Abstract Body (Do not enter title and authors here): Background: The first-ever genetically modified porcine-to-human cardiac xenotransplantation was performed in January 2022 at the University of Maryland, with the recipient surviving 60 days.

Aim: Characterize trends in conduction delay observed over the 60-day post-operative course of the first-ever porcine-to-human cardiac xenotransplant recipient.

Methods: Daily 12-lead ECGs were evaluated for presence and type of conduction delay. Over the post-operative period, 93 ECGs were obtained. ECGs that could not be assessed for conduction delay due to presence of significant artifact (2 ECGs) or paced rhythm (12 ECGs) were excluded from evaluation.

Results: During the 60-day postoperative period, the xenotransplant exhibited alternating conduction block including nonspecific intraventricular conduction delay (NSIVCD), right bundle branch block (RBBB), incomplete RBBB, and left anterior fascicular block (LAFB). Nonspecific intraventricular conduction delay (NSIVCD) was observed on 6 days in total, occurring primarily within the first 9 post-operative days. After day 9, the conduction block alternated between bifascicular block with RBBB+LAFB, incomplete RBBB+LAFB, RBBB, and incomplete RBBB. The predominant pattern of conduction delay was bifascicular block with RBBB+LAFB, occurring over 32 days in this period. Presence of incomplete RBBB+LAFB was noted on 17 days in total. Isolated RBBB occurred on 2 days, and incomplete RBBB on 4 days. There was no evidence of high-grade atrioventricular block observed in this period.

Conclusion: Ventricular conduction in the post-operative period of the first porcine-to-human xenotransplant was characterized by an alternating conduction block with NSIVCD in the early postoperative period (through day 9), followed by predominantly bifascicular block with RBBB and LAFB or incomplete RBBB and LAFB. Atrioventricular conduction remained largely intact without evidence of high-grade AV block nor dependence on back-up pacing during the majority of our patient’s post-operative course.