Predictor difficult intubation


1.    Mallampati Evaluation:

The basis of the Mallampati evaluation is to judge the size of the tongue in relation to the oral cavity as the tongue must be displaced for adequate laryngoscopic view. The evaluation is performed with the patient in the seated position and tongue maximally protruded. While traditionally this test has been performed with the head in a neutral position and absent phonation, Mashour et al. found that neck extension on average increased mouth opening by over 1 cm and reduced grades compared to neutral neck position. This reduced the number of false positive difficult airway assessments. (1) The evaluation generally has poor to intermediate sensitivity (50-80%, somewhat better than most other evaluations) while the specificity is often found to be in the high 90s.1-4 Combining Mallampati with thyromental distance or other airway evaluations can improve its accuracy at predicting difficult intubations at the cost of a small reduction in specificity. (2,4)

2.    Thyromental Distance:

Thyromental distance (TMD) can be used to estimate mandibular space. During laryngoscopy the tongue is displaced into the mandible and a short TMD suggests inadequate space for the tongue.4 The evaluation is performed with the patient’s neck fully extended and mouth closed. The distance between the thyroid prominence and the bony point of the mentum/most anterior aspect of the chin is measured. (3,5) Used alone, thyromental distance is not a very accurate predictor of difficult intubation (sensitivity 15-70%).2, 3, 6, 7 Using the ratio of height to thyromental distance was found to increase the predictive value of the evaluation. (5) Tripathi et al found that using the smaller Mac 2 blade in patients with short TMD (under 5 cm) resulted in less failed intubations, less intubation attempts, significantly greater percentage of easy views, significantly shorter intubation time and significantly less use of intubation aids such as bougie or stylet. (8)

3. Upper lip bite test:

The ease of laryngoscopy is significantly affected by mandibular mobility, and the ULBT is a measure of this

  • Class 1= lower incisors can bite upper lip above vermilion line
  • Class 2= lower incisors can bite upper lip below vermilion line
  • Class 3= lower incisors cannot bite upper lip

This evaluation has been found to be fairly specific without great sensitivity. Investigators determined its utility lies in predicting likely easy intubations rather than those that may be difficult. (7)


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  1. George A Mashour, Warren S Sandberg Craniocervical extension improves the specificity and predictive value of the Mallampati airway evaluation. Anesth. Analg.: 2006, 103(5);1256-9
  2. Nkihu A Merah, David T Wong, Dorothy J Ffoulkes-Crabbe, Olusola T Kushimo, Christopher O Bode Modified Mallampati test, thyromental distance and inter-incisor gap are the best predictors of difficult laryngoscopy in West Africans. Can J Anaesth: 2005, 52(3);291-6
  3. J C Tse, E B Rimm, A Hussain Predicting difficult endotracheal intubation in surgical patients scheduled for general anesthesia: a prospective blind study. Anesth. Analg.: 1995, 81(2);254-8
  4. Toshiya Shiga, Zen’ichiro Wajima, Tetsuo Inoue, Atsuhiro Sakamoto Predicting difficult intubation in apparently normal patients: a meta-analysis of bedside screening test performance. Anesthesiology: 2005, 103(2);429-37
  5. Banjong Krobbuaban, Siriwan Diregpoke, Sujarit Kumkeaw, Malin Tanomsat The predictive value of the height ratio and thyromental distance: four predictive tests for difficult laryngoscopy. Anesth. Analg.: 2005, 101(5);1542-5
  6. A R el-Ganzouri, R J McCarthy, K J Tuman, E N Tanck, A D Ivankovich Preoperative airway assessment: predictive value of a multivariate risk index. Anesth. Analg.: 1996, 82(6);1197-204
  7. Zahid Hussain Khan, Mostafa Mohammadi, Mohammad R Rasouli, Fahimeh Farrokhnia, Razmeh Hussain Khan The diagnostic value of the upper lip bite test combined with sternomental distance, thyromental distance, and interincisor distance for prediction of easy laryngoscopy and intubation: a prospective study. Anesth. Analg.: 2009, 109(3);822-4
  8. Mukesh Tripathi, Mamta Pandey Short thyromental distance: a predictor of difficult intubation or an indicator for small blade selection? Anesthesiology: 2006, 104(6);1131-6