The quantitative aspect of photoplethysmography revised

Area of Science:

• Biomedical Engineering
• Physiological Measurement

Background:

• Photoplethysmography (PPG) is widely used to measure physiological parameters.
• Understanding the transfer function of PPG is crucial for accurate data interpretation.

Purpose of the Study:

• To investigate the overall transfer function of PPG devices.
• To assess PPG output in relation to red cell density and intravascular volume.

Main Methods:

• In vitro assessment of PPG as a red cell densitometer.
• In vivo experiments correlating PPG output with air plethysmography measurements.
• Analysis of PPG output as a function of red cell density and intravascular volume.

Main Results:

• PPG output demonstrated a logarithmic relationship with red cell density, consistent with the Lambert-Beer theorem.
• In vivo, PPG output was found to be a logarithmic function of digital intravascular volume.
• A monologarithmic function was proposed to describe the combined effects of stasis on intravascular volume and hematocrit.

Conclusions:

• The study elucidates the non-linear transfer function of PPG devices.
• PPG output can be described by logarithmic and monologarithmic functions under different physiological conditions.
• Linearization of PPG electronics using an exponential amplifier is suggested for improved accuracy.