Synchronized biatrial arrhythmias: Anatomic differences in biatrial versus bicaval orthotopic heart transplants drive arrhythmogenic sequelae.

A 37-year-old male with a history of familial Titin (TTN) gene mutation and orthotopic heart transplant (OHT) 20 years prior presented for evaluation of persistent atrial arrhythmias, including atrial fibrillation and atrial flutter. Despite the lower risk for atrial fibrillation in OHT patients due to surgical anastomosis isolating the pulmonary veins, biatrial OHT, which preserves the native atrial connection (unlike bicaval OHT), allows for arrhythmogenesis on both the donor and recipient atrial sides. This patient's case illustrates a unique arrhythmic mechanism where electrical reentry between residual and transplanted atrial myocardium resulted in multiple distinct atrial tachycardias. Intracardiac electrograms revealed sinus rhythm along with atrial tachycardia, particularly in the posterior left atrium and the right atrium. High-density voltage mapping identified significant scarring along the anterior and lateral right atrium, likely due to the prior recipient-to-donor right atrial anastomosis. A narrow gap of viable tissue indicated electrical reconnection across the suture line, facilitating multiple micro-reentrant circuits. Radiofrequency ablation (RFA) was performed to interrupt this excitable gap, isolating the native and donor atrial tissues. The RFA lesion set transected the anastomosis site and extended to the cavo-tricuspid isthmus, reestablishing electrical isolation. This case highlights the importance of understanding the biatrial OHT anatomy for targeted electrophysiologic interventions, selective placement of reference electrodes/catheters for mapping, and the need for careful consideration of electrical reconnection across surgical anastomoses to prevent arrhythmogenesis.