Differences in Mortality and Use of Revascularization in Black and White Patients With Acute MI Admitted to Hospitals With and Without Revascularization Services FREE

Racial differences in care after acute myocardial infarction (AMI) have been widely reported.¹ These differences appear most marked for the use of invasive and costly technologies, such as coronary revascularization,^2- 4 although studies have documented similar benefits of post-AMI coronary revascularization in white and nonwhite patients.⁵

The factors underlying racial disparities are complex and may reflect differences in clinical presentation^6- 8 or medical decision making.⁹ More recently, studies have focused on systematic differences in the quality of care of hospitals and practitioners used by white and minority patients.^10- 12 These studies suggest that the different access to high-quality providers (ie, guideline-compliant, low-mortality hospitals) may be an important underlying cause of disparities.

Although ischemic heart disease is a common reason for inpatient hospitalization, less than one fifth of US acute care hospitals provide coronary revascularization,¹³ and a significant proportion of Medicare beneficiaries hospitalized with AMI are initially admitted to hospitals that do not provide either coronary artery bypass graft (CABG) surgery or percutaneous coronary intervention (PCI).¹⁴ Thus, to understand racial disparities in the use of coronary revascularization and long-term outcomes, it is important to examine differences in patterns of care among patients admitted to hospitals that do and do not offer revascularization. However, relatively few studies have examined such patterns and their potential impacts on disparities.^15- 16 Thus, the goal of this study was to assess racial differences in patterns of care and mortality for a contemporary national cohort of patients with AMI who were initially admitted to hospitals with and without revascularization services.

We used Medicare Provider Analysis and Review (MedPAR) data files for calendar years 2000 to 2005, which were obtained from the Centers for Medicare & Medicaid Services (CMS). The MedPAR files contain data on all Medicare fee-for-service hospitalizations, including demographic information; patient ZIP code and state of residence; primary and secondary diagnoses and procedures, as defined by International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes; admission source (eg, transfer from another hospital, emergency department); admission and discharge dates; disposition at hospital discharge; and a 6-digit unique hospital identifier. In addition, part A files are matched quarterly to the Medicare Enrollment database to incorporate dates of death after hospital discharge.

The study also used CMS Medicare Denominator files to determine patients' enrollment in Medicare part B before admission and the US Census 2000 Summary File (available at http://factfinder.census.gov) to determine ZIP code–level socioeconomic measures (eg, median household income). Rural-urban commuting area codes (available at http://www.ers.usda.gov/Data/RuralUrbanCommutingAreaCodes) were used to classify patients' residential ZIP codes into urban, large rural, small rural, and remote small rural areas. Before we conducted this study, we obtained approval with a waiver of consent from the University of Iowa institutional review board.

The patient cohort was drawn from 1 264 134 Medicare beneficiaries 68 years and older, identified in the MedPAR files with a primary diagnosis of AMI (ICD-9-CM 410), who were admitted between January 1, 2000, and June 30, 2005, to 4657 US hospitals. Of these patients, 699 926 (57.6%) were admitted to 1169 hospitals that performed at least 2 CABG and PCI procedures in Medicare beneficiaries during the year of patient discharge (ie, full-revascularization hospitals), and 515 998 (42.4%) were admitted to 3488 hospitals not offering full revascularization services. Patients who were admitted for AMI, PCI, or CABG within the previous 3 years were excluded to limit the sample to patients with initial episodes of myocardial infarction. Thus, the sample was restricted to patients aged 68 years and older at the AMI admission to ensure at least 3 years of previous Medicare data. Finally, the analysis focused on comparisons between 85 069 (6.7%) black and 1 130 855 (89.5%) white patients admitted to 4627 US hospitals, excluding 48 211 (3.8%) patients of other racial groups or for whom race data were missing. Vital status was available on all patients through March 31, 2006.

For each patient, we created a longitudinal record, including the index admission, transfers to other acute care hospitals, and subsequent hospitalizations or coronary revascularization procedures. Transfer was defined as admission to a second acute care facility within 1 day of discharge from the admitting hospital, with the additional criterion that either the discharge disposition from the admitting hospital indicated transfer to another acute care facility or the admitting source for the second facility indicated transfer from another acute care facility. Admissions to acute care facilities occurring more than 2 days after discharge from the admitting hospital were considered readmissions.

Main outcome measures included transfer to another acute care hospital with revascularization services, for patients admitted to hospitals without full revascularization; coronary revascularization with either PCI or CABG within 30 days of the index admission date; mortality within 30 days, 90 days, and 1 year after admission; and readmissions within 30 days for AMI or other forms of ischemic heart disease (ICD-9-CM codes 410-414 and 786.5). Transfers were further characterized as occurring within 2 days of the index admission date or more than 2 days after admission. One-year mortality was available for all patients except those admitted April to June 2005 (n = 46 504).

Race was identified in hospital discharge abstract data available in the MedPAR files and was categorized as either black or white. Other demographic and socioeconomic covariates included age (categorized as younger than 70, 70-74, 75-79, 80-84, and 85 years and older), sex, median household income (based on residential ZIP code–level data), urban vs rural residence (defined according to rural-urban commuting area codes), enrollment in Medicare part B during the month before admission, and distance from the admission hospital to the nearest hospital offering revascularization (based on distance between the ZIP code centroids of the 2 hospitals). Clinical variables included comorbid conditions (defined by ICD-9-CM secondary diagnosis codes, with previously defined algorithms for administrative data^17- 18), AMI location (categorized into anterior or lateral, inferior or posterior, subendocardial, or other unspecified locations, defined by the fourth digit of the primary ICD-9-CM code), severity indicators (use of an intra-aortic balloon pump or nonoperative mechanical ventilation on the day of admission), and year of discharge. Data on prescribed medications at discharge were not available.

Clinical and socioeconomic characteristics, rates of transfer, rates of revascularization with either CABG or PCI, and rates of death were compared for black and white patients with the χ² test and t test. The relationship between race and the likelihood of transfer, revascularization, and death was then evaluated with Cox proportional hazards models to adjust for differences in baseline sociodemographic characteristics, distance to the nearest revascularization hospital, comorbidity, and markers of disease severity. Comorbid conditions were selected into the model if they were significant (P<.001) in bivariate analyses. We used analysis of Schoenfeld residuals to test for possible violations of the proportional hazards assumption and in the case of mortality models to correct for such departures from the assumption of proportionality. Mortality models included interaction terms between race and 4 periods. This approach enabled us to estimate separate hazard ratios for black patients for the 4 discrete intervals. Although the interaction between race and time was also significant in revascularization models, the hazard of receiving revascularization was consistently lower for black patients at all points; therefore, we chose to present only the average 30-day hazard. All models were calibrated with a robust sandwich covariance matrix estimate to accommodate potential clustering of patients within hospitals. Mortality and revascularization analyses were conducted separately for patients admitted to hospitals with and without full revascularization services.

P values were 2 sided. Statistical significance was defined with a conservative criterion of P<.01. All analyses were performed with SAS statistical software version 9.1 (SAS Institute Inc, Cary, NC).

Compared with white patients, black patients were younger, more likely to be female, and more likely to reside in urban areas or areas with lower median household income (Table 1). These differences were consistent in hospitals with and without revascularization services. Black patients were more likely to have congestive heart failure, diabetes, peripheral vascular disease, renal failure, weight loss, and dementia but were less likely to have arrhythmia or chronic obstructive pulmonary disease. Black patients were also more likely to have subendocardial infarctions and less likely to have anterior, lateral, inferior, or posterior infarctions. Among patients in hospitals without revascularizations, black patients were admitted to hospitals that were closer on average to hospitals with revascularization services.

Among patients admitted to hospitals without revascularization services, 30.6% (n = 157 954) were transferred to a second acute care hospital, and of these, 36.6% (n = 57 771) occurred within the first 2 days of admission. Nearly all transfers (98.2%; n = 191 876) were to a hospital with revascularization capability. In unadjusted analyses, black patients admitted to hospitals without revascularization services were less likely than white patients (P<.001) to be transferred to a hospital with revascularization services within 2 days (7.4% vs 11.5%) and within 30 days (25.2% vs 31.0%) of admission.

At 30 days, 49.1% (n = 343 490) of patients admitted with AMI to hospitals with revascularization and 25.4% (n = 130 973) of patients admitted to hospitals without full revascularization had received coronary revascularization procedures. For patients admitted to hospitals with full revascularization services, 96.0% (n = 329 735) of these procedures were performed at the admitting hospital, 1.8% (n = 6090) at a transfer facility, and 2.2% (n = 7665) during subsequent admissions. For patients admitted to hospitals without full revascularization services, 8.5% (n = 11 081) of these procedures were performed at the admitting hospital (PCI only), 83.1% (n = 108 876) at a transfer facility, and 8.4% (n = 11 016) during subsequent admissions.

Overall, black patients had lower rates of revascularization at 30 days compared with white patients (27.8% vs 39.9%; P<.001). Rates of revascularization were lower in black patients than in white patients for total revascularization procedures, as well as for either PCI or CABG. Results were consistent for analyses of patients admitted to hospitals with and without revascularization services and for analyses limited to patients who were admitted to hospitals without revascularization services and who were eventually transferred (Table 2).

In Cox proportional hazards analyses adjusting for patient sociodemographic factors, clinical characteristics, distance, and urbanicity, the likelihood of transfer for black patients admitted to hospitals without revascularization was 22% lower compared with that of white patients (hazard ratio, 0.78; 95% confidence interval, 0.75-0.81; P<.001). The adjusted hazard ratio of undergoing coronary revascularization was also lower in black patients compared with white patients, in analyses of patients admitted to hospitals with and without revascularization, and in analyses limited to transfer patients (Table 2). These differences were consistent for total revascularization and for PCI and for CABG when examined separately.

Despite lower revascularization rates, black patients were less likely to be readmitted for ischemic heart disease during the 6 months after AMI than white patients (12.7% vs 13.2%; P<.001). The difference in readmission rates was smaller for black and white patients admitted to hospitals with revascularization (12.2% vs 12.6%; P = .01) than for patients admitted to hospitals without full revascularization (13.4% vs 14.1%; P = .001).

Overall, unadjusted 30-day mortality was similar for black and white patients (18.1% vs 18.1%; P = .85) but was higher (P<.001) for blacks at 90 days (25.4% vs 24.0%), 180 days (30.9% vs 28.3%), and 1 year (37.1% vs 33.3%). In separate analyses of admissions to hospitals with and without full revascularization, mortality rates were consistently higher for black patients admitted to hospitals with revascularization services; black patients admitted to hospitals without revascularization services had lower mortality at 30 days but higher mortality rates at 6 months and 1 year, regardless of transfer status (Table 3).

In models adjusting for demographic and clinical covariates, the hazard ratio of death for black patients relative to white patients was lower during the first 30 days after AMI in analyses of patients admitted to hospitals either with or without revascularization and among transfers. However, hazard ratios in black patients were significantly higher during days 31 to 90, 91 to 180, and 180 to 365 (Table 4). Further adjusting for receipt of revascularization as a time-dependent covariate decreased the magnitude of the hazard ratios but did not affect the significance of the differences (Table 4), with the exception of mortality after 90 days in patients admitted to hospitals without revascularization and mortality in transferred patients between 91 and 180 days.

Using claims data for Medicare beneficiaries with AMI who were admitted to hospitals with and without full coronary revascularization services, this study found that black patients had overall lower 30-day rates of coronary revascularization than white patients. Additionally, black patients were less likely to be transferred from hospitals without revascularization to hospitals with full coronary revascularization services and less likely to receive revascularization even after transfer. These differences persisted after accounting for sociodemographic factors, clinical characteristics, and distance to the nearest revascularization hospital.

Black patients also had lower adjusted rates of death 30 days after admission for AMI but had higher rates of death thereafter in analyses of patients admitted to hospitals with and without revascularization services and in analyses restricted to patients transferred from hospitals without revascularization services. Although some of the differences in mortality could be explained by the lower rates of coronary revascularization, differences in the risk of longer-term death generally remained statistically significant even after adjustment for the receipt of revascularization.

The current findings complement previous studies of racial disparities in the care of patients after AMI, which have shown that black patients are less likely to receive acute reperfusion therapy after AMI,^19- 21 to be referred for coronary angiography after AMI,^22- 23 and to undergo PCIs or surgical revascularization in the subacute phase of myocardial infarction.^19,23- 25 These differences are evident even in analyses focusing on ideal clinical candidates for revascularization and adjusting for clinical predictors.^2,19,23

Although data on disparities in the use of coronary interventions are abundant, less is known about how differential patterns of hospital care may contribute to population-based disparities. Such data are critical to understanding sources of disparities in cardiovascular care, given recent arguments for regionalization of coronary revascularization and other high-technology services.

In general, studies examining the influence of the organization of the health care delivery system on AMI outcomes have been restricted to few institutions, interventional hospitals, or immediate treatment strategies^26- 29 and did not address important questions about outcomes in the general population. Little information exists on the impact of availability of revascularization services on racial variations in AMI care. Several earlier studies evaluating the influence of the type of hospital admission on revascularization rates for black and white patients found lower procedure use for blacks independent of admitting hospital type.^30- 31 One study using 1993 discharge records from acute care hospitals in New Jersey concluded that use of cardiac invasive procedures after AMI was lower in black patients, regardless of the services available, but that, despite lower procedure rates, 1-year mortality rates did not differ between blacks and whites.³² However, the study was limited to 1 state registry and did not investigate differences in pattern of care beyond first admission for AMI.

To our knowledge, only 1 study has examined disparities in hospital transfer after initial admission for AMI.¹⁶ This study used data from the National Registry of Myocardial Infarction 2, a voluntary registry of patients from 1674 participating hospitals, and found that black patients were less likely to be transferred to another hospital after admission for AMI. Although the registry includes data abstracted from patients' medical records and a broader set of clinical risk factors than is available from administrative data, participating hospitals tend to be larger and more likely to offer coronary revascularization than US hospitals in general. Moreover, unlike the MedPAR data used in the current study, the registry does not contain data on patient outcomes after hospital discharge.

Thus, the current study adds to our understanding of racial differences in care after AMI. First, it examines patterns of care for patients admitted to hospitals with and without full revascularization capabilities and specifically examines hospital transfer patterns among patients admitted to hospitals without full revascularization services, who accounted for more than 40% of study patients. Among such patients, transfer is a major determinant of access to coronary revascularization after AMI. Second, the study was conducted using Medicare data and thus minimizes potential confounding in the delivery of hospital-based care because of differences in health insurance. Third, the study accounted for the potential impact of differences in distance to the nearest revascularization hospital, a factor that previous analyses have often ignored. Finally, the current study examined the impact of racial differences in AMI care on posthospital mortality. Similar to those in earlier studies,^22,33 black patients in the current study had lower early mortality, a finding that could be partially explained by the lower rates of transfer and procedure use after AMI, which carry an intrinsic risk of death. This mortality advantage disappears once the observation period is extended beyond 30 days, consistent with several earlier studies.^2,5 Although the higher risk of long-term mortality may reflect potential adverse effects of lower rates of early revascularization in black patients, receipt of revascularization may be a marker for other aspects of follow-up care, such as more aggressive management of cardiac risk factors. This latter possibility is particularly salient, given recent studies that have questioned the benefits of revascularization after AMI in certain clinical subgroups.³⁴

The study has several limitations. First, unmeasured aspects of medical decision making that may affect the process of care, including patient and family preferences, cannot be ascertained from administrative data. Although previous studies suggest that the proportion of patients refusing myocardial reperfusion is small and does not vary by race,^19,23 patient preferences may play a more prominent role in more delayed revascularization treatments. Similarly, although a previous single-center study found no racial difference in physician recommendations for coronary revascularization,³⁵ data on physician decision making in AMI treatment are scarce.

Second, the study was unable to account for important clinical indications for transfer and revascularization. For example, we were unable to adequately differentiate between patients with ST-segment elevation AMI (STEMI) and NSTEMI. Nevertheless, our analyses accounted for anatomical infarction site as a proxy for STEMI and NSTEMI. We were also unable to capture the use of thrombolytics in the MedPAR data, which has been shown to differ for black and white patients,¹⁹ and were unable to examine the potential impact of economic factors, such as the availability of supplemental health insurance.

Third, the development of risk-adjustment models based on administrative data has inherent limitations, and the reliability of individual diagnosis codes may vary across study hospitals; such data may not capture important long-term prognostic factors, such as body mass index, smoking, and follow-up medication treatment. Last, the analysis was restricted to elderly Medicare fee-for-service beneficiaries and thus may not be generalizable to younger patients or to patients enrolled in Medicare managed care.

Despite these limitations, the current study provides evidence that racial differences in the use of revascularization after AMI are of similar magnitude for patients admitted to hospitals with and without full revascularization capability and persist even for patients transferred from hospitals without full invasive cardiac services to hospitals providing these services. These differences could be due to unmeasured clinical or socioeconomic factors, patient preferences, and unmeasured aspects of medical decision making but are unlikely to be related to differences in access to hospitals performing revascularization procedures. Although differences in revascularization may reflect overuse of procedures in white patients, the receipt of revascularization could also explain some of the differences in longer-term mortality in black patients and may represent a broader marker of differences in post-AMI care between black and white patients.

Thus, as data on the benefits of revascularization in different patient subgroups continue to emerge, efforts to standardize post-AMI treatment with evidence-based protocols and aggressive risk-factor management are essential to eliminating racial differences in care for AMI and other coronary syndromes.