Photoplethysmographic Waveform and Pulse Rate Variability Analysis in Hyperbaric Environments.

The main aim of this work is to identify alterations in the morphology of the pulse photoplethysmogram (PPG) signal, due to the exposure of the subjects to a hyperbaric environment. Additionally, their Pulse Rate Variability (PRV) is analysed to characterise the response of their Autonomic Nervous System (ANS). To do that, 28 volunteers are introduced into a hyperbaric chamber and five sequential stages with different atmospheric pressures (1 atm; descent to 3 and 5 atm; ascent to 3 and 1 atm) are performed. In this work, nineteen morphological parameters of the PPG signal are analysed: the amplitude of the PPG pulse; eight parameters related to pulse width; eight parameters related to pulse area; and the two slopes of the PPG pulse. Also, classical time and frequency parameters of PRV are computed. The relative change of all the parameters was calculated with respect to the initial baseline state at 1 atm. Notable widening of the pulses width is observed in the four stages analysed, together with a decrease in the PPG amplitude at the last ascent stages. The PPG area increases with pressure, with no significant changes when the initial pressure is recovered. These changes in PPG waveform may be caused by an increase in the systemic vascular resistance as a consequence of vasoconstriction. This phenomenon suggests a sympathetic activation, causing a vasoconstriction in the skin circulation, especially in the extremities. However, the PRV results show an augmented parasympathetic activity and a reduction in the parameters that characterise the sympathetic response. So, only a sympathetic activation is detected in the peripheral region, as reflected by PPG morphology. The information regarding the ANS and the cardiovascular response that can be extracted from the PPG signal, as well as its compatibility with wet conditions make this signal the most suitable for studying the physiological response in hyperbaric environments, including scuba diving activities.