Preliminary investigation of partial optical path-lengths in pulse oximetry

Pulse oximetry, a ubiquitous method for determining a person’s oxygen delivery, has recently been criticized for its possible inaccuracy and bias, especially in cases of low oxygen saturation or darker skin tones. Typical pulse oximetry methods utilize an empirical calibration, which may lead to these issues. Alternatively, expressions to compute arterial oxygen saturation from pulsatile optical signals can be derived based on modified Beer-Lambert law. The key known parameter in the modified Beer-Lambert law expression is the ratio of optical path-lengths at infrared and red wavelengths. In this work we utilized a simple Monte Carlo model to investigate this ratio of path-lengths. The results suggest that the path length in the annulus surrounding blood vessels is what should be considered in the modified Beer-Lambert law derived expression. Furthermore, since our preliminary results show that this ratio of path-lengths depends on oxygen saturation, the linearity assumptions of modified Beer-Lambert law may not be valid for pulse oximetry. Future work will utilize the model herein to further investigate what parameters and considerations are important to pulse oximetry.