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

Significance: 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.

Aim: 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.

Approach: 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.

Results: 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.

Conclusions: 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.