Abstract
This article presents an overview of the advancements that have been made in the use of photoplethysmography (PPG) for unobtrusive sleep studies. PPG is included in the quickly evolving and very popular landscape of wearables but has specific interesting properties, particularly the ability to capture the modulation of the autonomic nervous system during sleep. Recent advances have been made in PPG signal acquisition and processing, including coupling it with accelerometry in order to construct hypnograms in normal and pathologic sleep and also to detect sleep-disordered breathing (SDB). The limitations of PPG (e.g., oxymetry signal failure, motion artefacts, signal processing) are reviewed as well as technical solutions to overcome these issues. The potential medical applications of PPG are numerous, including home-based detection of SDB (for triage purposes), and long-term monitoring of insomnia, circadian rhythm sleep disorders (to assess treatment effects), and treated SDB (to ensure disease control). New contact sensor combinations to improve future wearables seem promising, particularly tools that allow for the assessment of brain activity. In this way, in-ear EEG combined with PPG and actigraphy could be an interesting focus for future research.
Highlights
Introduction to PhotoplethysmographyPhotoplethysmography (PPG) is an unobtrusive small device that is able to measure numerous physiological functions
An automated algorithm adapted from an algorithm developed for ECG analysis to detect a characteristic heart rate (HR) pattern related to sleep apnea episodes, called auto-correlated wave detection with adaptive threshold (ACAT), was applied on PPG pulse interval data obtained from 41 patients undergoing diagnostic PSG
The authors stressed the possibility of applying PPG-based sleep staging during home sleep apnea testing (HSAT), as PPG is included in such portable monitoring devices (PM), in order to increase the accuracy and clinical assessment of obstructive sleep apnea (OSA) (e.g., rapid eye movement (REM)-related OSA)
Summary
Photoplethysmography (PPG) is an unobtrusive small device that is able to measure numerous physiological functions. Due to the fact that sleep follows an observed cardiovascular and respiratory pattern modulated by the autonomic nervous system, PPG is able to catch these changes and allow for the extrapolation of sleep staging and abnormal breathing pattern data. This data extraction process is complex, as analysis relies on mathematical models that should be tested, trained, and validated on a dataset before a potential clinical application [1]. PPG with other wearables, and, to assess its potential medical applications
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