Well that was a bit of a gap. A quick post on the return about something we all know we know but could do with unknowing – simple stuff on the paediatric airway. 

I don’t think my computer embeds a knowledge of anything. Or my phone. Everything is up for updating. I know because all too frequently it reminds me it’s time for some sort of update to patch this or patch that, or share all my data with everyone, always.

My brain meanwhile is quite a bit more stubborn. It seems a little bit contrary that we know that every single time we access a memory we rewrite it, and yet updating a fact is sometimes very hard to do. Facts we spent long hours embedding are incredibly hard to update.

Just take the paediatric airway and that accursed conical shape.

The Story We Knew We Knew

The version of the story I was always told was that the epiglottis is a bit like the letter “U”, the epiglottis is a little bit up there instead of straight ahead and the airway would accommodate a cone.  The common version of this holds that the cricoid ring is the narrowest part of a little person’s airway and all our focus relating to tubes and leaks and the airway should be set upon this.

The problem with this it appears to date back to a bit of research that could probably do with a refresh. That story didn’t start some time in the 80s. Or the 70s. Or even the 20s. It was actually in the 90s. The 1800s version.

I’ve written a bit on this before but the origin of the cone story comes from a guy with some plaster to pour and a small series of cadaveric airways. The problem being that when you isolate the airway from everything that supported it and pour some plaster into it that might not be the most accurate way of representing the actual dimensions of a non-stretched airway.

Well, I say non-stretched but of course there’s one bit of the airway that the plaster won’t stretch. The cricoid ring. That’s the bit that is fixed.

In fact that original paper which informed later work included only 15 specimens with age ranges from 4 months-14 years. Interestingly all of them shared that narrowest part of the airway being the cricoid feature. That plaster really stretches things up I guess.

Newer Tech

Now in 1897 plaster might have been pretty cutting edge. The thing is we should be able to do better now. Technology has sort of progressed a bit. TV comes in colour. Wait I mean TV is a thing that exists. Actually TV is pretty much obsolete for younger kids and you can buy self-lacing shoes from Nike.

Anyway, technology improves.

So now we have a range of different ways of trying to assess the airway in kids who have an airway that is still with them. Wani et al. have published something just this month in Pediatric Anesthesia which looks at this.

The short version is that they obtained CT scans of kids under 1 and looked at the dimensions of the airway. So if the cone theory holds this would demonstrate a progressive narrowing of the airway to the level of the cricoid, right?

Well, no

It doesn’t show that because it turns out that the airway of these little people isn’t that different to the slightly older kids Wani et al. published on last time (although they had the chance to include kids down to 1 month in that study the average patient was around 2 years old and they could be up to 10 years old and still manage to get their scans looked at).

When you look at the airway in a couple of dimensions, from front-to-back (known henceforth as “anteroposterior” or “AP”) and transverse (which will mysteriously be labelled “transverse”  from this point forward), there is a distinct difference:

• The narrowest part of the airway in this study was the transverse measurement at the subglottic region.
• The AP dimension is consistently bigger than the transverse one because the airway is elliptical.
• The ellipse does tend towards being more circular at the level of the cricoid ring (because the AP measurement decreases a little from the subglottic area to the cricoid ring) but the transverse measurement at the cricoid ring still exceeds that subglottic area.

Keep in mind that they aren’t looking at the actual narrowest bit of the airway here (I suspect their image slices of 2 mm wouldn’t have let them look in that much detail, but the narrowest part of the airway is really the vocal cords.

When it comes to the subglottic area measurements are we sure they are talking sense? Well the last time they looked in older kids they came to a similar conclusion. The airway in that stretch is an elliptical shape and the cricoid ring is not narrower than the subglottic region.

1897 turns out to have been a misleading time for learning things about kids’ airways. You can only use what you have at the time I guess.

Things to Consider

So have you ever been boxed around the ears with cricoid facts? Or even just been gently told that the resistance you feel to passing an endotracheal tube is just at that point where you’ve got to the cricoid ring.

Then it’s time for an update. You don’t even have to wait for the download and do a restart. Adjusting a couple of thoughts can’t be that tough, right?

There are a few points to consider during that reboot I think, some of which are mentioned in the paper itself:

1. The Ellipse Might Matter More Than You Think At First

This point is well made in the paper. Cast your mind to the last time you were told that uncuffed tubes were the bees knees. If bees had knees that were plastic tubes of different diameters I guess.

Remember that mystical ability to evaluate the appropriate-sized leak. Well if you now consider that the AP diameter of the airway at the subglottic level is actually a pretty decent size compared to that transverse span (about 25% bigger) then at that point where you hear a leak how do you know you’re not still up against the walls of the trachea at the subglottic level? I don’t know that you can and that surely runs the risk of the lateral airway wall putting up with lots of rough treatment.

That Cuff Thing

“Who cares?” you might well say. “I use cuffed tubes.” And plenty of people do just use cuffed tubes which is probably excellent. Some will even do that old trick where you inflate the cuff just until you hear the leak stop.

Well what this study should also remind us is that using a cuffed tube requires really careful consideration of what to do with that cuff. Certainly it suggests you really have to use a pressure manometer to establish that you’re not over inflating. If you’ll still have a leak with the cuff at appropriate pressures it might just be that you can still have a leak via those AP windows. Inflate a little more to stop that and you’ve now made that lateral wall pressure much higher.

We might have to make more of a point of prioritising adequate oxygenation and ventilation rather than degree of leak alone. So if there’s a small leak and everything else looks good, pursuing a completely leak-free set up (at least with the same sized tube) may just increase the risk that you’ll be abusing that delicate little trachea just to chase the silence.

3. The Cricoid Still Matters

It’s not just the airway anatomy version of letting all concerned parties down gently that should make us consider the cricoid ring. Even if it is more cylindrical and not the narrowest stretch, the cricoid is still a fixed ring. If you cause problems at that level that might induce swelling, where does that end up? It can’t swell easily into the surrounding tissues. It can swell into the lumen.  Cause swelling in that spot and your resistance to airflow becomes a much bigger issue.

The Bigger Picture

Now you could propose an argument along the lines of

“The exact level that it’s narrow doesn’t matter. As long as I pay attention to sizing and I’m careful to use the cuff right, the effect is the same.”

And I can sort of see that line of reasoning.

At the same time I think our care of patients should probably be based on the most up to date understanding available on the basics like anatomy. It is worth understanding that the first bit of resistance you are likely to note as the tube passes is at the subglottic region. If the main area of irritation of the airway around extubation has been at that subglottic region then maybe the role of CPAP is more evident. That leak you can hear doesn’t mean you’re not up against the mucosa.

More importantly understanding the evolution of our knowledge of the anatomy in this spot and in these patients is an excellent reminder that we should be wary of treating the facts we’ve always been taught as monuments. We should definitely stop teaching trainees things that aren’t backed up by current research. As techniques to explore seemingly simple questions get better some of our fixed concepts need to have the pin taken out of them so we can look again.

There’s a pretty fair chance that Kids Airway V 2.0 still has a few updates left in it yet.

Notes:

That picture of the iron rings was put in the Creative Commons bit of flickr by Ian Burt and appears unaltered.

Oh, and if this sort of stuff interests you there is a bit somewhere around here where you can put your email so you’ll get an update each time a post turns up. Or share it around I guess.

Obviously it’s worth going to the source literature and doing more reading around.

The paper that brought this topic up is here:

Wani TM, et al. Upper airway in infants – a computed tomography-based analysis. Pediatr. Anesth. 2017;27:501-5.

The one in older kids is this one:

Wani TM, et al. Age-based analysis of pediatric upper airway dimensions using computed tomography imaging. Pediatr Pulmonol. 2016; 51:267-71. 

A broader review of paeds airway anatomy updated with modern stuff can be found here (I really find this a good read):

Tobias JD. Pediatric airway anatomy may not be what we thought: implications for clinical practice and use of cuffed endotracheal tubes. 201525:9-19.