Table 2.

Frequency-domain variables of heart rate variability [38].

Variable	Description	Frequency Range
LF	Power in low-frequency range	0.04–0.15 Hz
HF	Power in high-frequency range	0.15–0.4 Hz
LF/HF ratio	Sympathovagal balance
LFnu	LF power in normalized units: (LF/(total power − VLF)) × 100
HFnu	HF power in normalized units: (HF/(total power − VLF)) × 100
Total Power	The variance of NN intervals over the temporal segment	Approximately ≤ 0.4 Hz
VLF	Power in very low-frequency range	≤0.04 Hz