Exploring the bias: how skin color influences oxygen saturation readings via Monte Carlo simulations

Our goal is to understand the root cause of reported oxygen saturation ( ${\mathrm{SpO}}_2$ ) overestimation in heavily pigmented skin types to devise solutions toward enabling equity in pulse oximeter designs.

We aim to gain theoretical insights into the effect of skin tone on ${\mathrm{SpO}}_2\text{-}R$ curves using a three-dimensional, four-layer tissue model representing a finger.

A finger tissue model, comprising the epidermis, dermis, two arteries, and a bone, was developed using a Monte Carlo-based approach in the MCmatlab software. Two skin tones-light and dark-were simulated by adjusting the absorption and scattering properties within the epidermal layer. Following this, ${\mathrm{SpO}}_2\text{-}R$ curves were generated in various tissue configurations, including transmission and reflection modes using red and infrared wavelengths. In addition, the influence of source-detector (SD) separation distances on both light and dark skin tissue models was studied.

In transmission mode, ${\mathrm{SpO}}_2\text{-}R$ curves did not deviate with changes in skin tones because both pulsatile and non-pulsatile terms experienced equal attenuation at red and infrared wavelengths. However, in reflection mode, measurable variations in ${\mathrm{SpO}}_2\text{-}R$ curves were evident. This was due to differential attenuation of the red components, which resulted in a lower perfusion index at the red wavelength in darker skin. As the SD separation increased, the effect of skin tone on ${\mathrm{SpO}}_2\text{-}R$ curves in reflection mode became less pronounced, with the largest SD separation exhibiting effects similar to those observed in transmission mode.

Monte Carlo simulations have demonstrated that different light pathlengths within the tissue contribute to the overestimation of ${\mathrm{SpO}}_2$ in people with darker skin in reflection mode pulse oximetry. Increasing the SD separation may mitigate the effect of skin tone on ${\mathrm{SpO}}_2$ readings. These trends were not observed in transmission mode; however, further planned research using more complex models of the tissue is essential.