2019 Volume 8 Pages 14-22
Measurement of instantaneous orthostatic heart rate change has been used as a screening method for care or rehabilitation for the elderly, because it allows evaluation of the autonomic nervous system. In this study, we developed a photoplethysmogram (PPG) measurement system on the forehead. The system has five sets of green light PPG sensors to measure instantaneous orthostatic pulse rate changes and is less prone to noise caused by body movement artifacts. We aimed to verify the effectiveness of the proposed method by comparing the accuracy of pulse rate measurements at the forehead and at the wrists. In the experiment, 11 young and healthy subjects were recruited, and were asked to wear an aging simulation kit during the experiment to simulate the standing-up movement of an elderly person. The pulse rates from forehead PPG, pulse rates from wrist PPG, and heart rates from electrocardiogram were measured simultaneously. The accuracy of pulse rate was evaluated by two indices: ERb, the average of error rates for 10 s before standing up, was adopted as an index without body motion; ERs, the average of error rates within 10 s before and after standing up and including the standing-up duration, was adopted as an index with body motion. Using these indices, statistical analyses including one-way ANOVA for contact pressures and paired t-test were conducted. For the error rates estimated from wrist PPG, a significant difference was found between ERb and ERs under most conditions, and ERs was higher than ERb. Meanwhile, for the error rates estimated from forehead PPG, no significant differences were found between ERb and ERs under most conditions. In addition, the ERs was significantly different between wrist PPG and forehead PPG, especially at the center of the forehead at low contact pressure. Therefore, we confirmed that pulse rates measured by PPG placed at the center of the forehead with low contact pressure are highly accurate compared with those measured by wrist PPG. The proposed method is thus proven effective for monitoring instantaneous orthostatic pulse rate changes.