ABSTRACT
Introduction Fetal arrhythmias constitute approximately 10% of referrals to perinatal cardiology centers. The vast majority of arrhythmias are atrial and ventricular ectopy, which are benign and require no treatment. Sustained bradycardia is unusual, and in large series, comprises only about 5% of all arrhythmia referrals.1,2 Bradycardia can be due to primary abnormalities in the cardiac conduction system or secondary to fetal or maternal conditions3 (Table 40.1). The most likely diagnosis of the bradycardic fetus depends on fetal heart rate and on gestational age at presentation. For example, a 32-week fetus with the new onset of a fetal heart rate of 70-80 bpm is more likely to have blocked atrial bigeminy (BAB) or the functional 2° atrioventricular (AV) block of long QT syndrome than anti-Ro/SSA antibody mediated AV block. The same fetal heart rate in a 15-week fetus is most likely to be AV block due to an abnormal conduction system and complex structural defects. Sinus bradycardia between 20 and 32 weeks can be secondary to any of the causes listed.
