The mechanism for the prolongation of P-R interval associated with advancing age is undefined. Using a high-resolution ECG (Marquette MAC-I) to signal average 512 cardiac cycles, we examined 185 healthy volunteers aged 20-83 years from the Baltimore Longitudinal Study of Aging with normal rest and exercise ECGs and a resting P-R interval <210 ms. Among the 161 subjects with visible His bundle activity, P-R interval increased with age (p < .001). This increase was due entirely to prolongation of the interval between the P wave onset and His bundle potential, i.e., the P-H interval, (p < .001) with no age-associated change in the H-V interval, p = NS. The P-H interval prolongation with age was localized to the P-R segment proximal to His bundle activation (p < .001). In a separate group of 7 asymptomatic older men (mean age = 71 yr), with first-degree atrioventricular (A-V) block on standard ECG (mean PR = 238 ± 14 ms), the P-H interval (193 ± 21, vs 136 ± 18 ms, p < .001) and proximal P-R segment (82 ± 19) vs 33 ± 15 ms, p < .001) but not the H-V interval (45 ± 11 vs 40 ± 9 ms, p = NS) were longer than in 25 age-matched men without A-V block. Thus, the modest age-associated prolongation of the P-R interval is localized to the proximal P-R segment, probably reflecting delay within the atrioventricular junction. A similar but more striking delay in the proximal P-R segment is responsible for first degree A-V block in apparently healthy older men.
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