Photoplethysmography Signal Behavior in Relation with External Stimuli: Temperature and Compresion Force

The photoplethysmography technique is a very dependent optical measurement both internal body physiological mechanism and extern body physic stimuli. The internal stimuli are represented by the vasomotor activity of the cardiovascular network rather than the external ones such a temperature variation, measurement site position with respect to the heart level, applied pressure between measurement site and optical sensor properties. Therefore, the associated signal obtained by converting light information into electrical nature is highly sensitive to the body external stimuli as long as it triggers the internal one. The signal is affected in the terms of the morphology amplitudes and offset with respect to the reference level. In order to analyze these changes the classical optical model of photoplethysmography is the start point in this paper. The model is based on the light scattering modulation due to the change in the blood volume takes place in the measurement site. The last technology advancement such as video capillaroscopy gives insights about the photoplethysmography origin. By analyzing the source of origin step by step in the various internal or external stimuli we found that the optical signal is highly affected where an interpretation without context can get different results. These finding suggest that the photoplethysmography sensor need to be accompanied at least by the temperature and position sensors. This design approaching is helpful in the wearable device which records the health data of the users.