Context: Prolongation of the electrocardiographic PR interval, known as firstdegree atrioventricular block when the PR interval exceeds 200 milliseconds, is frequently encountered in clinical practice. Objective: To determine the clinical significance of PR prolongation in ambulatory individuals. Design, Setting, and Participants Prospective, community-based cohort including 7575 individuals from the Framingham Heart Study (mean age, 47 years; 54% women) who underwent routine 12-lead electrocardiography. The study cohort underwent prospective follow-up through 2007 from baseline examinations in 1968-1974. Multivariable-adjusted Cox proportional hazards models were used to examine the associations of PR interval with the incidence of arrhythmic events and death. Main Outcome Measures: Incident atrial fibrillation (AF), pacemaker implantation, and all-cause mortality. Results: During follow-up, 481 participants developed AF, 124 required pacemaker implantation, and 1739 died. At the baseline examination, 124 individuals had PR intervals longer than 200 milliseconds. For those with PR intervals longer than 200 milliseconds compared with those with PR intervals of 200 milliseconds or shorter, incidence rates per 10 000 person-years were 140 (95% confidence interval [CI], 95-208) vs 36 (95% CI, 32-39) for AF, 59 (95% CI, 40-87) vs 6 (95% CI, 5-7) for pacemaker implantation, and 334 (95% CI, 260-428) vs 129 (95% CI, 123-135) for all-cause mortality. Corresponding absolute risk increases were 1.04% (AF), 0.53% (pacemaker implantation), and 2.05% (all-cause mortality) per year. In multivariable analyses, each 20-millisecond increment in PR was associated with an adjusted hazard ratio (HR) of 1.11 (95% CI, 1.02-1.22; P=.02) for AF, 1.22 (95% CI, 1.14-1.30; P<.001) for pacemaker implantation, and 1.08 (95% CI, 1.02-1.13; P=.005) for all-cause mortality. Individuals with first-degree atrioventricular block had a 2-fold adjusted risk of AF (HR, 2.06; 95% CI, 1.36-3.12; P<.001), 3-fold adjusted risk of pacemaker implantation (HR, 2.89; 95% CI, 1.83-4.57; P<.001), and 1.4-fold adjusted risk of all-cause mortality (HR, 1.44, 95% CI, 1.09-1.91; P=.01). Conclusion: Prolongation of the PR interval is associated with increased risks of AF, pacemaker implantation, and all-cause mortality.
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