Repolarization predictors of fetal long QT syndrome

BACKGROUND: Diagnosis of fetal long QT syndrome (LQTS) using fetal magnetocardiography (fMCG) is straightforward in cases of overt QTc prolongation accompanied by LQTS rhythms; however, cases of isolated QTc prolongation can be challenging. OBJECTIVE: The purpose of this study was to characterize repolarization in normal and phenotype-positive LQTS fetuses with the goal of using additional parameters of repolarization to improve the accuracy of fMCG diagnosis of LQTS. METHODS: fMCG recordings were taken from 37 phenotype-positive fetuses with confirmed LQTS and 132 normal controls. The normal fetuses were grouped into those with T and U waves and those with only T waves. We compared the repolarization characteristics of normal fetuses with only T waves with those of LQTS fetuses. We also compared the repolarization characteristics of normal fetuses with T and U waves with those of LQTS fetuses with 2-component T waves. RESULTS: Late-peaking T waves were strongly associated with LQTS (35/37 [95%]). No normal fetuses showed both QTc prolongation (QTc >500 ms) and a late-peaking T wave. U waves were seen in 11 normal fetuses (8%) and resulted in waveforms that often mimicked those of the 19 LQTS fetuses with 2-component T waves. However, in normal fetuses the polarities of the T and U waves were the same, whereas in LQTS fetuses with 2-component T waves the polarities of the components usually were opposite. CONCLUSION: A late-peaking T wave in association with QTc prolongation is a distinctive, reliable indicator of fetal LQTS. U waves confound assessment of QTc; however, normal U waves usually can be distinguished from LQTS T waves based on polarity.