Photoplethysmography (PPG) has evolved into a versatile, non-invasive method for continuous cardio- vascular monitoring over the past two decades. Despite its widespread use for assessing cardiovascular health and monitoring vital signs, PPG{\textquoteright}s clinical utility is compromised by its sensitivity to factors like the contact force (CF) exerted by the sensor and individual physiological variations. This work introduces a novel Multi-parametric Sensor System designed explicitly for multi-wavelength PPG (MW-PPG) signal characterization, utilizing a unique Multi-modal MW-PPG dataset collected in vivo. The research focuses on the dynamic responses of PPG signals to variations in CF, revealing that blood vessels closest to the skin are most affected, with their recovery dependent on the viscoelastic properties of the tissue and specific anatomical features like bone structure in the fingers. The amplitude of the AC component of the signals increases with CF as blood vessel walls compress but decreases as CF rises and arteries begin to occlude, resulting in complex waveform changes that vary with both wavelength and tissue properties. The DC component strongly correlates with CF levels, indicating its potential use as an indirect measure of CF. The study also demonstrates that the response of MW-PPG signals to CF is wavelength-dependent, with shorter wavelengths affected first due to shallower penetration. A distinct hysteresis pattern at the 631 nm wavelength was identified, and tonometric signals aligned with deeper wavelength PPG signals, challenging existing assumptions about their relationship. Furthermore, the research establishes an optimal range of normalized CF between 0.2 and 0.6 to ensure signal stability, emphasizing that signal quality indices (SQIs), such as skewness and kurtosis, remain consistent. Normalized CF values over absolute values are recommended. This adjustment could significantly refine measurement precision, accommodate interindividual variability, and enhance the consistency and reliability of signal quality across diverse populations. Additionally, it facilitates more uniform comparisons between studies, improving the re- producibility of findings in the field. In light of the findings from this research, robust methods for PPG signal acquisition are proposed, addressing factors frequently overlooked in the current state of the art. These methods include developing innovative devices, measurement techniques, and a novel calibration scheme. A preliminary version of a measurement device designed for concept testing and validation of these advancements has also been developed.
Lambert Cause, J 2024, 'A comprehensive multi-parametric PPG sensor platform targeting an in-depth understanding of the impact of the contact force', Vrije Universiteit Brussel, Universidad de Oriente - Santiago de Cuba.
Lambert Cause, J. (2024). A comprehensive multi-parametric PPG sensor platform targeting an in-depth understanding of the impact of the contact force. [PhD Thesis, Vrije Universiteit Brussel, Universidad de Oriente - Santiago de Cuba].
@phdthesis{23c89fe69dc14763820546047d049b41,
title = "A comprehensive multi-parametric PPG sensor platform targeting an in-depth understanding of the impact of the contact force",
abstract = "Photoplethysmography (PPG) has evolved into a versatile, non-invasive method for continuous cardio- vascular monitoring over the past two decades. Despite its widespread use for assessing cardiovascular health and monitoring vital signs, PPG{\textquoteright}s clinical utility is compromised by its sensitivity to factors like the contact force (CF) exerted by the sensor and individual physiological variations. This work introduces a novel Multi-parametric Sensor System designed explicitly for multi-wavelength PPG (MW-PPG) signal characterization, utilizing a unique Multi-modal MW-PPG dataset collected in vivo. The research focuses on the dynamic responses of PPG signals to variations in CF, revealing that blood vessels closest to the skin are most affected, with their recovery dependent on the viscoelastic properties of the tissue and specific anatomical features like bone structure in the fingers. The amplitude of the AC component of the signals increases with CF as blood vessel walls compress but decreases as CF rises and arteries begin to occlude, resulting in complex waveform changes that vary with both wavelength and tissue properties. The DC component strongly correlates with CF levels, indicating its potential use as an indirect measure of CF. The study also demonstrates that the response of MW-PPG signals to CF is wavelength-dependent, with shorter wavelengths affected first due to shallower penetration. A distinct hysteresis pattern at the 631 nm wavelength was identified, and tonometric signals aligned with deeper wavelength PPG signals, challenging existing assumptions about their relationship. Furthermore, the research establishes an optimal range of normalized CF between 0.2 and 0.6 to ensure signal stability, emphasizing that signal quality indices (SQIs), such as skewness and kurtosis, remain consistent. Normalized CF values over absolute values are recommended. This adjustment could significantly refine measurement precision, accommodate interindividual variability, and enhance the consistency and reliability of signal quality across diverse populations. Additionally, it facilitates more uniform comparisons between studies, improving the re- producibility of findings in the field. In light of the findings from this research, robust methods for PPG signal acquisition are proposed, addressing factors frequently overlooked in the current state of the art. These methods include developing innovative devices, measurement techniques, and a novel calibration scheme. A preliminary version of a measurement device designed for concept testing and validation of these advancements has also been developed.",
author = "\{Lambert Cause\}, Joan",
year = "2024",
language = "English",
school = "Vrije Universiteit Brussel, Universidad de Oriente - Santiago de Cuba",
}