Fetal Supraventricular Tachycardia Complicated by Hydrops Fetalis: A Case Report of Complete Reversal With Transplacental Flecainide Therapy

Fetal supraventricular tachycardia (SVT) is the predominant sustained fetal tachyarrhythmia and, when persistent, can progress to tachycardia-induced myocardial dysfunction and nonimmune hydrops fetalis (NIHF), a condition associated with substantial perinatal morbidity and mortality.Although digoxin has historically served as first-line therapy, flecainide demonstrates superior transplacental bioavailability and higher cardioversion rates in hydropic fetuses, in whom placental edema may impair digoxin transfer.We report a case of fetal SVT complicated by hydrops fetalis at 30 weeks' gestation that achieved complete intrauterine reversal following maternal flecainide therapy.A 31-year-old woman was referred after fetal echocardiography demonstrated SVT with a ventricular rate of approximately 300 beats per minute and 1:1 atrioventricular conduction, alongside fetal ascites and pericardial effusion consistent with NIHF.Oral flecainide was initiated and subsequently up-titrated because of persistent fetal tachycardia.Sinus rhythm was restored on treatment day 10, followed by complete sonographic resolution of hydrops by 32+6 weeks' gestation.Labor was induced at 38 weeks, resulting in vaginal delivery of a clinically stable male infant.Postnatally, prophylactic propranolol was started, but recurrent SVT on days 4 and 13 of life required escalation to flecainide and adenosine cardioversion before discharge in sinus rhythm.This case illustrates that regression of hydrops, rather than restoration of sinus rhythm alone, represents a more clinically meaningful marker of fetal cardiovascular recovery and underscores the importance of timely transplacental antiarrhythmic therapy and structured postnatal surveillance, given the well-documented risk of arrhythmia recurrence in infants with a history of fetal SVT complicated by hydrops.