Seasonal variations in ventricular repolarization and tachyarrhythmias in hibernating brown bears (<i>Ursus arctos arctos</i>)

In humans, hypothermia prolongs ventricular repolarization and associates with sustained ventricular tachyarrhythmias. In bears, body temperature drops during hibernation similar to moderate human hypothermia, yet they rarely face fatal outcomes during the winter. This suggests protective adaptations in bear electrophysiology. We studied seasonality in ursine ventricular repolarization by analyzing >1 year electrocardiogram (ECG) recordings from loop recorders implanted in 57 free-ranging Eurasian brown bears. In sinus rhythm, bears exhibited significantly longer RR, QT, and T_peak-T_end intervals (2441 ± 470, 508 ± 50, and 53 ± 8 ms, respectively) during hibernation than in the active period (649 ± 323, 232 ± 39, and 29 ± 5 ms, respectively). Optimal heart rate correction of QT interval (QT/RR^0.435) demonstrated significant prolongation during hibernation. QT and T_peak-T_end intervals remained longer during hibernation than in the active period, even when comparing ECGs with similar RR intervals in the two periods. Ventricular fibrillation occurred in four bears shot during licensed hunting in summer, which led to death. In conclusion, seasonal variations in ventricular repolarization in bears appear, at least partially, independently of heart rate. Compared to humans, ventricular repolarization is slower but more homogeneous. These findings, combined with the absence of fatalities during hibernation, support the theory of protective electrophysiological adaptations in bears. Insights into the underlying mechanisms have biomimetic potential for human therapy.