Multi-Wavelength Photoplethysmography Device for the Measurement of Pulse Transit Time in the Skin Microvasculature

The purpose of this study was to develop a multiwavelength photoplethysmography (MWPPG) device for studying the skin microvasculature. Microvasculature, in general, is an important part of the cardiovascular system because of its role in microcirculation, which manages tissue nutrient and oxygen supply, and where much of the peripheral resistance occurs. The developed device utilises the fact that the penetration depth of light into the skin is depended on the light wavelength. Thus, an MWPPG device utilising multiple different wavelengths of light allows the studying of blood vessels at different depths. The device proved to produce good quality signal making it possible to compute the small time differences between the pulses measured at different wavelengths, and hence enabling the computation of pulse transit times in the skin microvasculature (PTT_M). The order of the computed PTT_M values are in line with the theory of how deep each wavelength of light penetrates into the skin. The results could be used in hemodynamic monitoring.