Clinical studies on the sick sinus syndrome.

1) Overdrive suppression is clinically useful to evaluate depressed cardiac automaticity in the sick sinus syndrome. 2) His bundle electrograms revealed frequent association of atrioventricular conduction defects with the depressed automaticity in this syndrome. 3) Especially in BTS, marked intra-atrial conduction defect was observed, and this conduction defect may play a role for development of re-entrant tachycardia. 4) Pathologic changes in the sick sinus syndrome are supposed to involve the whole atrial tissues including not only the S-A nodal region but the A-V node and its approaches.