Evaluating Vascular Depth-Dependent Changes in Multi-Wavelength PPG Signals Due to Contact Force

Area of Science:

• Biomedical Engineering
• Physiological Measurement
• Cardiovascular Monitoring

Background:

• Photoplethysmography (PPG) is a non-invasive technique for cardiovascular assessment.
• Multi-wavelength PPG (MW-PPG) improves efficacy by utilizing multiple light wavelengths.
• The influence of contact force (CF) on MW-PPG signals remains understudied.

Purpose of the Study:

• To investigate the specific effects of contact force variations on MW-PPG signals.
• To analyze signal responses across different wavelengths and CF levels.
• To explore the potential of PPG signal components as indirect CF indicators.

Main Methods:

• Simultaneous recording of five PPG wavelengths (470-940 nm), CF, skin temperature, and tonometric measurements.
• Analysis of raw signals and PPG DC/AC components under varying CF.
• Investigation of signal evolution during increasing and decreasing CF phases.

Main Results:

• Individual variability in MW-PPG signal responses to CF was observed.
• Hysteresis and wavelength-dependent effects of CF on PPG signals were demonstrated.
• The PPG DC component correlated with CF trends for most wavelengths, suggesting potential as an indirect indicator.

Conclusions:

• Biomechanical properties and inter-individual variations are crucial in PPG signal formation.
• The arterial pressure wave is proposed as a key factor influencing PPG signals.
• Further validation is required for the practical application of CF indicators derived from PPG.