SignificanceOur goal is to understand the root cause of reported oxygen saturation (SpO2) overestimation in heavily pigmented skin types to devise solutions toward enabling equity in pulse oximeter designs.AimWe aim to gain theoretical insights into the effect of skin tone on SpO2-R curves using a three-dimensional, four-layer tissue model representing a finger.ApproachA 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, SpO2-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.ResultsIn transmission mode, SpO2-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 SpO2-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 SpO2-R curves in reflection mode became less pronounced, with the largest SD separation exhibiting effects similar to those observed in transmission mode.ConclusionsMonte Carlo simulations have demonstrated that different light pathlengths within the tissue contribute to the overestimation of SpO2 in people with darker skin in reflection mode pulse oximetry. Increasing the SD separation may mitigate the effect of skin tone on SpO2 readings. These trends were not observed in transmission mode; however, further planned research using more complex models of the tissue is essential.
Pulse oximeters’ varying performance based on skin tones has been highly publicised. Pulse oximeters tend to overestimate oxygen saturation values for people with darker skin (occult hypoxemia). The study aimed to construct a test bench to assess commercially available home based pulse oximeters. A laboratory simulator was used to mimic different oxygen saturation values (~70% to 100%). Four synthetic melanin filters were used to reproduce the effects of varying melanin attenuation levels. Three commercially available pulse oximeters (Biolight, N=13; ChoiceMMed, N = 18; MedLinket, N = 9) were reviewed and their response documented. All pulse oximeters’ responses under the effects of melanin attenuation did not change across various simulated oxygen saturation values. This does not match with the clinically observed data and one reason is that the light scattering due to tissue had not been fully replicated in the test bench. To investigate this further a Monte Carlo simulation of light propagation through the finger has been developed considering pulsatile flow and different skin tones. In reflection mode, the simulations highlight differences in measured R value and oxygen saturation with change in skin tone in the epidermal layer. However, in case of transmission mode, no change in the measured R value and oxygen saturation was observed. Further validation of these results from simulations is required to help us design pulse oximeters that are reliable and equitable for all users, regardless of skin tone.
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