Repolarization adaptation to rapid change in heart rate in human models - a review.

Hysteresis is a ubiquitous phenomenon and a salient feature of the adaptation of cardiac ventricular repolarization (VR) duration to changes in heart rate (HR), an expression of ultra-rapid cardiac memory. Against a background of a handful of previous studies, this review focuses on non-invasive electrophysiological assessment of the adaptation of VR duration and heterogeneity (aka dispersion) to changes in HR. Four different modalities were used: atrial pacing (incremental and step up/down), ventricular pacing (step up/down), and atropine-induced continuous HR increase in healthy subjects and patients who either had permanent pacemakers or were scheduled for ablation of supraventricular tachycardia or had long QT syndrome type 1 (LQT1). Vectorcardiography according to Frank, with orthogonal leads X, Y, and Z, was used for signal recording and beat-to-beat analysis. The RR interval (instantaneous HR) was the input. VR duration was assessed by the QT and QT_peak intervals and VR dispersion by T amplitude, T area, and the ventricular gradient. The main results were that independent of modality, VR duration adaptation follows a mono-exponential pattern, is reproducible, and at a stable HR it takes 2-3 min to reach steady state. In contrast, VR dispersion adaptation is more rapid and roller-coaster-like, presumably due to local differences in adaptation time. In LQT1 patients, VR duration adaptation time is reduced giving less time for electro-mechanical adaptation and coronary perfusion at HR increase. In conclusion, the patterns of adaptation of VR duration and VR dispersion differ, and further studies might provide information on these phenomena of both pathophysiological and therapeutic relevance.