Background

A common cause of anaesthesia related morbidity and mortality is difficult or failed intubation. Difficulty in intubation is usually associated with difficulty in exposing the glottis by direct laryngoscopy. This involves a series of maneuvers like extending the head, opening the mouth, displacing and compressing the tongue into the submandibular space and lifting the mandible forward. The ease or difficulty in performing each of these maneuvers can be assessed by one or more parameters.

Extension of head at the atlanto-occipital joint can be assessed by simply looking at the movements of the head, measurement of sternomental distance or by using devices to measure the angle₁. Mouth opening can be assessed by measuring the distance between upper and lower incisors with the mouth fully open. The ease of lifting the mandible can be assessed by comparing the relative position of the lower incisors in comparison with the upper incisors after forward protrusion of the mandible₂. The measurement of mento-hyoid distance and thyromental distance provide a rough estimate of the submandibular space₃₄₅₆. The ability of the patient to move the lower incisor infront of the upper incisor tell us about jaw movement. The classification provided by Mallampati et al₇ and later modified by Samsoon and Young₈ help to assess the size of tongue relative to oropharynx. Indirect laryngoscopy can be used assess the concealment of the larynx by the base of the tongue₉. It also helps in visualizing the structures above the glottis.Abnormalities in one or more of these parameters may help predict difficulty in direct laryngoscopy₁₀.

Initial studies tried to compare individual parameters to predict difficult intubation with mixed results₇₈₁₁. Later studies have attempted to create a scoring system₁₂₁₃ or a complex mathematical model₁₄₁₅. This study is an attempt to verify which of these factors are significantly associated with difficult exposure of glottis and to rank them according to the strength of association.

Methods

The study was conducted after obtaining institutional review board approval. Four hundred and ninety eight, ASA I adult patients, scheduled for various elective procedures under general anesthesia, were included in the study after obtaining informed consent. Patients with gross abnormalities of the airway were excluded from the study.

All patients were assessed the evening before surgery by a single observer. The details of airway assessment are given in Table 1.

Figure 1

Table 1: Method of assessment of various airway parameters (predictors)

Direct laryngoscopy with Macintosh blade was done by an anesthetist who was blinded to preoperative assessment. Glottic exposure was graded as per Cormack-Lehane classification₁₆(Fig 1):

Figure 2

Figure 1: Cormack-lehane grading of glottic exposure on direct laryngoscopy

GRADE 1: Most of the glottis visible
GRADE 2: Only the posterior extremity of the glottis and the epiglottis visible
GRADE 3: No part of the glottis visible, only the epiglottis seen
GRADE 4: Not even the epiglottis seen

Grades 1 and 2 were considered as ‘easy’ and grades 3 and 4 as ‘difficult’.

Results

Glottic exposure on direct laryngoscopy was difficult in 40 (8%) patients. The frequency of patients in various categories of ‘predictor’ variables is given in Table-2

Figure 3

Table 2: The frequency analysis of predictor parameters

Indirect laryngoscopic assessment could not be done in 66 (13.3%) patients due to excessive ‘gag reflex’. This parameter was excluded from analysis.

Statistical analysis was done with SPSS software version 17.0. In view of the dichotomous nature of the ‘dependent’ variable and multiple ‘predictor’ or ‘independent’ variables, ‘binary logistic regression’ was employed to analyze the significance and strength of association between the predictors and outcome. All the ‘independent’ variables are entered into the equation in the beginning and they were eliminated one by one at each step. When elimination of any variable produced any significant change to the model (p < 0.5) it was retained in the equation.

The predictors having significant association to the outcome are:

Degree of head extension (grades 2&3)
Mento-hyoid distance (less than 4 cm)
Modified Mallampati class (grades 3&4)

The strength of association was calculated by ‘odds’ which is an exponential of the correlation coefficient in the equation. The ‘odds’ of having a difficult glottis exposure was highest with ‘degree of head extension’ (odds 11.1) followed by ‘mentohyoid distance ’ (odds 3.4) and ‘modified mallampati class’ (odds 2.5)

Figure 4

Table 3: Ranking the predictors based on odds ratio

Discussion

Direct laryngoscopy is the gold standard for tracheal intubation. There is no single definition of difficult intubation. Difficult glottic view on direct laryngoscopy is the most common cause of difficult intubation. The incidence of difficult intubation in this study is similar to that of others.

As for as the predictors are concerned there are wide variations. Restriction of head and neck movement and decreased mandibular space have been identified as important predictors in other studies as well₁₇. Mallampati classification has been reported to be a good predictor by many but found to be of limited value by others_171819202122. Interincisor gap, forward movement of jaw and thyromental distance produce variable results in previous studies₄₂₃₂₄. Even though thyromental distance is a measure of mandibular space, it is influenced by degree of head extension. Indirect laryngoscopy was mentioned as a predictor of difficult intubation in an earlier report₉, but the practical applicability is limited in view of high incidence of ‘gag’ response.

There have been attempts to create various scores in the past. Many of them could not be reproduced by others or of limited practical value₂₅₂₆. Complicated mathematical models based on clinical and/or radiological parameters have been proposed by few in the past₂₇₂₈₂₉. But these are difficult to understand and tough to follow in clinical setting. Many of these studies consider all the parameters of equal importance.

Instead of trying to find ‘ideal’ predictor(s), score or model, we simply arrange them in an order based on the strength of association with difficult intubation. Restricted extension of head, decreased mento-hyoid distance and poor Mallampati class are significantly associated with difficult intubation. The ‘odds’ of a patients with restricted head extension is 11 times compared to those with normal head extension. Similarly patients with decreased mento-hyoid distance and poor Mallampati class have ‘odds’ of 3.4 and 2.5 respectively compared to those with normal value or grades in the respective categories. In other words patients with decreased head extension have much higher probability of having a difficult intubation compared to those with abnormality in other parameters. The type of equipments needed to manage can be chosen according to the parameter which is abnormal. For example in a patient with decreased mandibular space, it may be prudent to choose devices which do not involve displacement of the tongue like Bullard laryngoscope or Fibre-optic laryngoscope. Similarly in patients with decreased head extension devices like McCoy Larngoscope are likely to be more successful.

Conclusion

There is no perfect predictor of difficult intubation. Certain abnormalities have much higher probabilities of having a difficult airway compared to others. Restriction of head extension, decreased mento-hyoid distance (mandibular space) and poor Mallampati oropharyngeal class have significant association with difficult glottis exposure in the same order mentioned.

Correspondence to

Dr Vasudevan A, No-2, Type-IV, JIPMER campus, Dhanvantri nagar, Puducherry, India- 605006. Email: jipmervasu@gmail.com