Influence of Acute Mental Stress on the Forehead Photoplethysmographic Signal Waveform

The decreased environment temperature can cause vasoconstriction and drops the skin perfusion, which lowers finger photoplethysmographic (PPG) signal amplitude and signal to noise ratio. The forehead is relatively insulated from external temperature changes due to the presence of the skull and the scalp. Therefore, this site is less affected by temperature compared to other peripheral PPG signal registration sites. The aim of this study was to characterize the changes in the forehead PPG and second derivative of PPG (SDPPG) waveform parameters related to arterial stiffness for the acute mental stress assessment. The PPG signals were recorded from 42 subjects during eyes open and arithmetic stress test. The signals from 34 subjects (18 females and 16 males) were included to the post-processing and analysis of PPG waveform arterial stiffness related parameters. The changes in the forehead PPG waveform parameters PPGb, PPGAI, Snorm, and SDPPG waveform parameters b/a, c/a, d/a, e/a, and AGI were statistically significantly different (p < 0.05) between eyes open and stress test states. The calculated parameters indicated the increase in arterial stiffness due to the induced stress. However, c/a changed in the opposite direction than expected. In summary, the results indicate that the changes in the forehead PPG and SDPPG parameters could be used similarly to the finger PPG signal for the stress assessment. Nevertheless, further studies are needed.