Infant with 2:1 atrioventricular block after surgery for congenital heart disease: What’s the mechanism? Christopher L. Johnsrude, MD, MS From the Division of Pediatric Cardiology, Department of Pediatrics, University of Louisville School of Medicine, Louisville, Kentucky, and Pediatric and Adult Congenital Heart Disease Arrhythmia Service, Norton Children’s Hospital, Louisville, Kentucky. Introduction Surgery for congenital heart disease (CHD) results in injury to the atrioventricular node/His bundle (AVN/HB), causing postoperative AV block (AVB) in 0.7%–3% 1 and junctional ectopic tachycardia (JET) in up to 27%. 2,3 Primary risk factors for these arrhythmias include young age at surgery, specific surgical procedure, prolonged bypass/aortic cross- clamp times, and abnormal electrolytes. 2,3 Such arrhyth- mias can impose acute hemodynamic compromise, prolong hospitalization, and necessitate medical or surgical inter- vention. Surface electrocardiograms (ECGs) facilitate diagnosis of these arrhythmias and guide clinical management. For most patients, JET responds to nonpharmacological measures or medications, 2,3 often resolving within a few days. Advanced AVB is managed with temporary pacing; a permanent pace- maker is recommended if present beyond postoperative day (POD) 7–10. 4 JET or AVB usually occur as isolated arrhyth- mias, but can manifest in the same patient. Reported herein is a newborn with CHD whose early postop- erative course was unremarkable until POD 6, when she devel- oped new-onset 2:1 AVB and other arrhythmias, likely due to a seemingly rare mechanism, successfully treated with flecainide. Case report This full-term newborn underwent surgical repair of atrial and ventricular septal defects and aortic coarctation on day of life 2. Early postoperative rhythm was sinus with intact AV conduction and right bundle branch block (Figure 1A), common after VSD closure. Recovery was uneventful until POD 6, when continuous telemetry first recorded periods of 2:1 AVB (Figure 1B), prompting consideration for a perma- nent pacemaker. Review of telemetry confirmed frequent isolated second- degree AVB and prolonged periods of 2:1 AVB, and other new arrhythmias. Frequent premature junctional (PJCs) or ventricular complexes (PVCs) were recorded, as were blocked premature atrial complexes (PACs) (Figure 1C). That evening, she developed sustained tachycardia triggered and terminated by PACs, with abrupt onset and stable rate (w180 beats/min), QRS morphologies identical to sinus beats, and no VA dissociation (Figure 2A and 2B). P waves during tachycardia were low-amplitude; 12-lead ECGs were not obtained. One or multiple mechanisms could explain these arrhyth- mias, manifesting in the setting of evolving postoperative inflammation, resolving perioperative ischemia, or another condition; serum electrolytes were normal. Second-degree AVB could reflect interrupted conduction from injury to the AVN/HB, manifesting a bit later than usual. Late- coupled PACs could reflect abnormal atrial automaticity, blocking antegrade owing to the same AVN/HB injury. The sustained tachycardia suggested a reentrant mechanism, such as AVN or AV or atrial reentry. Rapid conduction dur- ing tachycardia suggested robust antegrade AV conduction, tough to square with intermittent AVB. The PJCs/PVCs might be caused by abnormal automaticity involving the AVN/HB, or sinus beats conducting in a 1:2 (“double- fire”) fashion. 5,6 Of these possibilities, 2 mechanisms—HB extrasystoles (HBEs) and 1:2 AV conduction—might explain most of these arrhythmias, neither implicating intrinsic AVN/HB conduction disease. Referral for a pacemaker was deferred and oral proprano- lol 0.5 mg/kg administered every 6 hours to treat tachycardia. After no effect on any of the above-mentioned arrhythmias for 36 hours, propranolol was exchanged for oral flecainide 1 mg/kg every 12 hours. Second-degree AVB and tachy- cardia did not recur after the first flecainide dose, and PACs and PJCs became rare, completely resolving within 24 hours. She was discharged home on POD 12 and continued on flecainide over 3 months of follow-up. Serial ECGs and 24-hour Holter monitors recorded no arrhythmias. KEYWORDS AV block; Congenital heart surgery; His bundle extrasystoles; Infant; postoperative arrhythmia (Heart Rhythm Case Reports 2022;8:27–30) Funding Sources: The authors have no funding sources to disclose. Dis- closures: The authors have no conflicts of interest to disclose. Address reprint requests and correspondence: Dr Christopher L. Johnsrude, Asso- ciate Professor, Division of Pediatric Cardiology, Department of Pediatrics, University of Louisville School of Medicine, Suite 113, KCPC, 571 S. Floyd St., Louisville, KY 40202. E-mail address: cljohn02@louisville.edu. 2214-0271/© 2021 Heart Rhythm Society. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). https://doi.org/10.1016/j.hrcr.2021.10.007