Monte Carlo investigation of the effect of blood volume and oxygen saturation on optical path in reflectance pulse oximetry

Despite the clinical importance of pulse oximetry, the precise nature of the interaction of light with tissue, which underlies the technique, is not yet fully understood. The limitations of the method with regard to its accuracy in conditions of compromised perfusion and/or low blood oxygen saturations are well documented but have only partly been resolved. Results from a static monolayer Monte Carlo model of optical path and reflectance at two wavelengths most commonly used in pulse-oximetry (660 and 940 nm) through skin tissue, containing different volume fractions of blood with a range of oxygen saturations, are presented. Results exhibited differences in mean optical path (MOP) between the two wavelengths, with differences generally increasing with increasing tissue oxygen saturation and decreasing blood volume. As an example, in a typical sensor configuration, the MOP of red light traveling through skin containing 7.5% blood volume fraction with mean oxygen saturation of 60% was 58% higher than that for infrared. The results presented should contribute to further understanding of the effect of physiological conditions such as anemia, ischemia and hypoxemia on the accuracy of pulse oximetry readings.