Abstract
Beyond its use in a clinical environment, photoplethysmogram (PPG) is increasingly used for measuring the physiological state of an individual in daily life. This review aims to examine existing research on photoplethysmogram concerning its generation mechanisms, measurement principles, clinical applications, noise definition, pre-processing techniques, feature detection techniques, and post-processing techniques for photoplethysmogram processing, especially from an engineering point of view. We performed an extensive search with the PubMed, Google Scholar, Institute of Electrical and Electronics Engineers (IEEE), ScienceDirect, and Web of Science databases. Exclusion conditions did not include the year of publication, but articles not published in English were excluded. Based on 118 articles, we identified four main topics of enabling PPG: (A) PPG waveform, (B) PPG features and clinical applications including basic features based on the original PPG waveform, combined features of PPG, and derivative features of PPG, (C) PPG noise including motion artifact baseline wandering and hypoperfusion, and (D) PPG signal processing including PPG preprocessing, PPG peak detection, and signal quality index. The application field of photoplethysmogram has been extending from the clinical to the mobile environment. Although there is no standardized pre-processing pipeline for PPG signal processing, as PPG data are acquired and accumulated in various ways, the recently proposed machine learning-based method is expected to offer a promising solution.
Highlights
Photoplethysmography (PPG) is a non-invasive method for measuring blood volume changes in a microvascular bed of the skin based on optical properties, such as absorption, scattering, and transmission properties of human body composition under a specific light wavelength (Challoner, 1979)
To be eligible for inclusion in this review, the primary requirement was that an article needed to focus on signal characteristics, waveform analysis, noise reduction, peak detection, waveform reconstruction, or quality assessment of PPG
As seen in previous studies, most PPG pre-processing techniques rely on frequency domain filtering, which is effective in removing noise in a range that does not overlap with the core frequency of PPG
Summary
Photoplethysmography (PPG) is a non-invasive method for measuring blood volume changes in a microvascular bed of the skin based on optical properties, such as absorption, scattering, and transmission properties of human body composition under a specific light wavelength (Challoner, 1979). In 1937, Hertzman found that the amount of light detected by back scattering after irradiating light to the skin was significantly changed according to cardiac activity. He suggested that PPG was a technique for measuring blood volume changes in a specific area irradiated with light (Hertzman, 1937, 1938). PPG records the amount of light transmitted or reflected by the change in concentration of substances in the blood and the optical path according to pulsation, which can be explained by the Beer–Lambert law that defines the attenuation of light intensity by the extinction coefficient, concentration, and optical path length of a medium when light passes through it (Beer, 1851). The total absorbance depends on the skin structure
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