While pulse oximeters have traditionally been utilized for measuring arterial saturation (SpO2), the widespread adoption of pulse oximetry has led to its expanded applications, including diagnostic uses like detecting hypoxemia during the COVID-19 epidemic. Recent research has identified unexpected errors in off-label applications, with discrepancies in diagnostic efficacy based on race or skin pigmentation. The positive bias in SpO2 measurements, particularly in the critical SpO2 range of 85-90%, poses a significant concern for devices intended for SpO2 measurement. This study employs Monte Carlo simulations to model transmittance mode pulse oximetry to investigate whether documented racial and pigmentation-based biases can be attributed to epidermal melanin. The research aims to elucidate key mechanisms that may guide the improvement of technology. Specifically, the study explores sensitivity to the epidermal region under conditions with and without surface reflection, shedding light on pulse oximeter function in diverse device designs. Results demonstrate a 26% increase in the total detected signal from the epidermis when transitioning from low to high surface reflection at 660 nm, and a 22% increase in the total sensitivity of the system to the epidermis at 940 nm with surface reflection. These findings suggest that heightened sensitivity to the epidermis region leads to more pronounced spectral effects, especially with reflective sensors, potentially due to reflected photons re-entering the tissue through the epidermis.
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