Exploiting Multiwavelength Pulse Transit Time to Improve the Calibration of RGB-Camera-Based SpO 2 Measurement

Recent studies investigated the feasibility of using an RGB camera to estimate peripheral oxygen saturation (SpO2), i.e., “Ratio of Ratios” by red (R) and green (G) channels and calibration to SpO2. This approach faces the challenge of RoR-SpO2calibration due to different skin penetration depths between R and G wavelengths among individuals. This paper proposed a novel concept of using multi-wavelength pulse transit time (MW-PTT) measured between the photoplethysmographic (PPG) signals from R and G channels to improve the SpO2calibration. Specifically, we used MW-PTT as a metric to prune the local skin regions with a larger MW-PTT delay to ensure that only the PPG signals stemming from similar skin layers (with limited MW-PTT delay) can be used for SpO2measurement. In this way, we reduced the impact of different skin penetration depths between R and G wavelengths for SpO2calibration. Experiments were conducted on 20 healthy adults in the lab and 17 patients in the intensive care unit (ICU) to validate the effectiveness of the proposed algorithm. The results show that, among lab subjects, the 95% confidence interval for the RoR–SpO2regression is 50% narrower when using the MW-PTT corrected RoR feature than the existing RoR method, indicating improved inter-subject consistency. For ICU patients, the average mean absolute error (MAE) of estimated SpO2across all patients was reduced from 3.08% to 1.37% by using the MW-PTT corrected method, demonstrating a marked improvement in the accuracy of calibration.