Photoplethysmography : methodological studies and applications

Sammanfattning: Photoplethysmography (PPG), an optical non-invasive technique for measuring skin perfusion changes, was investigated and evaluated. The method was compared with laser Doppler flowmetry (LDF) when measuring perfusion changes in human fingers and forearms. The results showed that both PPG and LDF reflect changes in skin perfusion under certain conditions related to the sample volume of PPG. This volume is in turn related to the wavelength and the probe type used. In addition, an increased sensitivity to perfusion changes can, in some vascular beds, be obtained when using a shorter wavelength (560 nm). The DC-coupled PPG does not provide a consistent measure of skin perfusion changes induced by local temperature variations.Optical properties of blood in motion were studied in perfusion models. Light reflection and transmission was found to be dependant on several parameters such as blood volume, viscosity, red blood cell orientation and haematocrit. The results also indicate that a shorter wavelength can be used to extract information about blood volume changes, whereas a longer wavelength primarily reflects changes in red cell orientation.A fibre optic sensor for monitoring of respiratory- and heart rates from the same fibre optic probe has been developed. Blood perfusion changes, synchronous with the heart- and the respiration rates, were measured by analysing reflected light from the skin surface. The new sensor has the advantages of being totally non-invasive. It can be positioned anywhere on the skin surface and is also insensitive to electromagnetic disturbances.Pulse oximetry utilizes the PPG signal for monitoring of the arterial oxygen saturation. The pulse oximeter signal was studied under various blood flow conditions, simulated in an in vitro model. The results indicate that blood flow conditions may affect the accuracy of the instrument.