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Preoxygenation

General Surgery · Final Professional

Definition

Preoxygenation (denitrogenation) is the administration of 100% oxygen to a patient before induction of anaesthesia, to replace the nitrogen in the lungs (functional residual capacity) with oxygen. This builds an oxygen reservoir that delays the onset of hypoxaemia during the apnoea that follows induction and while the airway is being secured.

Technique & Importance

The patient breathes 100% oxygen through a well-sealed face mask for about 3 minutes (or several vital-capacity breaths in an emergency) until the end-tidal oxygen is high. Preoxygenation prolongs the safe apnoea time, giving valuable extra minutes to intubate before oxygen saturation falls — crucial if intubation proves difficult, and especially important in patients who desaturate quickly (children, pregnancy, obesity, critical illness).

CLINICAL PEARL

Preoxygenation is essential before any rapid sequence induction and in any patient likely to be difficult to intubate or to desaturate fast (obese, pregnant, children, sick) — it buys the time that can prevent hypoxic harm.

Physiological Basis

The effectiveness of preoxygenation rests on the functional residual capacity acting as an oxygen store: replacing the nitrogen that normally fills it with oxygen means that, after the patient stops breathing at induction, the lungs hold a large reservoir from which the circulation can continue to draw oxygen. In a healthy adult this can extend the time to significant desaturation from around a minute to several minutes, whereas patients with a reduced FRC or high oxygen demand — the obese, pregnant, children and the critically ill — exhaust the store quickly and desaturate fast, which is precisely why unhurried preoxygenation matters most in them.

CLINICAL PEARL

Preoxygenation buys time: filling the FRC with oxygen turns a minute of safe apnoea into several, and it matters most in the very patients who desaturate fastest — the obese, the pregnant, children and the critically ill.

DANGER / REMEMBER

The end-point of preoxygenation is best judged not by time alone but by a high end-tidal oxygen concentration (indicating that most of the nitrogen has been washed out of the lungs), and a tightly sealed mask is essential, since even a small leak entrains room air and undoes the denitrogenation.

Extends safe apnoea time from about 1 minute to several.

KEY POINT

Key points TO remember

Preoxygenation/denitrogenation = 100% O₂ before induction to fill the FRC with oxygen.
Builds an oxygen reservoir that prolongs safe apnoea time after induction.
~3 minutes of tidal breathing (or vital-capacity breaths) via a sealed mask.
Vital before RSI and in rapid desaturators (obese, pregnant, children, critically ill).

EXAM TIP

Sources: Morgan & Mikhail’s Clinical Anesthesiology; Miller’s Anesthesia; Ajay Yadav’s Short Textbook of Anaesthesia.

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