Prenatal presentation of fetal bradycardia and long QT syndrome

Fetal bradycardia may be defined as a ventricular rate more than 2 SD below the gestational-age-specific mean. Sinus bradycardia, functional 2:1 atrioventricular block (AVB) and/or polymorphic ventricular tachycardia (VT) are recognized as potential presentations of fetal long QT syndrome (LQTS). The objectives of this study were to describe the associations with LQTS and outcomes of fetuses presenting with isolated sinus bradycardia, 2:1 AVB or VT.

This was a retrospective review of all cases presenting with sinus bradycardia, non-immune mediated 2:1 AVB or VT on referral to our tertiary fetal cardiology center between January 2018 and November 2023. Cases with maternal anti-Ro/anti-La antibodies, blocked atrial ectopic beats or major congenital heart disease were excluded. Data were collected on patient demographics, fetal echocardiography findings, clinical genetic results, family screening and postnatal outcome. The left ventricular isovolumetric relaxation time (LVIRT) and LVIRT normalized as a proportion of cycle length (N-LVIRT) were derived using pulsed-wave Doppler tracings to estimate the QT interval.

In total, 22 fetuses were included in the study, with a presenting rhythm of sinus bradycardia in 16, 2:1 AVB in two, both sinus bradycardia and 2:1 AVB in two and VT in two. None had a family history of LQTS. Genetic testing was performed in 14 cases of which 12 were tested prenatally. There were 13 cases with a positive LQTS genotype (eight KCNQ1, one KCNE1, two KCNH2, one SCN5A and one CALM2). Median presenting fetal heart rate (FHR) and FHR Z-score in sinus rhythm was 120 (range, 100-139) bpm and -3.04 (range, -4.98 to -0.91), respectively. In the 13 cases with genetically confirmed LQTS, N-LVIRT and LVIRT were persistently above threshold in two and six cases, respectively.

The most common finding in fetuses with persistent sinus bradycardia without major congenital heart disease or autoimmune disease is LQTS. Prenatal genetic testing enables tailored parental counseling and management of the pregnancy. Although N-LVIRT and LVIRT were not above the threshold values in most cases, they remain important measures for investigation in future studies. © 2026 International Society of Ultrasound in Obstetrics and Gynecology.