Preoxygenation & Apneic Oxygenation Before Intubation

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Updated July 2026  |  More ICU nursing guides →

The single most important thing that happens before an intubation is not the tube — it is filling the patient's lungs with oxygen first. Preoxygenation and its partner, apneic oxygenation, are what turn a frightening 60-second apneic window into a calm, unhurried one. They are also almost entirely nursing- and respiratory-therapy-driven: the provider is focused on the airway, and it is the bedside team that gets the sat as high as possible and keeps it there. Understanding the physiology lets you do it deliberately rather than just "slapping on a mask."

What preoxygenation actually does

Room air is about 21% oxygen and 78% nitrogen. The functional residual capacity — the air that stays in the lungs after a normal breath — is therefore mostly nitrogen, which does nothing useful. Preoxygenation is denitrogenation: by breathing a high fraction of oxygen for a few minutes, you wash the nitrogen out and refill that reservoir with oxygen. A well-preoxygenated healthy adult can then tolerate several minutes of apnea before the saturation falls, because there is a lungful of oxygen to draw on. A poorly preoxygenated patient desaturates in under a minute. The goal is a period of high-FiO2 breathing — ideally three minutes, or eight vital-capacity breaths if time is short — targeting an end-tidal or pulse-ox reading as close to 100% as the patient can reach.

The tools, from good to best for the sick patient

The practical upgrade most units make is the flush-rate non-rebreather: a standard NRB at 15 L/min still entrains room air and delivers less oxygen than people assume, but opening the flowmeter fully ("flush," often 40–70 L/min) delivers a much higher effective FiO2 with no new equipment.

Apneic oxygenation: oxygen with no breathing

Here is the elegant part. Even when a paralyzed patient is not breathing, the alveoli keep absorbing oxygen into the blood — roughly 250 mL/min is consumed — while very little CO2 comes back the other way. That creates a slight negative pressure that draws gas down from the upper airway. If you have oxygen sitting at the nose, it gets pulled into the lungs passively, without a single breath. This is apneic oxygenation, and in practice it means: leave a nasal cannula running at 15 L/min under the mask during preoxygenation, and — crucially — leave it on while the provider is laryngoscoping. The mnemonic many units use is NO DESAT (Nasal Oxygen During Efforts Securing A Tube). It can meaningfully extend the safe apnea window and is essentially free.

Position matters as much as the device

A supine, flat patient preoxygenates poorly because the abdominal contents push the diaphragm up and collapse dependent lung. Whenever the clinical situation allows (no suspected spinal injury, no severe hypotension), preoxygenate the patient sitting up, or in reverse Trendelenburg 20–30 degrees. The head-elevated position increases functional residual capacity, improves the reservoir, and often improves the laryngoscopic view as well. For the obese patient this is not optional — ramping to align the ear with the sternal notch is one of the highest-value moves you can make.

Why the shunt patient is the hard one

Preoxygenation works by filling ventilated alveoli. In a patient with shunt physiology — pneumonia, ARDS, pulmonary edema, atelectasis — blood is flowing past alveoli that are collapsed or fluid-filled and never sees the oxygen you are delivering. These are the patients who sit at 88% no matter how much oxygen goes on the face, and they desaturate almost instantly once apneic. The answer is positive pressure: NIV or a PEEP valve recruits some of those collapsed units so there is actually lung to fill. If a patient will not preoxygenate above the low 90s on a non-rebreather, that is the signal to escalate to NIV before the attempt — and to warn the provider that the safe window will be short. Communicating that expected short window is itself a nursing contribution.

Preoxygenation is where a calm intubation is won or lost. For the full sequence around it, see the ICU nurse's role in RSI; for keeping the blood pressure up through the attempt, peri-intubation hemodynamics; and for the failed first pass, the difficult airway.