Pre-Ox Like a Pro!

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Steve Haywood
Former Respiratory Therapist working to de-mystify the ventilator through hands-on workshops and online education. Additional interests include HFNC in CHF and COPD, Medical Education, and Tupperware.
Picture of Sharleen Yuan
Sharleen Yuan

She is a resident in the Emergency Medicine/Internal Medicine program at the University of Maryland Medical Center. She’s a researcher in Forensic Psychology and Biomedical Neuroscience, however, her greatest accomplishment is the ability to rap most of Eminem’s repertoire of music. Sharleen is the infographic wizard of CriticalCareNow.


Oxygen in correlates to oxygen out. End Tidal Oxygen (EtO2) measurements can determine the fraction of oxygen exhaled by a patient. The Difficult Airway Society recommends pre-oxygenation until the exhaled oxygen fraction is at least 87% (1). EtO2 devices are not widely available. Fortunately, two studies with similar methodologies looked at pre-oxygenation techniques and the EtO2 achieved by various methods (2,3). The results are summarized in the following infographic. 


A flush rate non-rebreather (NRB) or a BiPAP were the only devices that achieved an EtO2 level that achieved the recommended level. Of note, some of the sites in this paper defined “flush rate” as only 19LPM while others defined it as 50-70LPM. If all sites had used the higher flow rate, the EtO2 measurements may have been higher. 

Additionally, the patients randomized to the Bag-Valve-Mask (BVM) group received assisted respirations “at the discretion of the physician”. While we have demonstrated that a BVM can be used to pre-oxygenate without squeezing the bag here, another study has shown that adding assisted breaths to pre-oxygenation decreases the incidence of hypoxic events (4).

Finally, High Flow Nasal Cannula (HFNC) and BiPAP measurements were taken as the lowest achieved during the intubation attempt and two minutes after the intubation attempt. The numbers likely would have been higher if the measurement was taken only at the initiation of the intubation attempt.

The Debrief

  • A non-rebreather at flush rate is a cheap, easy intervention that leads to high levels of EtO2. 
  • A BiPAP can also achieve high levels of EtO2, although, if it is not already being used, it is more cumbersome and costly to set up only for the purpose of pre-oxygenation. 
  • If BiPAP is needed for pre-oxygenation, most modern ventilators have a non-invasive mode that can be used to deliver positive pressure breaths through a mask. The mask that comes with the BVM can be attached to the circuit and held tightly against the patient’s face. Once the patient is intubated remember to change the vent back to invasive mode before attaching to the ETT.
  • HFNC and BVM do achieve high levels of EtO2, although not quite to the recommended levels. 
  • If using an NRB, always turn the flow to flush as 15LPM is inadequate for preoxygenation.


  1. Frerk C, Mitchell VS, McNarry AF, Mendonca C, Bhagrath R, Patel A, O’Sullivan EP, Woodall NM, Ahmad I; Difficult Airway Society intubation guidelines working group. Difficult Airway Society 2015 guidelines for management of unanticipated difficult intubation in adults. Br J Anaesth. 2015 Dec;115(6):827-48. doi: 10.1093/bja/aev371. Epub 2015 Nov 10. PMID: 26556848; PMCID: PMC4650961.
  2. Caputo ND, Oliver M, West JR, Hackett R, Sakles JC. Use of End Tidal Oxygen Monitoring to Assess Preoxygenation During Rapid Sequence Intubation in the Emergency Department. Ann Emerg Med. 2019 Sep;74(3):410-415. doi: 10.1016/j.annemergmed.2019.01.038. Epub 2019 Mar 14. PMID: 30879700.
  3. Vourc’h M, Baud G, Feuillet F, Blanchard C, Mirallie E, Guitton C, Jaber S, Asehnoune K. High-flow Nasal Cannulae Versus Non-invasive Ventilation for Preoxygenation of Obese Patients: The PREOPTIPOP Randomized Trial. EClinicalMedicine. 2019 Jun 5;13:112-119. doi: 10.1016/j.eclinm.2019.05.014. PMID: 31528849; PMCID: PMC6737343.
  4. Casey JD, Janz DR, Russell DW, Vonderhaar DJ, Joffe AM, Dischert KM, Brown RM, Zouk AN, Gulati S, Heideman BE, Lester MG, Toporek AH, Bentov I, Self WH, Rice TW, Semler MW; PreVent Investigators and the Pragmatic Critical Care Research Group. Bag-Mask Ventilation during Tracheal Intubation of Critically Ill Adults. N Engl J Med. 2019 Feb 28;380(9):811-821. doi: 10.1056/NEJMoa1812405. Epub 2019 Feb 18. PMID: 30779528; PMCID: PMC6423976


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