Table 2 Summary of features extracted from the PPG.

PPG morphology

Notch amplitude (\(\hbox {N}_{amp}\))

Amplitude of the dicrotic notch (see Fig. 1)

Reflective index (RI)³⁸

Amplitude of the diastolic peak (see Fig. 1)

\(\Delta\) T⁴¹

Time from systolic peak to diastolic peak (see Fig. 1)

Crest Time (CT)¹⁵

Time from onset to systolic peak (see Fig. 1)

T_Sys⁴²

Time in systolic phase (see Fig. 1)

T_Dia⁴²

Time in diastolic phase (see Fig. 1)

T_Ratio⁴²

T_Sys/T_Dia

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

\(\hbox {Width}_{25}\)⁴¹

Width of the PPG at 25 % of its amplitude (see Fig. 1)

\(\hbox {Width}_{50}\)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

\(\sum _{n=2}^{N}FFT^2(f_n)/\sum _{n=1}^{N}FFT^2(f_n)\)

Inflection and harmonic area ratio (IHAR)⁴⁰

(1-NHA)/ IPA

Skewness²³

–

Kurtosis²³

–

VPG morphology

\(Sys_\mu\)¹⁸

Mean of VPG in the systolic phase

\(\hbox {Sys}_\sigma\)¹⁸

Variance of VPG in the systolic phase

\(\hbox {Dia}_{\mu }\)¹⁸

Mean of VPG in the diastolic phase

\(\hbox {Dia}_\sigma\)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

\(\frac{b - c - d - e}{a}\)

slope_b-c⁴²

Slope of a straight line between b and c, normalised by a (see Fig. 1)

slope_b-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

\(\hbox {A}_{g1-4}\), \(\sigma _{g1-4}\), \(\mu _{g1-4}\)

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 index_R (Gauss AI_R)⁴⁸ \(^{\dagger }\)

\(\frac{A_{g1} - A_{g2}}{A_{g1}}\)

Gaussian reflection index_R (Gauss RI_R)⁴⁸ \(^{\dagger }\)

\(A_{g3}/A_{g1}\)

Gaussian approximation of left ventricular ejection time (Gauss LVET)⁵¹

See reference for definition

\(\hbox {Gauss}_{Sys/Dias}\) *

\(\sum (g_s)/\sum (g_d)\)

\(\hbox {Gauss}_{A4/A1}\) *

\(A_{g4}/A_{g1}\)

\(\hbox {Gauss}_{\sigma 4/A1}\) *

\(\sigma _{g4}/A_{g1}\)

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