Monte Carlo simulation study of pulse oximetry overestimation

Pulse oximetry is the standard non-invasive method for monitoring blood oxygen saturation (SaO_2), yet systematic overestimation in hypoxemic patients (<90%) poses significant risks by potentially masking critical conditions. We hypothesize that wavelength-dependent differences in photon pathlengths through tissue drive this error. To investigate, we developed Monte Carlo simulations modeling photon propagation through an optically realistic human finger. The simulated tissue incorporated blood vessels comprising 5% of the volume, with SaO₂ levels ranging from 60–100%, analyzed at standard (660/940 nm) and alternative (760/840 nm) wavelength pairs. Our analysis revealed that in standard transmission, photon pathlengths at 660 nm were up to 45% longer than at 940 nm. This discrepancy directly causes the systematic overestimation observed clinically. Conversely, the alternative pair showed a reduced discrepancy of 10%. Importantly, measuring at the iso-pathlength point (IPL), a specific point on the tissue’s surface where the OPL remains nearly constant across varying scattering coefficients, reduced these variations to less than 5%, regardless of SaO_2. While standard transmission causes the Ratio-of-Ratios calibration curve to distort, the IPL method yields a curve overlapping closely with the theoretical model. These findings provide mechanistic insights into pulse oximetry bias and demonstrate that IPL measurements could significantly improve accuracy for hypoxemic patients.