Pulse-oximeter is described as the most important technological proceeding for monitoring the patients' safety during anesthesia, after surgery and in emergency. This opinion was widely confirmed in the 1990s when pulse-oximeter has been definitively introduced in the standard for base monitoring in the OR and has been proposed for routinary use also in the ICU. In this paper we consider the importance, in the cardiovascular, respiratory and brain parameters monitoring, of continuous oximetry of mixed venous blood (SVO2) and blood venous saturation in the internal jugular vein (SjvO2).