Fetal heart rate predictors of long QT syndrome

Fetal long QT syndrome (LQTS) is associated with complex arrhythmias including torsades de pointes and 2° atrioventricular block. Sinus bradycardia has also been associated with fetal LQTS, but little is known of this rhythm manifestation. Our purpose was to characterize the fetal heart rate (FHR)/gestational age (GA) profile of fetal LQTS.

We ascertained fetal LQTS subjects by family history (Group 1) or fetal arrhythmia referral (Group 2). We compared FHR in LQTS subjects versus normal fetuses. To identify FHR predictors of LQTS, we calculated a bradycardia index as % of LQTS FHR recordings either ≤110 beats per minute (obstetric standard) or ≤3(rd) percentile for GA. Among 42 LQTS subjects, 26 were in Group 1 and 16 in Group 2. There were 536 normal fetuses. The bradycardia index was only 15% for FHR ≤110 beats per minute, but 66% for FHR ≤3rd percentile for GA. Ten fetuses with complex arrhythmias also had severe and sustained sinus bradycardia throughout gestation. Identifying a fetal proband in Group 2 resulted in LQTS diagnosis in 9 unsuspected members of 6 families.

FHR varies by GA in both normal and LQTS fetuses. Postnatal evaluation of neonates with FHR ≤3(rd) percentile for GA may improve ascertainment of LQTS in fetuses, neonates, and undiagnosed family members.