[Electrophysiological mechanisms of tachyarrhythmias associated with the sick sinus syndrome (author's transl)].

Atrail programmed stimulation (APS) and His bundle recordings were performed in 24 patients with assessed sick sinus syndrome, 14 of whom had previously suffered from bouts of spontaneous paroxysmal tachycardias (Group I) and 10 with no history of previous similar complication (Group II). APS reproduced re-entry arrhythmias in 13 cases of Group I (92%). In 6 cases, junctional reciprocating tachycardia was elicited and the atrioventricular node function curve showed a typical steplike profile, consistent with longitudinal dissociation of the node itself as a cause of reciprocation. In 7 cases with previous paroxysmal atrial tachycardias, APS elicited intra-atrial re-entry arrhythmias. Absolutely no arrhythmia was elicited by APS in the 10 cases of Group II. High incidence of basal prolongation of conduction intervals was observed in both Groups (11 cases in Group I, 6 cases in Group II). Intra-atrial and/or atrio-nodel conduction delays were recognized in all the cases with intra-atrial re-entry arrhythmias. These results suggest that so called bradycardia-tachycardia syndrome should be regarded as an independent clinical form from the sick sinus syndrome, sharing with the latter the sinusal disfunction only. Re-entry mechanisms seems to be mainly responsible for the inititation of paroxysmal tachyarrhythmias: re-entry in the atrium, based on stable conduction delays, is probably correlated with scattered lesions of the same degenerative process affecting the sinus node. On the contrary, the frequent association of sick sinus syndrome with junctional paroxysmal tachycardia and its comples dissociation mechanism of the a-v node, raises the suspicion that, in such cases, two completely different processes are merely coexisting: a degenerative lesion of the sinus node and an anatomical or functional anomaly of a separate portion of the conduction system.