TABLE 1.

Definitions and calculation methods of the extracted features.

Features	Calculations	Definitions
(1) Pulse transit time R-foot	PTTrf	Time delay from the R-peak of ECG to the foot of PPG
(2) Pulse transit time R-middle	PTTrm	Time delay from the R-peak of ECG to the peak of the first derivative of PPG
(3) Pulse transit time R-peak	PTTrp	Time delay from the R-peak of ECG to the peak of PPG
(4) Ascending time	AT	Time span between the foot and peak of PPG in ascending edge
(5) Ascending slope	AS = IFP/AT	Ratio of PPG peak-foot point intensity difference in ascending edge (IFP) to ascending time
(6) first part of AS	FAS = IFM/TFM	Ratio of PPG M point intensity (IFM) to the duration from the foot point to M point of PPG (TFM)
(7) Ascending area	$\text{SYSAREA}={\sum }_{i=F}^P\left(I_i-I_F\right)$	Area under the ascending portion of the PPG waveform
(8) Descending time	DT	Time span between the peak and foot of the PPG in Descending edge
(9) Descending slope	DS = IPF′/DT	Ratio of PPG peak-foot point intensity difference in descending edge (IPF’) to descending time
(10) First part of DS	FDS = IM′F′/TM′F′	Ratio of PPG M′ point intensity (IM’F’) to the duration from the M′ to foot point of PPG (TM’F’)
(11) Descending area	$\text{DIAAREA}={\sum }_{i=p}^{F^{\prime }}\left(I_i-I_{F^{\prime }}\right)$	Area under the descending portion of the PPG waveform
(12) PPG intensity ratio	PIR (Ding X.-R. et al., 2016)	Ratio of PPG peak intensity to foot intensity
(13) Pulse width	PW	Time span between the M point and M′point of PPG
(14) Heart rate	HR	Time span between two adjacent peaks of PPG
(15) K value	K (Miao et al., 2020)	PPG characteristic value

Point M (and its corresponding point M′ in descending edge) represents the maximum derivative point of PPG; ${\sum }_{i=F}^P\left(I_i-I_F\right)$ represents the cumulative sum of intensity difference of PPG from point foot (F) to point peak (P) in ascending edge; ${\sum }_{i=P}^{F^{\prime }}\left(I_i-I_{F^{\prime }}\right)$ presents the cumulative sum of intensity difference of PPG from point peak (P) to point foot (F′) in descending edge. PPG, photoplethysmogram.