Green Light Photoplethysmography as a Substitute for Heart Rate Variability Monitoring

This study evaluated the validity of using pulse rate variability (PRV) obtained from green light photoplethysmography (PPG) as an alternative index of heart rate variability (HRV). The pulse rate (PR) obtained from PPG is almost identical to the heart rate (HR) obtained from electrocardiography (ECG), and PPG is a simple, low-cost optical measurement method. Several studies have confirmed the usefulness of PRV in place of HRV obtained from near-infrared (NIR) PPG. Recently, the use of green light PPG devices has increased compared to NIR PPG devices. Although the difference in wavelength affect the amplitude of the PPG signal, no study has examined the suitability of green PRV for autonomic nervous system (ANS) monitoring. Therefore, we evaluated the validity of green PRV in this study. We compared green PRV with HRV and NIR PRV in eight healthy volunteers. Green and NIR PPG signals and ECG were measured simultaneously with the subjects resting. The subjects were then asked to immerse one hand in an isothermal bath at 10 degrees C. The power spectra indices (LF, HF, and LF/HF) were calculated from the HRV as well as green and NIR PRV as HRV and PRV indices in the frequency domain. Both green and NIR PRV correlated strongly with the HRV [green, r = 0.994 (rest), 0.985 (cold); NIR, r = 0.992 (rest), 0.972 (cold)], and the green PRV indices in the frequency domain were not significantly different from the HRV indices in the frequency domain. In addition, our results indicated that the accuracy of PRV indices obtained from green light PPG were comparable in accuracy to those obtained from NIR PPG. Consequently, we propose that green PPG is an acceptable alternative for use in ANS monitoring.