TABLE 3.

Heart rate and heart rate variability features.

Abbreviation	Domain	Description
NN mean	Time	Mean of normal-to-normal peak intervals (NN)
NN CV	Time	Coefficient of variation of NN
NN SD	Time	Standard deviation of NN
NN median	Time	Median of NN
NN range	Time	Difference between maximum and minimum of NN
RMSSD	Time	Root mean square of consecutive differences in adjacent NN
CVSD	Time	Coefficient of variation of consecutive differences in adjacent NN
SDSD	Time	Standard deviation of consecutive differences in adjacent NN
NN50	Time	Number of interval differences greater than 50 ms
NN20	Time	Number of interval differences greater than 20 ms
pNN50	Time	Percentage of interval differences greater than 50 ms
pNN20	Time	Percentage of interval differences greater than 20 ms
HRV HR mean	Time	Heart rate mean
HRV HR SD	Time	Heart rate standard deviation
HRV HR min	Time	Heart rate minimum
HRV HR max	Time	Heart rate maximum
VLF	Frequency	Power spectral density in very low frequencies (0.003–0.04 Hz)
LF	Frequency	Power spectral density in low frequencies (0.04–0.15 Hz)
HF	Frequency	Power spectral density in high frequencies (0.15–0.40 Hz)
Total power	Frequency	Total power spectral density; sum of VLF, LF, and HF
LF/HF	Frequency	The ratio of LF and HF
LFnu	Frequency	LF normalized to the sum of LF and HF
HFnu	Frequency	HF normalized to the sum of LF and HF
Triangular index	Geometrical	Number of all NN divided by the maximum of the NN density distribution
CSI	Non-linear	Cardiac sympathetic index
mCSI	Non-linear	Modified cardiac sympathetic index
CVI	Non-linear	Cardiac vagal index
SD1	Non-linear	Poincaré plot, SD1
SD2	Non-linear	Poincaré plot, SD2
SD2/SD1	Non-linear	SD2 to SD1 ratio