  Significance: Pulsatile blood Oxygen Saturation (SpO2 ) 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: Develop an optical pulse oximetry method based on Dual-Ratio (DR)
measurements to suppress individual anatomical features on SpO2.
  Approach: Design 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 (MC) simulations and
in vivo experiments.
  Results: Different melanosome volume fraction in the epidermis, a surrogate
for skin tone, cause changes in the recovered SpO2 on the order of 1%.
Different heterogeneous pulsatile hemodynamics cause greater changes on the
order of 10%. SpO2 recovered with DR measurements showed less variability than
the traditional Single-Distance (SD) transmission method.
  Conclusions: For the models and methods considered here, SpO2 measurements
are more strongly impacted by heterogeneous pulsatile hemodynamics than by
melanosome volume fraction. This is consistent with previous reports that, the
skin tone bias is smaller than the observed variation in recovered SpO 2 across
individual people. The partial suppression of variability in the SpO2 recovered
by DR suggests promise of DR for pulse oximetry.
