Developing a monitoring psychological stress index system via photoplethysmography

The purpose of this work was to assess the psychological stress index (PSI) by using oxyhemoglobin saturation by SpO₂ i.e., the plethysmograph (PPG) signal], which can be measured easily and conveniently. We use the plethysmograph amplitude (PPGA) and the heart-beat interval (HBI) extracted from the SpO₂ waveform to obtain the stress index and to quantify it from 0 to 100. The respiration rate can also be extracted from the heart-rate interval, as shown in our previous research. Therefore, the PPG signal can display the heart rate, SpO₂ waveform, SpO₂ concentration, respiration rate, and psychological stress index. This methodology was tested on nine volunteer Taiwanese students under the stress of an English presentation. The experiment, which takes 30 min for each student, was separated into three stages: preparation, presentation, and discussion, and relaxation. The PSI values of these three stages were 49 ± 10, 60 ± 11, and 56 ± 10, respectively. The results were very successful, and showed PSI values changing from low, to high, and back to low during these three stages. In the near future, we hope to implement this system into a robotic wheelchair in order to monitor the PSI of elderly patients in a nursing home, and to evaluate their psychological condition based on this method.