Table 2.

Heart rate variability (HRV) frequency-domain measures [15]

Variable	Units	Description	Frequency range
Analysis of short-term recordings (5 min)
5-min total power	ms2	The variance of NN intervals over the temporal segment	≈≤0.4 Hz
VLF	ms2	Power in VLF range	≤0.04 Hz
LF	ms2	Power in LF range	0.04–0.15 Hz
LF norm	nu	LF power in normalized units LF/(total power-VLF)×100
HF	ms2	Power in HF range	0.15–0.4 Hz
HF norm	nu	HF power in normalized units HF/(total power-VLF)×100
LF/HF		Ratio LF (ms2)/HF (ms2)
Analysis of entire 24 hours
Total power		Variance of all NN intervals	≈≤0.4 Hz
ULF		Power in the ULF range	≤0.003 Hz
LF		Power in the VLF range	0.003–0.04 Hz
VLE		Power in the LF range	0.04–0.15 Hz
HF		Power in the HF range	0.15–0.4 Hz
α		Slope of the linear interpolation of the spectrum in a log-log scale	≈≤0.04 Hz

VLF: very low frequency band, LF: low-frequency band, HF: high-frequency band, ULF: ultra low frequency band