Sick sinus syndrome

Definition

A disorder of the sinoatrial node (now usually called sinus node dysfunction) in which its ability to generate and conduct impulses is impaired. ECG manifestations include inappropriate, persistent sinus bradycardia (a sinus rate below 60 bpm by the traditional definition; the 2018 ACC/AHA/HRS bradycardia guideline uses below 50 bpm), sinus pauses or arrest (the guideline counts pauses of more than 3 seconds), sinoatrial exit block, junctional or ventricular escape rhythms, and chronotropic incompetence (an inability to raise the heart rate in proportion to activity, in many studies defined as failing to reach 80% of the expected heart rate reserve during exercise). In the tachycardia-bradycardia (tachy-brady) variant, sinus or ectopic atrial bradycardia or pauses alternate with atrial tachycardia, atrial flutter or atrial fibrillation, and a pause may follow when the tachycardia stops. It is most often related to age-dependent, progressive degenerative fibrosis of the sinus node and surrounding atrial myocardium.

Clinical noteNeither a slow sinus rate nor a pause over 3 s is diagnostic on its own; correlating the patient's symptoms with the slow rhythm or pauses is considered the gold standard of diagnosis.