Dr I H Wilson,
Department of Anaesthesia, Royal Devon & Exeter Hospital, Exeter EX2 5DW.

Dr Andreas Kopf,
Department of Anaesthesia, Benjamin Franklin Medical Centre, Free University of Berlin, Hindenburgdamm 30, 12200 Berlin-Lichterfelde, Germany.

Introduction		Planning anaesthesia
Predicting difficult intubation		Case histories
Preparing for intubation		Further Reading

During routine anaesthesia the incidence of difficult tracheal intubation has been estimated at 3-18%.

Difficulties in intubation have been associated with serious complications, particularly when failed intubation has occurred. Occasionally in a patient with a difficult airway, the anaesthetist is faced with the situation where mask ventilation proves difficult or impossible. This is one of the most critical emergencies that may be faced in the practice of anaesthesia. If the anaesthetist can predict which patients are likely to prove difficult to intubate, he may reduce the risks of anaesthesia considerably. This paper reviews clinical techniques used for predicting difficulties in intubation and suggests different approaches to manage these patients.

There have been various attempts at defining what is meant by a difficult intubation. Repeated attempts at intubation, the use of a bougie or other intubation aid have been used in some papers, but perhaps the most widely used classification is by Cormack and Lehane [1] which describes the best view of the larynx seen at laryngoscopy (figure 1).

Class I: the vocal cords are visible Class II the vocals cords are only partly visible Class III only the epiglottis is seen Class IV the epiglottis cannot be seen.

This should be recorded in the patient's notes whenever an anaesthetic is administered so there is a record for future use. 
 
Predicting Difficult Intubation

Tracheal intubation is best achieved in the classic "sniffing the morning air" position in which the neck is flexed and there is extension at the cranio-cervical (atlanto-axial) junction. This aligns the structures of the upper airway in the optimum position for laryngoscopy and permits the best view of the larynx when using a curved blade laryngoscope. Abnormalities of the bony structures and the soft tissues of the upper airway will result in difficult intubation.

History and examination

Pregnant patients, those suffering from facial/maxillary trauma, those with small mandibles or intra-oral pathology such as infections or tumours are all more likely to present difficulties during intubation.

Patients who suffer with rheumatoid disease of the neck or degenerative spinal diseases often have reduced neck mobility making intubation harder. In addition spinal cord injury may result from excessive neck movements during intubation attempts. Poor teeth and the inability to open the mouth are obvious other factors as are obesity, and inexperience on the part of the anaesthetist.

Specific Screening Tests to Predict Difficult Intubation.

A history of successful or unsuccessful intubation during previous anaesthesia is obviously significant.

There a number of specific clinical assessments that have been developed to try to identify patients who will prove difficult to intubate. Mallampati suggested a simple screening test which is widely used today in the modified form produced by Samsoon and Young [2]. The patient sits in front of the anaesthetist and opens the mouth wide. pThe`patient is assigned a grade according to the best view obtained (figure 2).

View obtained during Mallampati test: 1. Faucial pillars, soft palate and uvula visualised 2. Faucial pillars and soft palate visualised, but uvula masked by the base of the tongue 3. Only soft palate visualised 4. Soft palate not seen. Clinically, Grade 1 usually predicts an easy intubation and Grade 3 or 4 suggests a significant chance that the patient will prove difficult to intubate. The results from this test are influenced by the ability to open the mouth, the size and mobility of the tongue and other intra-oral structures and movement at the craniocervical junction.

Thyromental distance

This is a measurement taken from the thyroid notch to the tip of the jaw with the head extended. The normal distance is 6.5cm or greater and is dependant on a number of anatomical factors including the position of the larynx. If the distance is greater than 6.5cm, conventional intubation is usually possible. If it is less than 6cm intubation may be impossible [3].

By combining the modified Mallampati and thyromental distance, Frerk showed that patients who fulfilled the criteria of Grade 3 or 4 Mallampati who also had a thyromental distance of less than 7cm were likely to present difficulty with intubation [4]. Frerk suggests that using this combined approach should predict the majority of difficult intubations. A 7cm marker can be used (eg a cut off pencil or an appropriate number of examiners fingers) to determine whether the thyromental distance is greater that 7cm.

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 pre-operative prediction of difficult intubation. A sternomental distance of 12.5cm or less predicted difficult intubation [5].

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. 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.

Protrusion of the mandible is an indication of the mobility of the mandible. If the patient is able to protrude the lower teeth beyond the upper incisors intubation is usually straightforward [6]. If the patient cannot get the upper and lower incisors into alignment intubation is likely to be difficult.

Wilson et al [7] studied a combination of these factors in a surgical population assigning scores based on the degree of limitation of mouth opening, reduced neck extension, protuberant teeth and inability to protrude the lower jaw. Although their method can predict many difficult intubations, it also produces a high incidence of false positives (someone who is assessed as a likely difficult intubation, but who proves easy to intubate when anaesthetised) which limits its usefulness.

X-ray studies

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.

Preoperative assessment

A combination of the above tests is better than using only one. The modified Mallampati, thyromental distance, ability to protrude the mandible and craniocervical movement are probably the most reliable.

Most patients without indicators of difficult intubation will prove easy to intubate under anaesthesia although occasional difficulties will occur. The majority of difficult intubations will be predicted by clinical assessment, but the tests will wrongly predict difficult intubation in some patients who will subsequently prove straightforward. 

(Continued ...)

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