Airway assessment

 

List:

·        Theories

·        Approach

·        Tests

·        Syndromes

 

Theories:

Three axis theory

·      Oral

·      Pharyngeal

·      Laryngeal

Two column theory

·      Posterior-facing curve from mouth to pharynx

o  Flattened by extension of head

·      Anterior-facing curve from pharynx to larynx

o  Flattened by flexion of lower C spine

·      Point of inflection at base of epiglottis

 

Approach to assessment:

Are there any red flags?

·      Airway infection

·      Airway cancer

·      Airway trauma

·      Airway radiation

·      Congenital abnormality

·      Previous difficult airway

Will procedures be hard?

Bag-mask ventilation:

·      Elderly

·      No teeth

·      Beard

·      ↓Jaw protraction

·      Obese

·      OSA (i.e. STOPBANG)

LMA insertion:

·      Small mouth

LMA ventilation:

·      Obese

·      Stiff neck

·      Stiff lungs

·      Abnormal upper airway

Laryngoscopy/intubation

·      ↑MP

·      ↓TMD, TMH

·      ↓IID

·      ↓Neck ROM

·      Obesity

·      OSA (i.e. STOPBANG)

FONA:

·      Obesity

·      Abnormal anatomy

Extubation:

·      Difficulty with intubation

·      Narcotic concentration

·      Airway swelling

What are the stakes?

·      Does surgery interfere with the airway?

·      What is the aspiration risk?

·      What is the patient’s cardioresp reserve?

Are there any red flags?

·      Airway infection

·      Airway cancer

·      Airway trauma

·      Airway radiation

·      Congenital abnormality

Will procedures be hard?

·      Bag-mask ventilation? (elderly, obese, no teeth, beard, snorer)

·      LMA insertion? (small mouth)

·      LMA ventilation? (obese, stiff neck, stiff lungs, abnormal anat)

·      Intubation? (↑MP, ↓TMD, ↓IID, stiff neck, not edentulous)

·      FONA? (obesity, abnormal anatomy)

·      Extubation?

What are the stakes?

·      Does surgery affect the airway?

·      What is the aspiration risk?

·      What is the patient’s cardioresp reserve?

 

Airway tests:

 

Difficult laryngoscopy:

 

Sensitivity

Specificity

Mallampatti

50%

80%

Thyromental distance (6cm)

37%

89%

Mouth opening (<3cm)

22%

94%

Upper lip bite

67%

92%

Thyromental height

83%

99%

(Cochrane review 2018 and Anesth Analg 2013)

 

Commentary:

Problem 1: rareness

·      Failed intubation rare (0.05-0.3%)

·      Low prevalence = high negative predictive value anyway

Problem 2: accuracy

·      Specificities (“Will this be an easy airway?”) are high

·      Sensitivities (“Will this be a difficult airway?”) are low
(exception: thyromental height test 83%)

Problem 3: applicability

·      Sensitivity and specificity are considerations when deciding whether to perform a test

·      We perform airway examination universally; hence predictive values are more pertinent

·      Airway examinations assess for the anterior larynx, but not for other kinds of difficult airway (e.g. tumour, radiation)

Problem 4: combination

·      There are many eponymous combined scores

·      They are cumbersome

·      They are better than single tests but still not that good

·      e.g. High MP/short TMD

·      e.g. Wilson Airway score (weight, beard, receding mandible etc.)

Upshot

·      Examination is poorly predictive of difficult intubation

·      Important to retrieve past anaesthesia notes

·      Important to make plan ABCD for every single anaesthetic

 

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