Table 5 Waveform features

Time-domain features (Fig. 7—part 4)

1

Crest time (CT)

The interval between the left valley (LV) and ESP [41]

2

Diastolic time (DT)

The interval between ESP to the right valley (RV) [84]

3

FN

ESP to DN

4

FirstPeakHeight

The height of first peak

5

HeightDifferenceFirstPeakToSecondPeak　(FS)

ESP to DN

6. ~ 8

Inflection point area (IPA)

A1

A2

The diastole to systole area ratio is calculated from the area under the curve from IP to RV (A2) relative to the area from LV to IP (A1) (L. [88]). The inflection point (IP), defined as the last point before DP where the first derivative shifts from positive to negative, is crucial in calculating the IPA. Points E or DN demarcate the diastolic and systolic areas [11, 75], with A1 approximated as a polygon formed by LV, ESP, DN, and DNB, and A2 as a triangle comprising IP, DN, and RV, simplifying the calculation process

9

IP

The ratio of second peak is not obvious so the inflection point is used as the second peak

10

IP slope

The slope of the wave at inflection point

11

Peak-to-peak time (PPT)

The interval between the early systolic peak [ESP, also known as the left peak (LP) or the first peak (FP)] and the diastolic peak (DP). The right valley (RV) is the lowest point in the BBI, and the preceding BBI's RV is termed the left valley (LV). The diastolic peak [DP, or second peak (SP)], is the first point between ESP and RV with a zero first derivative and a negative second derivative. If no second peak exists between ESP and RV, indicating a consistently negative slope from LP to RV, the DP is identified as the point with the minimum second derivative between ESP and RV

12

Notch height (NH)

The lowest point between ESP and DP is the dicrotic notch (DN)

13

Notch time (NT)

The interval between LV to the dicrotic notch (DN). In cases without a second peak, IP is designated as DN

14

Notch to valley time (NVT)

The interval between DN to right valley (RV)

15

RCA

The ratio of CT to NT

16

RDA

The ratio of NT to the combined duration of CT and DT

17

Reflection index (RI)

The ratio of the diastolic peak height (DPH) to the early systolic peak height (ESPH)

18

Stiffness index (SI)

The ratio of body height to PPT [6] [18]

19

SR

DP to RV

20

SPH

The second peak height

21

SecondPeakSlope

The slope at second peak

Curvature features (acceleration PPG, Fig. 7—part 4)

22

A

The first local maximum of the second derivative, determined by the point with the largest second derivative before ESP

23

B

The first local minimum of the second derivative, determined by the point with the smallest second derivative before ESP

C

The second local maximum of the second derivative, determined by the point with the largest second derivative point B and point E. (Not included in the data analysis.)

D

The second local minimum of the second derivative, determined by the point with the smallest second derivative between point B and point E. (Not included in the data analysis.)

24

E

The third local maximum of the second derivative, determined by the point with the largest second derivative between ESP and point F

25

F

The third local minimum of the second derivative, determined by the point with the smallest second derivative between the ESP and RV

26

G

The fourth local maximum of the second derivative, determined by the point with the largest second derivative between point F and point H

27

H

The fourth local minimum of the second derivative, determined by the point the smallest second derivative between point F and RV

28. ~ 32

BA, EA, FA, GA, HA

|B/A|, |E/A|, |F/A|, |G/A| and |H/A|. Given that B/A, E/A, F/A, G/A and H/A are negative, we converted them to their absolute values for a more intuitive interpretation

33

Aging index (AI)

BA– EA. AI is usually defined as BA—(CA + DA + EA) [7]. However, in this study, AI is defined as the difference between BA and EA only, due to difficulties in identifying points C and D with poor signal quality and the fact that the values of C and D are close to zero

Frequency-domain features (Fig. 8)

34. ~ 39

PSDi

The ith relative power spectrum density. The continuous PSD curve is segmented using the midpoints between each peak. We define the strength of the ith harmonic (\(PS{D}_{i}\), starting from 1) as the area between the ith midpoint and the (i + 1)th midpoint. Since the absolute values of each raw \(PS{D}_{i}\) is influenced by the quality of the signal, the values are normalized as the relative values: \(PSDi\, = \,\frac{{raw{ }PSD_{i} }}{{\mathop \sum \nolimits_{j = 1}^{6} raw{ }PSD.j}}\)

40

NHA

\(1-PSD1\)[8]

41

IHAR

1-NHA/IPA [8]