Predictors of difficult endotracheal intubation (by direct laryngoscopy)

Prior difficult intubation

Interincisor (intergingival in edentulous patients) gap <4 cm

Thyromental distance <6 cm

Sternomental distance <12 cm*

Head and neck extension <30 degrees from neutral

Mallampati oropharyngeal classification class III or IV^¶

Inadequate mandibular protrusion (inability to place lower incisors in front of upper incisors)* or Upper Lip Bite Test Class III^Δ

Neck circumference >40 cm*

Lack of sub-mental compliance (hard and noncompliant)*

Physical findings predictive of difficult endotracheal intubation. The greater the number of positive findings, the more likely intubation by direct laryngoscopy will be difficult. The highest positive predictive value comes from a history of difficulty with intubation, or findings of a short thyromental distance or decreased range of motion of the neck.

* Also predicts difficult video laryngoscopy (in addition to large tonsils and epiglottis and history of Cormack and Lehane grade 3 or 4 at direct laryngoscopy).
¶ The Mallampati classification is based on the clinician's ability to view oropharyngeal structures with the patient’s mouth open and the tongue protruded, as follows:

Class I: The entire tonsillar pillars, uvula, hard and soft palates are visualized
Class II: Partial uvula and soft palate are visualized
Class III: Only the soft palate is visualized
Class IV: No visualized structures beyond the tongue

Δ For the Upper Lip Bite Test, the patient is asked to cover the upper lip with the lower incisors, and results are graded as follows:

Class I: The patient can fully cover the upper lip with lower incisors
Class II: The patient can partially cover the upper lip with lower incisors
Class III: The patient cannot reach the upper lip with lower teeth

Information from:

Tremblay MH, Williams S, Robitaille A, Drolet P. Poor visualization during direct laryngoscopy and high upper lip bite test score are predictors of difficult intubation with the GlideScope videolaryngoscope Anesth Analg 2008; 106:1495.
Aziz MF, Healy D, Kheterpal S, et al. Routine clinical practice effectiveness of the Glidescope in difficult airway management: an analysis of 2,004 Glidescope intubations, complications, and failures from two institutions. Anesthesiology 2011; 114:34.
Hung OR, Pytka S, Morris I, et al. Clinical trial of a new lightwand device (Trachlight) to intubate the trachea. Anesthesiology 1995; 83:509.
Hung OR, Pytka S, Morris I, et al. Lightwand intubation: II--Clinical trial of a new lightwand for tracheal intubation in patients with difficult airways. Can J Anaesth 1995; 42:826.
Shiga T, Wajima Z, Inoue T, Sakamoto A. Predicting difficult intubation in apparently normal patients: a meta-analysis of bedside screening test performance. Anesthesiology 2005; 103:429.
Detsky ME, Jivraj N, Adhikari NK, et al. Will This Patient Be Difficult to Intubate?: The Rational Clinical Examination Systematic Review. JAMA 2019; 321:493.

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Predictors of difficult endotracheal intubation (by direct laryngoscopy)