Preoxygenation

Preoxygenation is the preparation phase before airway instrumentation intended to maximize the body’s oxygen reserves, extending the safe duration of apnea and reducing the risk of rapid desaturation during intubation attempts.

The physiological principle is straightforward. The lungs’ functional residual capacity (FRC) — the volume of gas remaining at the end of a normal breath — serves as an oxygen reservoir during apnea. On room air (21% oxygen), FRC contains relatively little oxygen. By breathing 100% oxygen for 3-5 minutes (or taking 8 vital-capacity breaths of 100% oxygen), the patient replaces nitrogen in FRC with oxygen, dramatically increasing the available reservoir.

In a healthy adult, adequate preoxygenation can extend the safe apnea time from approximately 1 minute (on room air) to 6-8 minutes. This window allows a clinician to attempt laryngoscopy, reposition, and reattempt if the first pass fails — without the patient desaturating to dangerous levels.

Preoxygenation is less effective — and more critical — in populations with reduced reserves:

• Pediatric patients — smaller FRC relative to metabolic rate, faster oxygen consumption, shorter safe apnea times
• Obese patients — reduced FRC from abdominal compression, increased oxygen consumption, rapid desaturation
• Pregnant patients — reduced FRC and increased metabolic demand
• Critically ill patients — shunt physiology, increased metabolic demand, limited response to supplemental oxygen

Techniques include:

• Tight-fitting non-rebreather mask at high flow — standard approach for cooperative patients
• Bag-valve-mask with PEEP valve — recruits collapsed alveoli and improves FRC
• Apneic oxygenation — high-flow nasal cannula continued during laryngoscopy, providing passive oxygen flow during apnea
• Patient positioning — head-up or ramped positioning improves FRC, particularly in obese patients

Preoxygenation is not optional. It is the single most important preparation step before any planned airway intervention. A failed first intubation attempt in a well-preoxygenated patient is manageable. A failed first attempt in an inadequately preoxygenated patient may become a CICO emergency.

Related terms

• Oxygenation — the process that preoxygenation supports
• CICO — the emergency adequate preoxygenation helps prevent
• Airway — the passage being instrumented