High-flow nasal oxygen (HFNO) at 60 L/min with FiO2 1.0 is used for pre-oxygenation before RSI in an obese patient. Compared to standard facemask pre-oxygenation, the primary advantage of HFNO during the apnoeic phase of RSI is:

• A Maintenance of positive end-expiratory pressure effect that delays airway closure and provides apnoeic mass-flow oxygenation ✓
• B Higher inspired FiO2 delivery than facemask (HFNO delivers FiO2 >0.9 vs ~0.8 for facemask)
• C Mechanical dilation of the upper airway preventing laryngospasm during paralysis
• D Active CO2 washout from the hypopharynx improving the carbon dioxide–oxygen gradient

Explanation

HFNO at high flows generates 2–5 cmH2O PEEP in awake patients through resistance at the nasopharynx, which helps maintain FRC and delays atelectasis. During the apnoeic period after induction/paralysis, the continued high-flow O2 delivered to the nasopharynx creates a mass-flow oxygen reservoir that sustains oxygenation by apnoeic diffusion: oxygen moves from the high-partial-pressure pharynx down a pressure gradient into alveoli while CO2 accumulates more slowly. This extends the safe apnoea time significantly (especially valuable in obese patients and those with pre-existing hypoxaemia). While HFNO achieves high FiO2, standard well-sealed facemask at 15 L/min can also achieve FiO2 >0.9; the unique advantage of HFNO is the apnoeic oxygenation mechanism during the intubation attempt.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.