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Make the Diagnosis: The Difficult Tracheal Intubation

Prior Probability of a Difficult Intubation

The clinical examination prior to tracheal intubation is intended to evaluate the likelihood that the clinician will easily accomplish the task, or whether the procedure will be difficult. Among patients who require intubation, approximately 10% (95% CI, 8.2-12) will prove difficult.

Population of Hospitalized Patients Among Whom a Difficult Intubation Should Be Anticipated

The operator should anticipate that all intubations could prove difficult. Depending on the emergency of the situation, the observer might have time only to perceive anthropometric features that increase the likelihood of a difficult intubation. In less emergent situations, the operator has time to both interview the patient about prior intubations and time to examine the patient.

Assessing the Likelihood of a Difficult Intubation

The upper lip bite test assesses the range of movement of the mandible by asking patients to bite their upper lip with their lower incisors. The most useful predictor of a difficult intubation is when the patient’s lower incisors do not extend beyond the vermilion border of the upper lip (see Table 107-1 and Figure 107-1). The hyomental distance is the distance between the hyoid bone and the mentum (the protruding part of the chin). A difficult intubation is predicted by a shorter hyomental distance (< 3 to 5.5 cm). Retrognathia (commonly referred to as an overbite) refers to a shortened distance between the angle of the jaw and tip of the chin; the appearance of a shortened mandible or a measured distance < 9 cm is considered retrognathia, and affected patients are more likely to have a difficult intubation.

Table 107-1Useful Findings for Assessing the Likelihood of a Difficult Intubation

The Wilson score has been the most frequently studied composite measure.1 The score incorporates body weight, cervical spine and jaw mobility, and the appearance of the jaw or teeth (see Figure 107-1). Studies have used either a threshold of ≥ 2 or ≥ 3. The widely used modified Mallampati score is not as accurate as the Wilson score.

Reference Standard Tests

Difficult intubation is most commonly defined with a Cormack-Lehane grade of 3 or 4. This scale describes the visibility of the vocal cords during laryngoscopy.2 In grade 3, the epiglottis can be seen, but the vocal cords cannot be seen. In grade 4, the epiglottis cannot be visualized. The Intubation Difficulty Scale (IDS) is a second measure that describes difficult intubation.3 The IDS combines the features of the Cormack-Lehane scale with the number of intubation attempts, operators involved, advanced airway adjuncts used, and the need for increased lifting force ...

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