Airway assessment

Author: Keith Greenland

Hi, My name's Dr Keith Greenland I'm a senior staff specialist in the Department of Anaesthesia and Perioperative Medicine at the Royal Brisbane and Women's Hospital. I'm also Associate Professor at the Hong Kong University in the Department of Anaesthesiology. This talk is actually about a structured and logical approach to the perioperative airway assessment of patients having anaesthesia. Before we actually get onto that approach one needs to get back to look at the work that's already been done by the college American College of Anaesthesiologists and particularly Johnathan Benumof in the states and they highlighted a number of tests that we commonly do.

These are just examples of those for instance voluntary jaw thrust, anterior protusion of the mandibular teeth in front of the maxillary teeth, in a way simulating what happens with directly laryngoscopy; mouth opening, to ensure adequate function of the TMJ joints and the endpoint there being at least three centimetres usually correlates with good laryngoscopy; thyromental distance the distance being, dependent on which literature you might be reading somewhere between 5 and 7 centimetres or three normal ordinary sized finger breadths submandibular space compliance looking at the compliance of the submandibular space and how it actually will perform during direct laryngoscopy. This is particularly relevant for patients who for instance who've got Ludwig's angina where there is infection of the tissues or the patients who've had radiotherapy to the submandibular space for laryngeal Ca (cancer). Both those cases there is a failure of the tissues to move normally during laryngoscopy and therefore leading to difficult laryngoscopy. Mallampati Mallampati I and II correlating with Cormack and Lehane I and II generally III and IV Malampati, corresponding with C+L III and IV C+L III and IV narrowness of the palate leading to problem with viewing of the cords during laryngoscopy length of the neck again a qualitative assessment really where there is no quantitative index available thickness of the neck, the short neck, brawny patients now we have semiquantitative measurement somewhere over 42 centimetres will indicate probably difficulty in laryngoscopy and then finally, range of movement of the neck generally whether it can achieve the sniffing position which we all use during laryngoscopy if it's available. The problem is I think nowadays that we have a problem with understanding the fact that laryngoscopy assessment is not a simple procedure like turning on a light switch.

It's not like testing your anaesthetic machine, if it works it works, if it fails it fails. Direct laryngoscopy, to predict whether it's difficult or easy is not that straightforward. One of the best articles on this was an editorial written by Steve Yentis in the UK and what Steve looked at was in this editorial was predicting the difficult intubation whether it was a worthwhile exercise or a pointless ritual. Now Steve looked at it in as far as if there was a test for instance A to B which defined whether the intubation was going to be easy or difficult it would be fine if it was like turning on a light switch, but it's not there is an overlap fundamental to that, leads to the fact that what is easy for one person is not easy for another so even the terms of what is difficult airway difficult laryngoscopy difficult intubation difficult bag mask, these are all very relative terms and so they don't lend itself to a simple or even a series of simple tests which you can do at a bedside. So therefore there is an overlap on top of that is the fact that if you have for instance under the bell shaped curve, the large one here of normal laryngoscopy and you overlap the difficult laryngoscopy and if you happen to have a test, for simplicity, A3 to B3 that will pick up every difficult laryngoscopy and difficult intubation then you're going to have alot which will be not difficult. Now this is probably appropriate one needs to actually make sure if you can, to pickup one hundred percent of the difficult patients and if it's not difficult well there is not a great deal lost the main worry is actually not picking up the difficult airway and being unprepared for it. So he does point out the fact that in essence the assessment of laryngoscopy and intubation preoperatively is fraught with problems, but in his conclusion what he did say was if I can quote, "I dare to suggest that attempting to predict difficult intubation is unlikely to be useful - does that mean one shouldn't do it at all? To this I say no, for there is another important benefit of this ritual: it forces the anaesthetist at least to think about the airway", and that really is the function of this talk, is to look at the assessment process not focusing on one particular test or even one series of tests but really taking a bird's eye view of airway assessment and better understanding what you're doing every day with normal laryngoscopy with a view towards understanding when things go wrong, why it's going wrong.

To that end what I propose is actually breaking up the process of laryngoscopy and intubation and looking at three anatomical columns. Posterior column which centres on the cervical spine Anterior column is the mandible and submandibular space and the final column is the airway passage itself. Therefore, an assessment of an airway prior to intubation and laryngoscopy needs to actually touch on all three columns to be fully effective. In another way we can say that the posterior column's, static position of the head and neck what I'm saying there is that the position of the head and neck is positioned, whether in the sniffing position or if it's in the neutral position, no pillow with a vertical gaze, as in manual in-line stabilisation of the neck during trauma management or even hyperextension of the head and neck during suspension laryngoscopy that ENT surgeons perform when they're doing microlaryngoscopy. So the static positioning is, really focuses around the function of the cervical spine.

Airway assessment

The Anterior column is what I call the "dynamic process" so that's after you've positioned the patient in the appropriate position and you put a laryngoscope in and lift up the mandible, submandibular space and compress these tissues up out of the way to hopefully provide a line of sight and finally there is the Middle column which is the airway passage itself. As I said, the cervical spine, the posterior column, the function really focuses on the ability to extend the occipito-atlanto-axial complex particularly from an anaesthetist's point-of-view where we stand behind the patient generally during laryngoscopy the extension of that joint is critical. The Anterior column, the dynamic process really as I said before, focuses on the mandible and submandibular space. Now if I look at that in a little bit more detail the issues here revolve around four. First of all volume the bony landmarks of this anterior column is the mandible itself. Now if the volume is small when you are lifting up the mandible and compressing the submandibular tissues up out of the way, if you have a retrognathia or micrognathia, small jaw and you haven't got enough volume to compress the tissues out of the way and therefore leading too difficult laryngoscopy.

The next component is if there is a poor compliance of those tissues. As mentioned before radiotherapy to the submandibular space from a laryngeal Ca (Cancer) or infection of the submandibular space as in Ludwig's angina these conditions lead to poor compliance of the tissues so when during the dynamic process of lifting up the laryngoscope blade you can't compress the tissues up out of the way and again you get a failure of good laryngoscopy and intubation. Apart from the actual lifting up the jaw they can be actually tethered at the TMJs (temporomandibular joints). So if you have TMJ dysfunction from rheumatoid or other ankylosing conditions then the anterior complex, what I call the "Anterior complex" is tethered backwards and fails to move forwards and you fail to get a good view. The other condition is calcification of the stylohyoid ligament if there is bilateral calcification of this ligament then again you have tethering of the anterior complex. This is a rare condition it's only written up in a few letters in the literature and therefore probably, it needs further investigation in that regard. More commonly than anything else it is the low volume states and the low compliance states that cause problems on a frequent basis to the anaesthetist. So let's focus on particularly the low volume states initially This is what I call the "Anterior complex" this is the bony landmarks the mandible and the laryngeal apparatus below that, and above it the maxillary teeth.

So during the dynamic process of laryngoscopy we put a laryngoscope into the mouth and you lift the mandible upwards and as I said you lift the tissues, the submandibular tissues up and compress them up out of the way. Now to assess any three dimensional object you need to look at all three dimensions it is not enough to look at one dimension and think that you understand that particular object. If you need a "line of sight" then we need to assess first of all the incisor to hyoid distance now that corresponds very nicely to the thyromental distance that we do every day. The TMJ to incisor distance corresponds very nicely with the length of the jaw and the TMJ to TMJ corresponds to the narrowness of the palate.

So having performed this assessment the three dimensions of this inverted triangular pyramid then we've got a better idea of the volume state of the anterior complex, however there is what I call absolute and relative retrognathia. If you have a patient with a micrognathia or retrognathia, a small jaw that's an absolute reduction in volume however if you've got a patient with buck teeth prominent maxillary teeth than what you need to do even if the jaw is normal is that you need to prognath that jaw, the mandible further forward than you normally have to, to overcome the obstruction of the buck teeth. So despite the fact that you have a normal volume in the population you have a relative retrognathic condition because you need to actually do more work to get a view.

Also the problem may occur if you have a large amount of tissue in that if you have a normal jaw you have still a relative retrognathia or micrognathia as in acromegalic who is got a large tongue then the volume may be normal in the population but it's still is small because the tissues to be compressed is of a large volume. So to look at this three dimensional object a short mandible retrognathic is a patient who is going to have problems with their airway. A short incisor to hyoid distance often correlating, actually many times with micrognathia as well leads to problems and finally the narrow palate where there is inadequate volume from that will cause a problem for the anaesthetist If we look at the compliance, the low compliance state now so we're looking at patients who've got normal volume but the tissues that have got to be compressed are of low compliance. This gentleman's had a biopsy of a lesion of his tongue and he had to return to theater with a hemorrhage into his tongue his tongue's now very low compliance state and laryngoscopy here is going to be quite difficult other conditions as I said radiotherapy to the submandibular space and Ludwig's angina There the tissues are non-compliant, normal volume but low compliance the TMJ dysfunction limited mouth opening this is going to cause problems with laryngoscopy as well as the calcification of the stylohoid ligament note in both these cases you've got tethering of the movement of that inverted triangular pyramid very rare conditions as I said before, the low volume state or the low compliance state much more common but I mention these two for completeness.

Let's look at the Posterior column, Static phase now this part of the talk is actually another part is involved with a talk I'll be giving later but just to briefly look at this static positioning. This patient's affected here by ankylosing spondylitis patient's with manual in-line neck stabilisation following trauma these patients are from the anaesthetists point-of-view are poorly positioned We talk about three axis alignment theory to look at the position of the patient that's the "oral axis", "pharyngeal axis" and "laryngeal axis". As I said we'll be talking about that in another talk but what I'd like to talk about is actually Two-curves you got the "Primary curve", the oropharyngeal curve and the "Secondary curve" which is really the pharyngo glotto tracheal curve and you have a "line of sight". Now this position is an MRI, in fact of myself with the airway configuration overlayed on it. Now this position is actually quite relevant to the anaesthetist because there is no pillow here and it's a vertical gaze, this actually corresponds with manual in-line neck stabilisation.

What's important, if you're going to draw a straight line is this line. This is a line immediately above the glottis, the supraglottic axis if you will and that is, in the manual in-line neck stabilisation or neutral position, is upsloping. So that when you put a laryngoscope in and flatten the oropharyngeal or primary curve you often have still a grade III intubation the glottis is above the epiglottis therefore you need for instance a bougie to overcome that last part of that line and get successful intubation or you need external laryngeal pressure to swing that part of the axis downwards so you get a view. When we do head elevation this is not the sniffing position yet this is head elevation, vertical gaze we find that actually there is flattening of the secondary curve and the last past of the primary curve.

So your vestibule or the supraglottic axis is now horizontal, if not starting to become down sloping so you now have a successful laryngoscopy. Finally you get extension of the head after a head lift the sniffing position this now actually causes flattening of both curves. Not complete flattening, you need a laryngoscope in the mouth to get final flattening of the primary curve to get intubation. But again you have a vestibule that's either horizontal or down sloping so you have successful laryngoscopy. The hyper-extended position I include here because ENT surgeons regularly perform laryngoscopy in this position during suspension laryngoscopy the head is extended back there is no head elevation in fact there's often a bolster underneath the shoulders to improve the extension. In this situation you have a very flat line-of-sight, hence the ENT surgeons sit down when they do there suspension laryngoscopy.

But you have still an upsloping vestibule, upsloping vestibule axis, therefore they require external laryngeal pressure to get a view of the glottis. But really what is important here is the assessment whatever position you do, is the assessment particularly of the occipito-atlanto-axial complex and the distance between the spine of the first cervical vertebrae and the occiput or the distance between the spines of the first and second. This is where the extension is very critical and this is why this particular complex is important in the overall assessment of the cervical spine. Just to take a point here, you see with all this talk about the different types of positioning and dynamic movement of laryngoscopy, terms such as anterior larynx really become irrelevant because an anterior larynx says nothing about how you perform laryngoscopy or what was the solution. It is just simply a term that says, well it was difficult you can say that an anterior larynx is the same as a Cormack and Lehane III or IV it doesn't really help the next anaesthetist perform laryngoscopy because in the end anterior larynx could be due to a number of causes causes for instance on the left ankylosing spondylitis a posterior column problem or an anterior column problem of two people on the right a short thyromental distance or a short mandible or patient with low compliance of the submandibular space so these patients actually therefore all have anterior larynx and I'd rather people didn't used these sort of terms but actually when they do their assessment either preoperatively or obviously during the procedure their assessment needs to go beyond these terms to fully understand what the problem is. Finally the "Middle column", middle column is assessment of the airway passage. These conditions such as foreign body in the airway, epiglottitis, peri-laryngeal tumours.

Assessment of these this particular area of airway management really revolves around a good history and examination above all else. The patient's ability to walk around to actually communicate effectively to lie flat, whether they're stridorous whether orthopnoeic, these are all just clinical assessment and is very very important. Investigations that can be done are nasopharyngoscopy often done by ENT surgeons, but I promote it within my department as a way of my registrar's assessing the middle column. Nasopharyngoscopy can be done very easily just in the semi sitting position with a local local anesthetic in the back of the nose and you can assess the quality of the airway with a simple scope that goes no further then the back of the tongue. There are problems with it of course there is the issue of the patient, the patient's not anaesthetised the patients often sitting up so things are going to change when they are anaesthetised and lying flat but it does give the anaesthetist some information about say a perilaryngeal tumour that needs to be assessed on an ENT list. The other assessment is some sort of imaging, soft-tissue X-Rays, MRIs and CTs.

This again is often done in a patient who is not anaesthetised, that is done often with the patient in the supine position and again, like nasopharyngoscopy doesn't give you the complete picture it gives you a little bit more information but above all else the history and examination is critical. So if you're going to look at understanding the laryngoscopy assessment than I'd look at it as far as a model of direct laryngoscopy the static phase posterior column and really assessing the occipito atlanto axial complex the dynamic phase is really the anterior column the volume of the submandibular space looking at absolute reduction in that space micrognathic or retrognathic patients and looking at the three dimensions so, short incisor to hyoid distance short TMJ (temperomandibular joint)to incisor distance and short TMJ to TMJ. Relative reductions can occur with prominent or buck teeth or a large tongue.

Compliance of the submandibular space low compliance really remains qualitative assessment unfortunately. Range of movement of the TMJs and that looks at limited mouth opening and also poor mandibular protrusion in front of the maxillary teeth. Range of movement of the stylohyoid ligament here we've got reduced movement anteriorly.

The airway passage as I just mentioned, the middle column we're looking at airway problems basing it around the history and examination, both imaging and nasopharyngoscopy. So once we've performed all these tests well then we have a very good idea about the airway itself. All of these can't be done in a bedside test but if they are applied clinically to the appropriate scenario then they can be helpful but needs to, like Steve Yentis suggested, we need a bird's eye view and a structured logical approach to airway assessment rather than focusing on one or two tests that will actually probably let us down anyway. As Albert Einstein said, "Everything should be made as simple as possible but not simpler". I hope I've provided an overview of airway assessment which will provide a better structured and logical approach.

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