Mental Stress Evaluation Method Using Photoplethysmographic Amplitudes Obtained from a Smartwatch

Abstract

In recent years, the number of patients with mental illness due to mental stress has steadily increased. Hence, the evaluation of mental stress in daily life is necessary to prevent mental illness. The conventional method for mental stress evaluation adopts biological information such as respiration, heart rate, saliva composition, and photoplethysmograms (PPGs). Among them, PPGs can be measured daily by a smartwatch. Therefore, we examined the feasibility of evaluating mental stress using PPG obtained by a smartwatch. In the mental stress evaluation, we focused on the photoplethysmographic amplitude (PPGA), which is a characteristic point of PPG. This amplitude can evaluate mental stress even with intermittent data, which makes it suitable for evaluating smartwatches when the obtained data are limited owing to body movements. In contrast, the accuracy of obtaining PPGA from a smartwatch might be low owing to various factors such as changes in blood flow or skin temperature. Hence, in this research, we proposed a method for obtaining a highly accurate PPGA. This method adopts PPGA via fast Fourier transform (FFT). In addition, matching the changes in PPGA for a short period of time is considered a challenge. Consequently, PPGA was calculated at regular time intervals. In the experiment, measurements from a photoplethysmograph and smartwatch were taken simultaneously. The measurement positions of the photoplethysmograph and smartwatch were at the left fingertip and left arm, respectively. After the measurements, the correlation coefficient between the photoplethysmograph and the PPGA of the smartwatch, to which this method was applied, was obtained. Accordingly, it was confirmed that PPGA can be obtained via the proposed method with high accuracy.