The Effects of External Pressure on Multi-Wavelength Photoplethysmography Signals

A device consisting of a multi-wavelength photoplethys-mography (MWPPG) sensor, pressure sensor and pressure generating system was developed to measure blood volume changes at different depths of the skin while changing the external pressure. The signals recorded at shorter wavelengths of light (blue and green) showed signs of blood vessel occlusion/opening at lower pressure readings than signals recorded at longer wavelengths of light (red and infrared). Similarly, the maximum oscillation amplitudes for the shorter wavelength channels occurred at lower pressure levels than for the longer wavelength channels. The oscillogram shapes and slopes also differed between the channels, shorter wavelength channels having had smaller slopes than the longer wavelength channels. In addition, the time differences between the pulse waveform feet of the different channels, computed relative to the infrared channel, increased together with the applied external pressure. The results indicate that MWPPG signals can provide new information about the vasculature by probing the tissue at different depths, and that contact pressure is a factor to consider in MWPPG applications. However, more research is needed to better understand the signals, especially how much overlapping information the longer wavelength channels carry with respect to the shorter wavelength channels.