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The American Society of Anesthesiologist’s (ASA) Closed Claims Project reports that difficult intubation leading to death or brain injury account for 9% of all claims. Some were classified as preventable. Preoperative evaluation with medical, surgical, and anesthetic history as well as physical examination and radiographic study evaluation minimizes the chances of unrecognized difficult intubation. No single factor reliably predicts difficult airway management. The more the predictors of difficulty in a given patient, the greater the likelihood of difficult airway. Once difficult intubation is recognized, practitioners may prepare additional equipment, modify induction agents, and secure backup support as necessary.


  • Difficult mask ventilation is an inability to face mask ventilate the patient.

  • Difficult laryngoscopy is an inability to visualize the vocal cords after multiple laryngoscopy attempts.

  • Difficult intubation is encountered when multiple attempts are required to intubate the trachea.

  • Failed intubation is the inability to place an endotracheal tube despite multiple attempts.

These definitions presume best operator and optimized positioning (ie, sniff position).


Although no single criterion envisages difficulty, a history of difficult airway is the single best predictor of future difficulty. Consequently, a thorough anesthetic history should include previous airway concerns. Interval change in the patient’s medical history or condition, such as new oral or pharyngeal pathology, significant weight or height gain (ie, previous surgery as a child), cervical spine injury, or pregnancy discounts previous airway success.

  1. Mallampati/Samsoon–Young Scoring—The Mallampati/ Samsoon–Young scale classifies airways according to the base of tongue to overall open mouth ratio. The underlying premise is that during direct laryngoscopy, the base of the tongue obscures the view of the larynx. Thus, a higher ratio would suggest a greater likelihood of difficult laryngoscopy. This test is performed with the patient’s head in the neutral position without phonation. A class I view includes the entire uvula, hard, and soft palates; class II—only a partial uvula view in addition to the hard and soft palates; class III—hard and soft palates with base of uvula visible; and class IV—hard palate only is visualized. Classes III and IV are associated with a higher incidence of difficulty with intubation (see Chapter 64).

  2. MacroglossiaMacroglossia predicts difficult intubation as a large tongue is difficult to be completely displaced by a rigid laryngoscope into the submandibular space.

  3. Thyromental distance—Thyromental distance is the distance between the thyroid cartilage and the mentum of the mandible. It is normally greater than 6.5 cm; thyromental distance predicts difficulty with intubation when less than 6 cm. This measurement suggests that the mandibular size is measured with the head extended at the atlanto-occipital joint.

  4. Mandibulohyoid distance—The mandibular–hyoid distance predicts a large, hypopharyngeal tongue blocking visualization of the glottic opening; hence, direct laryngoscopy and intubation difficulty is increased. This distance should be greater than 4 cm.

  5. Neck circumference—A short, thick neck with ...

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