Photoplethysmography

Photoplethysmography is a simple, noninvasive optical technique used to detect volumetric changes in blood in the microvascular bed of tissue. It is commonly employed to monitor cardiovascular function and perfusion.

Principle and components: A light source irradiates the skin, and a photodetector measures the amount of light either transmitted through or reflected by the tissue. The detected signal contains a constant component from static tissue and a pulsatile component caused by the heartbeat as arterial blood volume in the illuminated region changes. The pulsatile (AC) component is superimposed on the baseline (DC) signal. Using multiple wavelengths, especially red and near-infrared light, the technique can also estimate blood oxygen saturation by comparing light absorption at different wavelengths.

Configurations: Transmissive PPG places the sensor on opposite sides of a thin tissue region (e.g., fingertip or earlobe), while reflectance PPG places the detector on the same side as the light source, suitable for various body sites. Green light is often favored in wearable devices for its strong pulsatile signal in peripheral sites, whereas infrared light provides deeper tissue penetration.

Signal processing and metrics: The PPG waveform is analyzed to derive heart rate and, with advanced processing, pulse rate variability and indices of vascular function. When two wavelengths are used, the ratio of absorbed light enables estimation of SpO2. Motion artifact reduction and filtering are important for reliable measurements.

Applications and limitations: PPG is widely used in clinical monitoring, general wellness devices, and research into hemodynamics. Limitations include sensitivity to movement, ambient light, sensor contact, skin tone, and peripheral perfusion, which can affect accuracy, particularly for SpO2.