Heart rate variability (HRV) has been analyzed for detecting various pathological conditions and autonomic functions [1]. HRV is typically derived from an electrocardiogram (ECG) signal by computing the variations in time intervals between two R peaks over a period of time. The autonomic nervous system (ANS) is divided into sympathetic nervous system (SNS) and parasympathetic nervous system (PNS), the power spectrum in the high frequency (HF) region (0.15–0.4 Hz) of the HRV corresponds to parasympathetic (vagal) activity alone, and the power spectrum in the low frequency (LF) region (0.04–0.15 Hz) of HRV corresponds to both sympathetic and vagal activity [2], however, the LF power in normalized unit (n.u.) is used for estimating the sympathetic activity [1,2]. Therefore, the ratio of normalized power into these two frequency bands of HRV spectra is considered as a marker of sympathovagal balance [1,2...

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