Table 1.

Description of photoplethysmography (PPG) features.

ID	Features	Description
Time-domain (Geometrical) features
1	Mean_HR	Mean of instantaneous heart rate
2	Mean_VP	Mean of the time from Valley to Peak
3	Mean_PV	Mean of the time from Peak to Valley
4	Mean_NNI	Mean of the time from Peak to Peak
5	Mean_VVI	Mean of the time from Valley to Valley
Time-domain (Statistical) features
6	RMSSD	Root mean square of successive differences of NN interval
7	SDNN	Standard deviation of NN interval
8	SDSD	Standard deviation of successive differences of NN interval
9	Range_NN	Difference between the maximum and minimum NN interval
10	NN50	Number of interval differences of successive NN interval greater than 50 ms
11	pNN50	Corresponding percentage of NN50
12	NN20	Number of interval differences of successive NN interval greater than 20 ms
13	pNN20	Corresponding percentage of NN20
14	CVSD	Coefficient of variation of successive differences equal to the RMSSD divided by Mean_NNI
15	CVNNI	Coefficient of variation equal to the ratio of SDNN divided by Mean_NNI
Frequency-domain features
16	LF	Total energy of NN interval in the low frequency band (0.04–0.15 Hz)
17	HF	Total energy of NN interval in the high frequency band (0.15–0.4 Hz)
18	LF/HF ratio	Ratio of LF power to HF power
19	Total_Power	Total energy of NN interval
20	nLFP	Normalized low frequency power
21	nHFP	Normalized high frequency power
Nonlinear (Geometrical) features
22	SD1	Standard deviation for T direction in Poincare plot
23	SD2	Standard deviation for L direction in Poincare plot
24	SD12	Ratio between SD2 and SD1
25	CSI	Cardiac Sympathetic Index
26	CVI	Cardiac Vagal Index.