Impact of Sympathetic Activation in Imaging Photoplethysmography

Photplethysmography (PPG) is known to reflect changes in sympathetic tone. This contribution investigates the behaviour of imaging photoplethysmography (iPPG) upon sympathetic activation. To that end, we assessed the impact ofa distal painful stimulus on the facial iPPG and contralateral finger PPG waveforms. Our results show that alternating components of both signals behave differently. As expected, the alternating component of the finger PPG signal shows a significant and persistent decrease upon stimulus (p < 0.001). The alternating component of the iPPG signal shows only a slight decrease followed by a fast increase (p < 0.01). The found behaviour might be explained by different degrees of sympathetic responsiveness in the extremities and in the face. Sympathetic activation increases cardiac output and triggers general vasoconstriction. Extremities show highly pronounced vasoconstriction which decreases the alternating component. The facial vasoconstriction is comparatively small. As a result, local vasoconstriction might cause a short-term decrease followed by the contrary effect, namely an increase in the alternating component, driven by an increased systemic pulse pressure. Our finding has relevance for the interpretation of iPPG signals and the design of future use cases beyond remote heart rate assessment. In particular, care should be taken when expectations on the finger PPG are to be transferred to iPPG.