I could be wrong, but I was under the impression that above 6L/min a nasal cannula starts to have turbulent rather than laminar flow which reduces its effectiveness. To be honest, I'd love to be backed up or shot down on this as I don't fully understand the rationale. Also, high flows through the small apeture of an NC are going to feel less than pleasant on an awake patient.
I agree that this is a poorly understood aspect of 02 treatment, with limited research (though most show (as well as my limited experience) that patients prefer this OVER other methods of NIPPV in terms of comfort). It's one of the reasons for the post.
What also spurred this on was a situation I experienced on call in the cardiac ICU.....
Had a pt on high flow (15L at "100% Fi02"). I saw her early p.m. and she was sitting there talking to her friend. The nasal cannula was kind of crooked and thus not performing a good seal. But, she was saturating fine and looking pretty good sitting there chatting it up, in no distress at all.
SOOOOO, I get a friggin page at 4 a.m. from the RT asking to place an order for an ABG (they should have just ordered the damn thing but whatever). PRIOR ABG's while this woman was on high flow always showed Fi02=100%.
So, I said, listen, if we're gonna do an ABG, let's put her on a non-rebreather at like 60% for 1-2 hours and then draw the gas..... I wanted to see where she would be with a more "accurate" method of Fi02, which could be reasonably relied upon in lieu of lackluster studies demonstrating the actual Fi02 of high flow NC's.....
Later, at around 5:30 a.m. I was approached by this bitchy nurse whom I've never even seen. She goes "so, I hear you have some new "theories" on weaning from high flow" or something very similar to that. I'm like, WTF? and told her what I thought.
I know this sounds petty, but part of it was lack of sleep and the fact that she approached me so antagonistically at 5:30 a.m., which to her was her 9-10th hour of work, but my 23rd in a row..... So, I argue with her that I keep seeing ABG's with patients on high flow, with the cannula all crooked and crap, whilst talking (thus mouth breathing) to people. Then, the ABG's are coming back with P02's of 60% while the ABG reads "100% Fi02".
So, I insisted that it was very unlikely that the patient was actually receiving 100%, and thus we shouldn't be too suprised to see lackluster P02's coming back on "100% 02..... She goes on to INSIST that this never happens and that they ALWAYS put the FLOW RATE on such ABG's..... Well, later I pull up all the recent ABG's on that patient and show her how they ALL said "Fi02=100%" for which she didn't apologize or even seem to acknowledge.
She then hands me THIS study (after I alleged that I've seen many instances of non-EBM being practiced in my 3 short months as a resident.... for which she asserted that they NEVER practice non-EBM.....
). (She didn't think it was funny when I responded "oh, EBM eh?" "you mean the way you've always done it here in the unit?")
Here's the study: Note n=10 healthy volunteers as well as the reference to OTHER studies showing either the same or MORE Fi02 with mouth closed.... (in fairness those were also small studies).
Delivered oxygen concentrations using low-flow and high-flow nasal cannulas.
Wettstein RB, Shelledy DC, Peters JI.
Department of Respiratory Care, The University of Texas Health Science Center at San Antonio, Mail Code 6248, 7703 Floyd Curl Drive, San Antonio TX 78229-3900, USA. [email protected]
Comment in:
Respir Care. 2005 May;50(5):594-5.
Abstract
INTRODUCTION: Nasal cannulas are commonly used to deliver oxygen in acute and chronic care settings; however, there are few data available on delivered fraction of inspired oxygen (F(IO(2))). The purposes of this study were to determine the delivered F(IO(2)) on human subjects using low-flow and high-flow nasal cannulas, and to determine the effects of mouth-closed and mouth-open breathing on F(IO(2)).
METHODS: We measured the pharyngeal F(IO(2)) delivered by adult nasal cannulas at 1-6 L/min and high-flow nasal cannulas at 6-15 L/min consecutively in 10 normal subjects. Oxygen was initiated at 1 L/min, with the subject at rest, followed by a period of rapid breathing. Gas samples were aspirated from a nasal catheter positioned with the tip behind the uvula. This process was repeated at each liter flow. Mean, standard deviation, and range were calculated at each liter flow. F(IO(2)) during mouth-open and mouth-closed breathing were compared using the dependent test for paired values, to determine if there were significant differences.
RESULTS: The mean resting F(IO(2)) ranged from 0.26-0.54 at 1-6 L/min to 0.54-0.75 at 6-15 L/min. During rapid breathing the mean F(IO(2)) ranged from 0.24-0.45 at 1-6 L/min to 0.49-0.72 at 6-15 L/min. The mean F(IO(2)) increased with increasing flow rates. The standard deviation (+/- 0.04-0.15) and range were large, and F(IO(2)) varied widely within and between subjects. F(IO(2)) during mouth-open breathing was significantly (p < 0.05) greater than that during mouth-closed breathing.
CONCLUSIONS: F(IO(2)) increased with increasing flow. Subjects who breathed with their mouths open attained a significantly higher F(IO(2)), compared to those who breathed with their mouths closed.
Again, when you read this study, read it carefully, and notice FIRST how even THIS study shows max Fi02 of 81% at 15L (far from 100%). But, note also the controversy between mouth closed versus mouth open breathing.
***I'll admit that the only valid point that this nurse made (aside from challenging me which pissed me off and thus spurred me to look at this issue carefully to the extent that I may seriously look at initiating my OWN study regarding this issue...) was that regardless of what VALUE of Fi02 associated to the ABG, the real UTILITY (she didn't speak in these terms but you know) was to TREND Sp02 and P02's. I'll grant that one.
cf
