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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.

suMMARY The purpose of this study was to examine factors associated with the development of complete left bundle-branch block and the prognosis in a group of people not in hospital, who had no clinical evidence of ischaemic or valvular heart disease. Twenty-nine cases of left bundle-branch block without clinical evidence of ischaemic heart disease were noted in the Manitoba cohort of 3983 men under observation since 1948. The most frequent electrocardiographic finding before development of left bundle-branch block was a normal electrocardiogram; left ventricular hypertrophy though infrequent, was the only abnormality significantly more common than in the rest of the group. The development of left bundle-branch block was associated with distinct leftward shift of the frontal plane mean QRS axis. The most frequent clinical cardiovascular event observed after development of the block was sudden death without previous clinical evidence of ischaemic heart disease. The five-year incidence of sudden death as the first manifestation of heart disease was 10 times greater in men with left bundlebranch block than in those without it.Our knowledge of the natural history of complete left bundle-branch block has come almost entirely from patients in hospital with heart disease, because there are comparatively few data from apparently healthy populations. Because

The Manitoba Follow-up Study (MFUS) is Canada's longest running study of cardiovascular disease and ageing. The MFUS cohort consists of 3983 men recruited from the Royal Canadian Air Force at the end of World War II. At entry to the study, 1 July 1948, their mean age was 31 years, with 90% between ages 20 and 39 years. All study members were free of clinical evidence of ischaemic heart disease. The protocol of MFUS was to obtain routine medical examinations from these men at regular intervals over time. The research goal of the study was to examine the role that any abnormalities detected on routine electrocardiograms from apparently healthy men might play in the prediction of subsequent diagnoses of cardiovascular disease. Over the course of 65 years, about 35% of the cohort has documented evidence of ischaemic heart disease. The research focus was expanded in 1996 to explore the roles of physical, mental and social functioning in support of healthy and successful ageing. On 1 July 2013, 429 original cohort members were alive with a mean age of 92 years. Collaborative research with others outside the in-house team is welcomed.