Table 2.

Summary of features extracted from the PPG.

Category	Feature notation and publication	Description or formula
PPG morphology	Notch amplitude (${\text{N}}_{\mathit{amp}}$)	Amplitude of the dicrotic notch (see Fig. 1)
	Reflective index (RI)38	Amplitude of the diastolic peak (see Fig. 1)
	$\mathrm{\Delta }$ T41	Time from systolic peak to diastolic peak (see Fig. 1)
	Crest Time (CT)15	Time from onset to systolic peak (see Fig. 1)
	TSys42	Time in systolic phase (see Fig. 1)
	TDia42	Time in diastolic phase (see Fig. 1)
	TRatio42	TSys/TDia
	Slope transit time (STT)50	Slope of straight line from onset to peak (see Fig. 1)
	Stress-Induced Vascular Response Index (sVRI)39	$\mu$Dias/$\mu$Sys
	A140	Area under PPG in systolic phase (see Fig. 1)
	A240	Area under PPG in diastolic phase (see Fig. 1)
	Inflection point area (IPA)40	A2/A1
	${\text{Width}}_{25}$41	Width of the PPG at 25 % of its amplitude (see Fig. 1)
	${\text{Width}}_{50}$41	Width of the PPG at 50 % of its amplitude (see Fig. 1)
	Pressure index (PI)43	$\frac{t(N)-t(S)}{t(N)-t(W)}\times h$
	Normalised harmonic area (NHA)40	${\sum }_{n=2}^{N}FF{T}^2(f_n)/{\sum }_{n=1}^{N}FF{T}^2(f_n)$
	Inflection and harmonic area ratio (IHAR)40	(1-NHA)/ IPA
	Skewness23	–
	Kurtosis23	–
VPG morphology	$Sy{s}_{\mu }$18	Mean of VPG in the systolic phase
	${\text{Sys}}_{\sigma }$18	Variance of VPG in the systolic phase
	${\text{Dia}}_{\mu }$18	Mean of VPG in the diastolic phase
	${\text{Dia}}_{\sigma }$18	Variance of VPG in the diastolic phase
APG morphology	$\frac{b}{a}$, $\frac{c}{a}$, $\frac{d}{a}$ & $\frac{e}{a}$46
	Ageing index (AGI)46	$\frac{b-c-d-e}{a}$
	slopeb-c42	Slope of a straight line between b and c, normalised by a (see Fig. 1)
	slopeb-d42	Slope of a straight line between b and d, normalised by a (see Fig. 1)
	PPG AI47	PPG augmentation index $d_{-2}/b_{-2}$
Gaussian decomposition	${\text{A}}_{g1-4}$, ${\sigma }_{g1-4}$, ${\mu }_{g1-4}$	Amplitude, variance and mean of the four decomposed Gaussians
	Gaussian augmentation index (Gauss AI)12	$max(g_s)-A_{g3}$ (see Fig. 1)
	Gaussian reflection index (Gauss RI)12	$\sum (g_s)-\sum (g_3)$
	Gaussian reflected wave transit time (Gauss RTT)48	${\mu }_{g3}-{\mu }_{g1}$ (see Fig. 1)
	Gaussian augmentation indexR (Gauss AIR)48 ${}^{†}$	$\frac{A_{g1}-A_{g2}}{A_{g1}}$
	Gaussian reflection indexR (Gauss RIR)48 ${}^{†}$	$A_{g3}/A_{g1}$
	Gaussian approximation of left ventricular ejection time (Gauss LVET)51	See reference for definition
	${\text{Gauss}}_{Sys/Dias}$ *	$\sum (g_s)/\sum (g_d)$
	${\text{Gauss}}_{A4/A1}$ *	$A_{g4}/A_{g1}$
	${\text{Gauss}}_{\sigma 4/A1}$ *	${\sigma }_{g4}/A_{g1}$
PCA	PPG PCA 1-352	First 3 principal components of PPG beat
	VPG PCA 1-352	First 3 principal components of VPG beat
	APG PCA 1-352	First 3 principal components of APG beat

*Indicates features that, to the authors’ knowledge, have not been previously implemented for BP estimation. ${}^{†}$ Note that there are two Gaussian indices referenced reflection index and augmentation index. We refer to the second set using the subscript R to reflect the authors: Rubins et al. ${\mu }_{\text{Sys}}$ and ${\mu }_{\text{Dias}}$ represents the mean of the PPG during systole and diastole respectively. h is the height of the participant. t is the normalised time since the pulse onset. FFT is the fast Fourier transform. $g_i$ refers to the $i^{\text{th}}$ Gaussian; $g_s$ = $g_1$ + $g_2$ representing the systolic wave; $g_d$ = $g_3$ + $g_4$ representing the diastolic wave. $\sum (g_i)$ is the area under the $i^{\text{th}}$ Gaussian.