 and 1987, 1067 consecutive patients undergoing isolated CAB were referred to 1 surgeon practicing primarily single and another surgeon maximizing multiple IMA grafts (clinical practice trial). A 207-patient subset with multiple IMAs underwent postoperative graft angiography at 1 to 32 weeks to define initial IMA patency. Patients were followed-up yearly, and the groups were analyzed as (I) surgical strategy (surgeon operating) (single=413 versus multiple=654), (II) ultimate operation performed (single=418 versus multiple=449), or (III) single versus multiple coronary systems revascularized with IMAs (single=490 versus multiple=377). Advantages of this study design were that an entire referral population was examined, multiple IMAs were applied to the entire spectrum of baseline patient risk, 15-year follow-up provided a complete prognostic picture, and the subgroups were potentially comparable at baseline. In all 3 analyses, single and multiple groups were statistically similar with respect to baseline, operative, and immediate postoperative variables. Early IMA patency was 98.5% (333/338 grafts patent), validating the quality of IMA procedures. Unadjusted and adjusted 15-year outcome analyses for I, II, and III for death, myocardial infarction, percutaneous coronary intervention, redo coronary bypass, and the composite of all events identified multiple versus single as a significant predictor of outcome for the composite end point in adjusted analysis III (hazard ratio=0.808; 95% CI, 0.689 to 0.948; P=0.009), because of a 5% to 10% absolute reduction in each of the outcome variables at 15 years. Moreover, >50% reduction in reoperation rate was observed at 15 years in every analysis.
Conclusions— At 15-year follow-up, multiple IMA grafting was associated with a 19.2% adjusted risk reduction in death and cardiac events, caused by decreases in all adverse end points and fewer reoperations. These data indicate that the clinical advantages of maximizing IMA conduits are significant. Based on this information, it is suggested that multiple IMA grafting to 2 coronary systems should be applied liberally to patients with noncardiac risk profiles predictive of long-term survival.
Key Words: coronary artery bypass • multiple internal mammary artery grafting • bilateral internal thoracic artery • coronary artery disease
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Introduction |
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Routine use of the left internal mammary artery (IMA) as a graft to the left anterior descending coronary artery has been standard for >2 decades.1–4 However, studies comparing multiple to single IMA grafting have yielded mixed results, preventing definite conclusions.5–15 Most analyses have been observational, have exhibited significant selection biases, and have required major statistical modeling to compare clinically disparate subgroups. Additionally, because a single IMA graft provides a strong survival advantage by itself, it would take a large number of patients and lengthy follow-up to show an incremental benefit of multiple IMA grafting. To test the hypothesis that multiple as compared with single IMA grafting improves clinical outcomes, a prospective trial was undertaken in 1984. In an attempt to minimize the effects of selection bias, patients were allocated prospectively to 2 surgeons in the same practice and over the same period of time, with divergent management philosophies (one practicing primarily single and the other multiple IMA grafting). Negative intermediate-term results (mean follow-up=4 and 7 years) were published in 1990 and 1993.5,16 The goal of this article is to report the 15-year outcomes of this patient cohort using this specific approach to methodology, which is termed the clinical practice trial.
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Methods |
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Patient Population
Over a 3-year period between July 1, 1984 and June 30, 1987, 1067 consecutive patients undergoing isolated coronary artery bypass at Duke University Medical Center were allocated prospectively to 2 faculty surgeons with differing philosophies of IMA use, one using primarily single IMA grafts and the other maximizing multiple IMA grafts.5 At the time, true equipoise existed about the superiority of one approach over the other. Indications for surgery were stable or progressive angina, unstable angina, or acute evolving myocardial infarction (MI). Each surgeon had similar previous clinical results and used the same operative technique during the study period. Briefly, cardiopulmonary bypass was instituted with single venous cannulation, systemic hypothermia (24°C to 28°C), and aortic venting. Cold potassium crystalloid cardioplegia and topical hypothermia were used for myocardial protection. Distal coronary anastomoses usually were performed during a single arrest period, and proximal anastomoses were completed during reperfusion and rewarming. Both surgeons used frequent sequential vein grafts. All distal anastomoses were constructed with running 7-0 or 8-0 polypropylene suture. All vessels
1.5 mm in diameter, with a 50% stenosis, were grafted; coronary endarterectomy was avoided. The 2 surgeons had similar previous referral profiles, operative selection philosophies, and surgical outcomes. In theory, patient distribution to the care of each surgeon was based solely on random clinical consultation. During the study period, one surgeon (single) practiced an operative strategy of predominant single IMA grafting with adjunctive saphenous veins, whereas the other surgeon (multiple) attempted to maximize distal IMA anastomoses with frequent bilateral, sequential, and free IMA grafts. From 1 to 32 weeks postoperatively, 207 patients with multiple IMA grafting underwent coronary graft angiography to assess early patency. Characteristics of this angiographic cohort have been presented elsewhere.17
Baseline and Operative Data
All preoperative prognostic data were acquired prospectively in the process of generating an automated operative note. Data quality was monitored by the surgeons (in completing a legal document) and by databank personnel. When baseline patient characteristics were compared between the single and multiple surgeons, no significant differences were noted in either anatomic extent of coronary artery disease or baseline risk factors predicting mortality.5 Briefly, 89% versus 91% (single versus multiple) had multivessel or left main disease, and 45% versus 46% had acute presentation (operation from the coronary care unit or cardiac catheterization laboratory for unstable angina or acute ischemia18) (Table 1); 8.2% had reoperations in single and 7.2% had reoperations in multiple (P=0.53) IMA groups. The similarity of baseline characteristics supported the validity of this type of clinical patient allocation for comparison of 2 therapies. Individual surgeon operative variables, such as aortic cross-clamp time and pump time per distal anastomosis, were statistically similar, as were mean number of distal anastomoses per patient (single=3.3 versus multiple=3.5) (all p=NS). Thirty-day mortality (single=3.9%, multiple=1.8%) and complication rates also were statistically similar between the 2 surgeons (p=NS). Significant differences were observed, however, in IMA use. More detailed data are reported elsewhere,5 but the single surgeon used single IMA grafts in 74% of multivessel disease patients, whereas the multiple surgeon used multiple IMA grafts in 71% of multivessel disease patients (P<0.05), again suggesting a fair test of the hypothesis.
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Follow-Up
During the 15-year follow-up period, information was collected by the Duke Cardiovascular Databank on all-cause death and cardiac complications. These data were obtained prospectively by mailed questionnaires or telephone interviews at 6 months, 1 year, and annually thereafter. Data from clinical visits and rehospitalizations also were documented to ascertain follow-up and end point determination. Outcomes were assessed by: (1) all-cause mortality; (2) nonfatal MI; (3) percutaneous coronary intervention; (4) redo coronary bypass; and (5) a composite of these 4 end points.
Data Analysis
Three separate analyses were undertaken. Analysis I was based on "surgical strategy," with the surgeon to whom the patient was referred designating the single or multiple IMA surgical strategy group. However, for technical reasons, not all patients in the multiple strategy group could be revascularized with multiple IMAs, and multiple IMAs were performed occasionally in ideal patients in the single strategy group. Therefore, analysis II was performed, based on the actual operation the patient ultimately received (single versus multiple IMA grafting). Finally, analysis III was based on whether single or multiple coronary systems were revascularized with IMAs. Patients who received multiple IMA grafts to a single coronary system (such as with sequential grafts) were considered single IMA patients in this analysis. For all 3 analyses, baseline demographic, clinical history, operative variables, and immediate postoperative variables were assessed as percentages for discrete variables and as median, 25th, and 75th percentiles for the distributions of continuous variables. Cumulative event rates for each outcome as a function of time after the index surgery were calculated using the Kaplan-Meier method. Unadjusted 15-year event rates were generated in tabular form for each analysis and for each outcome.
Multivariable Cox proportional hazards models were applied to death and the composite outcome to determine long-term effects of single versus multiple IMA grafting after adjusting for minor differences in baseline characteristics. To find the significant adjustment variables for each Cox model and for each outcome, stepwise selection was used with the following characteristics: gender; age; history of peripheral vascular disease, cerebrovascular disease, chronic obstructive lung disease, previous percutaneous coronary intervention, redo coronary artery bypass grafting; diabetes; smoking history; hypertension; mild to moderate valvular disease; race; multivessel or left main coronary disease; acute presentation for the index surgery(versus elective); a coronary disease index incorporating both the location and the severity of stenosis and the affected number of diseased major epicardial vessels; and single versus multiple IMA graft strategy. These covariates were included in the final models if they entered at P
0.05, or if they were considered clinically important. Adjusted curves for survival and the composite end point were generated, and adjusted 15-year outcomes were tabulated. Two prespecified interactions between IMA strategy and either age or diabetes were tested in the adjusted multivariable models for the composite end point in all 3 analyses. In each model, the tests were considered significant if P0.05.
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Results |
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Analysis I: Single Versus Multiple Surgical Strategy
The surgical strategy analysis had 413 in the single IMA group and 654 patients in the multiple IMA group. It is presented to illustrate the raw data. Preoperative variables were statistically similar, except for a borderline higher incidence of diabetes in multiple and a clinically insignificant difference in ejection fraction (Table 1). No statistically significant differences were observed in unadjusted or adjusted mortality or the composite of mortality, MI, percutaneous coronary intervention, and reoperation to 15 years in analysis I (Figure 1). A significant difference was observed in unadjusted reoperation rate over 15 years (13.2% single versus 6.1% multiple; P=0.005), as well as in the adjusted value (Table 1). For comparison, variables identified as significant predictors of the composite end point in adjusted analysis I also are presented in Table 1. It should be noted that this analysis included all patients, even those with single vessel disease who were not candidates for multiple IMA grafting (10.6% single and 8.5% multiple), as well as patients receiving no IMA grafts for technical reasons (16% single and 3% multiple). Finally, no statistical treatment interaction was observed between multiple IMA grafting and age or diabetes (both P>0.05) in adjusted analysis I of composite outcome. In the 207-patient subset of the multiple IMA group undergoing routine angiography at 1 to 32 weeks postoperatively, early patency of IMA grafts was 98.5% (333/338),17 validating the quality of the IMA grafting.
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Analysis II: Actual Operation Performed
This analysis grouped patients according to the ultimate operation performed (single versus multiple IMA). Single-vessel disease patients were omitted from this analysis because they were not candidates for multiple IMA grafting; patients receiving only vein grafts also were omitted. All patients having >1 distal IMA anastomosis were considered multiple in this analysis, including patients having sequential IMA grafts to 1 coronary system. There were 418 patients in the single IMA group and 449 patients in the multiple IMA group. Baseline characteristics of these groups were similar except that the multiple IMA group contained slightly more diabetic subjects (Table 2). No statistically significant differences were observed in adjusted event rates for death or the composite end point to 15 years (Table 2, Figure 2). For comparison, multivariable predictors of composite outcome in adjusted analysis II are shown in Table 2. No statistical interaction of multiple IMA grafting with age or diabetes (both P>0.50) was observed in adjusted analysis II of composite outcome.
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Analysis III: Single Versus Multiple Coronary Systems Revascularized With IMA
In many patients, multiple IMA grafts were performed by sequential IMA conduits to a single coronary system. Although sequential grafts were considered multiple IMAs in analysis II, they may have affected prognosis uniquely. Thus, analysis III is the main focus of this article and addressed single versus multiple coronary systems being revascularized with an IMA. Again, single-vessel disease patients were eliminated from this analysis, as were patients receiving vein grafts only. There were 490 patients in the single coronary system group and 377 patients in the multiple group. Baseline characteristics of each group were similar except that the multiple coronary system group had slightly more smokers, hypertensive subjects, and diabetic subjects (Table 3). In analysis III, no statistically significant difference was observed in unadjusted or adjusted mortality, although the adjusted absolute mortality at 15-years was reduced by 5.6% (P=0.19) (Table 3, Figure 3). A statistically significant difference was observed in the adjusted composite end point of 15-year events in analysis III (65.7% in multiple versus 75.3% in single, log-rank P=0.0081). Once again, the adjusted redo coronary bypass rate was reduced by more than half in the multiple IMA group to only 3.4% at 15 years (P=0.0026). For comparison, other significant multivariable predictors of composite outcome in adjusted analysis III are shown in Table 3. No statistical treatment interaction was observed between single versus multiple and age or diabetes (both P>0.35) in determining the adjusted composite end point.
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Discussion |
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All biostatistical methods of assessing clinical outcomes can be categorized as using inductive reasoning, an outgrowth of logical philosophy. In discussing this topic, one needs to start from the principle that no form of inductive reasoning can deliver a demonstratively certain conclusion but produces an inference supporting a hypothesis at a given probability. Randomized and observational approaches to clinical research are useful, or even complementary, and the key to either is understanding advantages and limitations for a given question, performing meticulous analysis, and properly interpreting results.19,20 The randomization process minimizes treatment selection bias and produces subgroups that are comparable with respect to all baseline characteristics. This fact is especially important in studies of coronary surgery, but randomized trials have had a difficult time in accomplishing this for several reasons. They are expensive, often are poorly received by referring physicians, can experience difficulty recruiting adequate numbers of patients, and frequently contain patients at lower risk than the general population. If one therapy is known to be superior in any way, a survival study requires provisions for crossover, confusing the issue and reducing statistical power.
In contrast, retrospective observational studies can assess complete clinical experiences and provide a realistic view of actual clinical practice. Observational registries can link data entry with generation of procedural narratives and reduce the cost of clinical records. Data are acquired over time, on complete populations, and at negligible additional expense, although careful selection of prognostic variables and quality control are required. The disadvantage is that pre-existing biases can influence selection of therapy, producing subgroups that differ with respect to important baseline prognostic characteristics. Over the past 3 decades, multivariable models have been developed to compensate, to some extent, for baseline differences,21,22 but the magnitude of difference across which such models can accurately adjust is currently unclear.
The approach used in the present study, termed the clinical practice trial, also has advantages and limitations. The documented advantage was better comparability of baseline characteristics between groups. It seemed that 2 surgeons with similar operative philosophies working in the same practice and over the same period of time were referred patients that were similar with respect to baseline characteristics. In this way, the clinical practice trial resembled a randomized trial while retaining the advantages of observational studies. Thus, this study was potentially able to test the hypothesis of multiple IMA superiority over the entire spectrum of baseline patient risk, and with subgroups that were similar (obviating the need for excessive statistical modeling). The quality of IMA grafting was confirmed by the excellent patency rates observed in multiple IMA patients. However, a possibility still exists of surgeon-related differences in revascularization quality or other observational limitations that could have independently influenced long-term outcomes. All measures of preoperative, operative, and early postoperative results were statistically similar, though (whereas single versus multiple use was markedly different), arguing for a fair test of the proposed hypothesis. Finally, it is useful to approach clinical research questions in multiple different ways, and the method used in this article complements and potentially expands on previous analyses.
The positive results of the present study reinforce the findings of several recent articles,9,23 both in direction and magnitude. In the study by Lytle et al,9 the 
14% increment in reoperation-free survival at 12 years with bilateral internal thoracic artery (ITA) grafts might be closer to the 9.6% absolute improvement in composite outcome at 15 years in analysis III if their data had been adjusted for differences in baseline risk. In their propensity score analysis, quintile 3 had no difference in baseline characteristics, with 10-year survivals of 75% for bilateral ITA grafts and 74% for single ITAs (P=0.4). The 7% to 8% difference in 10-year survival in quintiles 4 and 5 occurred in the setting of significantly lower-risk bilateral ITA patients; however, the presented survival differences were not adjusted for baseline characteristics. Statistical adjustment might have reduced these differences and brought their results closer to those of the present study (5.6%). Finally, the increasing benefits of bilateral ITA grafts over time in the more recent article by Lytle et al23 also are evident in the present study (Figure 3).
In the article by Naunheim et al,6 no difference in 15-year survival was observed in a matched series of single versus multiple IMA patients from the early 1970s. However, a higher operative mortality in multiple IMA patients (9% versus 2%) may have biased this study from the outset. Analysis of hospital survivors showed only a marginal survival benefit at 15 years (P=0.05). In a similar case-matched study from Tokyo,10 multiple IMA did not alter the 7-year survival, but redo coronary bypass rates were significantly reduced. In 2 other case-matched and 1 comprehensive series,12–14 survival benefits of multiple IMA grafting could not be demonstrated, although trends were evident toward improved cardiovascular outcomes in many areas. It should be emphasized that few of these studies contained follow-up data beyond 10 years.
The present study documents clear clinical advantages of performing multiple IMA grafts to 2 coronary systems. The composite hazard of all-cause mortality and cardiac events was reduced by 19.2% at 15 years, a result that seems clinically, as well as statistically, significant. Although mortality was reduced by only 5.6%, this improvement was achieved with much lower cardiac event and reintervention rates. Reoperation, especially, was reduced to only 3.4% over 15 years in multivessel disease patients receiving multiple IMA grafts to 2 coronary systems. This result is impressive by itself, but the analysis also reinforces the propriety of addressing composite event rates rather than individual outcomes in studies such as these. Several other principles are evident. First, sequential IMA grafts did not seem to have the same prognostic significance as bilateral IMAs. This result is consistent with the approach of Lytle et al23 and suggests that bilateral IMAs, as a routine, should be placed to the 2 most important coronary systems. Second, the benefits of multiple IMA grafting in the present study did not become evident until after 7 to 10 years. Analyses (including the authors’ earlier articles) performed over shorter periods are unlikely to show a difference. At 15 years, the differences seem to be increasing, so that clinical benefits may be even greater in patients surviving longer.23 Third, because of this delayed benefit phenomenon, multiple IMAs may be most appropriate in patients with noncardiac risk profiles predictive of long-term survival. The contribution of preoperative comorbidities and advanced age to mortality become much more prominent in longer-term studies, as evidenced in Tables 1 to 3
. In fact, 96.5% of patients aged 70 years or older at the index operation in this series were dead by 17 years (unpublished results). It is possible that the clinical effects of multiple IMAs in elderly patients with high comorbidity are less, because of competing noncardiac risks and masking of multiple IMA benefits by earlier mortality from other causes.24For this reason, single IMA grafts perhaps should continue to be the primary approach in elderly patients with significant baseline comorbidity. However, it should be stated that objective selection criteria for multiple IMA use are, at present, unclear and await future studies. Finally, the results of the present analysis are complementary to and supportive of the clinical approach to coronary revascularization advocated by the Cleveland Clinic group23 and indicate that bilateral IMA grafts to 2 coronary systems should be performed much more frequently in selected patient subsets.
In conclusion, from multiple clinical studies of varying designs, a consistent picture is emerging of the magnitude of long-term clinical benefits that can be achieved by routine use of multiple IMA grafts. Outcomes are clearly better in multiple IMA patients, and the magnitude of difference seems more evident in patients with longer expected survival. Applying multiple IMAs to the entire coronary disease population in the present study improved 15-year absolute mortality by 5.6% while reducing coronary event and reintervention rates significantly. Performing multiple IMAs to 2 coronary systems was associated with a 19.2% reduction in the adjusted composite of death, percutaneous coronary intervention, MI, and redo coronary bypass at 15 years (P=0.009). These data justify routine multiple IMA use in most patients undergoing surgical coronary bypass, although precise selection criteria remain to be defined.
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Acknowledgments |
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This study was supported, in part, by a generous grant from St. Jude Medical.
The authors thank St. Jude Medical for their generous grant to the Duke Clinical Research Institute for statistical analysis. Also appreciated is the excellent editorial assistance of Tracey Rodgers.
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References |
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