Background: Evaluation of the neonatal QT interval is important to diagnose inherited arrhythmia syndromes and evaluate possible side effects of drugs. Accurate interpretation of an abnormal QT interval is dependent upon well-established reference values, obtained with modern technology on newborns with structurally normal hearts.
Objective: To describe the natural history of the QT interval during the first four weeks of life and to provide reference values for corrected and heart rate-specific uncorrected QT intervals, from a large general population sample with structurally normal hearts.
Methods: The Copenhagen Baby Heart Study is a prospective general population study that offered cardiac evaluation of all newborns in the Copenhagen area from 2016-2018. Eight lead electrocardiograms were obtained and analyzed with a computerized algorithm with manual validation. Echocardiograms were performed according to standard guidelines.
Results: We included 14,164 newborns (52% boys), aged 0-28 days, with normal echocardiograms. The median values (ms, 2-98%ile) for the corrected intervals QTc(Bazett), QTc(Hodges), QTc(Fridericia), and QTc(Framingham) were 419 (373-474), 419 (373-472), 364 (320-414), and 363 (327-405). During the first four weeks of life we observed a small decrease of QTcFramingham, and an increase of QTcHodges (both p<0.01), while QTcBazett and QTcFridericia did not change (p>0.05). Applying published QT interval cut-off values resulted in 5-25% of the newborns having QT prolongation. Uncorrected QT intervals decreased linearly with increasing heart rate. Sex and infant size did not affect the QT interval.
Conclusion: During the first four weeks of life QTcFramingham and QTcHodges showed minor changes, whereas QTcBazett and QTcFridericia were stable. The published QTc thresholds markedly overestimated the prevalence of newborns with QT prolongation. The QT interval was unaffected by sex and infant size. The presented reference values for corrected and heart rate-specific uncorrected QT intervals may facilitate a more accurate diagnosis of newborns with an abnormal QT interval.