Turn it (All the Way) Up: Flush Rate O2 for Pre-Oxygenation

Background: There has been a lot of buzz recently about the importance of pre-oxygenation in emergency airway management.  The recent publication of the ENDAO trial [2], a randomized clinical trial of ApOx vs no ApOx also emphasized this point.  In the review article accompanying this trial by John Sackles [3] he brought up the point that most patients in this study were intubated in less than 1 – 2 minutes.  In this scenario, preoxygenation alone would likely provide an adequate oxygen reservoir to prevent hypoxemia and that apneic oxygenation would only be helpful in the patients who exhausted their oxygen reserves (i.e. prolonged intubations). Although, apneic oxygenation has recently come into favor in emergency intubation, the issue that should maybe warrant greater consideration is proper preoxygenation. The optimal method of pre-oxygenation however, is often debated: bag-valve mask (BVM), nonrebreather (NRB), or simple face mask. 

What They Did:

  • Crossover trial with healthy volunteers (each volunteer was pre-ox with all 4 experimental techniques)
  • Patients pre-oxygenated for 3 minutes with:
    • NRB at 15L/min (NRB-15)
    • NRB with flush rate (>40L/min) oxygen (NRB-Flush)
    • BVM device with oxygen at 15L/min (BVM-15)
    • Simple mask with flush rate (>40L/min) oxygen (SM-Flush)
  • Flush rate oxygen achieved by rotating the flowmeter dial counterclockwise until it could not be turned farther (Engineer not affiliated with the study evaluated flush rate flow of oxygen through flowmeters and confirmed flush rate in this study was most likely 50 – 54L/min)


  • Primary Outcome: FeO2 in a single exhaled breath after 3 min pre-oxygenation (device accurate for measuring FeO2 within 1-3%)


  • ED staff at Hennepin County (These patients could also have history of well-controlled chronic respiratory disease and/or facial hair)


  • Symptomatic respiratory disease at participation
  • Smoking history greater than 5 pack-years
  • Pregnancy
  • Younger than 18 years


  • 26 subjects enrolled
    • Mean age: 31 years
    • Mean BMI: 24kg/m2
    • Mean baseline FeO2: 17.3% (95% CI 16.9 – 17.6%)
  • Critical Results


  • FeO2 measured with oxygen gas analyzer (Handi+ model R218P12) with manufacturer-reported accuracy within 1 – 3%
  • Oxygen gas analyzer calibrated with 100% oxygen before each preoxygenation trial (i.e. 4x per subject)
  • Each trial followed by 2 minutes of breathing room air and renitrogenation confirmed with FeO2 for each subject being measured between trials and returned to subject’s baseline value


  • Small, single center study without evaluation of patient oriented outcomes
  • Neither investigators nor subjects were blinded to preoxygenation device
  • Healthy volunteers used (i.e. results must be confirmed in a critically ill patient population)
  • Average BMI ~ 24 kg/m2 which may not generalize to the standard ED population depending on where you work
  • BVM may have had mask leaks which would underestimate the true efficacy of the BVM for pre-oxygenation
  • Study not designed to demonstrate superiority of NRB-Flush compared with NRB-15 or simple mask with flush rate oxygen
  • The value of flush rate oxygen may vary depending on the flowmeter model available at each institution


  • Important point made in this paper is that in a dyspneic, anxious, or agitated patient a tight mask seal may be difficult to achieve with a BVM.
  • BVM requires the use of a team member to hold the mask in place where a NRB does not (i.e. Use of NRB frees up a pair of hands)
  • Also many BVM devices lack a 1-way valve needed to achieve adequate oxygenation
  • How flush rate oxygen works in a non-sealed system: To provide near 100% inspired oxygen in a non-sealed system of oxygen delivery, the oxygen flow rate must exceed the inspiratory flow rate of the patient and the dead space nitrogen in the mask/upper airway must be flushed out between breaths.

Author Conclusion: “Preoxygenation with NRB-Flush was noninferior to BVM-15.  NRB with flush rate oxygen may be a reasonable preoxygenation method for spontaneously breathing patients undergoing emergency airway management.”

Clinical Take Home Point: Although the results of this study need to be confirmed in a critically ill patient population, it appears that flush rate oxygen via a NRB mask is non-inferior to BVM mask at 15L/min.


  1. Driver BE et al. Flush Rate Oxygen for Emergency Airway Preoxygenation. Ann emerg Med 2017; 69: 1 – 6. PMID: 27522310
  2. Caputo N et al. EmergeNcy Department use of Apneic Oxygenation Versus Usual Care During Rapid Sequence Intubation: A Randomized Controlled Trial (the ENDAO Trial). Acad Emerg Med 2017. [epub ahead of print]. PMID: 28791755
  3. Sackles JC. Maintenance of Oxygenation During Raid Sequence Intubation in the Emergency Department. Acad Emerg Med 2017 [epub ahead of print]. PMID: 28791775

Post Peer Reviewed By: Anand Swaminathan (Twitter: @EMSwami)

Cite this article as: Salim Rezaie, "Turn it (All the Way) Up: Flush Rate O2 for Pre-Oxygenation", REBEL EM blog, September 14, 2017. Available at: https://rebelem.com/turn-way-flush-rate-o2-pre-oxygenation/.

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