Apnoeic oxygenation
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I have comprehensive content from Miller's Anesthesia, Roberts and Hedges' Clinical Procedures in Emergency Medicine, and Morgan & Mikhail's. Let me compile the full answer.
Apnoeic Oxygenation
Definition
Apnoeic oxygenation is a physiologic phenomenon in which oxygen diffuses from the oropharynx/nasopharynx down into the alveoli during apnoea, driven by a net negative alveolar pressure — oxygen is consumed by tissues faster than carbon dioxide accumulates in the alveoli. Provided the airway is patent and oxygen is insufflated via the nose or mouth, oxygenation continues even without ventilation, substantially extending safe apnoea time beyond that achieved by standard face-mask preoxygenation alone. — Miller's Anesthesia, 10e
Physiological Mechanism
During apnoea, gas exchange depends on mass movement of gases along concentration gradients rather than cyclic pressure changes (the normal mechanism of breathing). — Morgan & Mikhail's Clinical Anesthesiology, 7e
The key drivers are:
| Factor | Effect |
|---|---|
| Oxygen diffuses across alveoli more readily than CO₂ | Net efflux of oxygen into blood exceeds CO₂ influx |
| Different blood solubilities of O₂ and CO₂ | CO₂ is far more soluble — carried in blood as bicarbonate, not just dissolved gas |
| High haemoglobin affinity for O₂ | Continued O₂ extraction from alveoli |
| Net result | Pressure gradient draws O₂ from nasopharynx → trachea → alveoli |
This is why oxygenation is maintained but CO₂ continues to rise (~3 mmHg/min under standard conditions, but only ~1.1 mmHg/min with THRIVE at high-flow). — Roberts and Hedges', Miller's
Clinical Techniques
1. Standard Nasal Cannula (NO DESAT technique)
- Flow rates: 5–15 L/min (up to 15 L/min)
- Place beneath the main preoxygenation device (face mask / BVM) before induction
- When preoxygenation device is removed for laryngoscopy, keep nasal cannula in place
- Increase to ≥15 L/min (or maximum) during intubation attempts
- Higher flow → higher FiO₂
- If nasal obstruction: insert a nasopharyngeal airway in one or both nares
- Limitation: not humidified — causes nasopharyngeal desiccation at high flows, though clinically insignificant at the durations used
2. High-Flow Nasal Cannula (HFNC) / THRIVE
- THRIVE = Transnasal Humidified Rapid-Insufflation Ventilatory Exchange
- Delivers warmed, humidified oxygen at up to 70 L/min
- Advantages over standard nasal cannula:
- Longer safe apnoea time
- Improves CO₂ clearance via nasopharyngeal gas washout → prevents severe respiratory acidosis
- Can be used as both a preoxygenation strategy (60 L/min for 3 min is equivalent to tidal-volume face-mask preoxygenation) and during apnoea itself
- In 25 patients with difficult airways: median apnoea time without desaturation to <90% was 14 minutes (range 5–65 min); mean CO₂ rise only 1.1 mmHg/min — Miller's Anesthesia, 10e
- ICU study showed HFNC superior to simple nasal cannula, though both groups had low rates of hypoxaemia — Roberts and Hedges'
Practical Protocol (Emergency/RSI Setting)
- Pre-intubation: place nasal cannula at 10–15 L/min before preoxygenation begins
- During preoxygenation: limit to 5–15 L/min if the awake patient finds higher flows uncomfortable
- At induction/paralysis: increase to ≥15 L/min (maximum)
- Do not remove the nasal cannula when the face mask is taken off
- Ensure upper airway patency during apnoea: jaw thrust, head-tilt/chin-lift, ensure patient is in intubating position
- Monitor SpO₂ continuously with a functioning pulse oximeter
Roberts and Hedges' Clinical Procedures in Emergency Medicine
Evidence
| Setting | Finding |
|---|---|
| OR (normal + morbidly obese patients) | Nasopharyngeal O₂ insufflation causes significant delay in desaturation; many subjects had no hypoxaemia after 6 min of apnoea |
| ICU (2016 RCT — Vourc'h et al.) | No difference in SpO₂ nadir between apnoeic oxygenation vs. no apnoeic oxygenation — but patients had been on supplemental O₂ for hours beforehand, limiting generalisability to ED/prehospital |
| ED/prehospital | Apnoeic oxygenation considered standard of care — population has less pre-treatment and shorter preoxygenation windows |
Important Limitation: CO₂ Accumulation
Apnoeic oxygenation maintains oxygenation but does not eliminate CO₂. Rising PaCO₂ leads to:
- Respiratory acidosis
- Sympathetic stimulation (tachycardia, hypertension)
- At extreme levels: cardiac arrhythmias, obtundation
At standard flow rates: CO₂ rises ~3 mmHg/min. THRIVE attenuates this to ~1.1 mmHg/min through nasopharyngeal CO₂ washout. After 8–10 minutes of apnoeic oxygenation, PaCO₂ typically reaches a clinically significant threshold — relevant in brain-dead testing (apnoea test).
Plum and Posner's Diagnosis and Treatment of Stupor and Coma; Miller's Anesthesia
Recent High-Level Evidence (2023–2024)
Three recent systematic reviews/meta-analyses are available:
- [Cochrane Review 2023 — White et al., PMID 37531462]: Oxygenation during apnoeic phase preceding intubation across prehospital, ED, ICU, and OR settings
- [Meta-analysis 2024 — Fuchs et al., Br J Anaesth, PMID 38030551]: Apnoeic oxygenation specifically in paediatric tracheal intubation
- [Network meta-analysis 2024 — Cristian de Carvalho et al., Br J Anaesth, PMID 38599916]: Comparative effectiveness of preoxygenation strategies
These provide the most current evidence base and may refine practice guidance for specific populations.
Summary: Apnoeic oxygenation exploits the asymmetry between O₂ consumption and CO₂ accumulation in the alveoli during apnoea to maintain SpO₂ via a simple nasal cannula or HFNC. It should be applied routinely in every tracheal intubation. THRIVE (high-flow humidified) offers the greatest extension of safe apnoea time and partial CO₂ buffering, making it particularly valuable in anticipated difficult airways and high-risk patients.
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