A Handheld Multispectral NIRS Device for Real-Time Baseline-Relative Monitoring of Tissue Oxygen Saturation

Tissue oxygen saturation (${StO}_{2}$) is clinically useful for assessing local perfusion, but many NIRS systems are expensive and bulky. We developed PhasorDetect, a handheld multispectral continuous-wave NIRS device designed for real-time, baseline-relative ${StO}_{2}$ monitoring.

PhasorDetect (8 wavelengths, 525-940 nm) was evaluated against the Hutchinson InSpectra during vascular occlusion tests in healthy participants (n = 14; both arms). InSpectra was placed over the thenar eminence, and PhasorDetect over the distal forearm. We assessed three complementary approaches: an analytical MBLL-based estimator, a baseline-drop regressor for continuous baseline-relative estimation, and an early-alert classifier for detecting baseline-relative declines. Model evaluation used arm-wise cross-validation to prevent leakage.

Continuous ${StO}_{2}$ regression (analytical and learning-based) matched the direction of ${StO}_{2}$ changes but did not consistently match drop magnitude across participants, consistent with limitations of coaxial reflectance measurements. We therefore reframed monitoring as baseline-relative early alerting and evaluated classifiers for $\ge$10% and $\ge$20% ${StO}_{2}$ drops. This approach yielded the most effective performance among the tested models.

PhasorDetect reliably tracks within-subject oxygenation dynamics during occlusion and reperfusion, supporting baseline-relative monitoring rather than absolute use.

A low-cost, portable multispectral NIRS platform can enable trend monitoring and early warning of perfusion compromise in settings in which conventional devices are impractical.