Variations of normal sinus node function in relation to age: role of autonomic influence.

An electrophysiological study of sinus node function, including measurements of resting heart rate, maximal corrected sinus node recovery time and sinoatrial conduction time, was performed in 30 patients, 12-79 years of age, without any clinical, electrocardiographic or electrophysiological evidence of sinus node disease. To analyse autonomic influences, variables were measured before and after sympathetic and parasympathetic blockade. No significant correlations were observed between age and electrophysiological measurements of sinus node function at the control study or after sympathetic blockade. In contrast, the electrophysiological parameters of intrinsic sinus node activity were correlated with age and showed a progressive lengthening of mean sinus cycle length, of maximal corrected sinus node recovery time and of sinoatrial conduction time. In addition, measurements after vagolysis suggest a progressive decrease of parasympathetic activity with increasing age. These data also indicate that the respective role of the two components of the autonomic nervous system vary with increasing age: parasympathetic activity predominates in younger subjects; sympathetic and parasympathetic tones are equilibrated in older subjects. The normal sinus node function represents an equilibrated system: in parallel with ageing of the intrinsic properties of the sinus node, parasympathetic activity decreases so that basal properties remain stable throughout life.