Figure 4.

Classes of photoplethysmogram (PPG) pulse wave shape: Typical changes in PPG pulse wave shape with age, from young (left) to old (right). As described by Dawber et al. (244): class 1 waves exhibit an incisura; class 2 show a horizontal on the line of descent; class 3 show a change in gradient on the downslope; class 4 shows no evidence of a notch. Pulse waves were measured using infrared reflection mode photoplethysmography, and were obtained from the Vortal dataset (245). Source: P. H. Charlton, “Classes of photoplethysmogram (PPG) pulse wave shape (https://commons.wikimedia.org/wiki/File:Classes_of_photoplethysmogram_(PPG)_pulse_wave_shape.svg)” (CC BY 4.0).