Methods: In MODEST, a study on patients with sick sinus syndrome who have an indication for pacing therapy, atrial pacing (AAI) is compared with dual-chamber pacing (DDD) combined with a novel algorithm developed to lower the number of ventricularly paced beats. The study aims to assess the impact of ventricular pacing on the development of atrial arrhythmias and to test the hypothesis that DDD pacing using the algorithm is associated with a higher rate of atrial arrhythmias as atrial pacing. Included will be patients with SSS and no high degree AV node disease except for patients with first-degree AV block ≤300ms. Patients will be followed stratified by their Wenckebach point (≥ 120 bpm versus < 120 bpm).