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.