Safe apnoea time

What “safe apnoea time” means

• Safe apnoea time = the time from stopping breathing (e.g., after induction) to when oxygen levels fall to an unsafe point.
• In practice, think: “How long until SpO2 starts to drop quickly?” (often defined as time to SpO2 90–92%).
• It is mainly about oxygen stores (in the lungs) vs oxygen use (by the body).
• It is not fixed: it varies hugely between patients and situations.

Why oxygen falls during apnoea (simple physiology)

• During apnoea, oxygen is still taken up by the body (oxygen consumption continues).
• If the lungs are filled with oxygen (good pre-oxygenation), there is a larger reservoir to draw from.
• If the lungs are partly filled with nitrogen/air (poor pre-oxygenation), the reservoir is smaller and desaturation happens sooner.
• Reduced functional residual capacity (FRC) shortens safe apnoea time: less “resting lung volume” = less oxygen store.
• Higher oxygen consumption shortens safe apnoea time: e.g., sepsis, fever, pregnancy, children.

Who desaturates quickly (high-risk groups)

• Obesity: reduced FRC and often airway difficulty, desaturation can be rapid.
• Pregnancy: reduced FRC + increased oxygen consumption, treat as high risk.
• Children (especially infants): high oxygen use and small FRC, very short safe apnoea time.
• Sepsis/fever/shivering: increased oxygen consumption.
• Lung disease (pneumonia, pulmonary oedema, severe COPD/asthma): shunt/VQ mismatch reduces effective oxygenation.
• Low starting SpO2 or anaemia does not directly reduce SpO2 reserve the same way, but low baseline SpO2 means less margin, anaemia reduces oxygen content even if SpO2 looks “OK”.

Pre-oxygenation (denitrogenation): the main tool

• Aim: replace nitrogen in the lungs with oxygen to maximise oxygen store.
• Standard approach: tight-fitting mask, 100% oxygen, calm breathing for 3 minutes.
• Alternative: 8 vital capacity breaths over ~60 seconds (less reliable in sick/anxious patients).
• Use a good seal: two-handed mask hold if needed, ask for help early.
• Check effectiveness: end-tidal oxygen (EtO2) if available, aim ≥ 0.85–0.90.
• If EtO2 not available: look for stable high SpO2 plus good technique (but SpO2 alone can be misleading).

Positioning: easy wins that extend safe apnoea time

• Head-up (20–30°) improves FRC and pre-oxygenation, especially in obesity and pregnancy.
• Ramped position for obesity: align external auditory meatus with sternal notch to optimise airway and ventilation.
• Avoid supine flat if high risk, unless clinically necessary.

Apnoeic oxygenation: “keep oxygen flowing” during laryngoscopy

• Nasal oxygen during apnoea can prolong time to desaturation (does not remove CO2).
• Typical approach: nasal cannula 5–15 L/min during pre-oxygenation and left on during laryngoscopy (local practice varies).
• High-flow nasal oxygen (if available and trained) can further extend safe apnoea time, but still requires a plan for ventilation and airway rescue.
• Apnoeic oxygenation is an adjunct, not a substitute for good pre-oxygenation and a clear airway plan.

Practical step-by-step for new starters (routine induction)

• Before induction: identify high-risk features (obesity, pregnancy, sepsis, lung disease, low baseline SpO2, predicted difficult airway).
• Position: head-up/ramped where appropriate, ensure monitoring and IV access ready.
• Pre-oxygenate: 100% O2, tight seal, 3 minutes tidal breathing, use two-handed technique if needed.
• Consider nasal oxygen for apnoeic oxygenation (especially if high risk).
• After induction: confirm ability to mask ventilate early, don’t persist with long laryngoscopy attempts.
• If SpO2 starts to fall: stop, ventilate with 100% O2, optimise airway maneuvers, call for help early.

When safe apnoea time is short: planning and escalation

• Have a clear “Plan A/B/C” for oxygenation and intubation before starting.
• Use the most experienced laryngoscopist early if high risk.
• Limit laryngoscopy attempts, prioritise oxygenation over intubation.
• Consider awake techniques or maintaining spontaneous ventilation if difficulty and rapid desaturation are both likely (discuss with senior).
• If difficulty ventilating: use airway adjuncts early (oropharyngeal airway, supraglottic airway) and follow local difficult airway guidance.

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