Journal of Postgraduate Medicine
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Year : 1991  |  Volume : 37  |  Issue : 1  |  Page : 40-3  

Difficult intubation--when can we predict it?

SK Kamath, SB Randive, MM Bhatt 
 Department of Anaesthesiology, K.E.M. Hospital, Parel, Bombay, Maharashtra.

Correspondence Address:
S K Kamath
Department of Anaesthesiology, K.E.M. Hospital, Parel, Bombay, Maharashtra.


Anaesthetists at times encounter difficult intubation in an apparently normal individual in spite of using the best possible positioning and most profound relaxation. This study was carried out to find out the most simple and easy measurement that may predict a difficult intubation. The study was conducted in three groups of patients of twenty each. The first group consisted of patients in whom intubation was easy. Second group consisted of anticipated difficult intubation. Cases in whom very difficult intubation was encountered were analyzed retrospectively and were included in third group. An attempt has been made to predict difficult intubation with the roentgenographic study of lateral view of mandible along with the measurement of distance from chin to thyroid cartilage. Of all the parameters, ratio of effective mandibular length to posterior mandibular depth and distance from chin to thyroid cartilage were found to be important to predict difficult intubation.

How to cite this article:
Kamath S K, Randive S B, Bhatt M M. Difficult intubation--when can we predict it?.J Postgrad Med 1991;37:40-3

How to cite this URL:
Kamath S K, Randive S B, Bhatt M M. Difficult intubation--when can we predict it?. J Postgrad Med [serial online] 1991 [cited 2022 May 23 ];37:40-3
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  ::   IntroductionTop

There are many anatomical factors like obesity, thick short neck, buck teeth etc. that warn the anaesthetist that he may encounter a difficult intubation. This prediction of difficult intubation helps anaesthetist concerned to take all precautions and keep ready things like long blade laryngoscope, stillette, various small size endotracheal tubes. In spite of all this at times anaesthetist face problems while intubation, which drains all blood out of him. These rare occasions made us undertake this study to find out factors, which may be responsible for these cases in whom, there are no obvious above-mentioned anatomical factors or pathological features such as tcmperomandibular joint ankylosis.

  ::   Material and methodTop

Sixty patients who were intubated under general anaesthesia were included in this study. They were divided into three equal groups as follows:

Group 1 consisted of 20 cases in whom intubation was easy.

Group 2 consisted of 20 cases with anticipated difficult intubation.

Group 3 consisted of 20 cases in whom intubation was found to be difficult without contributory anatomical factors. Intubation was said to be difficult when more than two attempts were required for intubation.

All the cases belong to ASA class 1. During pre-operative evaluation, anatomical factors predisposing difficult intubation like loose or missing teeth, short neck, protruding teeth, receding mandible were noted. In group 1 and 2 lateral roentagenograms of mandible were taken and following distances and angles were measured as in [Figure:1].

(i) effective mandibular length (EML): AC i.e. the distance between tip of lower central incisor to the mid-point of tcmperomandibular joint. (Fig. 1)

(ii) posterior mandibular depth (PMD): EF i.e. the width of mandibular joint between alveolar margin and lower border of mandible immediately behind the 3rd molar tooth.

(iii) anterior mandibular depth (AMD):AB i.e. the distance between the tip of lower incisor to the posterior border of the mandible.

(iv) mandibular angle: ADC i.e. the angle formed between the lower border of the body of mandible and the posterior border of the ramus of mandible. ADC.

Distance between thyroid cartilage to chin was measured in centimetres using a thread and scale. Using these parameters two ratios were calculated.

(1) Effective mandibular length AC


Posterior mandibular depth EF


(2) Effective mandibular length AC


Anterior mandibular depth EF

All the patients were induced with sleeping doses of 2.5% pentothal (5 mg/kg) and suxamethonium 2 mg/kg body weight. Intubation was attempted by qualified anaesthetist (with minimum 3 years' experience). Whenever more than 2 attempts were required intubation was considered as difficult and causes contributing to it were noted. Complications following intubation, if any, were also noted. The cases from group 3 were analysed in similar manner retrospectively.

  ::   ResultsTop

In Group 1 no difficulties were encountered during intubation.

All the patients from Group 2 could be intubated. However, in 4 cases, we had to make 4 attempts for intubation. In two of these cases nasotracheal intubation was carried out. For the rest 16 cases, 3 attempts were required for intubation. The responsible causes and their incidence are enumerated in [Table:1].

In Group 3, 14 cases required 4 attempts at intubation and in the rest, 3 attempts were needed. However all cases could be intubated.

In both Group 2 and 3 following measures were required for intubation, which are not used normally e.g. i) long blade laryngscope, ii) laryngeal back pressure by assistant, iii) two sizes smaller endotracheal tube than required, iv) stillette v) naso-tracheal tube.

Following intubation in 4 cases from Group 2 and 6 from Group 3 immediate oral trauma was observed with dislodgcment of loose incisors. Sore throat for 2 days was found in all the cases of difficult intubation.

Analysis of roentgenographic study carried out for all the 3 groups is depicted in [Table:2]

In short, an increase in mean EML was observed in cases from Group 2 (9.52 cm) and a decrease in Group 3 (9.08 cm) as compared to Group 1 (9.32 cm), though the difference was not significant. A definite increase in mean PMD was noted in patients from Group 2 and 3 (2.85 and 2.73 cm respectively) as compared to those from Group 1 (2.45 cm). There was a wide overlapping of values in 3 groups. Our study reveals that there is a slight but not significant difference in AMD measured in all 3 groups. No significant difference was observed in mandibular angle.

From these measurements (illustrated in [Table:2]) CML/AMD and CML/PMD ratios were calculated [Table:3].

These ratios were statistically compared thus eliminating factors like age, sex, height, weight and general body built. We found that the difference in CML/AMD ratios was not statistically significant, whereas difference in CML/PMD ratio was found to be statistically significant. (P<0.001), when Group 1 was compared with Groups 2 and 3.

Distances measured from thyroid cartilage to chin using thread and scale were as follows:

Group 1: 4.7-7 cm (Mean: 6.88 cm)

Group 2: 2.94.1 cm (Mean: 3.46 cm)

Group 3: 3-5 cm (Mean 3.97 cm)

  ::   DiscussionTop

The main aim of our study was to make the anaesthetist extra alert by predicting the cases of difficult intubation which one may have to face in an apparently normal individual.

The anatomical configuration of the mandible is very important in relation to difficult intubations and hence in 60 cases the mandibular configuration was analysed using roentgenography of lateral view of mandible.

Our study revealed that the average effective mandibular length was 9.32 cm, 9.52 cm, and 9.08 cm in Group 1, 2 and 3 respectively. The differences were not significant. A study by White and Kander2 also failed to show any difference in control and difficult intubation group. A definite increase in posterior mandibular depth was observed in patients from Group 2 and 3 in our study (2.85 and 2.73 cm respectively). These were the cases with difficult intubation. This observation is supported by White and Kander2. They found a significant increase in posterior depth of mandible in difficult intubation group (2.9-3.7 cm) as compared to normal (23-2.9 cm). We did not find any significant difference in anterior mandibular depths of 3 groups. White and Kander 2 in their study found that the AMD is relevant but to a much lesser extent. Their observations showed AMD values to be 18-5.1 cm in normal and 4.5-6.1 cm in difficult intubation group.

Ratio EML/AMD was found to be 2.91, 2.8 and 2.77 in Groups 1, 2 and 3 respectively. The difference was statistically insignificant.

White and Kander2 found this ratio to be 2.0-2.3 in normal and 1.6-2.3 in patients with difficult intubation with wide overlapping of values in two groups.

EML/PMD) ratio was also studied and was found to be 3.817 in Group 1, 3.42 in Group 2 and 3.35 in Group 3. When Group 1 was compared with Group 2 or 3 w.r.t. this ratio, the difference was found to be significant (P < 0.001). It was observed that difFiculties in intubation were encountered when this ratio was less than 3.74.

White and Kander2 also found this ratio important while comparing normal (33-4.5) and difficult intubation group (2.7-3.5).

There was no significant difference in mandibular angle determined for 3 groups; this finding is supported by White and Kander2. They found the mandibular angle to be 110?-115? in normal and 100?-121? in difficult intubation group.

All the above parameters required a lateral X-ray of mandible. So we tried to find out a simple measure i.e. the distance between the chin and thyroid cartilage. On an average, this distance was found to be 6.88 cm, 3.46 cm and 3.97 cm in Group 1, 2 and 3 respectively. Difficulties encountered while intubation in patients in whom the distance was less than 4.5 cm.

Akinyemi1 in 1980 has mentioned that the intubation becomes difficult when the distance is less than 5 cm.

From our study, we can conclude that intubation can be predicted difficult if chin-thyroid cartilage distance is less than 4.5 cm.


1 Akinyemi OO. Midline neck landmark and difficult laryngoscopy. Ind J Anaesth 1980; 28:231-234.
2White A, Kander PL. Anatomical factors in difficult direct laryngoscopy. Brit J Anaesth 1975; 47:468-474.

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