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Ann Thorac Surg 2003;76:464-470
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

The effect of left internal mammary artery utilization on short-term outcomes after coronary revascularization

^a Division of Cardiothoracic Surgery, Department of Internal Medicine at the University of Washington School of Medicine and the School of Public Health and Community Medicine, University of Washington School of Medicine, Department of Health Services of the University of Washington, Health Services Research and Development, Department of Veteran’s Affairs, Seattle, Washington, USA

Accepted for publication February 5, 2003.

^* Address reprint requests to Dr Aldea, Adult Cardiac Surgery, Division of CT Surgery, 1959 Pacific NE, AA115, Box 356310, Seattle, WA 98195, USA.
e-mail: aldea{at}u.washington.edu

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
BACKGROUND: The purpose of this study was to determine whether use of the left internal mammary artery (LIMA) during coronary revascularization influences short-term morbidity in all patients undergoing revascularization, as well as in patients over the age of 75 years, female patients, and patients with diabetes. The study also explored variability in the utilization of LIMA grafts across an entire state.

METHODS: Using the Clinical Outcomes Assessment Program (COAP) of the state of Washington, procedural outcomes were compared for patients receiving and patients not receiving LIMA grafts as part of revascularization procedures from January 1, 1999 to December 31, 2000. Mortality and major complications were examined, both as unadjusted rates and after adjusting for baseline patient risk factors.

RESULTS: A total of 16 centers performed 8,797 nonemergent coronary artery revascularizations, including 81.7% with LIMA grafts. The use of a LIMA graft was associated with a significantly lower mortality (3.7% No LIMA vs 1.6% LIMA), as well as decreases in ventricular arrhythmias, need for postoperative dialysis, need for transfusions, ventilator dependence, and length of hospital stay. These trends were true for the population as a whole as well as for all subgroups analyzed, and they persisted after correcting for differences in comorbid conditions. In addition, there was wide variability in the use of LIMA grafts from center to center in the state.

CONCLUSIONS: The use of LIMA grafts for coronary revascularization is associated with decreased mortality and morbidity. Despite these advantages, there is great variability in its application across the state of Washington.

	Introduction

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The widespread use of the left internal mammary artery (LIMA) as a conduit for coronary arterial revascularization established the standard by which all revascularization procedures can be judged [1–3]. The long-term outcomes associated with LIMA usage are clear and include higher patency than vein grafts, increased patient survival, and a lower rate of reintervention [4–8].

Despite these well-established advantages of LIMA usage, the usage of LIMA grafts is not universal. A recent evaluation of the database of the Society of Thoracic Surgeons reveals that about 81% of the coronary revascularization procedures in the United States each year include a LIMA conduit [13], either alone or in combination with a right internal mammary artery conduit (RIMA).

Begun in 1997, the Clinical Outcomes Assessment Program (COAP; see Appendix) of the state of Washington was an effort by the Washington State Health Care Authority in collaboration with the medical community to improve quality of care in the use of procedures relating to cardiovascular disease [15]. Since its inception, COAP has developed a comprehensive database that includes all of the cardiac revascularization procedures performed in the state. By carefully analyzing the data from COAP from 1999 to 2000, the current study was performed to assess the application of LIMA utilization across the state and determine the patterns of practice across the different surgical sites in the state.

Specifically, this study will examine the incidence of LIMA utilization and the variability of its application in high-risk revascularization subgroups: women, patients over age 75 years, and patients with diabetes, and in the population as a whole. With nearly 100% capture of all revascularization procedures in the state over the study period, specific practice patterns and biases can be minimized, allowing geographical generalizations superior to those obtainable with single institutional studies. Through analysis of these practice patterns, it will be possible to achieve LIMA utilization goals that will provide a measure of quality assurance throughout the state.

	Material and methods

Top Abstract Introduction Material and methods Results Comment References

 
Patient population
Between January 1, 1999, and December 31, 2000, there were 10,429 coronary artey bypass graft (CABG) procedures performed in 16 Washington state hospitals, all of which participate in COAP. Of these hospitals, there is one academic medical center, one Veteran’s Administration hospital, and 14 communtiy hospitals. The age and gender distribution of the enrolled patients is representative of all patients undergoing a first, nonemergent, isolated coronary revascularization in the US as a whole. Race data were not collected in the database. The racial distribution, given the high rate of capture, certainly reflects the racial distribution in the state as a whole. Patients undergoing these procedures were identified from hospital records, including operating room logs. This analysis excluded patients with a history of prior CABG (n = 753), as well as those who underwent emergent or salvage surgery (n = 614). In addition, patients who underwent a concomitant valve procedure were excluded from analysis. After applying these exclusion criteria, there were 8,960 procedures available for analysis. Of these, 8,797 cases had known LIMA status.

To assess the reliability of case ascertainment for 1999, the number of procedures performed in COAP was compared with the number in the Washington State Comprehensive Hospital Abstract Reporting System (CHARS), a hospital discharge abstract database. There was reasonable agreement between the two sources, with 5,365 isolated CABG operations listed in the CHARS database for the year 1999, and 5,067 in COAP, giving a 94% capture rate for that year.

Study variables
The COAP CABG database contains an extensive amount of information, all of which was not used in this study. Since its inception, the database has been modified annually by adding and removing data elements. The method of data collection varies by center, with some doing prospective data collection and others using chart review to complete the case report form. Data are transmitted electronically or by hard copy to a central contractor responsible for constructing a single database. Within COAP, there are ongoing efforts to improve the case report forms and to ensure data validity and reliability.

In addition to obtaining age and gender, the following medical history variables were assessed: diabetes mellitus, chronic obstructive pulmonary disease, cerebrovascular disease, peripheral vascular disease, hypertension, congestive heart failure, smoking, aortic or mitral valve disease, prior myocardial infarction, and prior percutaneous cardiac intervention.

Information related to the period immediately preceding CABG was also obtained. Variables obtained included the most recent creatinine level (mg/dL) before the procedure, the most recent ejection fraction during the current admission and before the procedure, the number of diseased vessels, the location of coronary artery stenoses more than 70%, the use of an intraaortic balloon pump before the procedure, and whether myocardial infarction evolved within 24 hours of hospital admission. The type of angina was defined as no angina, stable, or unstable, and the Canadian Cardiovascular Society Classification of angina was also assessed. Elevated creatinine was defined as more than 2.0 mg/dL and low ejection fraction as less than 30%.

Variables describing the procedure itself were also collected, and included surgical priority, defined as elective, urgent, emergent, or salvage, and graft type, including saphenous vein, left and right internal mammary artery, radial artery, and gastroepiploic artery. The use of off-pump CABG was also determined.

This study examined several outcomes that were reported in the COAP database. These included hospital mortality, postprocedure Q-wave myocardial infarction, ventricular arrhythmia, dialysis, stroke, return to the operating room, ventilator time more than 24 hours, and postprocedural length of stay. Data regarding the transfusion of red blood cells, whole blood, fresh-frozen plasma, cryoprecipitate, and platelets were also collected. Information about blood products and ventilator time was collected only in the year 2000.

Statistical methods
Baseline characteristics of the patients, who did and did not receive LIMA grafts, as well as outcomes, were compared with the ² statistic for categorical variables and the two-sample t test for continuous variables. Stepwise logistic regression was used to identify factors that predicted the use of LIMA grafts. Logistic regression analysis was used to assess the association between LIMA use and outcome, including hospital mortality and postoperative length of stay more than 7 days, for the groups as a whole as well as the following subgroups: women, men, patients with left anterior descending/left main coronary artery lesion, patients without left anterior descending/left main coronary artery lesion, patients with diabetes mellitus, and patients without diabetes mellitus.

The strategy for assessing the association between LIMA use and outcome was to allow covariates to enter first and then force in LIMA usage. Covariates that entered the model were those in the COAP risk adjustment model (unpublished data) for hospital mortality, and included age, gender, myocardial infarction less than 24 hours from surgery, ejection fraction less than 30%, preoperative use of an intraaortic balloon pump, and surgical priority. In addition, history of diabetes mellitus, chronic obstructive pulmonary disease, and site of the procedure, three variables associated with hospital mortality in multivariate analysis, were added to the list of covariates. This same set of covariates was used in the analyses of mortality and postoperative length of stay for all patients and for the six subgroups.

As in most extensive data collection efforts, missing data are problematic. For the variables used in risk adjustment, the proportion of missing values was as follows: ejection fraction less than 30% (17%), creatinine more than 2.0 mg/dL (26%), intraaortic balloon pump (9%), chronic obstructive pulmonary disease (3%), myocardial infarction less than 24 hours (4%), and diabetes mellitus (< 1%). To increase the numbers available for analyses and reduce bias, missing data for these items were imputed, such that the missing category for each variable was assigned to the more prevalent category. Sensitivity analysis was used to validate this method. Missing ejection fraction was assigned to the greater than or equal to 30% category, missing creatinine was imputed to the less than or equal to 2.0 category, and individuals with missing intraaortic balloon pump status were assumed not to have had this intervention.

	Results

Top Abstract Introduction Material and methods Results Comment References

 
From 1999 to 2000, 8,960 patients underwent nonemergent primary CABG as an isolated procedure in the state of Washington. Of these, 1,433 patients (16.2%) underwent CABG without LIMA and 7,364 patients (83.7%) underwent CABG with LIMA. In the remaining 163 patients (1.8%), the revascularization conduit was not known. Table 1 demonstrates differences in the preoperative risk variables between the two groups of patients.

Table 2 demonstrates the impact of LIMA usage on postoperative morbidity and mortality. Patients undergoing CABG with LIMA had a lower observed mortality (1.6% vs 3.7%), and were less likely to have ventricular arrhythmias, require postoperative dialysis, or return to the operating room for bleeding or to the cardiac catheterization lab for other diagnostic procedures. In addition, these patients were less likely to require transfusion or to remain on a ventilator more than 24 hours. The overall length of hospital stay for patients undergoing CABG with LIMA was 0.9 days shorter (5.6 vs 6.5 days).

Women represent 27.3% of all patients undergoing CABG in the state of Washington. Again, as in the general population, women undergoing CABG with LIMA had a lower observed mortality, lower incidence of ventricular arrhythmias, lower requirement for dialysis, and a shorter length of stay (6 vs 6.8 days, Table 4).

Diabetic patients represent 29.1% of all patients undergoing CABG in the state of Washington. In patients with diabetes undergoing CABG with LIMA, there was a trend toward lower morbidity with fewer myocardial infarctions, fewer ventricular arrhythmias, a lower need for dialysis, and a shorter length of hospital stay (Table 4). In addition, there was a statistically significant reduction in mortality (p < 0.0001).

There was significant variability in the use of LIMA across the state, with an overall rate of 84% but a range of 70% to 92%, representing a 1.3-fold difference in incidence (Fig 1). Only three centers (20%) fell below the national average of 81%, as determined by the Society of Thoracic Surgeon’s database [14]. In high-risk subpopulations, the degree of variability was even more profound. In women, the use of LIMA grafts ranged from 54% to 93%; in patients over the age of 75 years, from 58% to 91%; and, in diabetic patients, from 72% to 97% (Table 5). Of note, centers that had a high usage of LIMA grafts in all cases tended also to have a higher incidence of LIMA usage in the higher risk subgroups (Table 5).

View larger version (81K): [in this window] [in a new window]   Fig 1. Percent left internal mammary artery (LIMA) use by hospital as compared with the national mean. The percentage of LIMA use per institution is represented by each of the bars, with the black line showing a comparison with the national mean as obtained from The Society of Thoracic Surgeons database.

	Comment

Top Abstract Introduction Material and methods Results Comment References

Evaluation of data derived from the COAP database of the state of Washington, which encompasses all interventional revascularization procedures performed in the state, demonstrates that the use of LIMA bypass grafting is associated with better short-term outcomes in all patients, including lower observed mortality, decreased length of stay, lower incidence of ventricular arrhythmias, lower incidence of transfusion, and lower incidence of ventilator dependence. In addition, these benefits hold true for high-risk subgroups, including women, patients over the age of 75 years, and diabetic patients, despite differing risk profiles. The overall incidence of acute complications in patients undergoing CABG with LIMA was significantly lower, even in high-risk groups than in patients undergoing CABG without a LIMA graft. Of note, patients undergoing emergent surgery, patients with history of previous coronary revascularization procedures, and patients undergoing combined valve/coronary procedures were excluded. It would be expected that the outcomes would be similar for patients undergoing emergent surgery or combined coronary/valve procedures. The analysis of patients would be more complex, however, as many of these patients would not have a LIMA graft as a possible conduit due to previous use.

Whereas numerous studies have suggested that women undergoing coronary revascularization are at higher risk than men, there are few reports in the literature that define outcomes in women as related to LIMA utilization. However, the BARI trial noted a similar 5-year mortality for men and women, even though women have higher risk profiles [18]. The data from this study, however, clearly show improved short-term outcomes in women who received LIMA grafts. Despite this, women were the subgroup that was the least likely to receive a LIMA conduit.

Similarly, there is a paucity of literature that explores the effect of LIMA grafting on outcomes in diabetic patients, even though they comprise a significant percentage of the total number of patients requiring surgical revascularization. Morris and associates have noted that internal mammary artery grafting conveys a similar benefit for diabetic as for nondiabetic patients in terms of survival [19]. These data, however, do not reflect advances in surgical technique and improvements over the last 10 to 15 years. The data from the current study show decreases in a variety of short-term outcomes but, most importantly, a significant reduction in mortality associated with LIMA utilization in diabetics.

Given the large percentage of revascularization candidates who are of advanced age, it is understandable that the effect of LIMA utilization in the elderly has been better studied than in the other subgroups. The risk factors for operative mortality are well defined in the elderly [20], and multiple studies suggest that the use of the LIMA is beneficial in terms of both early and late survival [15, 21–23]. The current study confirms these benefits on short-term survival but also demonstrates decreased early morbidity related to LIMA grafting.

Previous work by the Northern New England Cardiovascular Disease Study Group has shown that internal mammary use is not always the standard, and shows great variability from surgeon to surgeon [24]. The data from the current study demonstrate similar variability from center to center. In addition, however, the current study shows clear benefits with regard to LIMA utilization in all of the major subgroups of coronary revascularization candidates that are independent of this variability.

The major limitation of this study is that it is not a prospective randomized study. As such, the decision to use the LIMA as a conduit was not controlled, and there is no way to account for differences in usage. The potential for confounding variables and some selection bias is unlimited. It is clear that subtle differences in patient presentation related to individual anatomic variation and individual patient risk profiles could significantly influence the surgical decision to use the LIMA as opposed to saphenous vein grafts. The data clearly demonstrate that surgeons do exercise these clinical differences and choices prudently, and, in fact, the use of LIMA is associated with a lower incidence of morbidity and mortality.

However, these differences in clinical variables and patient risk profiles between centers cannot explain the wide variability of incidence of LIMA use from 70% to 92% in all patients. These differences must therefore be rooted in clinical biases and perceptions, including the beliefs that use of the LIMA is disadvantageous in certain high-risk subgroups, that it might be technically more demanding, or that it might be associated with a higher incidence of acute morbidity.

The COAP database of the state of Washington was established in an attempt to empower physicians to continue to improve clinical outcomes in patients undergoing coronary revascularization. Given the data from this study, it is clear that further education in the state is necessary to show cardiac surgeons that the use of a LIMA graft, when carefully applied, is associated with an improvement not only of long-term outcomes but acute morbidity as well. LIMA usage appears to be a surrogate of short-term outcome and the long-term durability of coronary artery bypass grafting. Its use, therefore, should be considered as an independent quality indicator. By making this information available to all surgeons in the state of Washington, this study will hopefully encourage greater usage of LIMA grafts, at levels equivalent to the national mean in all patient subgroups, in an effort to improve clinical outcomes for coronary revascularization. ([9,10,11,12]

	Appendix

 
The Clinical Outcomes Assessment Program (COAP) is designed to facilitate collaborative quality improvement for cardiac revascularization in Washington State. Program activities are supported through participant fees. All data collection, analysis, and dissemination are conducted in accordance with Washington State RCW 43.70.510 and the University of Washington Human Subjects Review Board. The specific content of this manuscript may not reflect the opinions or conclusions of all members of the COAP organization.

The COAP organization is described on the COAP website (www.coap.org) including the Management Committee members, sub-committee leaders, and participating institutions. The following institutions contributed data to COAP that was used in this paper:

Deaconess Medical Center Spokane Northwest Hospital Seattle Overlake Hospital Medical Center Bellevue Providence General Medical Center Everett Providence St. Peter Medical Center Olympia Providence Medical Center Yakima Sacred Heart Medical Center Spokane St. Joseph Medical Center Tacoma St. Joseph Hospital Bellingham Southwest Medical Center Vancouver Swedish Health Services First Hill Campus, Seattle Swedish Health Services Providence Campus, Seattle Tacoma General Hospital Tacoma University of Washington Medical Center Seattle Veteran’s Affairs Medical Center Seattle Virginia Mason Medical Center Seattle

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