Key Clinical Topics in Anaesthesia Roger Langford, David Ashton-Cleary
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Airway assessment

 
Key points
  • Airway assessment is a dynamic process comprising many different elements
  • Bedside tests should not be used in isolation
  • The actual assessment is one aspect of a two-part process; equally important is the subsequent airway management plan
A thorough airway assessment is the cornerstone of any anaesthetic assessment, allowing the practitioner to
  • Prepare necessary equipment and the environment for airway management
  • Summon help if needed
The 4th National Audit Project, conducted by the Royal College of Anaesthetists and the Difficult Airway Society, highlighted that airway assessment (and subsequent planning) in critical incidents was poor and may have contributed towards negative outcomes.
The overall airway assessment is not only aimed at determining ease of intubation, but also that of bag-mask ventilation and laryngoscopy and follows a multimodal approach involving history, examination and investigation.
 
History
In addition to a previously documented difficult airway, there are various congenital and acquired conditions that may pose problems and should be sought during the preoperative assessment. Some examples include
  • Congenital syndromes
    • Down's syndrome
    • Pierre Robin syndrome
    • Treacher Collins syndrome
    • Mucopolysaccharidoses
  • Acquired
    • Orthopaedic/rheumatological
      • Rheumatoid arthritis
      • Ankylosing spondylitis
    • Tumour
      • Airway, neck or mediastinal masses, e.g. goitre
      • Previous radiotherapy to the neck
    • Traumatic
      • Dental, facial, laryngeal, tracheal trauma
      • Cervical spine injury
      • Acute burns
    • Infection
      • Croup and epiglottitis
      • Oral, pharyngeal, retropharyngeal abscesses
      • Ludwig's angina
    • Endocrine/other
      • Acromegaly
      • Diabetes
      • Obesity and obstructive sleep apnoea
      • Pregnancy
 
Examination
Examination comprises two parts: a general systemic examination followed by a focused airway examination. General patient features that are associated with difficult airways include
  • Presence of stridor
  • Short neck
  • Small or receding mandible (which may be disguised by a beard)
  • Obvious neck swellings, e.g. goitre
  • Obesity
  • Large breasts
  • Evidence of trauma or previous surgery
 
Specific airway examination
  • Teeth
    • Prominent teeth can increase difficulty of laryngoscopy
    • Although the presence of restorative dental work may not directly make the airway difficult, it may be a source of anxiety for the inexperienced laryngoscopist
    • Even though an edentulous mouth can make laryngoscopy easier, it may make bag-mask ventilation more difficult
  • Palate
    • High-arched palates are associated with a superiorly displaced tongue, limited 2space for laryngoscope insertion and a posterior oropharynx
    • Associated conditions include Marfan's syndrome, Pierre Robin syndrome, trisomy 21
  • Mouth opening
    • Is a marker of temporomandibular joint (TMJ) mobility and ease of laryngoscope insertion
    • A distance between the upper and lower incisors of <3–4 cm (or three patient finger breadths) is associated with increased difficulty
  • Mallampati class
    • Classes are based on the visibility of pharyngeal structures on mouth opening and tongue protrusion without phonation
    • Classes I, II and III were proposed initially with classes 0 (Shashtri) and IV (Samsoon and Young) added later
      • Class 0 = epiglottis seen on mouth opening and tongue protusion
      • Class I = tonsillar pillars, soft palate and uvula visible
      • Class II = tip of uvula masked by base of tongue
      • Class III = soft palate visible only
      • Class IV = hard palate visible only
    • Is a marker of several things
      • Adequacy of mouth opening for laryngoscope insertion
      • Size of tongue relative to oral cavity
      • Potential ease of tongue displacement
    • Mallampati class III or above is associated with increased difficulty
  • Prognathism
    • The ability to protrude the mandible
      • Class A = lower incisors protruded anterior to upper incisors
      • Class B = lower incisors in-line with upper incisors
      • Class C = lower incisors cannot reach upper incisors
    • Is a marker of TMJ mobility
    • Limited prognathism is associated with increased difficulty of bag-mask ventilation and laryngoscopy
  • Cervical mobility
    • The ability to flex/extend the atlanto-occipital joint
    • Is a marker of the ability to align three axes (oral, pharyngeal and laryngeal) and achieve the ‘Sniffing the morning air’ position
    • Neck extension <35° is associated with increased difficulty
  • Thyromental distance (Patil's test)
    • The distance from the mental process to the thyroid notch when head and neck are extended
    • Is a marker of the submental space and the ease of tongue displacement with the laryngoscope blade
    • A distance of <6 cm is associated with increased difficulty
  • Sternomental distance (Savva's test)
    • The distance from mental process to sternal notch when the head and neck are extended
    • Is a marker of head and neck mobility
    • A distance of <12 cm is associated with increased difficulty
  • Prayer sign
    • Inability to place both palms flat together
    • Is seen in diabetic patients and is a marker of limited joint mobility
    • It is thought that the same process affects the cervical spine, TMJ and larynx
None of these tests are sensitive or specific in isolation; however, when we amalgamate them into an overall airway assessment their sensitivity and specificity improve. The combination of Mallampati and thyromental distance has been suggested to have the highest discriminative power.
 
Prediction tools
There are several scoring tools that take these examination findings into account in order to predict likelihood of difficult intubations; however, the most widely cited is the Wilson risk score. A score of 3 or more predicts 75% difficult intubation (12% false positive) (Table 1).3
Table 1   Wilson risk score for predicted difficult intubation
Variable
Score
Weight
0 = <90 kg
1 = >90 kg
2 = >110 kg
Head and neck movement
0 = >90°
1 = ~90°
2 = >90°
Jaw movement [interincisor gap (IG) and subluxation (Slux)]
0 = IG >5 cm or SLux >0
1 = IG <5 cm or SLux = 0
2 = IG <5 cm or SLux <0
Receding mandible
0 = Normal
1 = Moderate
2 = Severe
Prominent teeth
0 = Normal
1 = Moderate
2 = Severe
 
Investigations
Other investigations can be used to ascertain underlying anatomy and function, and although they may not predict difficult intubation, they can still provide useful information, e.g. subglottic stenosis. Such techniques include:
  • Head, neck and chest X-rays
  • CT/MRI of neck and chest
  • Fibreoptic techniques, e.g. nasendoscopy or fibreoptic laryngoscopy
  • Flow-volume loops
Further reading
  1. Cook TM, Woodall N, Frerk C. Fourth National Audit Project. Major complications of airway management in the UK: results of the Fourth National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society. Part 1: anaesthesia. Br J Anaesth 2011; 106:617–631.
  1. Mallampati SR, Gatt SP, Gugino LD, et al. A clinical sign to predict difficult tracheal intubation: a prospective study. Can Anaesth Soc J 1985; 32:429–434.
  1. Shiga T, Zen'ichiro W, Inoue T, Sakamoto A. Predicting difficult intubation in apparently normal patients: a meta-analysis of bedside screening test performance. Anaesthesiology 2005; 103:429–437.
  1. Wilson ME, Spiegelhalter D, Robertson JA, et al. Predicting difficult intubation. Br J Anaesth. 1988; 61:211–216.
 
Related topics of interest
  • Awake intubation (p. 13)
  • Airway – difficult and failed intubation (p. 4)
  • Airway – the emergency airway (p. 7)
  • Airway – the shared airway (p. 10)