Therapeutic hypothermia (TH) is effective for neonatal hypoxic–ischemic encephalopathy (HIE). The combination of abnormal myocardial repolarization and fatal arrhythmia in patients with accidental hypothermia has prompted clinical validation of the proarrhythmic potential of TH. However, to our knowledge, there have been few clinical studies on myocardial depolarization and repolarization abnormalities caused by TH in neonates. Therefore, we investigated the effects of TH on neonatal myocardial depolarization and repolarization by capturing the waveform changes in electrocardiograms (ECGs) associated with body temperature (BT) before and after TH. We included three neonates with HIE diagnosed at birth who were treated with TH in our hospital. The heart rate, RR interval, P wave duration, PR interval, QRS duration, QT interval, corrected QT (QTc) interval by Fridericia’s formula, J point-T end (JT) interval, corrected JT (JTc) interval by Fridericia’s formula, T peak-T end (Tpe) interval, Tpe/QT, and QRS/QTc were calculated retrospectively using an ECG. The correlations of ECG parameters recorded concurrently with 33 samples in which BT measurements were confirmed were performed. BT and heart rate were positively correlated (R: 0.589, p = 0.0003). BT was negatively correlated with Tpe/QT (R: − 0.470, p = 0.0058), the QTc interval (R: − 0.680, p < 0.0001), and the corrected JT interval (R: − 0.697, p < 0.0001). TH does not affect atrial or ventricular depolarization but prolongs the ventricular repolarization process in a temperature-dependent manner.
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