. 2022 Jun 30;10:920946. doi: 10.3389/fpubh.2022.920946

Table 1.

Significant CVD-related predictors based on PPG fiducial points.

Signal	PPG Parameters	Measurement	Remarks	Significance
PPG	Stiffness Index (SI)	$S I = \frac{H}{D T}$	H = Subject height; DT, Delta Time (Time from systolic peak to diastolic peak).	Risk factor for coronary heart diseases such as hypertension, diabetes, and smoking is proportional to the stiffness index (26).
PPG	Reflection Index (RI)	$R I = \frac{D_{p e a k}}{S_{p e a k}} \times 100 %$	D_peak = Amplitude of diastolic peak from the onset. S_peak = Amplitude of systolic peak from the onset.	A good indicator for vascular assessment (27).
PPG	Augmentation Index (AIx)	$S I = \frac{S_{p e a k} - D_{p e a k}}{S_{p e a k}} \times 100 %$	D_peak = Amplitude of diastolic peak from the onset. S_peak = Amplitude of systolic peak from the onset.	AIx increases in older and CVD subjects (28).
PPG	Crest Time (CT)	CT = Systolic_time − Onset_time	CT, Time from onset to systolic	CT is affected by aging and arteriosclerosis (29).
PPG	Crest Time Ratio (CTR)	$C T R = \frac{C T}{C y c l e_{t i m e}}$	CT, Crest Time; Cycle_Time, Time from onset to next onset	CTR was lower in the non-diabetic subject than that in diabetic subjects (P = 0.004). Patients with diabetes have a higher risk of CVD (29).
APG	Ratio	$\frac{b}{a}$	Amplitude ratio	b/a increases as arterial stiffness increases (30).
APG	Ratio	$\frac{c}{a}, \frac{d}{a}, \frac{e}{a}$	Amplitude ratio	All ratios decrease as arterial stiffness increases (30).
APG	Vascular Aging (VA)	$V A = \frac{b - c - d - e}{a}$ or $V A = \frac{b - e}{a}$	Aging Index	Assessment of vascular aging and arteriosclerotic disease (30).