fotoplethysmography

Photoplethysmography, sometimes spelled fotoplethysmography in non-English contexts, is a simple, low-cost optical technique for detecting changes in blood volume in the microvascular bed of tissue. It uses a light source, typically LEDs, and a photodetector to measure how much light is transmitted through or reflected from tissue. The detected signal is proportional to the local blood volume.

The method relies on pulsatile arterial blood volume; each heartbeat causes a transient increase in light absorption, producing a waveform known as the photoplethysmogram (PPG). The signal contains a pulsatile AC component superimposed on a slowly varying DC component related to tissue, venous blood, and baseline absorption. When multiple wavelengths are used, the ratio of absorption can estimate arterial oxygen saturation (SpO2) in clinical devices.

Configurations include transmission mode (light on one side and detector on the opposite side, as in finger clips) and reflectance mode (detector near the light source, as in the wrist or forehead sensors). PPG sensors are widely integrated into clinical monitors and consumer wearables for continuous heart rate monitoring.

Applications include measuring heart rate and interbeat interval, estimating SpO2, and, with advanced processing, respiration rate, vascular changes, and autonomic function. In research, PPG is used to assess arterial stiffness and blood flow dynamics.

Limitations include susceptibility to motion artifacts, variations in skin pigmentation and tissue thickness, ambient light interference, and sensor pressure. Calibration and signal processing are essential for accuracy, and PPG provides indirect information about cardiovascular status rather than direct hemodynamic measurements.