Dual-ratio approach to pulse oximetry and the effect of skin tone.

Significance: Pulsatile blood oxygen saturation ( ${\mathrm{SpO}}_2$ ) via pulse oximetry is a valuable clinical metric for assessing oxygen delivery. Individual anatomical features, including skin tone, may affect current optical pulse oximetry methods.

Aim: We developed an optical pulse oximetry method based on dual-ratio (DR) measurements to suppress individual anatomical confounds on ${\mathrm{SpO}}_2$ .

Approach: We designed a DR-based finger pulse oximeter, hypothesizing that DR would suppress confounds from optical coupling and superficial tissue absorption. This method is tested using Monte Carlo simulations and in vivo experiments.

Results: Different melanosome volume fractions in the epidermis, a surrogate for skin tone, cause changes in the recovered ${\mathrm{SpO}}_2$ on the order of 1% in simulation and in vivo. Different heterogeneous pulsatile hemodynamics cause greater changes on the order of 10% in simulations. ${\mathrm{SpO}}_2$ recovered with DR measurements showed less variability than the traditional single-distance (SD) transmission method.

Conclusions: For the models and methods considered here, ${\mathrm{SpO}}_2$ measurements are strongly impacted by heterogeneous pulsatile hemodynamics. This variability may be larger than the skin tone bias, which is a known confound in ${\mathrm{SpO}}_2$ measurements. The partial suppression of variability in the ${\mathrm{SpO}}_2$ recovered by DR suggests the promise of DR for pulse oximetry.