The difficult airway is one where an airway expert has trouble achieving bag-mask ventilation or tracheal intubation. On even coarse inspection of the patient one can identify that their intubation is going to be difficult. The clinical features which one might use to make such an assessment are numerous and can be broadly summarised as “bearded, fat and without a neck”. In the ICU the intubations are frequently performed under what one might describe as “FUBAR conditions”, and so there is an additional non-anatomical layer of difficulty which consists of environmental and situational factors.
Historically, the college has only ever brought this up in early papers: Question 11 from the first paper of 2006 and the identical Question 3 from the second paper of 2002. The question put to the candidates was “outline how you would assess a patient for potential difficulty with endotracheal intubation”. This could have been worded better. Perhaps to ask “what clinical features can be used to identify a difficult larynoscopy”, or “what factors might make endotracheal intubation difficult for an ICU patient”, or “describe your approach to assessment of the airway, focusing on features which predict a difficult intubation”. Something like this might easily appear in some sort of cruel procedure viva, where the examiners segue into a discussion of intubating LMAs or CICO scenarios.
As far as important literature goes, the statement from the American Society of Anaesthesiologists Task Force (Apfelbaum et al, 2013) was my primary resource for this summary. Additional information was also derived from the extensive 2005 review by Gupta et al, which has an incredibly high information density.
Definition of the difficult airway
What does difficult mean? Some of us might find it difficult to intubate any patient whatsoever. The difficulty might have nothing to do with the patient’s airway, and be completely due to prevailing circumstances (eg. with the patient being crumpled into a crouching position behind the wheel of a smashed car). If one wished to sound clever, one might find the following quote from Chris Nickson suitable: “The difficult airway’ represents a complex interaction between patient factors, the clinical setting, and the skills and preferences of the practitioner”. This is also the statement from the American Society of Anaesthesiologists Task Force (Apfelbaum et al, 2013). The Task Force offers a prose definition of a difficult airway, which has been half-digested and regurgitated into point form:
The airway is difficult IF:
- It is difficult to bag-mask ventilate
- i.e. no seal, excessive leak, or excessive resistance due to obstruction
- It is difficult to intubate:
- None of the vocal cords are visible (Grade IV direct laryngoscopy)
- Multiple attempts are required for intubation
- The operator is a “conventionally trained anesthesiologist”
The 2013 ASA definition does not specify timeframes or the precise definition of what “difficult” means, but the old 2003 definition suggested that “difficult” means more than three attempts, or the whole process taking longer than ten minutes. Failure to bag-mask ventilate was previously defined as the inability of said “conventionally trained anesthesiologist” to maintain an oxygen saturation of over 92% while unassisted, i.e. without resorting to the use of the two-man technique.
The ASA might give the air of an authoritative resource for definitions, but in actual fact this definition is not widely accepted. Or rather, there are a whole range of similar-sounding variations. For instance, Williams et al (from his 2015 book, “Emergency Airway Management“ describes difficult airway in terms of “more than two attempts using the same blade” or the use of a bougie, or the use of any other sort of device after an initial failed intubation attempt. Huitink and Bouwman (2015) go even further to suggest that in fact there is no such thing as a “difficult airway” and that no predictive assessment is possible, given the variability in expertise among “conventionally trained anesthesiologists”, among equipment and among clinical scenarios.
Standards of airway assessment
Nørskov et al (2015) have recently published a retrospective survey of difficult airways. From 188,064 cases onl 3391 were “difficult” by the ASA definition. Of these, 93% were unanticipated. This is a scary figure. On a more encouraging note, of the airways which were anticipated as difficult, only 25% were actually difficult (presumably because extra work went in to preparing for a smooth intubation attempt). From this, one might conclude that preoperative airway assessments are not being done by lazy anaesthetists. Alternatively, one might conclude that the routine airway assessment is grossly unreliable in predicting difficult intubation, and ninety percent of the time it will spring on you unannounced, no matter which precautions you have taken. In this spirit of pointlessness, the following is a list of these unreliable clinical and historical features predictive of difficult intubation.
Firstly, an “anaesthetic history” should be fairly informative, as one would probably find a previous operation report where the anaesthetist, hands shaking from the experience, documented their harrowing ordeal. Thus:
- History of difficult intubation in previous attempts
- History of airway-altering changes since the last anaesthetic, eg. significant weight gain, C-spine surgery, head and neck radiotherapy, etc.
The following preoperative patient characteristics were associated with a difficult laryngoscopy or intubation:
- Obstructive sleep apnea
- History of snoring
Specific pathologies associated with difficult intubation, which may be available as history:
- Recent intubation (oedema, trauma, etc)
- Airway trauma
- Airway infection
- Mediastinal mass, eg. retrosternal goitre
- Ankylosing spondylitis
- Degenerative arthritis (i.e. of the C-spine or jaw)
- Subglottic stenosis
- Lingual hypertrophy (i.e. big fat tongue)
- Syndromic appearance:
- Treacher-Collins syndrome: Auricular and ocular defects, malar and mandibular hypoplasia
- Pierre Robin syndrome: micrognathia, macroglossia, cleft soft palate
- Down syndrome: small mouth; macroglossia
- Goldenar’s syndrome: malar and mandibular hypoplasia
- Kippel-Feil syndrome: congenital C-spine fusion
That’s obviously not an exhaustive list, but it is a handful of easily recalled causes which one might wish to keep up their sleeve for that time someone asks “what medical conditions are associated with a difficult intubation?”
There are probably hundreds of features which have been found to be associated with difficult intubation. It would be impossible (and counterproductive) to list them all here. Surely, if some sort of rare tropical parasite laid its eggs in your posterior pharynx, that would influence your laryngoscopy grade- but the college is unlikely to award a large amount of marks for mentioning this. Thus, there is a list of commonly seen features which predict difficult intubation:
- Level of consciousness (i.e. is the patient cooperative enough for an awake intubation?)
- Level of comfort (i.e. can the patient be positioned properly, or are they to short of breath?)
- Pregnancy (makes everything difficult)
- Syndromic appearance
Mouth, face and jaw
- Long upper incisors (“buck teeth”)
- No teeth (edentulous patients are easier to intubated, but harder to bag-mask ventilate)
- Prominent overbite
- Inability to “prognath”, i.e to voluntarily protrude the mandible
- Small mouth opening (3cm is the minimum to comfortably admit a laryngoscope blade).
- Mallampati score more than 2 (i.e. a barely visible uvula)
- Arched or narrow palate
- Compliance of the mandible and mandibular space (i.e. is it possible to manipulate it, or is it relatively fixed by some sort of disease process, eg. a submandibular abscess)
- Patency of nares: polyps, deviated septum etc.
Neck and posture
- Thyromental distance (“three ordinary finger breadths”, or 6cm)
- Mandibulo-hyoid distance of less than 4cm
- Sternomental distance of less than 12cm
- Thick short neck
- Restricted range of neck motion
The LEMON screening tool to predict difficult laryngoscopy
LEMON is a nice short way of remembering what to look for. It certainly seems to be reliable in predicting difficult intubation. I used this tool to create a easily memorable answer to the CICM SAQs for difficult intubation. It is also favoured with a LITFL entry, which is always a sign of high quality.
- Does the patient look like the stereotypical difficult intubation?
Evaluate: 3:3:2 rule
- 3 fingers width of mouth opening
- 3 fingers width of thyromental distance (from the thyroid cartilage to the mental process of the mandible, colloquially referred to as the chin)
- 2 fingers width of distance from the hyoid to the thyroid
- amount of pharynx which can be seen by opening the mouth
Obesity and obstruction
- Is the patient morbidly obese?
- is there some sort of obstruction, eg abscess?
- this determines how easy it will be to align the planes
Investigation of the difficult airway
There may be little relevance to most of these, as far as their prediction of a difficult airway is concerned. Perhaps in the pre-operative clinic the anaesthetist can afford to send some of the patients to have a neck CT or nasal endoscopy, and then make up some sort of clever-sounding plan for their anaesthetic. In the ICU, few intubations are totally elective, and therefore the value of investigations is largely for determining extubation readiness, particularly if the patient has had some sort of anatomical airway problem. The classical example of this is a submandibular abscess with no cuff leak.
Lateral neck Xrays in the neutral position can be used to make a series of measurements which predict intubation difficulty. Specific measurements include:
- mandibular length, from incisor to the TMJ
- Posterior mandibular depth, measured from the bony alveolus immediately behind the 3rd molar tooth to the lower border of the mandible
- Atlanto-occipital gap (the major factor which determines the limits of extension of the head) – seen as the space between C1 and the occiput
Nobody does lateral spine Xrays anymore, and so these parameters have fallen into disuse.
CT of the airway
This again is more related to planning of airway access prior to oral and maxillofacial surgery, particularly where nasal intubation access may be difficult (Grimes et al, 2014). The airway CT can be informative for many different reasons. Situations where it offers an advantages are listed below:
- Prediction of difficult nasal intubation (imaging of abnormal turbinate anatomy, for example)
- Assessment of subglottic or laryngeal stenosis (if you already know it exists)
- Surgical airway planning (eg. to determine the position of the thyroid isthmus, or to assess tracheomalacia)
Excellent pictures of subglottic stenosis and lung tumours can be found in the article by Boiselle et al (2002). Even with this relatively primitive pre-iPhone technology the multiplanar CT made possible a practice of "virtual bronchoscopy". In the modern era, fairly accurate virtual nasendoscopy can be performed, which has been used with good effect for real-life situations ( Ahmad et al, 2015). However, this practice - though it is non-invasive - it still requires radiation exposure, which is not ideal. It is therefore far from routine, and its efficacy is thus far represented only by case reports.
This technique has advantages in those situations where the patient is too uncooperative for an awake intubation. For instance, Nagamine et al (2007) report using 3D images of the airway of a young girl with Treacher Collins syndrome. The patient had unusual lopsided airway axis deviation, which was only identified on CT and which prompted some changes in head positioning, resulting in a relatively straightforward bronchoscopic intubation.