 In addition to the risks associated with hypoxemia, the monitoring of Arterial Oxygen Saturation also avoids the adverse effects associated with hyperoxia. Indeed oxygen leads to the formation of free radicals, which in physiological condition are regulated by antioxidant mechanisms. However, in anesthesiology, it is common to find pathological situations acute respiratory distress syndrome, shock, prolonged inflammatory state where these regulatory mechanisms are overwhelmed. Oxygen administration to supra-physiological concentrations leads to a formation of excessive free radicals, aggravating the existing injury including lung. 