16-year incidence of ischemic heart disease in the NHLBI twin study. A classification of subjects into high- and low-risk groups.

Prospective data from the National Heart, Lung, and Blood Institute (NHLBI) Twin Study were used to investigate the relationship of risk factors measured in 1970 to 1971 to 16-year incidence of ischemic heart disease (IHD) in 905 males born between 1917 and 1927. A newly developed methodology, tree-structured survival analysis (TSSA), was used to classify subjects into discrete subgroups that differed significantly in profiles of risk factors and incidence of IHD. On the basis of five characteristics--systolic blood pressure (SBP), high-density-lipoprotein cholesterol (HDL-C), 1-hour postload glucose levels, forced expiratory volume in 1 second (FEV1), and a family history score for heart disease--the TSSA algorithm partitioned the cohort into six discrete subgroups that formed three clusters of individuals with distinct IHD experience. Highest IHD incidence rates were experienced by a subgroup of 56 men with baseline SBP above 134 mm Hg and HDL-C levels lower than 33 mg/dL. No IHD events were observed in a subgroup of 117 men who had low SBP, high FEV1, and a negative family history of heart disease. Relationships to the twinning condition showed that for both zygosities, cotwin-pair members were in the same risk subgroup more often than expected; however, the overall difference between the frequency of monozygotic (MZ) and dizygotic (DZ) cotwins was relatively small. The highest MZ/DZ ratios of observed to expected cotwin-pair membership occurred in subgroups with the highest and lowest incidences of IHD, suggesting that these extremes of incidence are most likely to be genetically determined.