Photoplethysmography (PPG) has been extensively used for pulse oximetry monitoring in perioperative and intensive care. However, some components of PPG signal have been employed for other purposes, such as non-invasive hemodynamic monitoring. Perfusion index (PI) is derived from PPG signal and represents the ratio of pulsatile on non-pulsatile light absorbance or reflectance of the PPG signal. PI determinants are complex and closely interlinked, involving and reflecting the interaction between peripheral and central hemodynamic characteristics, such as vascular tone and stroke volume. Several studies have shed light on the interesting performances of this variable, especially for hemodynamic monitoring in perioperative and intensive care.

In the first section of this chapter the physiological and pathophysiological determinants of PI are exposed, along with relevant measuring techniques and potential limitations. Second, the existing data concerning the usefulness of PI in different clinical settings are presented and discussed. Lastly, we review known perspectives and identify new perceptions concerning the use of PI that should be explored regarding its utilization.