“…Initially, HRV analysis was introduced to investigate the vago-sympathetic balance and corresponding phenomena ( Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996 , later Task Force Society, 1996). However, the HRV features have been also shown as useful for detection of cardiac arrhythmias, as—technically speaking—they measure the irregularity of RR intervals (or heart rate) (e.g., Task Force Society, 1996 ; Bernston et al, 1997 ; Parsi et al, 2021a ). These features are calculated from the RR interval sequence (tachogram) and can be divided into three categories, i.e., time-domain (various statistical measures, such as mean or median value, standard deviation of normal interval (SDNN), root mean square of successive RR differences (RMSSD), and triangular interpolation index calculated from the histogram (TINN)), feature-domain (power and peak of very-low-, low-, and high-frequency bands, their ratios, etc.…”