Accuracy enhancement in reflective pulse oximetry by considering wavelength-dependent pathlengths

Objective.Noninvasive measurement of oxygen saturation (SpO₂) using transmissive photoplethysmography (tPPG) is clinically accepted and widely employed. However, reflective photoplethysmography (rPPG)-currently present in smartwatches-has not become equally accepted, partially because the pathlengths of the red and infrared PPGs are patient-dependent. Thus, even the most popular 'Ratio of Modulation' (R) method requires patient-dependent calibration to reduce the errors in the measurement ofSpO₂using rPPGs.Approach.In this paper, a correction factor or 'pathlength ratio'βis introduced in an existing calibration-free algorithm that compensates the patient-dependent pathlength variations, and improved accuracy is obtained in the measurement ofSpO₂using rPPGs. The proposed pathlength ratioβis derived through the analytical model of a rPPG signal. Using the new expression and data obtained from a human hypoxia study wherein arterial oxygen saturation values acquired through Blood Gas Analysis were employed as a reference,βis determined.Main results.The results of the analysis show that a specific combination of theβand the measurements on the pulsating part of the natural logarithm of the red and infrared PPG signals yields a reduced root-mean-square error (RMSE). It is shown that the average RMSE in measuringSpO₂values reduces to 1 %.Significance.The human hypoxia study data used for this work, obtained in a previous study, coversSpO₂values in the range from 70 % to 100 %, and thus shows that the pathlength ratioβproposed here works well in the range of clinical interest. This work demonstrates that the calibration-free method applicable for transmission type PPGs can be extended to determineSpO₂using reflective PPGs with the incorporation of the correction factorβ. Our algorithm significantly reduces the number of parameters needed for the estimation, while keeping the RMSE below the clinically accepted 2 %.