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Respiratory failure: Preoxygenation

  • Administration of 100% FiO2 prior to intubation to extend the “safe apnea time” (until SaO2 drops to ~88-90%) via denitrogenation of the lungs, which results in a large alveolar oxygen reservoir
    • Optimal preoxygenation (ETO2 of 80-90%+) results in a reservoir of ~3,000 mL of O2 in the lungs vs. ~450 mL when breathing room air in healthy patients (10+ minutes of O2 reserve vs 1-2 minutes with room air)
      • The majority (but not maximum) of the reservoir increase occurs with 4 deep breaths within 30 seconds – this may be all that a critically ill patient can tolerate before proceeding with intubation
    • O2 consumption during apnea is ~200-250 mL/min (~3 mL/kg/min) in healthy adults
    • SaO2 of 88-90% marks the inflection point on the O2-Hgb dissociation curve beyond which further decreases in PaO2 lead to a rapid decline in SaO2 (~30% every minute)
    • Can use standard mask or non-rebreather (+/- PEEP which recruits poorly ventilated alveoli, increases FRC, and prevents atelectasis)
    • In some critically ill patients, critical desaturation may occur immediately despite attempts at preoxygenation – if SaO2 and/or ETO2 do not improve after ~3 minutes of preoxygenation then further preoxygenation effort is unlikely to be of benefit and intubation should proceed
    • If respiratory drive is insufficient, provide positive pressure ventilation to preoxygenate (being mindful to not distend the stomach and increase aspiration risk)
  • Oxygenation can be further optimized via apneic oxygenation in addition to preoxygenation (i.e. high flow O2 via nasal cannula through patent airway while intubating the patient)
    • At least 15 L/min O2 (up to 60 L/min) via nasal prongs with head of bed elevated to 30 degrees (HOB elevation increases FRC and thus the lung reservoir)
    • This is merely an adjunct – it cannot compensate for ineffective preoxygenation
    • Can use this adjunct while preoxygenating the patient if they are able to tolerate it (and able to maintain a good mask seal), or add it after the patient is sedated

Updated definition 2020:

During intubation there is a period of time during laryngoscopy and ETT placement when a patient remains apneic. It is important to consider a patient’s risk of desaturation during this period of apnea, taking into account underlying respiratory pathology and expected difficulty of the intubation procedure. Pre-oxygenation aims to increase the concentration of oxygen in a patient’s FRC volume, as the FRC represents a patient’s oxygen reserve during periods of apnea. Increasing the concentration of oxygen in a patient’s FRC will prolong the time a patient can remain apneic without desaturating.

Three minutes of spontaneous breathing at 100% FiO2 during pre-oxygenation can extend apneic time to desaturation up to 10 minutes in patients with normal respiratory function. A similar increase in FRC oxygen concentration can be achieved with 8 deeps breaths over the course of 1 minute. An end tidal O2 concentration of >90% indicates adequate pre-oxygenation with a good mask seal.

In patients with respiratory failure, PEEP may need to be applied to improve oxygenation. PEEP is most effectively applied with NIV during pre-oxygenation. NIV helps to recruit alveoli, leading to more efficient gas exchange. It has been shown that patients with acute hypoxemic respiratory failure desaturate less during intubation if they are preoxygenated with NIV vs conventional oxygen therapy through a mask.


  1. Fong KM et al. Preoxygenation before intubation in adult patients with acute hypoxemic respiratory failure: a network meta-analysis of randomized trials. Critical care (London, England) vol. 2019; 23(1):319. PubMed Link