Table 2.

Heart rate variability feature parameters of ECG

Parameter	Units	Description
Time-domain
$\stackrel{-}{\mathrm{RR}}$	[ms]	The mean of RR intervals
SDNN	[ms]	Standard deviation of normal to normal RR intervals
$\stackrel{-}{\mathrm{HR}}$	[1/min]	The mean heart rate
SDHR	[1/min]	Standard deviation of instantaneous heart rate values
MinHR	[beats/min]	Min heart rate per minute
MaxHR	[beats/min]	Maximum heart rate per minute
RMSSD	[ms]	The root mean square of successive RR interval differences
NN50	[count]	Number of successive RR interval pairs that differ more than 50 ms
pNN50	[%]	NN50 divided by the total number of all NN intervals
HRV triangular index	–	The integral of the RR interval histogram divided by the height of the histogram
TINN	[ms]	Baseline width of the NN interval histogram
Frequency-domain
VLF power	[ms2]	Absolute power of VLF band
LF power	[ms2]	Absolute power of LF band
HF power	[ms2]	Absolute power of HF band
LF/HF	–	Ratio between LF and HF band powers
Total power (TP)	[ms2]	Total spectral power
Normalized LFP	–	LF/(TP-VLF)
Normalized HFP	–	HF/(TP-VLF)
Nonlinear
SD1	[ms]	Poincaré plot standard deviation perpendicular the line of identity
SD2	[ms]	Poincaré plot standard deviation along the line of identity
SD2/SD1	[%]	Ratio of SD2 to SD1
ApEn	–	Approximate entropy
SampEn	–	Sample entropy
${\alpha }_1$,${\alpha }_2$	–	Short-term and long-term fluctuations of detrended fluctuation analysis (DFA)