Fetal heart rate and the neonatal QT interval: Results from a large general population study

Fetal bradycardia has been reported in neonates with congenital long QT syndrome, but it remains unknown whether there is a general association between fetal heart rate (FHR) and the neonatal QT interval.

We examined whether first- (FHR1) and second-trimester FHR (FHR2) measurements were associated with the neonatal QT interval.

We investigated neonates from a large population-based cohort study with available electrocardiograms and FHR1 measurements. We also investigated a subgroup with FHR2 data. The neonatal corrected QT (QTc) interval was the primary outcome variable. Univariate and multiple regression analyses were also performed.

The study cohort included 7998 neonates (48.4% girls) with a median age of 11 days (interquartile range [IQR] 7-14) at electrocardiographic recording. At a median gestation age (GA) of 90 days, we found a median FHR1 of 159 beats per minute (IQR 155-164) and no association with the QTc interval (adjusted difference 0.407 × 10^-4; 95% confidence interval -0.656 × 10^-4 to 0.147 × 10^-3; P > .05). The median FHR2 (available in 2202 neonates; median GA 140 days) was 148 beats per minute (IQR 144-153), and lower FHR2 values were associated with longer QTc intervals (QTc using Fridericia's correction formula 367 vs 362 ms between outer quartiles). This association remained significant after multifactorial adjustment (adjusted difference -0.164 × 10^-3; 95% confidence interval -0.880 × 10^-4 to -0.110 × 10^-4; P < .05).

FHR2 is associated with longer neonatal QTc interval, whereas no association was observed between FHR1 and the QTc interval. These findings suggest a GA-dependent association, emphasizing the importance of considering GA when evaluating FHR and neonatal QT intervals.