doi: 10.1161/01.CIR.0000157728.49918.9F
(Circulation. 2005;111:1210-1216.)
© 2005 American Heart Association, Inc.
Cardiovascular Surgery |
Patient and Hospital Differences Underlying Racial Variation in Outcomes After Coronary Artery Bypass Graft Surgery
From the Division of Cardiology, Department of Internal Medicine (S.H.K.), and Division of General Internal Medicine, Department of Internal Medicine (M.S.V.S., G.E.R.), University of Iowa Carver College of Medicine, and the Center for Research in the Implementation of Innovative Strategies in Practice (CRIISP), Iowa City VA Medical Center (M.S.V.S., G.E.R.), Iowa City, Iowa.
Correspondence to Gary E. Rosenthal, MD, Division of General Internal Medicine, University of Iowa Hospitals and Clinics, SE618 GH, 200 Hawkins Dr, Iowa City, IA 52246. E-mail gary-rosenthal{at}uiowa.edu
Received September 21, 2004; revision received January 6, 2005; accepted January 11, 2005.
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Abstract |
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Background— Few studies have examined the association of race and outcomes after coronary artery bypass graft (CABG) surgery while controlling for both patient and hospital effects.
Methods and Results— We retrospectively analyzed data on a cohort of 566 785 white and 24 354 black Medicare beneficiaries 65 years old and older undergoing CABG in 1091 US hospitals from 1997 to 2000. Mortality and repeat revascularization rates were examined after sequential adjustment for patient and hospital differences by use of generalized estimating equations. Unadjusted mortality was higher (P<0.001) in black than in white patients at 30 (6.4% versus 5.2%), 90 (8.3% versus 6.6%), and 365 days (13.5% versus 9.8%) after surgery. Black patients were more likely (P<0.001) to undergo CABG at hospitals with the highest mortality (56% versus 47%) and at hospitals in the lowest volume quintile (24% versus 20%). Adjusted only for patient characteristics, mortality was 8%, 11%, and 25% higher in black patients at 30, 90, and 365 days. After adjustment for hospital effects, 30 and 90 day mortality was similar but 17% higher in black patients at 365 days. Racial differences in mortality were greater in men than in women. On adjustment for patient and hospital effects, repeat revascularization rates were similar in black and white patients.
Conclusions— Racial disparities in CABG outcomes are sensitive to the effects of sex and duration of postsurgical follow-up. The increasing disparity in outcomes as follow-up increased is consistent with the hypothesis that black patients have less access to secondary prevention and rehabilitation services after surgery.
Key Words: race • outcomes • bypass • mortality
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Introduction |
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Coronary heart disease (CHD) affects approximately 12 million people in the United States and accounts for more than half of cardiovascular deaths.1 Although mortality from CHD has decreased in the past 35 years, rates have declined differentially by race and sex. Mortality from heart disease has been consistently higher for men than women and higher in black patients than white patients. Between 1980 and the early 1990s, the percentage decline in CHD mortality was the lowest among black women and the highest among white men.2 In 2001, age-adjusted death rates for heart disease for black patients exceeded those for white patients by 29%.3
Several studies have demonstrated that the use of coronary artery bypass graft surgery (CABG) in patients with CHD varies according to sex and race, with lower rates among women and black patients.4–7 These relationships have held even after adjustment for the presence of specific clinical indications for CABG.6–11
Although several previous studies have examined the association of race and outcomes after CABG, the results have varied depending on the particular populations that have been studied (eg, patients enrolled in controlled trials versus population-based samples).12–15 Studies conducted in patients enrolled in clinical trials may provide more detailed information about clinical end points, but such results may not be generalizable to the larger population of patients undergoing revascularization in US hospitals. In addition, previous studies have often been limited to relatively short-term end points (eg, 30-day mortality) and have generally not accounted for differences in outcomes that may be a result of differential use of hospitals by black and white patients with different underlying quality.15
The present study was designed to examine race-related disparities in outcomes after CABG. The study examined 2 important end points: mortality and repeat revascularization after the index CABG surgery. For each end point, the study examined shorter-term (30-day) and longer-term (90-day and 1-year) outcomes. The study used national Medicare claims data to examine a broad population and used a multistep risk adjustment process that sequentially adjusted for patient and hospital-level differences. Finally, given the results of previous studies showing sex-related differences in outcomes, the present study assessed whether associations between race and outcomes were modified by sex.
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Methods |
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Data
The study used the Medicare Provider and Analysis Review (MedPAR) Part A files for 1997 to 2000 that were purchased from the Centers for Medicare and Medicaid Services (CMS). The MedPAR Part A public use files contain UB-92 hospital discharge abstracts for Medicare patients covered by the hospital care program (Part A) discharged from nonfederal acute-care hospitals and other types of facilities and have been used extensively in health services research.16–19 Data elements include demographic information; primary and secondary diagnoses and procedures, as captured by International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) codes; admission source (eg, transfer from another hospital, emergency room); admission and discharge dates; disposition at the time of hospital discharge; and a 6-digit unique hospital identifier. In addition, Part A files are matched quarterly to the Medicare Enrollment database to obtain dates of death for Medicare beneficiaries who died after hospital discharge. Follow-up was available for all study patients for a minimum of 1 year after the date of CABG surgery.
Patients
CABG procedures performed during 1997 to 2000 (n=715 820) were identified by ICD-9-CM procedure codes (36.10 to 36.19). For patients who underwent more than 1 CABG during the study period (n=7313), only the first CABG was included in the eligible sample. Patients undergoing concomitant valve surgery during the same admission (n=80 523); patients for whom race was listed as Hispanic (n=7885), Asian (n=4137), North-American Native (n=647), Other (n=8800) or Unknown (n=2894); and patients for whom zip-code–level median household income was not available were excluded (n=12 482), leaving a final sample of 591 139 patients who were identified in the database as being black (n=24 354) or white (n=566 785).
Patient Characteristics
Demographic and clinical variables included age, race, sex, admission priority (urgent, elective, or emergent), admission source (eg, emergency room, transfer from another acute-care hospital, or referral by physician), comorbid conditions, and clinical factors reflecting the acuity or difficulty of the CABG surgery (eg, concurrent acute myocardial infarction, CABG performed on the same day as a cardiac catheterization or coronary angioplasty, previous CABG surgery [ie, reoperation]).
Comorbid conditions were determined by use of previously established algorithms for mapping ICD-9 codes to 30 specific conditions (eg, diabetes, chronic obstructive pulmonary disease, congestive heart failure, cerebrovascular disease, peripheral vascular disease) that were developed by Jones et al20 and Elixhauser et al.21 These approaches specifically sought to limit the use of variables that could represent complications (eg, cardiac arrhythmias) that occurred during the incident hospitalization.
Socioeconomic status was assessed by linking zip-code–level data on median household income of persons age 65 years and over from 2000 US Census data to MedPAR data.
Study End Points
The 2 primary study end points were mortality and repeat coronary revascularization after the index CABG surgery. Mortality was measured as deaths occurring at 3 time points: (1) within 30 days of surgery or during the index hospitalization, (2) within 90 days of surgery, and (3) within 365 days of surgery. Repeat coronary revascularization was defined as repeat hospitalization for either CABG or percutaneous coronary intervention (PCI) within 30 days, 90 days, and 365 days of the index CABG surgery.
Measures of Hospital Performance
Two measures of hospital performance, hospital volume and risk-adjusted hospital mortality, were calculated on the basis of the patient-level data in the MedPAR data. Hospital volume was defined as the total number of CABG surgeries, without concomitant valve surgeries, performed on Medicare patients at a given hospital and was expressed as the average number of CABG surgeries per year. Hospital volume was then categorized into quintiles, such that each volume quintile included similar numbers of patients. Volume ranges for the 5 quintiles were 1 to 132, 133 to 214, 215 to 306, 307 to 467, and 
468.
Risk-adjusted mortality rates for each hospital were calculated by dividing the observed 30-day or in-hospital mortality by the predicted mortality rate for that hospital. The predicted 30-day or in-hospital mortality for each hospital was calculated by use of a logistic regression model that included white patients only because of differences across hospitals in the proportion of black patients. The covariates included patient characteristics that were independently related to mortality (see Statistical Analysis, below). Hospitals were then categorized into 6 groups based on ratios of observed to predicted mortality (< 0.50, 0.50 to 0.74, 0.75 to 0.99, 1.00 to 1.49, 1.50 to 1.99, and >2.00).
Statistical Analysis
Our analysis included 3 steps. First, we compared the prevalence of demographic and clinical risk factors in black and white patients and rates of the study end points in black and white patients by use of the 
2 test. Second, we developed patient-level risk-adjustment models for each end point. Third, we developed additional multivariable models that also adjusted for differences in hospitals used by black and white patients.
Development of Patient-Level Risk-Adjustment Models
Separate risk-adjustment models for each end point were developed by entering candidate patient risk factors associated (P<0.05) with each end point in bivariate analyses into stepwise logistic regression models. Patient risk factors that were independently associated (P<0.01) with outcomes in the logistic regression models were retained in the final set of risk-adjustment models. In these analyses, age was examined as a continuous variable; admission priority was expressed by use of indicator variables for emergent and urgent admissions (relative to elective admissions); admission source was expressed as indicator variables for patients transferred from another acute-care facility and admitted through the emergency room (relative to patients referred by a physician); comorbid conditions were included individually as indicator variables; socioeconomic status was measured by incorporating zip-code–level median household income as a continuous variable; and high-risk clinical conditions (eg, CABG on the same day as cardiac catheterization) were expressed as individual indicator variables. Variables included in the mortality and repeat revascularization models are shown in Table 1. After the identification of patient-level predictors, an indicator variable for black patients was added to the risk-adjustment models to determine the odds of the outcome in black patients relative to white patients. Models for each end point were estimated by use of generalized estimating equations22 with an independent working correlation matrix (which assumes that outcomes for all patients are independent) and robust methods for calculating the 95% confidence intervals for regression coefficients.23
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Adjustment for Hospital-Level Differences
To more directly account for the possibility that black and white patients are clustered within different hospitals, a second set of generalized estimating equation models were estimated that used an exchangeable correlation matrix, which assumes that outcomes for individual patients are equally correlated within hospitals but statistically independent across hospitals. This approach used the unique hospital identifier included in the Medicare data to directly account for differential use of hospitals by black and white patients. Differences in the odds ratios associated with black race between the first set of models adjusting for patient characteristics and the second set of models adjusting for patient characteristics and hospital differences is a reflection of the impact of hospital-level differences on the relative mortality in white and black patients.
We first tested for the significance of sex by race interaction. Analyses were then repeated by use of samples stratified by sex to obtain sex-specific risk-adjusted mortality and revascularization rates. All analyses were performed by use of SAS version 8.2 (SAS Institute Inc).
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Results |
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Black patients were younger (mean ages, 72.4 versus 73.8 years; P<0.001) and were less likely to have had a previous CABG (P<0.001) (Table 2). Black patients were also less likely to have chronic obstructive pulmonary disease and to undergo cardiac catheterization or angioplasty on the same day as CABG. However, black patients were more likely to be female and to have hypertension, diabetes, peripheral vascular disease, renal failure, and CHF. Black patients were also more likely to be admitted emergently and to have had an acute myocardial infarction. The average zip code-estimated median household income for black patients was lower than the average median household income for whites ($33 528 versus $42 909, P<0.001). Of the black patients, 49% resided in zip codes in the lowest quintile for median household income, compared with 19% of white patients (P<0.001); similarly, only 8% of black patients resided in zip codes in the highest quintile for median household income, compared with 21% of white patients (P<0.001).
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The median volume of CABG surgeries at hospitals in which black patients underwent surgery was lower than the median volume at hospitals in which white patients underwent surgery (n=237 versus n=256; P<0.001). Furthermore, a greater proportion of black patients underwent surgery at hospitals in the lowest volume quintile (24% versus 20%; P<0.001) (Figure 1). Black patients were also more likely to undergo CABG at hospitals with higher mortality rates than white patients. For example, a greater proportion of blacks underwent CABG at hospitals with ratios of observed to predicted mortality rates greater than 1.0 (56% versus 47%; P<0.001) (Figure 2).
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Unadjusted mortality was higher (P<0.001) for black patients than for white patients in analyses of 30-day or in-hospital mortality (6.4% versus 5.2%), 90-day mortality (8.3% versus 6.6%), and 365-day mortality (13.5% versus 9.8%). Similarly, unadjusted odds ratios for the 3 end points were 1.26, 1.29, and 1.44, respectively (Table 3).
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The racial difference in the odds of death decreased substantially after adjustment for patient-level factors. For example, in-hospital or 30-day mortality was only 8% higher (95% CI, 1% to 15%) in black patients relative to white patients, after adjustment for patient-level factors. Adjustment for hospital-level effects (ie, differences in hospitals used by black and white patients) further diminished differences in mortality, such that 30-day or in-hospital mortality was similar (OR=1.03; 95% CI, 0.97 to 1.09).
Although these patterns were similar for 90-day and 365-day mortality, the magnitude of the differences in mortality increased over time. For example, after adjustment for patient and hospital-level effects, the odds of death at 365 days were 17% higher (95% CI, 12% to 22%) in black patients.
For each of the 3 end points, interactions between sex and race were significant (P<0.001) in analyses adjusting for patient and hospital effects. In stratified analyses, differences in mortality by race were greater in men than in women (Table 3). After patient and hospital-level effects had been accounted for, the odds of death were similar in black women and white women at 30 and 90 days and were 10% higher (95% CI, 4% to 18%) in black women at 365 days. In contrast, the odds of death in black men, relative to white men, were 15% higher (95% CI, 6% to 25%) at 30 days, 16% higher (95% CI, 7% to 25%) at 90 days, and 28% higher (95% CI, 20% to 36%) at 365 days after adjustment for patient- and hospital-level effects. Racial differences in mortality were generally similar over the 4-year duration of the study, because analyses found no evidence of an interaction between race and year of surgery (P>0.11 for all years).
Overall repeat revascularization rates (CABG or PCI) for both black and white patients were 0.2% at 30 days, 0.7% at 90 days, and 2.2% at 365 days. An overwhelming majority of the repeat procedures represented PCI (eg, 93% of procedures within 365 days). Unadjusted rates of repeat revascularization in black and white patients were similar at 30 (P=0.18) and 90 days (P=0.96) but were higher in blacks at 365 days of the index CABG (2.5% versus 2.1%, P<0.001). Adjusted for patient and hospital-level effects, rates of repeat revascularization were somewhat lower at 30 days (P=0.05) and 90 days (P=0.32) and somewhat higher at 365 days (P=0.14) in black patients (Table 4). In separate analyses of men, racial differences in repeat revascularization rates increased over time, similar to the pattern seen for mortality (Table 4).
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Discussion |
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This study represents the largest contemporary analysis of racial variation in outcomes after CABG surgery. Using a generalizable national database, our analysis was unique in its longitudinal follow-up of patients to determine longer-term mortality and rates of repeat revascularization, its sequential adjustment for both patient- and hospital-level factors, and its evaluation of sex-specific differences in racial variation.
We emphasize 4 findings from this study. First, black patients were more likely to undergo CABG in hospitals with lower volume and with higher risk-adjusted mortality rates. This differential use in hospitals explained some of the differences between black and white patients in higher unadjusted mortality. Although these differences may reflect differences in geographic proximity of different hospitals, the differences may also reflect more fundamental difficulties in accessing lower-mortality hospitals.
Second, racial differences in mortality were more pronounced in men than in women, such that, even after adjustment for patient and hospital effects, mortality in black men, compared with white men, was significantly higher at all 3 time points evaluated. Although few previous studies have evaluated sex-specific differences in outcomes, the differences we observed in men and women suggest several potential hypotheses. It is possible that black men have lower access to health care than black women and may present relatively later in the course of illness. It is also possible that the quality of care received by black men within individual hospitals may be lower than that received by white men, whereas care received by white and black women is similar. Similarly, survival advantages after CABG, because of better postsurgical care and rehabilitation, may be limited to white men. Finally, differences may exist in the relative use of CABG and PCI procedures in black men and black women, such that percutaneous procedures are preferentially used in lower-risk black men, whereas CABG is preferentially used in black men with more severe forms of coronary artery disease or at higher risk of adverse outcomes.
Third, the study found that differences in mortality between black and white patients increased as the period of postsurgical follow-up increased. After adjustment for patient characteristics and hospital effects, mortality was similar at 30 and 90 days after surgery but was significantly higher for black patients 1 year after CABG. The temporal trends in mortality we observed are also consistent with a previous study by Gray et al,13 who reported no significant differences in 30-day mortality between black and white patients who underwent CABG in a single hospital during 1984 through 1992 but higher mortality for black patients 1 year after CABG. Furthermore, a single hospital study by Peterson et al24 found higher risk-adjusted mortality in black patients 5 years after CABG.
Finally, the study found that rates of repeat revascularization exhibited temporal patterns similar to those seen for mortality, with odds ratios that increased as the period of follow-up increased, although the magnitude of differences was less than those observed for mortality. To the best of our knowledge, our study represents the first study examining racial variation in repeat revascularization rates.
Several possible factors, both at patient level and at hospital level, could explain why the differences in mortality and repeat revascularization rates increased over time. Black patients had a higher prevalence of several comorbid conditions and were more likely to be admitted emergently, which could suggest that black patients undergoing CABG were sicker and presented much later in the course of the disease, which has been demonstrated previously.25–28 The higher rate of emergent admissions may also indicate that black patients have less access to primary care or to primary and secondary preventive care. Thus, after CABG, black patients may receive suboptimal longitudinal care encompassing recommendations for lifestyle changes and risk-factor modification. This possibility is consistent with a recent report that highlighted the differences in training and access to clinical resources of physicians providing primary care to black and white patients29 and with previous reports of the underuse of medical therapy and cardiac rehabilitation in black patients.30–33
Our findings are also consistent with those of other previous studies. For instance, the odds ratios for 30-day or in-hospital mortality that we observed were relatively similar to those of a previous analysis15 based on the Society of Thoracic Surgeons clinical database, and they confirm findings in this study of a lack of difference in unadjusted mortality between black and white women. However, this previous analysis did not evaluate long-term outcomes or account for differences in the types of hospitals used by black and white patients or for potential confounding by socioeconomic factors.
In interpreting our findings, several potential methodological limitations should be considered. Our analysis may be confounded by unmeasured differences between black and white patients in such factors as disease severity, functional status, preferences for care and compliance with recommended care, and social support. In light of our findings, such differences may be more pronounced in black men than in white men. Similarly, our analysis was based on administrative data, which do not include important prognostic variables, such as left ventricular ejection fraction, and which may be subject to variations across hospitals in the reliability of coding of specific comorbid and acute conditions. In addition, we relied on zip-code–level data to measure socioeconomic differences, as opposed to patient-level data, an approach that has been validated by previous studies.34–37 Finally, our analysis was limited to Medicare beneficiaries, and the generalizability to patients younger than 65 years is uncertain. Yet, more than half of CABG surgery in the United States is performed in persons 65 years old and older.1 Moreover, a recent analysis found that analyses based on Medicare patients only might yield unbiased estimates of outcomes in all patients.38 A further limitation is that Medicare claims files typically do not include information on patients enrolled in Medicare managed-care plans. Such enrollment may differ in black and white patients. However, managed care enrollees are younger and healthier than fee-for-service enrollees39 and are therefore likely to have lower rates of CABG surgery, so the impact of this limitation may be small.
In summary, the present study indicates that racial differences in mortality exist after CABG surgery. These differences may be more pronounced in men than in women, may increase as the period of follow-up increases, and may, in part, reflect differential use of high-mortality hospitals by black patients. In light of a recent Institute of Medicine report40 and the goals established in Healthy People 2010 to eliminate disparity and improve cardiovascular health,41 the present findings indicate that continued efforts are necessary to ensure equitability in access to appropriate preventive and rehabilitative services by black patients. Last, the findings indicate the importance of sequential adjustment for both patient- and hospital-level characteristics in evaluating racial disparities in outcomes.
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Acknowledgments |
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This research was supported, in part, by an award (HFP 04-149) from the Health Services Research and Development Service, Veterans Health Administration, Department of Veterans Affairs. Dr Rosenthal is a Senior Quality Scholar, Office of Academic Affiliation, Veterans Health Administration. Dr Konety was supported by a Cardiovascular Interdisciplinary Fellowship supported in part by National Institutes of Health/National Research Service Award grant 5-T32-HL-07121-28 from the University of Iowa Cardiovascular Research Center.
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Footnotes |
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The views expressed in this article are those of the authors and do not necessarily represent the views of the Department of Veterans Affairs.
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 L. L. Nguyen, N. Hevelone, S. O. Rogers, D. F. Bandyk, A. W. Clowes, G. L. Moneta, S. Lipsitz, and M. S. Conte Disparity in Outcomes of Surgical Revascularization for Limb Salvage: Race and Gender Are Synergistic Determinants of Vein Graft Failure and Limb Loss Circulation, January 6, 2009; 119(1): 123 - 130. [Abstract] [Full Text] [PDF]
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 C. W. Yancy, W. T. Abraham, N. M. Albert, R. Clare, W. G. Stough, M. Gheorghiade, B. H. Greenberg, C. M. O'Connor, L. She, J. L. Sun, et al. Quality of Care of and Outcomes for African Americans Hospitalized With Heart Failure: Findings From the OPTIMIZE-HF (Organized Program to Initiate Lifesaving Treatment in Hospitalized Patients With Heart Failure) Registry J. Am. Coll. Cardiol., April 29, 2008; 51(17): 1675 - 1684. [Abstract] [Full Text] [PDF]
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 N. Menachemi, A. Chukmaitov, L. S. Brown, C. Saunders, and R. G. Brooks Quality of Care Differs by Patient Characteristics: Outcome Disparities After Ambulatory Surgical Procedures American Journal of Medical Quality, December 1, 2007; 22(6): 395 - 401. [Abstract] [PDF]
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A. M. Davis, L. M. Vinci, T. M. Okwuosa, A. R. Chase, and E. S. Huang Cardiovascular Health Disparities: A Systematic Review of Health Care Interventions Med Care Res Rev, October 1, 2007; 64(5_suppl): 29S - 100S. [Abstract] [PDF]
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R. H. Jones The Year in Cardiovascular Surgery J. Am. Coll. Cardiol., May 16, 2006; 47(10): 2094 - 2107. [Full Text] [PDF]
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J. S. Rumsfeld and A. J. Epstein Racial Disparities in Cardiovascular Procedure Outcomes: Turn Down the Volume J. Am. Coll. Cardiol., January 17, 2006; 47(2): 425 - 426. [Full Text] [PDF]
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N. R. Kressin Separate but Not Equal: The Consequences of Segregated Health Care Circulation, October 25, 2005; 112(17): 2582 - 2584. [Full Text] [PDF]
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J. Skinner, A. Chandra, D. Staiger, J. Lee, and M. McClellan Mortality After Acute Myocardial Infarction in Hospitals That Disproportionately Treat Black Patients Circulation, October 25, 2005; 112(17): 2634 - 2641. [Abstract] [Full Text] [PDF]
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H. M. Krumholz The Year in Epidemiology, Health Services, and Outcomes Research J. Am. Coll. Cardiol., October 4, 2005; 46(7): 1362 - 1370. [Full Text] [PDF]
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R. O. Bonow, A. O. Grant, and A. K. Jacobs The Cardiovascular State of the Union: Confronting Healthcare Disparities Circulation, March 15, 2005; 111(10): 1205 - 1207. [Full Text] [PDF]
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