The airway

Published on 06/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

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TOPIC 1 The airway

Topic Contents

Predicting difficult airway management 1

Test: Bedside tests 1
View at laryngoscopy 4

Test: Cormack and Lehane view 4
Other investigations of the airway 5

Test: Soft tissue imaging of the lower airway 5
Test: Flexible nasendoscopy 5
Test: Flow volume loops 5
Test: Plain radiography 6

Predicting difficult airway management

Test: Bedside tests

Indications

Used to predict difficult facemask ventilation and tracheal intubation. They should form part of the routine clinical examination of the patient during the anaesthetist’s preoperative assessment, and should be performed in conjunction with a full history and physical examination as appropriate. The latter will not be discussed further in this topic.

Failed intubation and ventilation leading to serious morbidity or death is generally accepted as having an incidence of 1 in 10 000 to 1 in 100 000 of the general surgical population.

The most commonly used tests are listed below.

Mallampati test

First described in 1984 as a three-point scoring system. A modification by Samsoon and Young into a four-point system has generally been accepted.

How it is done

Keeping the head in a neutral position the patient is asked to open the mouth fully and protrude the tongue as far as possible, without phonation. Looking from the patient’s eye level the pharyngeal structures are inspected (Fig. 1.1). It has been suggested that the specificity and positive predictive value of the modified Mallampati test is improved by slightly extending the neck.

image

Fig. 1.1 Modified Mallampati classification.

Interpretation

The following structures are visible:

Class I – soft palate, fauces, uvula, anterior and posterior pillars
Class 2 – soft palate, fauces and uvula
Class 3 – soft palate and the base of the uvula
Class 4 – soft palate is not visible at all.

Management principles

A prediction of a difficult airway may trigger an alternative management strategy, for example inhalational induction, fibre-optic intubation or awake percutaneous approach under local anaesthesia.

Limitations and complications

• There is no universally accepted definition of what constitutes difficult airway management, which is itself subject to the expertise of the anaesthetist. It can be defined according to time taken, attempts made, hypoxaemia or requirement for a second dose of muscle relaxant.
• Because true failed airway management is very rare (1 in 3000 of the general surgical population), none of the commonly used bedside tests are reliably able to identify those at risk of difficult airway management without a high false negative rate.
• Used alone, therefore, the Mallampati test has limited accuracy for predicting the difficult airway and its use as a screening tool has been called into question. It is undoubtedly valuable when used in conjunction with other bedside tests.

Protrusion of the mandible

This gives an indication of the mobility of the mandible. This can be graded as follows:

Grade A – able to protrude lower teeth beyond upper incisors
Grade B – able to protrude lower teeth until they are level with upper teeth
Grade C – not able to oppose upper and lower incisors.

If the patient is grade A or B, intubation is usually straightforward. If the patient cannot get the upper and lower incisors into alignment intubation is more likely to be difficult.

Thyromental and sternomental distance

To obtain thyromental distance the patient is asked to fully extend the neck from neutral position. The distance from the mentum to the thyroid notch is measured. A distance of less than 6 cm is associated with difficult laryngoscopy.

Sternomental distance is measured from the sternum to the tip of the mandible with the head extended and is influenced by a number of factors including neck extension. It has also been noted to be a useful screening test for preoperative prediction of difficult intubation. A sternomental distance of 12.5 cm or less predicts difficult intubation.

Head extension

Keeping the head in a neutral position and the line joining the mentum to the angle of the mandible parallel to the floor, the patient is asked to maximally extend the head on the neck. The angle traversed by the mento-mandibular line is measured. Head extension of 35 degrees or more is normal (Fig. 1.2).

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Fig. 1.2 Head extension.

Specifically, extension at the atlanto-axial joint should be assessed by asking the patient to flex their neck by putting their head forward and down. The neck is then held in this position and the patient attempts to raise their face up testing for extension of the atlanto-axial joint. Since laryngoscopy is optimally performed with the neck flexed and extension at the atlanto-axial joint, reduction of movement at this joint is associated with difficulty.

Palm print test

This test is said to predict difficult direct laryngoscopy in diabetic patients, perhaps because it is a marker of small joint stiffness. The palm and fingers of the dominant hand of the patient are painted with ink. The patient then presses the hand firmly against a white sheet of paper on a hard surface. The less that the phalanges are able to touch the paper, the more difficult intubation is likely to be.

Mouth opening

Less than 4 cm is associated with difficult direct laryngoscopy.

Further investigations

These tests can be used in conjunction with other clinical data such as age, gender and body mass index (BMI) to further refine the prediction of difficult ventilation, intubation and laryngoscopy. A variety of these multivariate tests exist, all more sensitive and specific than a simple Mallampati score, but none have been proven to have overall superiority.

LEMON: Stands for look-evaluate-Mallampati-obstruction-neck. It has been used in the USA in a resuscitation setting.
Wilson’s risk index: Parameters include weight, head and neck movement, jaw opening and protrusion, buck teeth and receding mandible.
El-Ganzouri risk index: Evaluates mouth opening, thyromental distance, Mallampati, neck movement, jaw protrusion, body weight and history of difficult intubation.
Arne risk index: Devised and validated with patients undergoing ear nose and throat (ENT) surgery, the index scores according to Mallampati score, mouth opening, jaw protrusion, head and neck movement, history of difficult intubation and clinical symptoms.

View at laryngoscopy

Test: Cormack and Lehane view

Indication

While not strictly a test, it is used as a marker of view at direct laryngoscopy.

How it is done

The best view is obtained of the laryngeal inlet and immediately surrounding structures using direct laryngoscopy.

Originally it was divided into four grades, depending on the view (Fig. 1.3). A modified five-grade score has also been suggested, where grade 2 is divided into 2A (posterior border of cords seen) and 2B (only arytenoids and epiglottis seen).

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Fig. 1.3 Cormack and Lehane classification of the view at laryngoscopy; anatomy of the glottic opening seen from the pharynx.

Interpretation

Laryngeal view:

Grade I – vocal cords visible
Grade II – only posterior commissure or arytenoids visible
Grade III – only epiglottis visible
Grade IV – no glottic structure visible.

Grade III and IV laryngoscopic views are considered as difficult laryngoscopy as no part of the glottis is visible.

Limitations and complications

• View at laryngoscopy depends on the skill of the operator, and the range of alternative direct laryngoscopy equipment available.
• A poor (grade 3 or 4) view does not always predict difficult intubation, which is after all usually the goal during laryngoscopy.
• The scoring system does not give any guidance as to which alternative technique to facilitate endotracheal intubation would be most appropriate.

Other investigations of the airway

Test: Soft tissue imaging of the lower airway

Indication

Usually in the elective setting when a lower airway abnormality is suspected, either a mass protruding into the airway, or previous surgery or radiotherapy.

How it is done

Computed tomography (CT) or magnetic resonance imaging (MRI) imaging is performed of the upper thorax and neck. This is particularly useful if a mediastinal mass has been diagnosed on plain chest radiography, and the degree of tracheal involvement or invasion must be found.

Limitations and complications

• These investigations are not performed in the acutely unwell patient.
• Rapidly growing pathologies of the mediastinum can quickly render the investigations out-of-date.

Test: Flexible nasendoscopy

Indication

Before elective surgery when anatomical abnormality of structures above the vocal cords is suspected. This is sometimes performed before ENT surgery.

How it is done

It is usually carried out by ENT surgeons. After topical local anaesthesia, a small (<4 mm) endoscope is passed nasally and directed towards the vocal cords. Photographs are taken of abnormal structures, which can be used to plan anaesthesia and oxygenation for elective surgery.

Test: Flow volume loops

See Topic 2.

Test: Plain radiography

Various studies have been used to try to predict difficult intubation by assessing the anatomy of the mandible on x-ray. These have shown that the depth of the mandible may be important, but they are not commonly used as a screening test.

Patients with known conditions affecting the cervical spine, such as rheumatoid arthritis or ankylosing spondylitis should undergo lateral cervical spine radiography before intubation if at all possible. Flexion and extension views are particularly useful in this scenario (Fig. 1.4), both to detect poor neck mobility and consequent difficult direct laryngoscopy, and to pick up potentially unstable ligamentous pathology at the atlanto-axial joint.

image image

Fig. 1.4 Cervical (A) flexion and (B) extension views of a patient with rheumatoid arthritis. Note the metal pin at the atlanto-axial junction.

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