Reduce hypoxemia overestimation in pulse oximetry based on iso-pathlength: a Monte Carlo study

Pulse oximetry devices systematically overestimate blood oxygen saturation (SaO₂) in hypoxemic patients, potentially masking critical conditions. We hypothesize that this bias stems from wavelength-dependent differences in optical pathlength (OPL), which vary with SaO₂ levels. This Letter investigates the change in photon pathlength in tissue between different wavelengths and proposes the iso-pathlength (IPL) point, an angular position identifiable by constant intensity between media of varying scattering coefficients, as a solution. Using Monte Carlo (MC) simulations on a finger model (5% blood volume, 60-100% SaO₂), we analyzed the OPL ratio at various exit angles for the standard 660/940 nm wavelength pair and a closer pair with 760/840 nm. Analysis revealed that at the standard pulse oximetry transmission position, the OPL ratio for the 660/940 nm pair is highly variable, ranging from 1.25 to 1.45 across SaO₂ levels, while the OPL ratio for the alternative 760/840 nm pair changed from 1.05 to 1.12. In contrast, the OPL ratio at the IPL point remains stable, not exceeding 1.05 across all SaO₂ levels. These MC results provide mechanistic insight into pulse oximetry bias and demonstrate that the IPL technique could help in the detection of critical hypoxemia, where current devices are most prone to dangerous overestimation.