Fetal Atrial Flutter: Electrophysiology and Associations With Rhythms Involving an Accessory Pathway

Background Atrial flutter ( AF l) accounts for up to one third of all fetal tachyarrhythmias and can result in premature delivery, hydrops, and fetal death in 10% of cases; however, the electrophysiology of AF l in utero is virtually unstudied. Methods and Results In this observational study, we reviewed 19 fetal magnetocardiography studies from 16 fetuses: 15 fetuses (21–38 weeks’ gestation) referred with an echocardiographic diagnosis of AF l and 1 fetus (20 weeks’ gestation) referred with a diagnosis of tachycardia that was shown by fetal magnetocardiography to have transient AF l in addition to atrioventricular reciprocating tachycardia. Thirteen fetuses showed AF l during the fetal magnetocardiography session, including 4 that presented prior to the third trimester. Five fetuses had incessant AF l; all but 1 of the others with AF l showed additional significant rhythms. Specifically, AF l showed a strong association with rhythms involving an accessory pathway: atrioventricular reciprocating tachycardia, blocked reentrant premature atrial contractions, and ventricular preexcitation. The observed initiations and terminations of AF l most often involved reentrant premature atrial contractions. Spontaneous termination of AF l showed AF l cycle length oscillations. Nine fetuses with 2:1 AF l also showed periods of 4:1 conduction or variable conduction that oscillated between 2:1 and 4:1; however, 3:1 AF l was relatively rare. Conclusions Fetal AF l can occur as early as midgestation and is often accompanied by atrioventricular reciprocating tachycardia and other rhythms associated with an accessory pathway. The findings depict critical differences in the electrophysiology of AF l in the fetus versus the neonate.