Correlation of maternal flecainide concentrations and therapeutic effect in fetal supraventricular tachycardia

Background: Transplacental flecainide is an established therapy for fetal supraventricular tachycardia (SVT), but there is a paucity of data regarding the dose–response relationship. Objective: The purpose of this study was to investigate the relationship between maternal flecainide concentrations, arrhythmia control, and adverse fetal effects in fetal SVT. Methods: Fetuses with SVT treated with transplacental flecainide at our tertiary fetal cardiology unit between 1997 and 2012 were retrospectively studied. The maternal trough flecainide concentrations throughout treatment were collated, and clinical notes were reviewed to ascertain the response to therapy and fetal outcome. Results: Thirty-three fetuses were treated at a median (range) gestation of 28 weeks (20–38 weeks). Median fetal heart rate was 250/min (range 207–316/min). One patient was lost to follow-up, and this fetus was excluded from further analysis. In total, 25 of 32 fetuses (78%) converted to sinus rhythm. Median time to conversion to sinus rhythm was 3 days (range 2–12 days). Median flecainide concentration was 460 μg/L (range 250–866 μg/L) at conversion to sinus rhythm. Flecainide concentrations were not significantly different between responders and nonresponders (P = .849). Twelve of 14 hydropic and 13 of 18 nonhydropic fetuses converted to sinus rhythm with similar flecainide concentrations (P = .316). No fetus achieved cardioversion with a maternal serum flecainide concentration <250 μg/L. No fetus died while being treated with flecainide. Conclusion: The clinical response to flecainide appears good, even in hydropic fetuses. Trough maternal flecainide concentrations, once therapeutic, do not predict cardioversion in the fetus with SVT. Flecainide therapy appears both safe and effective for the fetus when monitored appropriately.