The assessment of the patient's airway is an integral part of the pre-operative workup. Its purpose is topredict potential problems, allowing a management plan to be developed ahead of time and avoid an unanticipated difficult airway.
Basically, the aim is to predict and therefore plan ahead for potential problems in two areas:
- Laryngoscopy and intubation, and more importantly ...
- Ventilation, i.e. oxygenation, which means ease of ...
- Bag-mask ventilation
- Oxygenation with a supraglottic airway as a rescue technique
- Oxygenation with a infraglottic airway/ FONA as a last resort
Predicting difficulties with ventilation and oxygenation is more important than predicting difficulties with laryngoscopy. Failed laryngoscopy or intubation is not inherently dangerous if you can oxygenate your patient.
In addition to identifying relevant patient factors, preoperative airway assessment should also take into account thetype of surgery plannedand its impact on thetype of anesthesia required. This is particularly important in operations with a'shared airway'. Communicate with your surgeon!
Airway management problems continue to be the single most common cause of morbidity and mortality attributable to anesthesia! In addition, these are problems that you own exclusively as an anesthesia provider. There is very little blame sharing in avoidable airway management mishaps.
The Australian and New Zealand College of Anaesthetists (ANZCA) has put together an excellent document on preoperative airway assessment.
ANZCA suggests a list of 9 essential questions or "core considerations" which need to be adressed during preoperative airway assessment:
- Is there information about any previous airway difficulties?
- Is there any altered cardiorespiratory physiology?
- What is the impact of the surgery on the airway?
- How difficult will it be to bag-and mask ventilate?
- How difficult is it to place a supraglottic airway?
- How difficult will it be to intubate the patient?
- How difficult will it be to perform an infraglottic airway?
- What is the risk of aspiration?
- How easy will they be to extubate safely?
Like any kind of patient evaluation preoprative airway assessment can be divided into historyandexamination, supported byclinical tests.
The Australian VORTEX appraoch website has a fantastic page on preoperative airway assessment and planning. Do yourself a favor andcheck it out!
HISTORY
Previous airway management problems
"The best predictor of future behavior is past behavior."
Look atold anesthetic records, if they are available, to see if problems with airway management were documented.In practice this means finding comments on ease or difficulty of bag-mask ventilation, laryngoscopy view, and any special airway techniques or equipment used previously. You might even consider requesting records from a different hospital if airway management is expected to be particularly challenging and time allows.
Ask the patient!Although it is unlikely that a patient will be able to provide details about why airway management was difficult in the past, statements such as ' they we're struggling to get the tube down' or similar should obviously ring alarm bells.
Previous interventions/ surgeries/ therapies potentially affecting airway management
Find out if the patient has had head/ neck surgery before such as laryngeal surgery, neck dissection, facial reconstruction, tracheotomy or c-spine surgery. A particular large red flag is the history of radiotherapy to the neck!
Consider whether a patient had any of these interventions or treatmentssincethe last documented 'uneventful intubation'.
Aspiration risk
Certain patients are at an increased risk of regurgitation and aspiration of stomach contents such as food particles or gastric acid because of conditions such as GERD, hiatal hernia, obesity, autonomic neuropathy, non-npo status, etc. While these conditions do not affect the mechanical aspects of airway management per se, an increased aspiration risk might lead you to consider specific airway management techniques, e.g. a rapid sequence induction, which in themselves have certain (higher) risks.
Obstructive sleep apnea (OSA)
OSA has been identified as an independent risk factor for difficult bag-mask ventialtion and intubation. Ask your patient if they had a formal diagnosis of OSA, which requires a sleep study, and if they use home CPAP.
You are likely to come across patients who have not definitiely been diagnosed with OSA but in whom you strongly suspect it. A screening score such as the STOP-BANG assessment, developed by the University Health Network in Toronto, CA, is helpful in identifying likely OSA cases and guide overall risk stratification.
These are the questions for the STOP-BANG score:
- Do you Snore loudly (loud enough to be heard through closed doors or your bed-partner elbows you for snoring at night)?
- Do you often feel Tired, Fatigued, or Sleepy during the daytime (such as falling asleep during driving or talking to someone)?
- Has anyone Observed you Stop Breathing or Choking/Gasping during your sleep?
- Do you have or are being treated for High Blood Pressure?
- Body Mass Index more than 35 kg/m2?
- Age older than 50?
- Neck size large? (Measured around Adam's apple)
- Gender = Male?
Low Risk: Yes to 0 - 2 questions
Intermediate Risk: Yes to 3 - 4 questions
High Risk: Yes to 5 - 8 questions or Yes to 2 or more of 4 STOP questions + male gender
or Yes to 2 or more of 4 STOP questions + BMI > 35kg/m2
or Yes to 2 or more of 4 STOP questions + neck circumference 17 inches/ 43cm in male or 16 inches/ 41cm in female
Obesity
Obesity has several adverse effects on bag-mask ventilation and laryngoscopy, which are discussed in the page 'Special patient circumstances' - 'The obese patient'.
Altered cardio-respiratory physiology
Like an increased aspiration risk, cardio-respiratory disease does not affect the mechanical aspects of airway management per se. It might affect you ariway management plan though as you have to anticipate a range of issues such as reduced tolerance of apneic time during intubation or reduced tolerance of tachycardia/ hypertension as a sympathetic response to laryngoscoopy/ intubation.
EXAMINATION & CLINICAL TESTS
Like any diagnostic tool, an ideal method of preoperative airway assessment should havehigh sensitivity and specificity, while maintaining a high predicitve value. Disappointingly, no single test for airway assessment meets these requirements!
Click here for an excellent editorial on predicting the risk of difficult intubation, and the problems all tests suffer with regards to sensitivity, specificity, and predictive value.
More on predicting risk below ...
This is not an exhaustive list of tests. Others not mentioned here have been described and used.
Dental status
Get a short focussed history from the patient about previous dental surgery, cosmetic procedures such as veneers or implants for the upper and lower front teeth (incisors, canines and pre-molars), and removable/ non-removable dental 'hardware'.
Dental 'hardware' comprises a range of devices, most of which do not influence airway management at all. However, it is advisable, for medico-legal purposes, to document their presence in case of iatrogenic damage, especially as upper incisors, canines and pre-molars are concerned:
- Caps or crowns (they are the same thing): Usually made from porcelain the devices are glued/ bonded on top of to cosmetically cover defects or just for better overall aesthetic appearance.
- Bridges: These are devices that 'bridge' the gap between two teeth with one oar more artifical teeth. They are permanently attached, and are sometimes also called 'fixed partial dentures'. Since bridges are not removable they are of limited relevance to airway management and hence assessment.
- Dentures and partial dentures: These are removable, unlike bridges. On what to do about dentures, see below ('The edentulous patient').
- Retainers/ retainer bars and braces: These are orthodontic devices than are used in the course of treatment for teeth mal-alignment. Braces are fixed to a row of teeth either at the outside or inside surfaces. Braces that are fixed to the outside of the teeth can injure lips on the inside during airway interventions such as oral airway insertion or laryngoscope placement. Retainers are either of the removable or non-removable type and are used after orthodontic treatment to preserve therapy results over time.
Other, more levevant findings that need to be appreciated and documented are:
- Loose teeth: These constitute a foreign body-aspiration risk should they become dislodged at any point in time during a general anesthetic or airway management in general. Make sure you mention to the patient that they might lose the tooth perioperatively!
- Missing teeth, damaged/ chipped teeth: Like most dental 'hardware', they do not affect airway management per se, but their presence needs to be documented for medico-legal purposes, especially for upper front teeth.
- Protruding/ 'buck' teeth: This term desribes large protruding upper central incisors, which have the same effect on laryngoscopy as a high-arched palate (see below).
To document issues with a particular tooth use the universal numbering system, which starts the count from the first right upper molar. Therefore the right upper central incisor is number '8', left upper central incisor is '9', the left lower central incisor is '24', the right lower central incisor is '25', and so on.
The British Journal of Anesthesia has a nice review article 'Dental knowledge for anesthetists'.
The edentulous patient
Edentulous patients are often much more difficult to bag-mask ventilate, because after removing upper and lower dentures the facial contours, especially when head-tilt and chin-lift are applied as basic airway maneuvers, do not fit an anesthetic facemask very well. Contributing to this is the fact that loss of teeth often leads to significant mandibular atrophy.
It therefore makes sense to leave (full) dentures in-situ for bag-mask ventilation (Conlon et al., 2007). It might not be quite so straightforward with smaller partial dentures. The smaller the device, the higher the risk of it becoming dislodged and 'lost', migrating into the hypopharynx causing airway obstruction before intubation or immediately after extubation!
On the other hand, the edentulous patient is often easier to intubate (with dentures removed), simply because there are no teeth in the way and an atrophic mandible does to need as much forward displacement with the laryngoscopy to get a good view.
Being edentulous has been indentified as an independent risk factor for difficult mask ventilation.
Mallampati score
This is assessed by asking the patient (in a sitting or upright position) to open his/ her mouth and protrude the tongue maximally without phonation, with the observer being at eye level with the patient. Visibility of faucial pillars, soft palate and uvula inside the patient's mouth will result in a score between one and three.
TIP: To assess a Mallampati score: Unless your patient is standing up you need to sit down, squat or kneel ... and do not ask the patient to say 'ahhh'!
Mallampati in his original paper from 1985 demonstrated the relationship between his scoring system and ease of laryngoscopy. Most patients with a score of one would have a grade 1 Cormack-Lehane view during laryngoscopy, most patients with a score of two would have a grade 2 view and most patients with a score of three would have a grade 3 view. The grade 4 view was later added to the original 3-point scale, which is why the Mallampati score is sometimes referred to as the modified Mallampati score.
The Mallampati test gives and idea of the relationship between tongue size and mobility and size of the oral cavity. This relationship is important because it somewhat predicts how easily the tongue can be displaced by a laryngoscope blade during intubation.
[pictures with examples]
A significant added benefit of doing a Mallampati score assessment is that it focusses the examiner’s attention on the anatomy of the oral cavity including dental status and patients' ability to open their mouth.
Mouth opening/ inter-incisor gap (IIG)
The interincisior gap (IIG) is the distance in cm between the patient's upper and lower incisiors with maximal mouth opening.
Similarly to the measurement of thyromental distance (TMD) (see below), measurement of IIG is usually done by finger breadths in clinical practice rather than a tape measure.
Ask the patient if they can insert 2 and then 3 fingers between their upper and lower incisiors. Appreciate that finger breadth measurement can yield widely variable results depending on the size of the fingers. Use your own fingers for more consistent measurement from patient to patient and if you do, make a one-time effort to measure your personal finger breadths and remember the cm values for two and three fingers!
Which values are concerning or re-assuring?
IIGs of >5cm for intubation and >4cm for insertion of a supraglottic airway have been suggested to have a fairly high predicitve value for success.
An IIG of less than 3.5cm is below a range that is considered 'normal'. A significantly reduced IIG of less than 2.5cm will make insertion of a LMA Unique impossible. Below 1.5cm (one finger breadth in a majority of people) insertion of a (video)laryngoscope blade will be impossible.
Mouth opening/ IIG can be reduced for mechanical or functional reasons. A functional impariment of mouth opening (e.g. due to TMJ pain/ discomfort) may improve with general anesthesia and muscle relaxation.
Thyromental distance (TMD)
Thyromental distance is the distance from mental prominence (most anterior part of the bony chin) to the tip of the thyroid cartilage (thyroid notch or 'Adam's apple') during maximum head extension with the mouth closed.
Like the Mallampati score described above, thyromental distance (TMD) is supposed to be a measure of the ease of tongue displacement with a 'standard' laryngoscope (not videolaryngoscope) blade during intubation. The bigger the TMD the larger the mandibular or submandibular space into which the tongue is pushed with the laryngoscope.
How is this measured in practice?
Measurement and documentation is in 'cm'. However, you won't see many providers getting a tape measure out and applying this to a patients neck. In practice any anesthesiologists measure with finger breadths, which will obviously vary between folks with slender digits or sausage fingers! Make a one time effort and measure the width of your index, index plus middle, index/ middle/ ring, and index/ middle/ ring/ pinkie fingers and remember those values.
When assessing TMD do a quick visual inspection of the anterior neck and exterior laryngeal landmarks at the same time. If your airway assessment is concerning for difficulty a gentle palpation of the thyroid notch and cricothyroid membrane locations are justified.
Which values are concerning or re-assuring?
Precisely defined TMD cut-off values for predicting easy or difficult airway management don't exist or are at least disputed. However, a TMD of 6.5cm or greater is rarely associated with difficulty. Conversely, a TMD below 6cm suggests that direct conventional laryngoscopy might be impossible.
It has been suggested that correction for height and weight might improve the generally low positive predcitve value of TMD when used in isolation.
A significant added benefit of doing a TMD assessment is that it focusses the examiner’s attention on the (surface) anatomy of the anterior neck and larynx.
Temporomandibular joint (TMJ) mobility
There are two test being used in clinical practice which assess TMJ mobility. The direction of mobility we are interested in is forward subluxation of the mandible, which is what occurs with the 'jaw thrust' basic airway maneuver and the mandible 'lift' during laryngoscopy. The two tests are:
1. Upper lip bite test (ULBT)
For the upper lip bite test the patient is asked to bite their upper lip (duh!). The degree to which a patient can bite their top lip is graded 1 to 3.
2. Jaw protrusion
To assess jaw protrusion (prognathism) ask your patient to open their mouth fully and then push their lower jaw forward. Observe if the lower incisors can be advanced in front of the upper ones.
Anterior mandibular subluxation can be limited due to issues not related to the temporomandibular joint. Soft tissue fibrosis/ skin scarring over the anterior neck due to radiation therapy for example can significantly reduce TMJ mobility without there being a joint abnormality as such.
Neck mobility
Neck mobility is the term for the range of motion in the cervical spine and the occipito-atlanto joint. To be precise, occipito-atlanto (C0-C1) joint mobility really determines head as opposed to c-spine mobility, but for the purpose of airway assessment we usually assess neck and head mobility together at the same time.
Reduced neck (and head) mobility, particularly reduced extension, is predictive of difficult standard laryngoscpopy and intubation. A normal range of motion for the neck in the combined extension and flexion ranges is 90 degrees or larger. Precise angle measurement are difficult to perform at the bedside. An alternative surrogate measure of neck/ head mobility is sternomental distance:
Sternomental distance
The definition for the sternomental distance (SMD) is the distance between the mental prominence, i.e. the lower border of the chin, and the sternal notch with full head extension and a closed mouth. A value <13 cm is often mentioned as predictive for difficult intubation. SMD is a measure of neck extension rather than the full neck range of motion.
Both the angle measurement of the flexion/ extension range and the sternomental distance measurement are rarely performed in actual clinical practice. The first one requires a goniometer and the second one a tape measure. Most practitioners usually ask the patient to flex their neck and head ("Can you bring your chin down to your chest?") and then extend their neck as much as possible. Rther than measuring angles or distances we generally rely on an 'eyeball' assessment of neck mobility.
Transnasal flexible endoscopic laryngoscopy (TFEL)
This procedure allows a visual assessment of the oropharynx, hypopharynx and larynx for pathologies (tumors, edema, abscesses, tonsillar hypertrophy, etc.) and gives a general idea of how 'open' these spaces are as a pathway for the laryngoscope blade, ET tube, and your visual line of sight towards the larynx.
It is a (surprinsingly) well-tolerated minimally-invasive procedure which can be performed in the OR or the preop area. ENT surgeons do TFELs all the time in the office/ clinic. Sedation is not required but topical local anesthesia is, and can easily be achieved with a nasal atomizer and 4% topical lidocaine. Ideally the patient should be sitting up facing the practitioner. A flexible fiberoptic bronchocope is passed through a nostril and once the tip of the scope has reached the nasopharynx the scope is flexed caudad, allowing a look 'down' onto the larynx. The patients should protrude the tongue as much as possible to improve the view.
Clearly TEFL is not a routine preoperative airway assessment tool but in can be useful in certain situations, particularly if there is a known (progressive) pathology in the oropharynx, hypopharynx or larynx. It is arguably an under-used exam technique.
TEFL has been evaluated as a tool for preoperative airway assessment (Gemma et al. 2020).
Other patient features
There are a number of anatomical features of the patients face and neck that can be visually assessed if not easily measuredandpredict difficult mask ventilation and/ or laryngoscopy. Since measurements and classifications are lacking, quantifying these features and assigning predicitve value to them is difficult. Noticing them in a patient and expecting challenges in airway management based in their presence comes down to clinical experience.
There is overlap with more quantifiable measurements dicussed above.
Facial hair/ beard
Facial hair can be problematic for two reasons:
- It can make bag-mask ventilation more challenging since it is more difficult to achieve a seal between mask and facial hair that between mask and skin
- A large beard can obscure facial features that are predicitive of difficult mask ventilation and/ or intubation such as a receding chin or a 'small' mouth opening
If you approach the subject gently, some patients might not mind at all to shave or at least trim a bushy beard preoperatively. This is worth considering especially when other concerning factors are present (e.g. obesity, sleep apnea, ...).
Bag-mask ventilation for patients with large/ bushy beards is discussed in the article 'Bag-mask ventilation'.
'Small' mouth
While inter-incisior gap describes mouth opening a 'small mouth' means a reduced oral orifice circumference. The two are related but not the same: A patient can have a reduced IIG without having a 'small mouth'.
High-arched palate
A high-arched palate can impair the laryngoscopy view. It is associated with an above normal overbite and relatively reduced mandibular protrusion (the patient might be able to protrude the mandible to the normal extent, but because of the overbite, might still not be able to score high for jaw protrusion or on the upper lip bite test).
Short neck
There is no objectively quantifiable measurement for a 'short neck' which has been evaluated in the context of airway assessment. However, the term commonly appears in the 'airway' literature. There might be a relation to TMD.
Receeding chin
A receeding chin has a similar effect as a high-arched palate, namely impairment of the laryngoscopy view. It is associated with (but not the same as) a reduced TMD.
Protruding/ 'buck' teeth
See above under 'Dental status'.
A possible sequence for the preoperative airway exam
It is probably fair to say that no two anesthesia providers will do their preoperative airway exam exactly the same way. One suggested sequence is outlined in the table below:
PREDICTING DIFFICULT INTUBATION AND VENTILATION- THE VALUE OF TESTING
Assessing the airway before surgery using some combination of the tests described above is standard clinical practice. But how good are these tests on their own or in combination to predict which patients will be truly difficult to ventilate and/ or intubate and which ones won't? Unfortunately there is no way to talk about tests and the value of testing without some basic statistical knowledge. Familiarize yourself again with the concepts of sensitivity, specificity, positive predictive value and negative predictive value!
What are the sensitivities, specificities and positive predictive values of the commonly used tests described above?
- Mallampati score: This is arguably the most recognized and most performed test for preoperative airway assessment. Depending on which validation study you read, sensitivity is between 40- 80%, specificity is between 50 - 85%, and the positive predictive value (PPV) is between 5- 20%.
- Inter-incisor gap (IIG): Mouth opening has a sensitivity of around 25- 45%, a specificity of around 95%, and a PPV of up to 25%.
- Thyromental distance (TMD): Depending on the study, sensitivity is quoted to be between around 5% and 90%, specificity ranges from roughly 25% to 80%, and PPVs from about 15% to 40% have been found.
- TMJ mobility: Sensitivity is up to 30% at best, specificity is 85% to 90%. Like for the other tests, the PPV is low being around 20%.
- Neck mobility:
It is immediately obvious that many tests have relatively low sensitivity although they are largely fairly specific. Even more concerning though is the fact that their positive predictive value is generally very low. This is because truly difficult intubations are comparatively rare. The PPV of a test always suffers when the condition it aims to predict is rare!
Combinations of tests perform somewhat better. For example, the Mallampati score together with the thyromental distance achieved around 80% sensitivity, 98% specificity and a PPV of almost 65% in one study (the PPV was so much higher because of the high specificity of this test combo).
Scoring systems
One example of a scoring systems for preoperative airway assessment is the Wilson risk sum score. Others include the Arne risk index or the El-Ganzouri risk index. Neither are widely used in clinical practice but apply more in research settings to describe patient or study populations:
Even with this combination of tests, while the sensitivity and specificity of the Wilson risk score are up to 55% and 90% respectively, the PPV is still only about 10%.
The 'value' of the preoperative airway assessment
Given that the positive predictive value of preop airway assessment test if overall low, even when used in combination, is there much point in doing any kind of assessment at all? Is it a waste of time? To quote from Yentis (2002): "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 for this reason we should encourage our trainees (and ourselves) to continue doing it. What we must do is dispel the myth that it actually helps us in predicting dificulty accurately, and have a clear idea about what we are going to do if a test suggests a patient may be difficult, knowing that they probably won't be."
SITUATIONAL AWARENESS
There are some easily overlooked aspects of preoperative airway assessment because they go beyond the more familiar medical history and examination discussed above and have more to do with logistics. They are nevertheless crucial to the development of your airway management plan, just as the assessments described above are.
- Where are you going to do your airway interventions? Operating room, ED, ward, ICU, out-of-hospital setting, ...? Are you familiar with the location?
- What equipment is available? What needs to be brought from some place else? How long will that take? Can the patient be adequately positioned for airway management (be particularly weary of CT scan, angiography tables that do not allow head elevation or reverse Trendelenburg positioning)?
- Who else is on your team? Do you know what their skillset is? What is yours, might you need help? Would help be available?
REFERENCES
Mallampati, S. R.; Gatt, S. P.; Gugino, L. D.; Desai, S. P.; Waraksa, B.; Freiberger, D.; Liu, P. L. (1985): A clinical sign to predict difficult tracheal intubation: a prospective study. In Canadian Anaesthetists' Society Journal 32 (4), pp.429–434. DOI: 10.1007/BF03011357.
Gemma, Marco; Buratti, Luca; Di Santo, Davide; Calvi, Maria R.; Ravizza, Alfredo; Bondi, Stefano et al. (2020): Pre-operative transnasal endoscopy as a predictor of difficult airway: A prospective cohort study. In European Journal of Anaesthesiology 37 (2), pp.98–104. DOI: 10.1097/EJA.0000000000001127.
