PPGs

PPGs, or photoplethysmograms, are noninvasive optical signals used to detect changes in blood volume in peripheral tissue. The term PPG refers to the measurement technique, and PPGs can denote multiple simultaneous signals or recordings from different sites.

Principle and signal characteristics: A light source illuminates the tissue and a photodetector measures changes in light absorption or reflection caused by the pulsing arterial blood. The resulting waveform contains a pulsatile component (AC) riding on a constant background (DC). By using multiple wavelengths, typically red and near-infrared, the ratio of pulsatile to nonpulsatile signals across wavelengths is used to estimate arterial oxygen saturation (SpO2).

Measurement configurations: PPG can operate in transmission mode, where light passes through a thin body part such as a finger or earlobe, or in reflection mode, where light reflects from deeper tissues and is captured at sites such as the wrist, forehead, or chest. Modern wearables commonly employ reflection-mode sensors at the wrist or finger.

Applications: PPG is widely used for heart rate monitoring and SpO2 estimation in consumer wearables and clinical devices. It supports pulse rate variability analysis, peripheral perfusion assessment, and sometimes estimates of blood pressure or arterial stiffness through derived metrics. PPG imaging and camera-based PPG extend the technique to remote or multi-site monitoring without contact.

Processing and limitations: Signal processing involves filtering, motion artifact reduction, and peak detection to extract heart rate and other metrics. Limitations include sensitivity to motion, ambient light, skin tone, and peripheral perfusion. PPG is noninvasive and inexpensive but does not directly measure blood pressure and often requires calibration and site-specific considerations for accuracy.

History and scope: Since its development, PPG has evolved from clinical instruments to ubiquitous wearable sensors and imaging methods, enabling continuous vascular and cardiovascular assessment across diverse settings.