Symptomatic 2:1 AV block in a young adult.

A 34-year-old man presented with fatigue and episodic dizziness and was found on surface ECG to have sinus rhythm with second-degree atrioventricular (AV) block manifesting as 2:1 AV conduction. Because 2:1 block precludes reliable Mobitz classification from the surface tracing alone, evaluation focused on identifying reversible causes and localizing the site of block with targeted testing and electrophysiology study. Extensive structural, inflammatory, and infectious evaluation was unrevealing, and provocative maneuvers (atropine/isoproterenol) failed to restore stable 1:1 conduction. Intracardiac recordings demonstrated absence of a His potential on nonconducted beats with normal AH/HV intervals on conducted beats, localizing the block proximal to the His bundle and supporting intrinsic AV nodal disease. Given persistent symptoms without a reversible etiology, the patient underwent dual-chamber pacemaker implantation with marked improvement in fatigue and dizziness.