Study variables
Patient demographics, medical history and medication were extracted as available. The ECGs were evaluated independently by cardiac electrophysiologists in a standardized manner. Disagreement was resolved by consensus. ECG interpretation included identification, interpretability and measurement of the longest QT interval in limb and chest leads, respectively, and the evaluation of the T-wave morphology in each available lead. T-wave morphology was categorized into flat, broad, late peaking, notched, biphasic or inverted. We then added up all limb and chest lead morphologies in order to evaluate the respective dominant morphology. For measurments, we used electronic calipers which we calibrated individually to each ECG for the QT interval, QRS duration and RR interval. The corrected QT (QTc) interval was calculated using Bazett’s formula QTc= QT/√RR if heart rate was ≤90 beats per minute (bpm) and Fridericia‘s formula QTc= QT/(RR^0.33) if heart rate was >90 bpm.