Definitions Acute respiratory failure describes any impairment in oxygenation or ventilation in which the arterial oxygen tension falls below 60 mm Hg (acute hypoxemia), the carbon dioxide tension rises above 50 mm Hg (acute hypercarbia, hypercapnia) and the pH drops below 7.35, or both. For patients who have underlying chronic respiratory failure, acute hypercarbia can be diagnosed by an incre...