Coronary disease surveillance in Olmsted County objectives and methodology

The community surveillance study of coronary heart disease (CHD) in Olmsted County, MN, is designed to estimate trends in myocardial infarction (MI) incidence, case fatality rate, and CHD mortality, while including all ages. A distinctive feature of this study is its ability to capture longitudinal data before and after index events via the medical record linkage system of the Rochester Epidemiology Project. The goal of this report is to describe the methods implemented to measure CHD trends, the implications of including elderly individuals on MI ascertainment and trends in prior CHD among persons with incident MI. The methods are based on standardized criteria involving the review of death certificate information and hospital records to identify CHD deaths, and incident MIs in Olmsted County. The medical record linkage system in place under the auspices of the Rochester Epidemiology Project was used to ascertain antecedent CHD and outcomes. Hospitalized MIs were screened from sampled events coded ICD9 codes 410-414 and classified using enzyme values, cardiac pain, and ECG coding. After screening 5,042 records, a cohort of 1,658 validated incident MIs was assembled 35% (575) among persons aged 75 years or greater. The proportion of MIs validated with cardiac pain and enzymes without Minnesota ECG coding was lower among the elderly than among persons less than 75 years of age (35 vs. 29%, respectively; P <.001). The proportion of events validated without requiring ECG coding decreased over time in both age strata (P for trend.001). Reliability analyses indicated excellent agreement in event classification. More than half of the incident MIs did not have antecedent CHD, and this proportion increased overtime. These data indicate that the elderly contribute approximately one-third of the cases of incident MI, underscoring the importance of including all ages to fully characterize the burden of CHD. Cases among elderly persons more frequently require ECG coding for validation, but standardized ascertainment procedures are feasible and reliable in all age groups. More than half of the incident MIs occurred among persons with no prior CHD, and this proportion increased over time. The combination of standardized methodology and of the longitudinal data via the record linkage system of the Rochester Epidemiology Project will allow reliable measures of CHD trends and help define preventive strategies.