Normal and abnormal plethysmographic digit waveforms

With photoplethysmography (PPG), infrared light is emitted onto a defined area of skin. More or less light is absorbed depending upon changes in blood volume, which is determined by measuring the light reflected from the skin. A normal PPG waveform consists of a short rise in the upstroke during systole (A), a gradual decline during diastole (B), and a dicrotic notch in the diastolic downstroke (C). Moderate arterial obstruction results in loss of the dicrotic notch, flattening of the upstroke and downstroke, and rounding of the peaks of the waveform. Severe obstruction flattens the waveform.