Key result
Blue and green reflectance PPG increases signal-to-noise ratio by ~8 dB versus red transmittance during motion.
Why the study?
Selecting the optimal light wavelength to prevent motion artifacts during photoplethysmography heart rate measurements had not been thoroughly examined.
Do specific light wavelengths and measuring modes in photoplethysmography improve signal-to-noise ratio and heart rate estimation accuracy during motion in healthy volunteers?
Cross-Sectional (n=12)
No
Do specific light wavelengths and measuring modes in photoplethysmography improve signal-to-noise ratio and heart rate estimation accuracy during motion in healthy volunteers?
Mean Difference: 7.8
Absolute Event Rate: 12% vs 4.2%
p-value: p=<0.001
Blue and green light in reflectance mode are the optimal settings for photoplethysmography to accurately measure heart rate during motion artifacts.
Blue/green reflectance PPG may aid motion-robust HR monitoring in wearables; leaves open optimal device design pending larger validation.
Photoplethysmography (PPG) is a simple method to measure various physiological indices, including heart rate (HR). To prevent motion artifacts, the optimal light wavelength for PPG measurements should be selected. However, this countermeasure has not been examined thoroughly. This study addressed PPG robustness against motion artifacts for different light wavelengths and measuring modes to accurately determine HR. Twelve healthy volunteers underwent motion artifact experiments during PPG measurements, in which they were asked to either remain still or wave their hands horizontally or vertically as fast and rhythmically as possible. Reflectance mode blue (RB), green (RG), red (RR), and near-infrared (RNIR) lights and transmittance mode red (TR) and near-infrared (TNIR) lights were evaluated for PPG signals acquired along with electrocardiogram (for reference HR) and hand acceleration measurements. The analysis revealed that the RB and RG PPG modes increased the signal-to-noise ratio by approximately 8 dB compared to TR PPG, and the HR obtained from both did not exhibit fixed or proportional bias, with a Pearson's correlation coefficient above 0.986. Furthermore, RNIR PPG was superior to TR PPG by approximately 4 dB, and its calculated HR did not show fixed or proportional bias, with a Pearson's correlation coefficient of 0.967. The RR, TNIR, and TR PPG modes showed comparable and inferior performance. Therefore, blue and green lights followed by near-infrared light in reflectance mode are the recommended settings to measure HR using PPG. These findings may serve as guidelines for researchers and engineers to improve PPG measurements and devices.
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Matsumura et al. (2020) conducted a cross-sectional in Healthy (n=12). Blue and green light reflectance mode PPG vs. Transmittance mode red light PPG was evaluated on Signal-to-noise (S/N) ratio (MD 7.8 dB, p=<0.001). Blue and green light reflectance mode PPG increased the signal-to-noise ratio by 7.8 and 7.7 dB, respectively, compared to transmittance mode red light PPG during motion.
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