PPG Signal Morphology-Based Stress Assessment

Mantas Rinkevičius1, Spyridon Kontaxis2, Eduardo Gil3, Raquel Bailón4, Jesus Lazaro5, Pablo Laguna6, Vaidotas Marozas7
1Kaunas University of Technology, 2Biomedical Research Networking Centre on Bioengineering, Biomaterials and Nanomedicine, 3University of Zaragoza, 4I3A, IIS, Universidad de Zaragoza, CIBER-BBN, 5University of Connecticut & University of Zaragoza, 6Zaragoza University, 7Professor


Aim. Stress is a healthy natural response to a perceived or actual threat. However, when stress is persistent, it decreases work productivity, increases the risk of diseases, and affects the quality of life. Stress is reflected in physiological variables such as blood pressure, velocity of forward and reflected pulse waves related to vessel stiffness, and heart rate, among others. This study analyses parameters derived from PPG signal morphology for mental stress assessment. Methods. A low-complexity algorithm is designed using the filtered second derivative of the PPG signal for estimation of three wave amplitudes A1, A2, and A3 (located at T1, T2, and T3, respectively), related to the forward pulse P1 and the reflections P2 and P3 from the renal and iliac sites in the central arteries, respectively. The time delay T12 between P1 and P2 and T13 between P1 and P3 are analyzed as surrogates of instantaneous vessel stiffness. Additional parameters are studied including the amplitude A1 and delay T1 related to systole, the diastolic interval Td estimated from dicrotic notch until the end of the pulse, and the pulse duration Tp. The data (11 subjects) contain a baseline (BL) and five different stages with induced stress: storytelling (ST), memory task (MT), stress anticipation (SA), a video display (VD), and arithmetic task (AT). Results. The most significant differences between BL and the stress stages are found for A1 and T1, where lower values are related to sympathetic activation, while Td shows better performance than T12, T13, and Tp. Conclusion. The analyzed PPG signal morphological parameters are related to stress-induced sympathetic activation, thus offering the potential to be used in wearable devices for unobtrusive monitoring and management of occupational stress, and prevention of cardiovascular diseases.