Sick sinus syndrome

Sick sinus syndrome (SSS) or sinus node dysfunction (SND) refers to any condition where the SA node is dysfunctional. This causes intermittent sinus bradycardia (< 40 bpm when awake), sinus pauses (pauses <3 seconds), sinus arrest (pauses >3 seconds), tachycardia-bradycardia syndrome, chronotropic incompetence, or SA block. Patients present with intermittent complaints of fatigue, dizziness, palpitations, syncope, etc.

SSS is usually idiopathic, due to SA node degeneration, but it can also be due to ischaemia or structural cardiac abnormalities like cardiomyopathy. It mostly affects elderly.

Patients often require implantation of a pacemaker.

Tachycardia-bradycardia syndrome

Tachycardia-bradycardia syndrome, usually called simply tachy-brady syndrome, refers to when a patient both has alternating atrial tachyarrhythmias (like atrial fibrillation) and symptomatic sinus bradycardia. This occurs in 50% of those with SSS. The atrial tachyarrhythmia may spontaneously convert to sinus rhythm, but because the sinus node is sick, there may be a pause before the sinus node kicks in, which can be symptomatic.

The syndrome can cause dizziness, syncope, palpitations, and chest pain. Pacemaker implantation is usually necessary, especially if the atrial tachyarrhythmia has a rapid ventricular response. The ideal treatment would be rate-limiting drugs, like beta blockers or calcium channel blockers, but these would worsen the sinus bradycardia when it occurs. Implantation of a pacemaker (which prevents symptomatic bradycardia) can allow for administration of rate-limiting drugs to reduce the rapid ventricular response of the tachyarrhythmia, alleviating the symptoms.