Sick sinus syndrome or tachycardia-bradycardia syndrome is characterized by alternating episodes of supraventricular tachycardia (atrial tachycardia, AF, common flutter) and episodes of bradycardia related to sinus node dysfunction. Symptomatology combines secondary clinical signs with tachycardia (palpitations, heart failure) and bradycardia (fatigue, cognitive disorders, dyspnea, lipothymia, syncope). This association suggests the presence of diffuse organic atrial lesions with degenerative impairment of the automatic cells of the sinus node but also fibrosis extending to the atrial myocardium and occasionally to the specific tissue of the atrioventricular junction explaining the putative concomitant observation of atrioventricular conduction disorders.

It is common to differentiate sick sinus syndrome (absence of a chronological link between tachycardia occurence and bradycardia occurence), post-tachycardia pauses (sinus pauses observed exclusively directly after termination of atrial arrhythmia) and vagal arrhythmias (episodes of arrhythmia observed solely after a sinus slowing of vagal origin).