Photoplethysmography. Part 2. Influence of light source wavelength.

When the microvascular blood perfusion in human skin is measured by photoplethysmography (PPG), infra-red light (800-960 nm) is normally used as the light source. The PPG signal, which consists of a pulsatile (AC) and a slowly fluctuating (DC) component, was studied at different optical wavelengths utilising optical fibres for guiding the light to and from the skin surface. Finger and forearm skin was examined and high and low skin blood perfusion was brought about by local water-induced temperature provocation. The analysis of the measurement results provided evidence that the use of shorter wavelengths in PPG (AC) for monitoring skin perfusion changes could be applicable. The use of different optical wavelengths also raises the possibility of recording perfusing changes at different depths in the superficial tissue. The sweat water content in stratum corneum of human skin will probably determine the total amount of reflected and backscattered radiation reaching the photodetector. This is important when the skin perfusion is changed by alterations in the environmental temperature conditions activating the sweat glands in tissue. Temperature-dependent optical characteristics of blood-free skin tissue may explain the limited ability of the DC component of PPG to monitor skin perfusion changes.