The natural history of primary first-degree atrioventricular heart block.

The long-term prognosis of first-degree heart block in the absence of organic heart disease has not been clearly defined. We addressed this question in a 30-year longitudinal study of 3983 healthy men. We identified 52 cases that were present on entry into the study and 124 incident cases during follow-up. The incidence rose steadily after age 40 and was 1.13 per 1000 person-years over the entire period. Two thirds of the cases had only moderate prolongation of the PR interval (0.22 to 0.23 second). We compared four age-matched controls with each case for histories of scarlet fever, rheumatic fever, diphtheria, smoking, blood pressure, and body-mass index. No significant differences (P greater than 0.05) were found. Likewise, mortality from all causes did not differ between cases and controls. Although somewhat higher rates of morbidity and mortality from ischemic heart disease were observed in the cases than in the controls, the differences were not significant. Progression to higher grades of heart block occurred in only two cases. In view of the prognostic findings and the rare occurrence of advanced degrees of heart block, we conclude that primary first-degree heart block with moderate PR prolongation is a benign condition. This conclusion may not apply, however, to persons with more marked prolongation of the PR interval, a very rare condition.