[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anaesthesia and ICU](https://durgical.com/en/anaesthesia/) › [The airway](https://durgical.com/en/anaesthesia/areas/airway/) › Preoxygenation and apnoeic oxygenation

# Preoxygenation and apnoeic oxygenation

- Technique

Breathing pure oxygen through a tight mask before the breathing stops, so the lungs hold an oxygen store that buys safe minutes during intubation.

Updated Nov 13, 2026

## Key points

- Goal: denitrogenate the functional residual capacity so it becomes an oxygen store that lengthens safe apnoea time.
- Endpoint: an end-tidal oxygen fraction of about 90% or more, after about three minutes of tidal breathing or eight deep breaths.
- A mask leak (beard, no teeth, poor seal) defeats preoxygenation: the end-tidal oxygen trace shows it.
- Head-up positioning and continuous positive pressure help obese and hypoxaemic patients; apnoeic oxygenation needs an open airway.
- Apnoeic oxygenation slows desaturation but not the rise in carbon dioxide.

On this page

- The basics Key points
- What it is
- Indications
- Equipment and preparation
- Step by step (and why)
- Who desaturates fast during apnoea, and why
- Next step
- Memory aid
- Elsewhere in DurgiPedia

- Clinical Complications

- Safety Contraindications and precautions

- Study Test yourself

- Connections Connections

## What it is

Preoxygenation replaces the nitrogen in the lungs with oxygen before induction of anaesthesia, so that the functional residual capacity (the air left in the lungs at the end of a normal breath) becomes an oxygen reservoir. When the patient stops breathing after induction and muscle relaxation, that reservoir keeps the haemoglobin saturated for longer: a healthy adult breathing room air desaturates within about a minute of apnoea, while a well preoxygenated one may tolerate several minutes. It is done with high-flow oxygen through a tight-fitting face mask, with the patient head-up, until the exhaled oxygen fraction shown by the gas analyser is about 90% or more. Apnoeic oxygenation adds oxygen through nasal cannulae during the apnoea itself: because the blood takes up oxygen faster than carbon dioxide returns to the alveoli, oxygen keeps flowing down an open airway into the lungs without breaths. It prolongs the safe time but does not remove carbon dioxide, which keeps rising. Obese, pregnant, critically ill and young patients have a smaller reservoir or a higher oxygen use and desaturate much faster, so they gain the most from careful technique.

## Indications

- Every induction of general anaesthesia, even when an easy airway is expected.
- Rapid-sequence induction, where mask ventilation is avoided and the oxygen store is all the patient has.
- Predicted difficult airway, so there is time for a second attempt or a rescue device.
- Patients who desaturate fast: obesity, pregnancy, children, sepsis, lung disease and critical illness.
- Emergency intubation outside the operating room, combined with apnoeic oxygenation by nasal cannula.

## Contraindications and precautions

- No absolute contraindication before intubation: the brief exposure to high oxygen is far safer than hypoxaemia.
- Beards, facial injuries, missing teeth or a nasogastric tube break the mask seal; plan a better seal or positive pressure.
- Agitated or confused patients may not tolerate the mask; calm explanation or light sedation may be needed, watching breathing.
- Positive-pressure techniques can inflate the stomach; use them with care when the stomach is full.
- During airway or head and neck surgery with diathermy or laser, high oxygen near the field is a fire risk.

## Equipment and preparation

- Oxygen supply, anaesthetic circuit or self-inflating bag with reservoir, and a valve for positive end-expiratory pressure.
- Face masks in several sizes and a strap or two-handed grip for a tight seal.
- Gas analyser showing inspired and end-tidal oxygen and the carbon dioxide waveform.
- Nasal cannula (standard or high-flow nasal oxygen device) for apnoeic oxygenation.
- Pulse oximetry, working suction, airway adjuncts and the intubation kit ready before starting.

## Step by step (and why)

- Sit the patient head-up or in reverse Trendelenburg — the diaphragm drops, the oxygen store grows and the mask fits better.
- Explain and apply the mask with a tight seal — any leak lets room air in and keeps nitrogen in the lungs.
- Give high-flow oxygen while the patient breathes normally for about three minutes, or eight deep breaths — this washes nitrogen out.
- Watch the end-tidal oxygen and the capnogram — about 90% end-tidal oxygen and a square wave confirm a good seal.
- Add continuous positive pressure or noninvasive ventilation if hypoxaemic or obese — it opens collapsed alveoli and enlarges the store.
- Place nasal cannulae with oxygen and leave them running through laryngoscopy — oxygen keeps entering an open airway during apnoea.
- Keep the airway open with a jaw thrust while apnoeic — mass flow of oxygen stops if the pharynx collapses.
- Agree a desaturation limit before starting — the team stops the attempt and reoxygenates when it is reached.

## Who desaturates fast during apnoea, and why

| Patient | Why | What helps |
| --- | --- | --- |
| Obesity | Smaller functional residual capacity, higher oxygen use | Ramped head-up position, positive pressure, apnoeic oxygenation |
| Late pregnancy | Uterus lifts the diaphragm, higher oxygen use | Left uterine displacement, head-up, careful seal |
| Infants and children | High oxygen use for body size | Fast, efficient technique; a calm induction |
| Critical illness, pneumonia, ARDS | Shunt through flooded alveoli; oxygen does not reach the blood | Noninvasive ventilation or high-flow nasal oxygen, positive pressure |
| Sepsis, fever | Higher oxygen consumption | Full preoxygenation and apnoeic oxygenation |

## Complications

- Rapid desaturation from a mask leak or a poorly prepared patient, often discovered only after induction.
- Absorption atelectasis: pure oxygen in poorly ventilated alveoli is absorbed and they collapse; recruitment and positive pressure help.
- Gastric insufflation when positive pressure is used, increasing the risk of regurgitation.
- Carbon dioxide still rises during long apnoea, causing acidosis that matters in raised intracranial pressure or pulmonary hypertension.
- A false sense of security: a well preoxygenated patient can be oesophageally intubated and keep a normal saturation for minutes.

## Next step

- Document the end-tidal oxygen reached and any difficulty with the mask seal.
- If the saturation falls during the attempt, stop, reoxygenate with mask or supraglottic airway, and call for help early.
- Call a senior before induction in patients expected to desaturate fast who also have a predicted difficult airway.
- After intubation, recheck oxygenation and recruit collapsed lung if the saturation stays low.

## Memory aid

The end-tidal trace tells the truth: if exhaled oxygen is not around 90%, the store is not full.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the aim of preoxygenation?

To replace the nitrogen in the functional residual capacity with oxygen, so the lungs hold an oxygen store that lengthens the time before desaturation during apnoea.

## How do you know preoxygenation is adequate?

The end-tidal oxygen fraction on the gas analyser reaches about 90% or more, with a normal capnogram showing a tight mask seal.

## How does apnoeic oxygenation work?

The blood takes up oxygen from the alveoli faster than carbon dioxide returns, creating a mass flow that draws oxygen from the nasal cannula down an open airway without breaths.

## Why does an obese patient desaturate faster during apnoea?

The functional residual capacity is smaller, so the oxygen store is smaller, and oxygen consumption is higher; the head-up position and positive pressure help.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anaesthesia/preoxygenation-and-apnoeic-oxygenation/)

## Elsewhere in DurgiPedia

- [Bag-mask ventilation](https://durgical.com/en/clinical-skills/bag-mask-ventilation/)
- [Starting non-invasive ventilation](https://durgical.com/en/clinical-skills/starting-non-invasive-ventilation/)
- [Oxygen therapy: devices, flows and safety](https://durgical.com/en/nursing/oxygen-therapy-devices-flows-and-safety/)
- [Lung volumes, capacities and spirometry](https://durgical.com/en/physiology/lung-volumes-capacities-and-spirometry/)

## Connections

## Related 5

- [Rapid-sequence induction and intubation](https://durgical.com/en/anaesthesia/rapid-sequence-induction-and-intubation/)
- [Tracheal intubation](https://durgical.com/en/anaesthesia/tracheal-intubation/)
- [The unanticipated difficult airway: plans A to D](https://durgical.com/en/anaesthesia/the-unanticipated-difficult-airway-plans-a-to-d/)
- [Intraoperative hypoxaemia: a stepwise approach](https://durgical.com/en/anaesthesia/intraoperative-hypoxaemia-a-stepwise-approach/)
- [Predicting a difficult airway: Mallampati, mouth opening, neck and jaw](https://durgical.com/en/anaesthesia/predicting-a-difficult-airway-mallampati-mouth-opening-neck-and-jaw/)

## Related conditions 4

- [Obesity](https://durgical.com/en/conditions/obesity/)
- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Acute respiratory failure (hypoxaemic and hypercapnic)](https://durgical.com/en/conditions/acute-respiratory-failure/)
- [Obstructive sleep apnoea](https://durgical.com/en/conditions/obstructive-sleep-apnoea/)

[← All of anaesthesia](https://durgical.com/en/anaesthesia/)

Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: [Oxygen Therapy](https://www.ncbi.nlm.nih.gov/books/NBK551617/) (StatPearls (NCBI Bookshelf), 2025); [Tracheal Rapid Sequence Intubation](https://www.ncbi.nlm.nih.gov/books/NBK560592/) (StatPearls (NCBI Bookshelf), 2022).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/anaesthesia/preoxygenation-and-apnoeic-oxygenation/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anaesthesia/preoxygenation-and-apnoeic-oxygenation/