Abstract Body (Do not enter title and authors here): Intermittent high-grade atrioventricular (AV) block is a rare bradyarrhythmia in the young. In the absence of structural conduction system disease, it may result from heightened vagal tone. We present a unique case of a young adult with coexisting incessant atrial tachycardia (AT) and high-grade AV block, both of which resolved following catheter ablation targeting the AT arising in the left atrial appendage (LAA).

Description of Case:
A 25-year-old obese male with a 10-year history of exertional fatigue, palpitations, and presumed sinus tachycardia was referred for evaluation. Baseline ECG showed AT at 103 bpm with negative P waves in I and aVL. Ambulatory monitoring for 13 days revealed frequent tachycardia to a max of 181, average 99 bpm, but also 277 episodes of high-grade AV block up to 3 nonconducted P waves, including daytime pauses up to 2.5 seconds. An electrophysiology study for symptomatic “near incessant” AT demonstrated a focal AT arising from the base of the LAA with epinephrine infusion. Radiofrequency ablation at this site (6 lesions) rendered the AT non-inducible. AV nodal function and His-Purkinje conduction were normal. Follow-up ECGs and 2 week monitors immediately and at 6 months post-ablation revealed inappropriate sinus tachycardia (IST), and no AT nor AV block.

Discussion:
This case illustrates a rare co-occurrence of focal LAA AT, intermittent high-grade AV block, and IST, for which LAA ablation targeting AT also eliminated AV block. Whether all the findings are linked due to an autonomic mechanism or whether the AT was serendipitously located near ganglionated plexi that were responsible for the AV block is uncertain. These findings support the emerging role of autonomic modulation with ablation in the management of high-grade AV block in the young.