FIO2 is defined as the percentage of oxygen that is commonly contained in the air or the concentration of oxygen in the air that the person normally breathes. Mechanical ventilation is a form of artificial breathing apparatus that assists the patient with breathing. The main aim of mechanical ventilation is to provide the patient with adequate oxygen. At the start, the patient on mechanical ventilation is provided with 100% of FIO2 to prevent hypoxemia and then slowly titrated down based on the partial pressure of oxygen from ABGs. High levels of oxygen must be titrated down in order to prevent barotrauma and oxygen toxicity. Under normal circumstances, FIO2 can be calculated by using the following formula: FiO2= 20% + (4 X oxygen liter flow)