Wandering Bundles With Sinus Arrhythmia

A 3-year-old boy with congenital ventricular septal defect underwent a closure procedure. Telemetry after the procedure reveals sinus arrhythmia with varying types of bundle branch blocks. Inverse decremental conduction in left posterior fascicle, related to preceding RP interval during sinus arrhythmia, underlies changes between right and left bundle branch blocks. (Level of Difficulty: Advanced.).