Coronary Heart Disease Incidence and Survival in African-American Women and Men: The NHANES I Epidemiologic Follow-up Study

Since a series of reviews and symposia in the early 1980s stimulated interest in and funding for studies of the clinical epidemiology of coronary heart disease in African-American persons [1-3], a considerable amount of new knowledge about the subject has accumulated. Many erroneous or outdated clinical concepts (for example, that coronary heart disease is rare or uncommon in African-American persons) have been put to rest [4-8]. In contrast to the many reports on white populations from the Framingham Study and other studies, the number of reports on the incidence of coronary heart disease from prospective, longitudinal studies of population-based cohorts of African-American women and men remains small [9]. Compared with white persons, African-American persons have higher rates of death from coronary heart disease at younger ages and lower rates after middle age; this makes examination of incidence data within age groups very important [1-5]. Such data are almost completely lacking, however, and several questions of vital importance to clinicians therefore remain incompletely answered. We analyzed data from a national cohort that was followed for 22 years to test the following hypotheses: 1) The incidence of first myocardial infarction or coronary heart disease or the rate of death from coronary heart disease in African-American persons differs from that in white persons, 2) these differences cannot be completely explained by standard risk factors for coronary heart disease and socioeconomic status, 3) survival after the first hospitalization for coronary heart disease among African-American persons differs from that among white persons, and 4) the incidence of coronary procedures done during or after hospitalization for coronary heart disease among African-American persons differs from that among white persons. Methods The first National Health and Nutrition Examination Survey (NHANES I) collected data from 1971 to 1975 on a nationwide probability sample of the civilian noninstitutionalized population of the United States (excluding Alaska, Hawaii, and reservation lands of American Indians) aged 1 to 74 years [10, 11]. Details of the plan, sample design, response rates, and the manner in which the study was conducted are published elsewhere, as are the procedures used to obtain informed consent and maintain confidentiality [10, 11]. Elderly persons, women of childbearing age, and persons residing in poverty areas were oversampled. The NHANES I Epidemiologic Follow-up Study is a longitudinal study of participants in NHANES I who were 25 to 74 years of age when the survey was conducted [12-17]. The personal interviews and physical and laboratory examinations of NHANES I provided the baseline data for the Follow-up Study. This analysis is based on four waves of follow-up data collection done in 1982 to 1984, 1986, 1987, and 1992. Information was obtained from an interview, medical records from health care facilities for the period between baseline and last follow-up interview, and death certificates for all decedents. A study that used both Medicare data and data from the follow-up study found that the follow-up study detected 80% of the hospital stays for coronary heart disease that were detected by either source [17]. This analysis included African-American and white persons who were 25 to 74 years of age at baseline for NHANES I. Of the 14 183 persons who were in this age group at baseline, 775 (5.5%) were lost to follow-up because they or their proxy was not interviewed during any follow-up wave or because no death certificate was available. Excluded from all analyses were 983 persons with unknown baseline history of heart disease, systolic blood pressure, serum cholesterol level, smoking status, family income, or level of education. Also excluded from analyses of coronary heart disease were 1019 persons who had a history of heart disease at baseline; these patients were defined as those who had ever been told by a physician that they had had a heart attack or heart failure or who had used any medicine, drugs, or pills for a weak heart during the 6 months before the baseline interview. After all exclusions, 11 406 persons examined at NHANES I survey locations 1 through 99 remained for analyses of coronary heart disease. The length of follow-up in the NHANES I Epidemiologic Follow-up Study for survivors who were free of coronary heart disease ranged from 8.0 to 22.1 years (median, 19.2 years). Incident cases of coronary heart disease met at least one of the following criteria: 1) the death certificate on which the underlying or primary cause of death was coded with ICD-9 [International Classification of Diseases, Ninth Revision] codes 410 through 414 or 2) there were one or more hospital stays during the follow-up period for which any discharge diagnosis was coded with ICD-9-CM (ICD-9, Clinical Modification) codes 410 to 414. An ICD-9-CM code of 410 served to identify incident cases of acute myocardial infarction that required hospitalization among the discharge diagnoses from the first recorded hospitalization in which coronary heart disease was the discharge diagnosis. Persons who were hospitalized for coronary heart disease that was not related to acute myocardial infarction were not considered to be at risk for subsequent incident cases of acute myocardial infarction requiring hospitalization. The date of occurrence was estimated as the date of first hospital admission with a diagnosis of coronary heart disease or the date of death for persons who died of coronary heart disease and whose hospital records did not list a diagnosis of coronary heart disease. For mortality analyses, cause of death was defined by the underlying cause of death: coronary heart disease with ICD-9 codes 410 to 414. Baseline Variables Blood samples were obtained, and frozen serum was sent to the Centers for Disease Control and Prevention for measurement of serum total cholesterol levels [10, 18]. The baseline questionnaire on medical history included questions about selected conditions and about medications used for these conditions during the preceding 6 months [10]. Ethnic group (race, designated black, white, and other) was determined by interviewer observation. A revised race variable was created during the 1982-1984 follow-up period to resolve discrepancies between the race observed by the NHANES I interviewer and the race reported by the respondent during the 1982-1984 follow-up period. Revised race codes were based on a case-by-case adjudication. At the beginning of the baseline physical examination, the physician measured the blood pressure with the examinee seated [10]. Information on smoking status was obtained at baseline for the subsample (approximately 50% of the whole) that underwent a more detailed baseline examination, as described elsewhere [10, 11]. For the remaining persons, information on smoking status at baseline was derived from responses to follow-up questions on lifetime smoking history or was imputed [19, 20]. The validity of this approach has been documented [19, 20]. Other baseline variables were measured as described elsewhere [10]. Statistical Analysis For the 25- to 54-, 55- to 64-, and 65- to 74-year-old age groups, approximate plots of cumulative probabilities of survival without coronary heart disease were based on Kaplan-Meier statistics calculated using the LIFETEST procedure in SAS software [21, 22]. Age-adjusted and risk-adjusted estimates of the risk for coronary heart disease in African-American persons relative to white persons were derived from Cox proportional-hazards regression models that were computed by using the PHREG procedure in SAS software [23, 24]. Separate analyses were done for groups categorized by age at baseline and sex. Results are shown for models that included age at baseline in single years and age plus other confounders, entered as continuous variables (systolic blood pressure, serum cholesterol level) or as indicator variables (history of current smoking, level of education < 12 years, family income < $5000). Modification of the effect of ethnic group according to age or education was assessed in regression models and is reported only if the modification was significant. In the incidence analyses, duration of follow-up was calculated as the time from the date of examination to the date of coronary event or, for participants who did not have a coronary event, the date of last follow-up interview or death. For analyses of coronary disease-related mortality, participants whose underlying cause of death was a condition other than coronary heart disease were censored on their date of death; survivors were censored on the date of the last follow-up interview with the participant or proxy [24]. For analysis of survival after first diagnosis of coronary heart disease, 1858 persons who were hospitalized at least once and had a discharge diagnosis of coronary heart disease (ICD-9-CM codes 410 to 414) were followed from the date of the first recorded hospitalization for which the discharge diagnosis was coronary heart disease until death or the last follow-up contact. Duration of follow-up ranged from 0 days to 20.4 years (median, 4.2 years). Kaplan-Meier curves were examined and Cox proportional-hazards regression models were computed to obtain estimates of risk for death from all causes for African-American persons relative to white persons; these models controlled for age at first hospitalization for coronary heart disease and a discharge diagnosis of acute myocardial infarction at the first hospitalization for coronary heart disease. Separate models were run for men and women, for admission ages 25 to 64 years and 65 years and older, and for all admission ages combined. Survivors were censored on the date of the last follow-up interview with the participant or proxy. For analysis of coronary procedures, incidence rates per 1000 person-years were computed among persons who we