Condensed Monte Carlo simulations applied to reflectance pulse oximetry

In reflectance pulse oximetry the ratio R/IR between the red and infrared intensity fluctuations, as measured at the skin surface, is used to estimate the arterial oxygen saturation. This ratio is influenced by light propagation in tissue, as measurements at several distances between light sources and detectors simultaneously show that R/IR depends on this distance. In the present study the influence of the estimated tissue properties on R/IR and its distance dependence are investigated by means of Monte Carlo simulations, a method to vary the optical properties without the need for a new Monte Carlo simulation. A three wavelength model has been introduced, because of secondary emission of the red LED. The influence of water absorption has been taken into account. The simulation results depend on the chosen optical properties. Results of R/IR for SaO2 equals 98% with properties from in vivo experiments agree much better with the measured values than the predictions based on in vitro data available in literature. The results show that the condensed Monte Carlo simulation is a valuable tool to gain insight in the principles of reflectance pulse oximetry: The model, assuming a homogeneous distribution of pulsations, is able to describe the experimental results for pulse sizes, R/IR and its distance dependence very well.