Three minutes of preoxygenation is a habit, not an endpoint. What determines apnoeic reserve is how completely nitrogen has been washed out of the functional residual capacity, and the measure of that is end-tidal oxygen — a fraction of 0.9 or above is the target, reached in some patients in well under three minutes and in others not at all in five.
The commonest reason the target is never reached is a leaking mask seal, and the capnograph will usually tell you: a trace that is flattened or shows an implausibly low end-tidal carbon dioxide means gas is being entrained from the room. Two hands on the mask, or a second pair, is worth more than another minute of a poor seal.
Position changes the reserve as much as the mask does. A head-up or ramped position raises functional residual capacity, and it matters most in the patients whose reserve is already smallest — obesity, pregnancy, advanced ascites, the third trimester. Nasal cannula oxygen left running underneath the mask and continued through the apnoeic period extends the time available at no cost and with no extra equipment.
Where the anaesthetic machine does not display end-tidal oxygen, the substitute is not a shorter clock but a longer one with a verified seal: tidal volume breathing for a full three minutes at a high fresh gas flow, with the reservoir bag seen to move.
- Preoxygenate to an end-tidal oxygen fraction of 0.9 or above rather than for a fixed time
- Check the capnograph trace for a leak — a flattened trace or implausibly low end-tidal carbon dioxide means a poor mask seal
- Use a ramped or head-up position, particularly in obesity, pregnancy or abdominal distension
- Run nasal cannula oxygen under the mask and continue it through apnoea to extend safe apnoea time
- Where end-tidal oxygen is not displayed, use a full three minutes of tidal breathing at high fresh gas flow with a verified seal
Why it matters
The reserve that matters during a difficult intubation was decided before the drug went in.
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