The QT interval of a body surface electrocardiogram (ECG) reflects the depolarization and repolarization processes across the ventricular myocardium 20.2. Ventricular repolarization and, hence the QT interval duration, fluctuate from one heartbeat to the next, giving rise to so-called QT interval variability. Under resting conditions during periods of stable heart rates, the beat-to-beat fluctuations in QT interval are rather small with a standard deviation of less than 5 ms. QT interval variability (QTV) is typically measured over either 5-minute time intervals or 256 heartbeats under quasi-stationary conditions. Although the role of the autonomic nervous system (ANS)-in particular, the sympathetic branch-in generating and/or modulating QTV has not been fully elucidated, it is likely that the ANS acts at several levels via different pathways. A substantial number of interventional studies, using different autonomic stimuli, reported an increase of QTV during periods of heightened sympathetic nervous system activity.