Skin Perfusion Studies: Historical Notes and Modern Measuring Strategies Using Non-invasive Photoplethysmographic Sensor Concepts

For adequate skin perfusion rhythmicity assessment, sensor technology must be used, which fulfills basic demands such as unobtrusiveness and continuous monitoring with spatial resolution photoplethysmography (PPG) and its modern camera-based imaging variant (PPGI) ideally meet these requirements, and they are generally accepted in the field of noninvasive medical diagnostics. PPG can work in reflective or transmissive mode and detects blood volume changes in the vascular plexus within the region of transilluminated tissue. The PPG and PPGI signals comprise a complex of pulsatile wave formations (AC) associated with cardiac, respiratory, and different nervous system activities, which are superimposed to a non-pulsatile baseline (DC) due to optical damping of bloodless tissue. Although most of the PPG applications in medical diagnostics are today devoted to recording cardiac rhythmicity, the strength and future of PPG lie in detecting the distributed and, in some cases, highly autonomous skin perfusion dynamics below the frequencies of the "central oscillators", namely the heartbeat and breathing. This chapter presents selected activities and results of the bilateral and interdisciplinary long-term cooperation between IIT Madras in Chennai and RWTH Aachen University in Aachen in this exciting research field of the dermal perfusion dynamics.