 Intubation may be necessary for a patient with decreased oxygen content and oxygen saturation of the blood caused when their breathing is inadequate ( hypoventilation ) , suspended ( apnea ) , or when the lungs are unable to sufficiently transfer gasses to the blood . Such patients , who may be awake and alert , are typically critically ill with a <unk> disease or multiple severe injuries . Examples of such conditions include cervical spine injury , multiple rib fractures , severe pneumonia , acute respiratory distress syndrome ( ARDS ) , or near @-@ drowning . Specifically , intubation is considered if the arterial partial pressure of oxygen ( PaO2 ) is less than 60 millimeters of mercury ( mm Hg ) while breathing an inspired O2 concentration ( <unk> ) of 50 % or greater . In patients with elevated arterial carbon dioxide , an arterial partial pressure of CO2 ( PaCO2 ) greater than 45 mm Hg in the setting of <unk> would prompt intubation , especially if a series of measurements demonstrate a worsening respiratory acidosis . Regardless of the laboratory values , these guidelines are always interpreted in the clinical context . 
