A Model for Waveform Dissimilarities in Dual-Depth Reflectance-PPG.

Area of Science:

• Biomedical Optics
• Physiological Measurement
• Vascular Physiology

Background:

• Pressure wave attenuation in vascular tissue causes dissimilarities in dual-wavelength reflectance photoplethysmography (PPG) signals.
• Understanding these dissimilarities is crucial for accurate PPG interpretation, especially concerning dermal and subdermal blood volume variations (BVVs).

Purpose of the Study:

• To model the wavelength-dependency of reflectance-PPG signal shapes using green (G) and red (R) wavelengths.
• To investigate the utility of phase parameters derived from dual-depth PPG signals for microvascular characterization.

Main Methods:

• Developed a model for wavelength-dependent reflectance-PPG signal shapes.
• Acquired reflectance-PPG signals in G (520-580 nm) and R (625-720 nm) from the fingers of nine healthy subjects.
• Utilized skin compression to perturb dermal and subdermal BVV contributions and validate the model.
• Quantified dual-depth signal dissimilarities using the phase shift ($\phi$) between dermal and subdermal BVVs and the observed phase shift (PS) between G and R PPG signals.

Main Results:

• The average phase shift ($\phi$) between dermal and subdermal BVVs was 37.6 degrees (95% CI [22.0, 53.2]).
• An average observed phase shift (PS) of 12.5 degrees (95% CI [7.8, 17.2]) was measured at uncompressed skin.
• These phase parameters effectively capture the dual-depth information inherent in PPG signals.

Conclusions:

• Phase parameters derived from dual-wavelength PPG signals can differentiate contributions from dermal and subdermal vascular beds.
• The findings suggest that these phase parameters hold significant potential for non-invasive microvascular characterization and diagnosis.