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Pre-oxygenation prior to rapid sequence intubation (RSI) is performed to prevent hypoxemia during endotracheal intubation.
An appropriate period of pre-oxygenation will potentially increase the amount of apnea time during intubation, however patients with certain critical illnesses (e.g., severe pneumonia) may desaturate faster than expected.
Apnea time can be increased by maintaining high-flow oxygen by nasal cannula (e.g., 15L), during application of the bag-valve mask and during the time of attempted endotracheal tube placement; this concept is known as apneic oxygenation.
Apneic oxygenation is based on the principle that when patients are apneic, alveoli absorb oxygen into the blood stream at a rate of approximately 250 mL/minute, creating a diffusion gradient from the pharynx (containing a high-density of oxygen from the nasal cannula) to a lower concentration of oxygen in the alveoli.
Although a patient’s oxygenation can be maintained longer using apneic oxygenation, its application does not remove the continuous buildup of CO2 in the alveoli during apena. Therefore, respiratory acidosis can result after a prolonged period of apneic oxygenation.
The complete article describing the physiology and practical applications can be found here....it's free! http://www.annemergmed.com/article/S0196-0644(11)01667-2/fulltext
References
Weingart, S and Levitan, R. Preoxygenation and prevention of desaturation during emergency airway management. Ann Emerg Med. 2012 Mar; 59(3):165-175.e1
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