Reflective Arterial Pulse Oximetry for New Measuring Sites and Long-Term Assessment of Dermal Perfusion

Until 2010, photoplethysmography (PPG) was almost exclusively used commercially as a measurement method for vital signs in medical diagnostics. Most of the conventional sensing systems were placed on the finger, for the non-invasive recording of cardiologically synchronous blood volume fluctuations and arterial oxygen saturation. However, as the new decade entered, a clear trend became apparent. New applications of PPG were developed and marketed for the first time in medical diagnostics as well as in other areas of life. In addition to medical diagnostics, the basic technology of PPG can now be found in health care, life science and other areas. It is clear that vital signs will become an integral part of the information age and that PPG, as a non-invasive technology, will always play a key role in data collection. The research activities presented in this chapter on the topic of new application sites of the PPG, conducted by the Chair of Medical Information Technology at RWTH Aachen University, are in the context of this evolutionary process. The described in-ear PPG system not only brings significant physiological advantages, but also opens up new possibilities outside of clinical routine. The validation through various studies carried out on the dynamics of skin perfusion and arterial oxygen saturation in the external auditory canal show fundamental findings and principles with regard to the in-ear PPG. The chapter thus places the PPG in the context of new fields of application: the research and development activities began with an initial vision and, in retrospect, are a piece of the puzzle in the evolution of photoplethysmography as it exists today. In addition, due to the general validity of the study results, this chapter can be an inspiration and basis for future developments and trends of PPG.