Effects of age on intrinsic heart rate, heart rate variability, and AV conduction in healthy humans

Heart rate, heart rate variability, and atrioventricular (AV) conduction were studied in 20 young (30 +/- 5 yr) and 19 older (69 +/- 7 yr) healthy men and women before and after single and double autonomic blockade (randomized order: atropine, 0.04 mg/kg i.v.; propranolol, 0.2 mg/kg i.v.). Basal R-R intervals did not differ, but older age increased P-R intervals (177 +/- 24 vs. 149 +/- 17 ms, P < 0.001) and decreased SD of R-R (43 +/- 17 vs. 70 +/- 18 ms, P = 0.001) and heart rate spectral content (area under the power vs. frequency curve from 0.04 to 0.32 Hz: 3.01 +/- 2.1 vs. 7.82 +/- 4.8 beats/min2, P < 0.009), as well as postural responses (R-R decreases of 107 +/- 80 vs. 250 +/- 72 ms, P < 0.002). Atropine decreased R-R intervals, SD of R-R, and high-frequency (0.24-0.32 Hz) spectral content less in elderly subjects compared with younger subjects. Propranolol increased R-R and P-R intervals equally in old and young and abolished low-frequency (0.04-0.12 Hz) increases with standing (P < 0.0008). After double blockade, R-R, P-R, and paced AV intervals were longer in old subjects. Mean values were as follows: R-R intervals, 859 +/- 176 vs. 677 +/- 106 ms, P < 0.001; P-R intervals, 179 +/- 23 vs. 149 +/- 17 ms, P = 0.0002; paced P-R intervals (500 ms), 251 +/- 39 vs. 215 +/- 47 ms; and AV block cycle length, 413 +/- 51 vs. 385 +/- 69 ms (multivariate analysis of variance, P < 0.03). After double autonomic blockade, heart rate variability was nearly eliminated in young and old (reduced > 98%, P < 0.0001). We conclude that age differences in heart rate variability can be explained by autonomic influences, but heart rate and AV conduction differences exist independently of beta-adrenergic and/or parasympathetic influences.