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

Abstract

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.

Keywords: Monte Carlo; melanin; occult hypoxemia; oxygen saturation; pulse oximeter; racial bias; skin color; transmission mode.

Fig. 1

Output geometry of the finger tissue model produced by MCmatlab, containing the epidermis, dermis, two arteries, and a bone.

Fig. 2

Positioning and placement of the light source and collector illustrated in (a) transmission mode and (b) reflection mode geometry. The source is represented by a black dot, and the detector is represented by a red ring.

Fig. 3

Normalized fluence rate distribution of the detected photons at ${\mathrm{SpO}}_2=90\%$ in transmission mode, shown at diastolic red and IR for light skin [(a), (b)] and for dark skin [(c), (d)], respectively. The color bar represents the magnitude of the distribution of detected photons through the finger tissue. Plots (e) and (f) show horizontal and vertical line profiles of the normalized fluence distribution, at $Z=0.12\mathrm{cm}$ and $X=0\mathrm{cm}$, respectively.

Fig. 4

Detected light intensities in transmission mode tissue geometry for light and dark skin plotted against oxygen saturation and shown in panels (a) and (b), respectively. The PI at red and IR for light and dark skin as a function of oxygen saturation is illustrated in panels (c) and (d). The $R$ ratio versus ${\mathrm{SpO}}_2$ curves for light and dark skin are presented in panel (e).

Fig. 5

Distributions of OPL at diastolic red and IR wavelengths, presented for light skin [(a), (b)] and dark skin [(c), (d)]. These distributions are based on a sample of 10,000 detected photons in each case.

Fig. 6

Normalized fluence rate distribution of the detected photons at ${\mathrm{SpO}}_2=90\%$ in reflection mode, shown at diastolic red and IR for light skin (a), (b) and for dark skin (c), (d), respectively. The color bar represents the magnitude of the distribution of detected photons through the finger tissue. Plots (e) and (f) show horizontal and vertical line profiles of the normalized fluence distribution, at $Z=0.12\mathrm{cm}$ and $X=0\mathrm{cm}$, respectively.

Fig. 7

Detected light intensities in reflection mode (central illumination and detection) tissue geometry for light and dark skin plotted against oxygen saturation and shown in panels (a) and (b), respectively. The PI at red and IR for light and dark skin as a function of oxygen saturation is illustrated in panels (c) and (d). The $R$ ratio versus ${\mathrm{SpO}}_2$ curves for light and dark skin are presented in panel (e).

Fig. 8

PI plots for light (a) and dark skin (b) as a function of ${\mathrm{SpO}}_2$ (%), along with its corresponding ${\mathrm{SpO}}_2\text{-}R$ curve (c), for a small SD distance.

Fig. 9

PI plots for light (a) and dark skin (b) as a function of ${\mathrm{SpO}}_2$ (%), along with its corresponding ${\mathrm{SpO}}_2\text{-}R$ curve (c), for a medium SD distance.

Fig. 10

PI plots for light (a) and dark skin (b) as a function of ${\mathrm{SpO}}_2$ (%), along with its corresponding ${\mathrm{SpO}}_2\text{-}R$ curve (c), for a large SD distance.

Fig. 11

(a) Median optical path length (OPL) and (b) median penetration depth (PD) in light and dark skin against varying source–detector (SD) separations at red and IR wavelengths for ${\mathrm{SpO}}_2=90\%$.