Evaluation of Microcirculation With Photoplethysmography (Pulse Oximetry) in Healthy Volunteers and Ventilated Patients

Photoplethysmography (BluePoint, Frankfurt, Germany) is the pulse wave measured by a pulse oximeter. It is obtained based on the absorbance of red/infrared light in a given tissue. The signal is made up of two components: 1) the alternating component (AC), which is the pulse wave itself, which is determined by the absorbance of pulsatile arterial blood. 2) The continuous component (DC), which represents the absorbance of venous blood and the rest of the tissues. Within these tests the investigators find vascular occlusion, elevation of the legs, raising or lowering the tested hand above/below the phlebostatic level, cold/heat test, etc. Each of them evaluates different aspects of circulation such as endothelial function, myogenic response to blood flow, perfusion pressure, autonomic reflexes, preload-dependence, or venous flow among other aspects of microcirculation.