The sinus node function: normal and pathological.

To determine the evolution with age, of extrinsic and intrinsic sinus node electrophysiological parameters and to assess the role of each component of the autonomic nervous system relative to age in patients with and without sick sinus syndrome, electrophysiological studies of sinus node function were performed in 223 patients subdivided into four groups according to the results of their electrophysiological testings: group I included patients with normal extrinsic and intrinsic sinus node function, group II patients with exclusive extrinsic sinus dysfunction, group III patients with exclusive intrinsic sinus dysfunction and group IV patients with extrinsic and intrinsic sinus node dysfunction. The electrophysiological study was performed twice: at basal state and after autonomic blockade. Whatever the sinus node function (normal or abnormal) the extrinsic sinus node electrophysiological variables did not correlate with age; inversely all the electrophysiological measurements of the intrinsic sinus node (normal or abnormal) lengthened progressively with age, suggesting an ageing phenomenon of the intrinsic sinus node throughout life. Moreover, the study of the percentage of chronotropy of the sinus node electrophysiological variables shows a predominance of vagal tone in young subjects, whereas sympathetic activity is most prominent in elderly patients with and without sick sinus syndrome. The sinus node (normal or pathological) represents an equilibrated system: the age-related modification of the autonomic nervous system counterbalances the senescence of the intrinsic sinus node in such a way that the basal electrophysiological characteristics remain stable throughout life.