An abnormality involving the generation of the action potential by the sinus node and is characterized by an atrial rate inappropriate for physiological requirements. manifestations include severe sinus bradycardia, sinus pauses or arrest, sinus node exit block, chronic atrial tachyarrhythmias, alternating periods of atrial bradyarrhythmias and tachyarrhythmias, and inappropriate responses of heart rate during exercise or stress. [pmid:17420362]