Left Bundle Branch Tachycardia: A Nightmare Confluence of Tachycardia and Bradycardia.

A frail nonagenarian presented with incessant wide complex tachycardia exhibiting a left bundle branch block (LBBB) morphology. The arrhythmia was transiently responsive to adenosine boluses but proved refractory to direct-current cardioversion, exhibiting a pattern of immediate reinitiation. Upon transient termination, the baseline electrocardiogram demonstrated sinus rhythm with varying degrees of LBBB aberrancy. Intracardiac electrogram analysis was used to narrow the differential diagnosis of the tachycardia mechanism. Crucially, post-termination recordings documented a rare phenomenon of alternating bundle conduction during bradycardia. This was characterized by a severely diseased infra-Hisian conduction system with an H-V interval of 306 ms-representing one of the longest documented intervals in the literature. This case highlights the capacity for extremely slow, albeit tenuous, conduction within a profoundly diseased conduction system.