Why pre-oxygenation matters

What is pre-oxygenation?

• Giving high-concentration oxygen before induction to replace nitrogen in the lungs with oxygen.
• Main aim: increase the oxygen store in the lungs (functional residual capacity, FRC) so oxygen levels fall more slowly during apnoea.
• Often called “denitrogenation”: washing out nitrogen so the lungs act like an oxygen reservoir.

Why it matters (the safety benefit)

• During induction and intubation there is usually a period of reduced breathing or no breathing (apnoea).
• Pre-oxygenation delays desaturation (drop in oxygen saturations), giving more time to: optimise mask ventilation, reposition, call for help, or change plan.
• It reduces the risk of hypoxic harm if laryngoscopy is difficult, the airway is unexpected, or ventilation is challenging.
• It is one of the simplest, highest-impact safety steps you can do before airway instrumentation.

Who needs extra care (high-risk groups)

• Obesity: lower FRC and faster desaturation; consider head-up positioning and longer pre-oxygenation.
• Pregnancy: increased oxygen consumption and reduced FRC; desaturates quickly—do not skip pre-oxygenation.
• Children: high oxygen consumption; can desaturate rapidly—ensure good mask seal and technique.
• Sepsis/fever, pain, agitation: increased oxygen demand; may need coaching and effective technique.
• Lung disease (e.g., pneumonia, COPD): may not achieve high end-tidal oxygen; consider adjuncts (PEEP/CPAP, NIV) and plan for shorter safe apnoea time.

How to do it (practical technique)

• Use 100% oxygen (or the highest available) via a well-fitting anaesthetic mask connected to a circle system or Mapleson circuit with high fresh gas flow.
• Aim for a tight mask seal: two-handed technique often works best, especially in beards, edentulous patients, or obesity.
• Position matters: head-up (ramped in obesity) improves FRC and pre-oxygenation effectiveness.
• Common method: tidal breathing for ~3 minutes with high oxygen flow and good seal.
• Alternative (when time-limited): 8 vital capacity breaths over ~60 seconds with high flow and good seal (less reliable if patient can’t cooperate).
• If available, use end-tidal oxygen (EtO2) to assess adequacy; a typical target is EtO2 ≥ 0.85 (85%) before induction.

Optimising pre-oxygenation in “first time” scenarios

• Anxious patient: explain simply (“This is filling your lungs with oxygen to keep you safe while you go off to sleep”); encourage slow deep breaths.
• Beard/poor seal: use two hands, consider water-soluble gel on mask edge, consider an oral airway, or use a supraglottic device if appropriate and trained.
• Edentulous: leave dentures in until asleep if safe; if removed, expect leaks—use two-handed seal and consider gauze in cheeks.
• Obesity: ramped position (ear-to-sternal-notch alignment), head-up tilt, consider PEEP/CPAP during pre-oxygenation if trained and appropriate.
• Critically unwell/hypoxic: consider CPAP/NIV pre-oxygenation and apnoeic oxygenation; involve senior help early and plan for rapid desaturation.

Apnoeic oxygenation (useful add-on)

• Oxygen can continue to diffuse into blood during apnoea if oxygen is delivered to the upper airway (e.g., nasal cannula).
• It can prolong time to desaturation but does not remove carbon dioxide—so it is not a substitute for ventilation.
• Consider nasal oxygen during laryngoscopy, especially in high-risk patients, if it doesn’t interfere with the plan.

How to know it’s working (signs of adequate pre-oxygenation)

• SpO2 should be high and stable, but SpO2 alone can be misleading (it may stay high even if lung oxygen stores are not maximised).
• If EtO2 monitoring is available: rising EtO2 towards target (commonly ≥85%) suggests effective denitrogenation.
• You should feel confident about mask seal: minimal leak, reservoir bag movement with breathing, and stable circuit pressures.