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 Adequate preoxygenation should be provided in all but the most emergent situations.

 The aim is to replace nitrogen in the lungs with oxygen. This increases the length of time before desaturation when the patient is apneic (‘apnea time’), providing a margin of safety in case ventilation and intubation become difficult.

 Preoxygenation can be performed using a facemask, continuous or bilevel positive airway pressure, or a high flow nasal cannula (HFNC) providing 100% oxygen at flows of at least 10 L/min. It typically requires approximately 3 minutes of normal tidal volume breathing to achieve an end‐tidal oxygen concentration of approximately 90%.

 Given normal functional residual capacity (FRC) of about 2 L, and an oxygen consumption rate of about 200–250 mL/min, a properly preoxygenated adult should have an apnea time of about 5–8 minutes before significant desaturation. Reductions in apnea time should be expected in conditions in which FRC is decreased (i.e. obesity, pregnancy, tense ascites) or conditions of increased oxygen consumption (i.e. sepsis, pregnancy, hyperthyroidism).