The assessment of the patient's airway is an integral part of the preoperative workup. Airway management problems continue to be the single most common cause of morbidity and mortality attributable to anesthesia, i.e. what is most likely to get you into trouble! The purpose of the preoperative airway assessment should therefore be to predict potential problems which might be encountered in the OR, allowing a management plan to be developed ahead of time.
Basically, the aim is to predict and therefore plan ahead for potential problems in two areas:
1. Standard laryngoscopy/ intubation
2. Ventilation, i.e. oxygenation
In addition to identifying relevant patient factors, preoperative airway assessment should also take into account the type of surgery planned and its impact on the type of anesthesia required. This is particularly important in operation with a 'shared airway'. Communicate with your surgeon!
Like any kind of patient assessment it can be divided into history and examination, supported by clinical tests.
"The best predictor of future behavior is past behavior."
Previous airway management problems
Look at old 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.
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, a statement such as ' they we're struggling to get the tube down' or similar should obviously ring alarm bells.
Previous interventions 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. Has the patient had radiotherapy to the neck?
Consider whether a patient had any of these interventions or treatments since the last 'uneventful intubation'.
EXAMINATION & CLINICAL TESTS
This is not an exhaustive list of tests. Others not mentioned here have been described and used.
Like any diagnostic test, an ideal method of preoperative airway assessment should have high sensitivity and specificity. No single test or score for airway assessment meets these requirements.
- Patient features
There are a number of anatomical features of the patients face and neck which predict difficult laryngoscopy, such as a small mouth (related to but not the same as 'poor mouth opening/ small interincisor gap'), an arched/ high palate, a short neck, or protruding/ 'buck' teeth. All these features are somewhat difficult to quantify.
- Mallampati score
As described in Mallampati's original paper from 1985, this is assessed by asking the patient (in a siting or upright position) to open his/ her mouth and protrude the tongue maximally. Visibility of faucial pillars, soft palate and uvula inside the patient's mouth will result in a score of one to three. A Mallampati score of four was later added.
Faucial pillars, soft palate and entire uvula can be visualized.
Faucial pillars and soft palate can be visualized. The uvula is masked by the base of the tongue.
Only the soft palate is visible.
The soft palate is not visible, i.e. only the hard palate can be visualized at the roof of the mouth.
Mallampati in his original paper 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.
- Mouth opening/ interincisor gap
Reduced mouth opening is associated with difficult laryngoscopy and intubation. But how do we differentiate between 'good' and 'poor' mouth opening without a tape measure? One helpful way to quantify mouth opening is to ask the patient whether he/ she can place three fingers between their upper and lower teeth (see picture below). Whereas three finger-breadths is ideal, anything less than two (around 3 cm) predicts an increased risk of difficult laryngoscopy.
- Thyromental distance
This should ideally be greater than 6.5 cm, which is about three finger-breadths.
- Mandibular protrusion
Asking patients to 'protrude their mandible' might just draw a blank. It might be easier to ask "can you bite your top lip?" or "can you show me your teeth and push your chin forward?".
Mandibular protrusion has been classified into three grades.
Yentis, S.M. (2002), “Predicting difficult intubation - worthwhile exercise or pointless ritual?”, Anaesthesia, Vol. 57 No. 2, pp. 105–109.
Mallampati, S.R. et al. (1985), "A clinical sign to predict difficult tracheal intubation: a prospective study", Canadian Anaesthetists Society Journal, Vol. 32, pp. 429-434.