Correction: Thanks to @amerelman for catching an error in the original post. This study was all about preoxygenation. In my mind, I was also making application to apneic oxygenation with high flow NC, and it made the summary unclear. It's fixed now. Thanks for the real-time Twitter correction. Power of #FOAMed
Written by Clay Smith
If using nasal cannula under a bag-valve mask seal, flow rates must be 15 L/min or more to overcome the small air leak and positively impact preoxygenation.
Why does this matter?
Apneic oxygenation with high flow nasal cannula (NC) is considered standard practice by some and ineffective by others. The literature has gone back and forth on its effectiveness. High flow NC can also help with preoxygenation. But having the NC tubing under the mask may impair the seal, cause a leak, entrain room air, and impair oxygen delivery. Is there an ideal flow rate that would overcome this to be of benefit for preoxygenation?
Overcoming leaks in RSI easier than FBI
This was a randomized crossover study with 40 healthy volunteers and varying configurations of NC and bag-valve mask (BVM) oxygen delivery set ups. They found that end tidal oxygen (EtO2) was lowest with the NC set at 0 L/min and 5 L/min. NC at 10 L/min was equivalent to BVM alone on EtO2 measurement. NC 15 L/min had the highest EtO2. What this means is that there is air leak and entrainment of room air from nasal cannula tubing under a mask seal, and it takes at least 10 L/min to compensate for this. At NC flow rates of 15 L/min, the leak was not only overcome but led to slightly better preoxygenation, primarily in the first minute of preoxygenation. The take home point is that if you are going to use NC for preoxygenation under a BVM, you need to maximize flow rates to >15 L/min to do any good.
Addition of Nasal Cannula Can Either Impair or Enhance Preoxygenation With a Bag Valve Mask: A Randomized Crossover Design Study Comparing Oxygen Flow Rates. Anesth Analg. 2018 Apr;126(4):1214-1218. doi: 10.1213/ANE.0000000000002341.
Life in the Fast Lane has an epic post on the subject of Apneic (or Apnoeic) Oxygenation. Read this!
Peer reviewed by Thomas Davis