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.