Is the sniffing position optimal for viewing the glottic opening?

Welcome back to Depth of Anesthesia. I'm David and I'm an anesthesia resident at the Massachusetts General Hospital. My guest today is Dr. Keith Baker. Dr. Baker is the current Vice Chairman of Education and was the previous residency program director here. Dr. Baker, welcome to the show.

Hi David, pleasure to be here. So for our case today, we're going to return to the patient that some listeners might be familiar with from our episode where we explore establishing mask ventilation before administering a paralytic. The patient is a 72-year-old man with a history of coronary artery disease, hypertension, obesity, and type 2 diabetes who's presenting for a laparoscopic umbilical hernia repair. He's 510 and 120 kg with a rather thick neck. On initial direct laryngoscopy by the resident, a grade 3 Cormac Lahane view is seen.

The resident re-establishes mask ventilation and removes the pillow that the patient's head is resting on to optimize the view of the courts. Dr. Baker, what do you think is the claim here? From what I'm hearing, it sounds like the concept is by removing a pillow or extending the patient's head that the resident will obtain a better view of the glottis. To our listeners, what is the level of your agreement with the claim that removing pillows or blankets will optimize your view of the glottic opening? And what is the level of evidence for what you believe?

We actually recently conducted a Twitter poll that asked how removing pillows or blankets would affect visualization. got about fifty-eight votes and forty percent voted that the view was improved and fifty five percent said the view was worsen This is pretty in line with what we saw at our recent MGH grant rounds. Dr. Baker presented this and we conducted a poll where we asked people about their level of agreement. At our institution, 44% of the respondents either strongly agreed or agreed, and 42% strongly disagreed or disagreed. Interesting findings. Well, it seems to be reflective of what happens in the operating room.

As you know, uh I spend many uh many days in the operating room with uh our trainees. And it is a very frequent finding that after a suboptimal glottic exposure during direct laryngoscopy, that the resident's first move is to begin to pull out blankets and pillows from beneath the patient's head in an effort to extend the patient's head further. And Dr. Baker, before we start reviewing some of the evidence, I think it'd be helpful to describe the sniffing position and whether that's the same as the position you would achieve. by removing pillows? Well the sniffing position itself is uh actually an old uh description of uh optimizing patients prior to direct laryngoscopy So it was promulgated by Chevalier Jackson, who was a famous thoracic surgeon.

His main interest was exposing the glottis for his surgical purposes. In short, there are two features that come together to form the snipping position. Firstly, is neck flexion, where essentially the head is brought well forward of the chest The second maneuver is to extend the head on the neck, which is to tilt the head back. If you imagine a person smelling something in the air, such as a freshly baked cookie or so, you know that someone's head goes forward and the nose tips up, and that's the combination of flexion and extension, sometimes called sniffing, though the sniffing position. after the famous sniffing of the morning air. I sense there's a claim related to this position.

Would it have anything to do with aligning the quote unquote axes? The initial descriptions of the three axes became quite well known to most uh trainees, myself included. And it made its uh entree into most textbooks, and you can read it there in almost any textbook about airway management, including those originating back in the eighties and earlier. In short, there are three axes that people concern themselves with, and the idea was that by appropriately flexing the head and extending the head on the neck, that one could align these axes and thereby take a look under direct visualization and see, with their eyes, the glottic opening. It turns out, however, that no matter what you do with the head, neck, and so forth, you can't actually align the three axes and see the glottis without some sort of distortion of tissues with m with a metallic blade or s at least if you want to look in a direct line of sight.

And I think that is reflected in this first study that we're going to be talking about. So this is a study called Study of the Sniffing Position by MRI. So this study is composed using eight normal adult humans. They are going to be awake, not anesthetized, and supine as one would be prior to the initiation of general anesthesia. They received an MRI of their airways under three conditions.

The positions are randomly assigned to three different positions. The first position is full head extension with no blankets or pillows beneath the head. This is the sort of position that many individuals end up in once a Laryngoscopus pulls the pillows or blankets from beneath the head and extends the head. The next position is neutral, where the head is flat on the surface but not extended. And the third position is the classic sniffing position, which has a 7 centimeter cushion under the head, which causes the neck neck to be flexed and the head to be extended So we go through a range of three positions, extended, neutral, and into snipping And the authors made a series of measurements correct on each MRI scan?

They did. For each scan in each individual, the three axes that are associated with the airway were measured. What were those axes? There were three axes. One was the axis of the mouth, which is along the hard palate.

The second was the pharyngeal axis. Which passes through the anterior portion of the atlas in C2. And the last was the laryngeal axis, which relates to the glottic exposure. Importantly, the line of vision is the straight line passing from the essentially the teeth to the posterior aspect of the laryngeal inlet. So it's can you see the larynx or not?

That's the line of vision. Got it, so what did they find? No matter how the volunteers' heads and necks were positioned, from extension to neutral to sniffing, there was no condition under which the three axes were aligned such that the line of visual visualization allowed exposure of the larynx. In other words, there's no way to bend the head and neck so that you can see the larynx at least not without a laryngoscope to displace tissue and expose the larynx. Alright, so for this study of the sniffing position by MRI, what's the punchline?

Well the punchline is that there's no way to align the three axes in a manner that allows you to see the larynx, at least not without a laryngoscope to displace tissue to expose it. In other words. The three axes don't show you the larynx when you align them because you can't actually align them. So now we'll move into a study that actually starts to look at some of the forces and positions required for direct laryngoscopic exposure of the vocal cords. This is one of the earliest studies and I know this is one of your favorites, Dr. Baker.

It's a favorite for a couple of reasons. Uh firstly it's was conducted by one of my colleagues, a surgeon here at the MGH. And importantly, he's a voice surgeon and his job is to operate on the larynx itself. And so he needs to get Superior exposure of the vocal cords in their entirety in order to operate on every element of the vocal apparatus. So the degrees to which he can expose the glottis are extraordinary and so I try and emulate many of the things that he does to ex to achieve the level of exposure that he does.

He really sets the bar high What did they actually look at in this study here? Well this study had three different elements. The element that's most germane to today's podcast relates to taking the patient and moving them through three different positions from extension to sniffing to flexed flexed That flex flex nomenclature relates to taking someone in the sniffing position who already has a flexed neck neck and then adding a flexed head on top of that So really it's from a sword swallowing extension all the way to head flexed up with the chin on the chest. It's quite a range of motion. And in this study, they graded the extent to which the vocal cords could be exposed as the person was put in extension, then to sniffing, and then into full flex -flexed position.

And this was one of the first studies that actually compared exposure to the glottis in different patients, right? It was. It the real element here was careful measurement about glottic exposure with very carefully controlled Positions of the head and neck for each of the patients. How do they rate the degree of vocal cord exposure? In this case, surgeons oftentimes use percentage of the Vocal cords that are exposed.

So 100% of the vocal cords means you can see the anterior commissure all the way down to the attachments at the retinoid processes Gotcha. So what were the findings of this study? Well there was another element to the study where they changed and used different width and size laryngoscope blades. In each case the laryngoscope blade was a straight blade, akin to the so called middler blade. The findings were blade independent, and in each case, more and more of the glottis was exposed as the patient was exposed in the extended to sniffing to flexed flexed position.

In other words, you could see more of the glottis in the flex flex than you could in the snipping, and more in the sniffing than you could in the extended position, irrespective of the laryngoscope blade that was chosen. Again, these are straight blades because that's the type of blade that laryngologists use when they perform surgery on patients. Alright, so what's the punchline for this study? Well I'd say in short, if you want to see more of the glottis than otherwise, use the flex-flex position to get maximum exposure of the vocal cords Sniffing position works pretty well, but least well, as far as exposure goes, is the extended head and extended neck position that is obtained by pulling pillows out from beneath the head of the patient So I think those studies are really interesting because I've you know in the year that I've been here I've really rarely seen people use the flex flex position.

What do you think are some of the barriers for using this actual position? Well there are two that I can think of. Firstly, most anesthesiologists don't even know to do it, so I think it's it's one of awareness in part. And secondly, it's pretty clear that as you go from extended to sniffing to flex flex, the amount of working room you have in the oral pharynx gets quite compressed. In other words, you don't have a lot of so-called working room in the mouth to install a breathing tube.

Oftentimes I'll see an Terrific exposure of the glottis and flex flex, but I don't have a lot of room to maneuver a tube into that glottis that I can see so well. And I think the actual process of attaining that view with the patient's head is a little bit difficult in itself because if you use your other hand, then there's nothing to hold the ET to. And I think there's a pretty striking photo in the study in which the patient's head is essentially resting on the laryngoscopist's belly Yeah, there are a couple different ways to achieve that flex flex arrangement. One is to lift the head up yourself with your free hand. The other, and you can see it in that photograph, is that the unique laryngoscope used by odolaryngologists and many surgeons is a Jackson intubating laryngoscope which has a very different arrangement and allows the uh I think exposure because of the torque arrangements of that blade versus a conventional Miller blade So I do think that there's a real mechanical advantage to the surgical laryngoscope versus the anesthesia laryngoscope.

Some people also ask an assistant to hold the head up while they are trying to laryngoscope the patient, and another group of a laryngologist or anesthesiologist will use their abdomen to keep the head in its position, which I find a little awkward, but some people have m have mastered. One of the few b benefits of a large gut, I suppose. Indeed. So let's move on now to our third study, and this study is titled Head Elevation Improves Laryngeal Exposure with Direct Laryngoscopy This study's handy for a couple of reasons. Firstly, it's done with patients, uh a large number, 167 of them actually.

Importantly, these are largely normal size patients who do not have known difficult airways. So you might consider them very typical patients coming for elective surgery. Also, the blade they chose for this study was a Mach III or a MAC IV at certain times, but essentially that's important because the prior study with the laryngologist was with a stray blade, so this is now with a conventional Macintosh blade. They had a couple exclusion parameters, correct? They did.

Of course, you couldn't have reflux, you couldn't be morbidly obese, you couldn't have poor dentition, or have a history of a difficult intubation, which really rules out a small subset of people that we know can be challenging How did the authors go about performing this study? They had three positions that were uh randomly assigned to the patient. One study was essentially a neutral position which had nothing underneath the head and the head had a slight extension. So this is akin to, I would say, probably neutral with a small extension. The second position was what's called sniffing position and that's where they elevated the head six centimeters with a little cushion underneath the head that was a measured six centimeters and there was a slight extension of the head.

So again There's some neck flexion, some slight head extension, and that is the sniffing position. And then they added a position called the elevated sniffing position, which had 10 centimeters uh a cushion beneath the occiput of the head again with slight head extension. So this is approaching the sniff sniff except they don't actually flex the head on the neck, they extend it slightly. So Essentially you're going from a flat position with the head against the table with no pillows or blankets and then elevating the head conventional sniffing and then further into this extended sniffing And they did these three different positions in every single patient, correct? And it was all in a random order?

That's right. So that's the best arrangement you could hope for. I thought it was interesting that they really went to great lengths to try to standardize this. They actually had a pre-study period of training where They essentially train these laryngoscopists to use the same amount of lifting force on the blade throughout the study just to try to minimize that as a variable Alright, so what were the results of this study? Well they they really mimicked the prior study uh with regard to exposure as the head goes from being uh flat to being elevated to being further elevated, which is to say that the extent of glottic exposure increased as the head was increasingly elevated Just like the prior study with the vocal surgeon or the laryngosc laryngologist found that when you go from extended To sniffing to flex flexed, you saw more and more glottic exposure.

This study showed the same thing in real patients with a Macintosh blade as an anesthesiologist might use. And so in fact the complete glottic exposure went from 14 % to 35 % to almost 45 % as the head was elevated from extended sniffing to extended sniffing. And what about the patients who constituted the difficult laryngoscopy group? They did find that in that small subset of people, and in all comers that's about 2% of people, who are difficult to expose a glottis. They found that as the patients were moved from extension extended to sniffing, that they could see a a decent view, at least something to intubate, in 74% of them In fact, two of them were even better visualized when they went to extended sniffing position.

So it did seem that there was a benefit From going from extended to sniffing and even extended sniffing in that particular subgroup of people. They also found that in no particular patient was the grade made worse as the patient went from extended to sniffing. to extended snipping. So it seemed to be no loss by doing this process. Gotcha. So what would you say is the punchline of this study?

Again, probably that if you want to expose the glottis, which is to say you want to see more of the vocal cords than you would otherwise, you're going to want to place the pizza the patient in at least the sniffing position, if not the extended sniffing position. They didn't actually measure intubation in this study, they measured visualization of the glottis. And so I think we get back to that point mentioned earlier that although you may see the glottis, If you have a smaller amount of working room, you may have some trouble putting a tube into the glottis, and you may need either a smaller tube or say a bougie to navigate the small working room that you may encounter.

For me and most people I'm familiar with, difficult intubation is synonymous with I can't see the glottis, I can't see the vocal cords Therefore, this concept of elevating the head and even further elevating the head beyond sniffing position really is an important element to getting a view that is what causes most of my difficult intubations. I rarely have a problem where I can see the cords and actually can't get a tube in after some little bit of rearranging of things. So really I think exposing the glottis is primarily the difficulty associated with difficult intubation. And so I think this this methodology of raising the head, going to sniffing and beyond, is probably in the s in the extended sniffing position or the flex flex position really is a way to solve a glodic exposure issue.

Very interesting. Now our next study looks at a very different population. It's in human cadavers And this is titled Head Elevated Laryngoscopy Position Improving Laryngeal Exposure by Increasing Head Elevation Yeah, this is a neat study for a couple of reasons. Firstly, uh as you mentioned, the patients were deceased. So they didn't need anesthesia.

In addition, the people who did the grading of the laryngoscopic view were blinded to the actual position of the patient Which is to say, these were all done with a videoscope attached to the head of the laryngoscopist, and then the videos were graded by an independent group who knew nothing about what was going on. In addition, the head position was measured carefully with protractors and so on. So the angles of the head were well known, and the elevation of the head above the bed was also very well known. Lastly, it's important to note that this study was also conducted with a straight blade laryngoscope. And the authors measured POGO, right?

Yes, the pogo or POGO simply means the percent of glottic opening which was exposed. So it's a curious term to denote percentage of the glottis seen. So if you see A score of 100, it means you see 100% of the glottis, which is from the anterior commissure, all the way back to the aretinoids. The entire glottic entrance is exposed So what did they find in these seven cadavers? Well, like the prior studies, they showed that as the head is raised from the bed or the table to higher and higher elevation beyond the table, which is to say going from extended or f going from neutral to sniffing to even higher or extended sniffing position. that the percent of the glottic opening that was exposed during laryngoscopy, it increased almost linearly from about twenty-five percent of the glottis being exposed when the patient was when their head was flat on the table to about eighty percent of the glottis being exposed when the head was fully elevated well off the table.

One thing the authors do mention is that They thought that fresh cadavers would likely approximate the mechanical response to laryngoscopy, but they did say that there could be unrecognized different responses to tongue compression or other aspects that could have affected their results. What do you think about that? I think that's perfectly uh reasonable and I say that because So I do think that the cadaveris can alter the the tissue distortion and so on in important ways. So for example, the voice surgeons I work with use some techniques that aren't available to most anesthesiologists. Firstly, our odolyngologists or voice surgeons suspend the patients with a thing called a suspension gallows, which is to say the patient's head and neck is literally suspended from the laryngoscope, which is to say that the weight of the head is entirely suspended by the laryngoscope, which causes significant tissue stretching, distortion and ultimately tearing in many patients.

So there is a the fossil pillows, for example, come under heavy tension, and as the head hangs there over the course of five minutes or so, those tissues will actually tear. Which allows pretty extraordinary glottic exposure of these patients and allows them to actually perform surgery on the anterior commissure with no difficulty at all You and I can't do that because we laryngoscope someone and place a tube within seconds, we don't hang them for minutes. All this is to say that cadavers have different stensibilities of their tissues. after they uh die for a variety of reasons and so it may alter what you see but I don't think that the head positioning itself would then change anything beyond that Gotcha.

So punchline for this study? Yet again, a straight blade and head-elevated uh head position seems to again expose more of the glottis and show you more of the vocal cords, if that's of interest to you. And again, I just go back to saying that uh most of the difficult intubations that I encounter really come down to seeing the glottis. And so if I want to see more glottis, Head elevated and extended head elevat head elevation seem to be something I would be wanting to uh achieve. Again, there can be times where you can see the glottis and the working room uh are diminished as you get into head elevated positions.

But at least you can see what you're doing and put at least a bougie into the airway uh and then put a tube over the bougie. Yeah, makes a lot of sense. Alright, so the last study we're going to be looking at is our largest study. So this is of 456 patients, and it's a randomized study where they compare this sniffing position with simple head extension in elective surgery patients. So this study was in fact a very representative study of patients coming for general surgery.

These were coming for orthopedic surgery, abdominal surgery, vascular surgery, and so on They were they had a total of four hundred and fifty-six patients in this study. Importantly, they were all orangoscoped with a MAC III blade And probably equally important, perhaps more importantly, these patients, although they were under anesthesia prior to laryngoscopy, they did not receive muscle relaxants for any of this during any of this exposure element of the study. They had topical anesthesia sprayed on them and they were intubated in that manner. And this was a prospective randomized crossover design where They have seven attending anesthesiologists participate in recruitment and induction, right? That's right.

So here you have a large number of patients, but a small number of anesthesiologists actually doing the exposure. So what was the actual process of randomization? They would take and essentially flip the the dice, uh they would flip the coin, so to speak, and the patients were either initially exposed or had laryngoscopy with the head in the neutral position meaning with the head flat on the table, or they were randomized to have the head first put in the sniffing position And then whatever position wasn't used, the secondarily the next position was chosen. Which is to say you were randomized either neutral, then sniffing, or sniffing, then neutral. And how do they assess glodic visualization?

In this case, they used uh the good old-fashioned Cormac and Lahane grading classification without without any external laryngeal manipulation. This is quite different than the POGO score or the other percentage scores where they're actually graded on a continuous scale from zero to a hundred percent. This is a classic one, one A and so forth. Gotcha So over the results of this study. All comers, for all patients, they did not find an important difference in glottic exposure between the head flat against the table in the neutral position, or in the sniffing position with their MAC3 blade.

However, in the subset of patients who were obese or who had limited neck extension, there was an advantage to the sniffing position. So you're saying that the distribution of Cormac Lahain grades was not different between the two groups. But when they did a subgroup analysis of those patients who had elevated BMI or limitations of head extension That did make a difference. That's correct. That's a good way to summarize it.

In short, on average, it doesn't matter Most people can be exposed to the point where an anesthesiologist can place a breathing tube. However, in the subset that provides a challenge to exposure, the sniffing position is the go-to move. Gotcha. Dr. Baker, would you be able to summarize some of the key points in the studies that we've explored today? So there are a couple of uh take-home messages here.

Firstly, we need to step way back from all of these studies. and recognized that each of them related to glottic exposure and not to actual intubation. So none of these studies addressed how easy it is to go from induction to intubation. They all really looked at induction to glottic exposure, which is to say showing you the vocal cords I've made the claim, or at least the inference, that most of my difficult innovations circulate around seeing the vocal cords. Given that that explains most of my difficult intubations, most of my concern relates to exposure of the glottis. Most of the studies have shown, whether you use a MAC 3 blade for some of the studies or straight blades in the other studies, that as you go from a head neutral or an extended position up into sniffing and then further beyond that with either extended sniffing or even flex flexed, that you're likely to see more of the vocal aperture as you do that process.

I also will tell you, but it's not shown in these studies, that oftentimes as you do that, the amount of working room you have in the oral pharynx or the mouth, it goes down, which is to say you can see the glottis, but there's not a lot of room to maneuver a tube into the uh trachea or into the glottis and then into the trachea. So when I have that limitation of working room My repair for that is to take a small profile item such as a bronchoscope or a bougie, install it into the tracheobronchial tree through the glottis that I can usually visualize and then put a small tube such as the 6O over that bougie into the tracheobronchial tree or into the trachea Another important finding is that for many patients, you can put the position of the patient in anything you'd like.

You can put them extended, you can turn their head to the left, you can turn their head to the right, you can do all sorts of things and still obtain a sufficient glottic exposure to get them intubated. So for the average person, I don't think it's critical how you start the the the the head position. However, when you get into difficult exposure circumstances, that's where I think these maneuvers such as head elevated, sniffing, flex flexed, and so on, I think that's where they pay dividends. Gotcha. I think that's a really important point you just made there.

And I think that's a great summary of some of the concept of concepts that we talked about today. So now that we've reviewed the evidence, let's return to our original claim. What is the level of your agreement with the claim that removing pillows will optimize your view of the glottic opening? And what is the level of evidence for what you believe? Dr. Baker, thanks for joining us today on the show.

We hope that this will spark some conversation for our listeners with your colleagues and really encourage you to review some of the studies that we reviewed today. Thank you for the opportunity to be on your show, David. As always, our show notes and references are available at Depth of Anesthesia. And please feel free to connect with us at depth of anesthesia at gmail. com or on Twitter. Thanks again everyone.

Stay hungry and keep asking questions.