PPG: the optical pulse waveform AI
Photoplethysmography (PPG) measures changes in detected light associated with changes in blood volume in tissue. Here the finger waveform provides the peripheral pulse used in timing and shape measurements. Its voltage is not a blood-pressure reading. For background, see Allen (2007), Photoplethysmography and its application in clinical physiological measurement.
Peak and rise mean different things
For each ECG-defined cycle, the PPG peak is the maximum waveform value; the rise marker is the maximum positive sample-to-sample slope. Rise does not mean the pulse foot. The delay from ECG to either marker includes processes before and during propagation of the peripheral pulse, so it is not a pure vascular transit time.
The model considers ECG-to-PPG rise and peak delays and the delay from Film0’s mechanical J peak to the PPG rise. PPG also contributes magnitude and shape descriptors. See waveform features.
Signal polarity, sensor contact, gain and waveform quality affect these measurements. Unit-range normalization removes overall offset and scale, but does not guarantee a physiologically equivalent waveform. The variability and alignment note explains this distinction.