The SA node is the dominant pacemaker of the heart. Sinus node dysfunction (also known as sick sinus syndrome) is a group of clinical disorders where the SA node is unable to generate the heart rate that is physiologically required.

Abnormal automaticity occurs when the sinus impulse generation fails. Sinus node dysfunction is brought about by disorders in conduction, automaticity, or both. Localized cardiac pathology, pharmaceuticals, toxins, and systemic diseases involving the heart are all potential contributors to abnormal automaticity.

Mechanisms of sinus node dysfunction include abnormal conduction or a delay in impulse transmission. Sometimes, the impulse is produced normally but is abnormally conducted to the neighboring atrial tissue. This may be caused by fibrosis, atherosclerosis, and inflammatory processes within the heart.

Numerous ECG anomalies are associated with SA node dysfunction, including sinus bradycardia, sinus pauses, sinus arrest, sinoatrial nodal exit block, and chronotropic incompetence (an abnormality where the heart rate cannot keep up with physiologic demands during exercise).

Sinus node dysfunction can co-occur with other supraventricular tachycardias such as atrial flutter and atrial fibrillation.