Low frequency variability in photoplethysmography and autonomic function assessment

The photoplethysmography (PPG) signal is composite in nature. It has low frequency (“DC”) components relating to respiration, vasomotion, blood pressure control, endothelial function and thermoregulation, as well as the high frequency components relating to the heart synchronous (“AC”) pulse waveform. There is a growing interest in the “DC” characteristics, their measurement at different sites across the body when made at rest or in response to a physiological challenge, and subsequent signal analysis methods. This chapter also considers beat-to-beat blood pressure tracking as well as pulse rate variability, and overviews a range of clinical applications including cardiovascular assessment, sleep science, pregnancy and maternity, pain, mental health and wellbeing, neurophysiological assessment, and studies in unique and challenging situations such as space and military research.