Sex Differences in Cardiac Catheterization after Acute Myocardial Infarction: The Role of Procedure Appropriateness

Context Women have cardiac catheterization after an acute myocardial infarction less often than men; however, the reasongender bias or clinical factorsis unknown. Contribution This large retrospective study of elderly Medicare patients compared numbers of indicated (guideline-recommended) and actual catheterizations performed within 60 days of acute myocardial infarction for women and men. Overall, women less often had cardiac catheterization than men but rates of the procedure did not differ between women and men with clear-cut guideline indications for catheterization. Implications Differences in catheterization use after acute myocardial infarction usually reflect appropriate clinical decisions rather than gender bias. The Editors Data on sex variations in use of cardiac catheterization after an acute myocardial infarction are mixed, with some studies reporting that the procedure is used less often in women (1-13) and others finding no sex differences in use of the procedure (9, 14-19). Despite these conflicting findings, some observers have suggested that sex variations in cardiac care are evidence of sexual discrimination by physicians (20, 21) and reflect a pervasive gender bias in the health care system (22). Although studies of cardiac catheterization use have examined the effect of clinical (1-6, 8, 10, 12, 13, 23), hospital (5, 8, 13), and payer characteristics on sex differences in cardiac procedure use (1-5, 8, 10, 13), few have investigated the contribution of procedure appropriateness to sex differences in procedure use. Without considering the appropriateness of cardiac catheterization for an individual patient, it is unclear whether sex variations in cardiac catheterization use reflect proper clinical practice based on a difference of need, undertreatment of women, overtreatment of men, or a combination of factors. The few studies that have examined sex differences in cardiac catheterization use after adjusting for appropriateness of the procedure have included small numbers of patients from single centers or other selected populations and have reported conflicting results (18, 24-26). Thus, the extent to which sex differences in cardiac catheterization use after an acute myocardial infarction may be attributed to sex differences in appropriateness of the procedure is unknown. We evaluated data from the Cooperative Cardiovascular Project (CCP), a set of medical record data abstracted for Medicare beneficiaries hospitalized with acute myocardial infarction between 1994 and 1996, to determine whether patient appropriateness for cardiac catheterization explained sex differences in procedure use. Specifically, we sought to evaluate whether sex differences in cardiac catheterization use were seen in patients with strong indications, equivocal indications, or weak indications for the procedure. Methods CCP and the Study Cohort The CCP has been described in greater detail elsewhere (27). Briefly, Medicare patients hospitalized with a primary discharge diagnosis of acute myocardial infarction (International Classification of Diseases, 9th Revision, clinical modification [ICD-9-CM], code 410.X, with the exception of readmissions code 410.X2) (28) were sampled, by state, for an 8-month period between January 1994 and February 1996. Among the 234 769 hospitalizations abstracted, we excluded patients who were younger than 65 years of age (n = 17 593), were not hospitalized with a chart-confirmed myocardial infarction (n = 31 186), or had transferred into the index hospital (n = 42 278). Patients were sorted by admission date, and readmissions during the study period were excluded (n = 23 773). We also excluded patients for whom American Hospital Association (n = 2363), 1990 U.S. Census (n = 6322), or Medicare Part A (n = 34 187) data were not available. Finally, we excluded 1765 patients hospitalized outside of the United States, 61 patients whose race or ethnicity were unknown, and 308 patients whose mortality status was unknown at follow-up. A total of 91 325 patients met one or more of these criteria; the remaining 143 444 patients constitute the study cohort. Appropriateness of Cardiac Catheterization Indications for having cardiac catheterization were evaluated after the acute phase of infarction (>12 hours after symptoms onset). Using the 1996 American College of Cardiology/American Heart Association (ACC/AHA) guidelines (29) and appropriateness criteria previously reported by Guadagnoli and colleagues (30), we classified patients into three groups (Table 1). The strong indication group consisted of patients in whom cardiac catheterization was generally recognized as beneficial, useful, and effective (ACC/AHA class I) (29). The equivocal group included patients in whom data on the effectiveness of the procedure were unclear (ACC/AHA class IIa [evidence may favor cardiac catheterization]; ACC/AHA class IIb [evidence may not favor cardiac catheterization]) or patients with uncomplicated myocardial infarctions (neither ACC/AHA class I, class II, nor class III). The weak indication group included patients who had conditions for which cardiac catheterization was considered unlikely to be effective (ACC/AHA class III). For patients who met more than one classification criterion (for example, a patient who had angina for more than 24 hours after admission and concomitant hepatic failure), classifications were prioritized by using the following order: weak indications (class III), strong indications (class I), equivocal indications (class IIa and class IIb); this was done to maximize specificity among patients classified as having a strong indication. Table 1. Cardiac Catheterization Appropriateness Criteria Cardiac Catheterization Use The principal outcome was the use of nonacute cardiac catheterization within 60 days of hospital admission, as determined by evaluating the hospital medical record and Medicare Part A billing records for ICD-9-CM procedure codes associated with cardiac catheterization (37.22, 37.33, 88.5388.57). We did not include cardiac catheterizations conducted within the first 12 hours of hospitalization because these were associated with the use of primary percutaneous transluminal coronary angioplasty. Statistical Analysis We compared men and women for differences in medical history, clinical presentation, comorbid conditions, physician, and hospital characteristics using chi-square and t-test analyses. We also compared men and women by the appropriateness criteria and the ACC/AHA cardiac catheterization classes used to create the appropriateness groupings. We compared the crude rates of cardiac catheterization use for men and women in the full study cohort and when stratified by procedure appropriateness (strong, equivocal, or weak). Logistic regression models were used to derive predicted and risk-standardized rates of cardiac catheterization for men and women, adjusting for differences in sociodemographic characteristics, medical history and clinical presentation, physician characteristics, and hospital data associated with cardiac catheterization use in previous analyses (31, 32). Patient sociodemographic characteristics included race and residential ZIP code measures of median household income; distribution of occupations; and percentage of population with a high school education or higher, as reported in the 1990 U.S. Census (33). We based medical history measures on clinical experience and previously identified predictors of procedure use. These measures included age, left ventricular ejection fraction, a previous myocardial infarction, congestive heart failure, smoking status, hypertension, diabetes, cerebrovascular disease, peripheral vascular disease, dementia, functional status, and immune system status (compromised or not compromised). We also accounted for the following characteristics ascertained at admission: systolic blood pressure, heart rate, Killip class, infarct location, renal dysfunction, Q-wave infarction, ST-segment elevation, microalbuminuria, anemia, and provision of immediate reperfusion therapy (thrombolytic agents or primary percutaneous transluminal coronary angioplasy within 12 hours of admission). Physician characteristics included age, sex, and specialty, as reported in the American Medical Association Masterfile (34). Hospital data were obtained from the 1994 American Hospital Association Survey of Hospitals (35) and included level of cardiac care facilities (none, catheterization laboratory, or coronary bypass surgery suite), ownership (public, not-for-profit, or for-profit), teaching status, number of patients with myocardial infarction who were treated, rural location (location outside of a Metropolitan Statistical Area), and U.S. Census division. To retain all variables in multivariable modeling, dummy variables were used to denote patients with missing data for left ventricular ejection fraction (n = 52 347), microalbuminuria (n = 37 683), weight (n = 12 056), functional status (n = 3325), anemia (n = 3232), systolic blood pressure (n = 748), and physician characteristics (n = 13 177). Risk-standardized rates of cardiac catheterization for men and women were compared in both the overall cohort and within procedure appropriateness groups. The interaction between sex and cardiac catheterization appropriateness was confirmed by the incorporation of terms representing an interaction between patient sex and cardiac catheterization appropriateness and the use of a likelihood ratio test comparing the multivariable logistic regression model incorporating patient sex and cardiac catheterization appropriateness terms with a model that did not include these terms. To adjust for sex-associated differences in patient refusal, we repeated our analyses and evaluated the composite end point of cardiac catheterization within 60 days of hospitalization or refusal of cardiac catheterization during hospitalization. We conducted secondary analyses by further stratifyi