Please note that I have sorted out this issue and the following rant is a rant on my thought process leading up to my eventual resolution looking at PEEP and high flow nasal cannula. 

I was trained, or maybe even made it up in my head, that for every 10L of flow increase on the high flow nasal cannula, HFNC, you get 1cmH2O of “PEEP”. Is this accurate? The short answer in my opinion is no. At least not the way you’re thinking about it. I’ve been digging pretty deep into the topic because although much data is suggestive of it, but I can’t find something that I can clearly understand. Maybe it’s just my lack of intelligence or lack of direct pulmonary training.

Positive end-expiratory pressure (PEEP) is defined by UpToDate as the alveolar pressure above atmospheric pressure that exists at the end of expiration. Therefore, we need to look at alveolar pressure directly. In particular, we need to look at extrinsic PEEP. Without a closed system, we cannot obtain that data. I have run into several papers cited below that discuss methodologies used to estimate what PEEP should be in the HFNC system. The 2009 Parke study looked at the mean nasopharyngeal airway pressure and deemed it to be 2.7cmH2O with a flow of 35L and the mouth closed. That’s not the alveolar pressure. The Corley study utilized electrical impedance tomography along with a transducer placed nasally that ran down into the esophagus that measured the airway pressure. With the flow in the study between 35-50L on the HFNC system, the authors found that there was an increase in the airway pressure by 3cmH2O. Is this what’s being considered as PEEP?

Lastly, Parke performed another study in CVICU patients where she and her team measured nasopharyngeal pressures at 30L, 40L, and 50L, and concluded that the high flow (HFNC) system provided 3-5cmH2O of PEEP. I guess that’s where the numbers I was taught came from. But in reality does that translate to PEEP? Do we just need to accept that we are comparing apples and oranges? Do we just need to change our language since we are just so comfortable of saying “PEEP” because we’re used to it on our ventilators? Am I just going to have to delete this post after I am exposed as being a moron when a number of people just comment about how silly am I that I do not know this stuff? Why are we even trying to compare the two? We know that pharyngeal pressure is increased by the HFNC system. That’s fine and dandy. Patients do well on HFNC when used in the correct setting. Plenty of data to support that. But this system uses flow rather than pressure and we are comparing apples to oranges. The three articles are all FREE! Links below.

Addendum: tonight is 9/24/19 and it’s 4:34 in the am. I am currently working a night shift. I have run into additional data that has provided me with some perspective as to the whole PEEP/Paw discussion. Parke performed a study that was published in 2015 using Electrical Impedance Tomography where there was a marked improvement in the end-expiratory lung volumes. Then Frat, the main author of the FLORALI trial, commented on the mechanism of how this happens by stating that the large nasal prongs create a resistance to the exhaled air by continuously pushing high flow air and in turn this causes positive pressure. The issue lies when the patient opens their mouth. This could be highly variable. Anyway, I still take issue with the numerical measurement of it. CLICK HERE for more articles on high flow.

Citations looking at high flow and PEEP

Parke R, McGuinness S, Eccleston M. Nasal high-flow therapy delivers low level positive airway pressure. Br J Anaesth. 2009 Dec;103(6):886-90. doi: 10.1093/bja/aep280. Epub 2009 Oct 20. PMID: 19846404; PMCID: PMC2777940.
Link to Article
Link to FULL FREE PDF

Corley A, Caruana LR, Barnett AG, Tronstad O, Fraser JF. Oxygen delivery through high-flow nasal cannulae increase end-expiratory lung volume and reduce respiratory rate in post-cardiac surgical patients. Br J Anaesth. 2011 Dec;107(6):998-1004. doi: 10.1093/bja/aer265. Epub 2011 Sep 9. PMID: 21908497.
Link to Article
Link to FULL FREE PDF

Parke RL, McGuinness SP. Pressures delivered by nasal high flow oxygen during all phases of the respiratory cycle. Respir Care. 2013 Oct;58(10):1621-4. doi: 10.4187/respcare.02358. Epub 2013 Mar 19. PMID: 23513246.
Link to Article
Link to FULL FREE PDF

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