Prediction and Identification of a Difficult Airway in Adults

Introduction

• A patient is considered to have a difficult airway if one or more healthcare providers skilled in airway management have difficulty mask-ventilating or intubating the patient using one or more standard techniques.
• Up to 20% of patients have a difficult airway.1 It is important to prepare for difficult airways because they are a major risk factor for increased apneic time and hypoxia, which can lead to anoxic injury.²
• Many patients with a difficult airway are classified as such because providers are unable to visualize their vocal cords during direct laryngoscopy (DL).
• The most common classification of DL views is the Modified Cormack-Lehane grading.³
  • Grade 1: Visualization of the full glottis
  • Grade 2:
    • 2a: Visualization of a partial view of the vocal cords
    • 2b: Visualization of only the arytenoids or the posterior part of the vocal cords
  • Grade 3: Visualization of only the epiglottis
  • Grade 4: Unable to visualize the glottis or epiglottis.
• Grade 2B and higher is associated with increased risk of a difficult intubation.
• Clinicians should always document the view achieved, particularly in patients with a difficult airway.

• There are many tools available to manage a difficult airway, and the ASA has an algorithm available to navigate which tools should be used when.
• The latest ASA Difficult Airway Algorithm can be found here: Link

Predictors of a Difficult Airway^2,4,5

• Please see the OA summary on difficult mask ventilation. Link

Anesthetic History

• Prior Difficult Airway:
  • The most significant predictor of a difficult airway is a prior history of difficult airway management. It is essential to inquire about the patient’s previous anesthetics and complications. However, just because a patient did not have a difficult airway previously does not exclude the possibility that the patient could have a difficult airway now, as circumstances, such as disease progression, new physiology, or acute trauma, may have changed.
  • Therefore, it is important to investigate changes that could indicate new difficulty with mask ventilating or intubation, such as weight gain, new dental crowns, interim neck radiation, trauma or surgery, or burns to the face or neck.
  • Inquiring about changes in phonation or swallowing, as well as snoring or shortness of breath, is also helpful.
• Importance of Documentation: If a patient had a difficult airway, this should be thoroughly documented in the patient’s chart. It is important to specify if mask ventilation, intubation, or both were difficult, and all techniques used – successful and unsuccessful. The patient should be notified. Some hospitals have a difficult airway letter for patients to keep for future surgeries.

Demographic and Past Medical History

• Key patient demographic features and certain medical conditions can predict a difficult airway. High-risk categories for difficult airway are listed in Table 1.

High Risk Categories

Past Medical History

• Increased age (>55 years old)
• Male sex
• History of obstructive sleep apnea/snoring
• Elevated body mass index (BMI): greater than 30
• Pregnancy (OA summary. Link)
• Prior history or current mediastinal mass

Acquired or Congenital Craniofacial and Cervical Abnormalities (Non-Exhaustive list)

• Mandibular hypoplasia
• Cervical vertebral abnormalities
• High arch palate or cleft palate
• Down syndrome (OA Summary. Link)
• Choanal atresia
• Tracheomalacia
• Pierre Robin sequence (OA Summary. Link)
• Treacher-Collins syndrome (OA Summary. Link)
• Hallermann-Streiff syndrome

• Routine diagnostic testing is not recommended for all patients before intubation.⁴ For patients who have clinical features suggestive of the risk factors suggested above, an x-ray of the head and neck, cervical spine computed tomography scan, and pulmonary function tests can be considered.

Bedside Prediction Tools

• Indicators of difficult mask ventilation and intubation can often be identified on physical examination; therefore, a thorough physical examination should be performed before induction. Below are some common indicators of a possible difficult airway.

Mallampati Score⁶

• The Mallampati score is a measure of the space available between the base of the tongue and the oropharyngeal cavity. A larger available space indicates easier intubation. This usually correlates with the patient’s tongue size, with a smaller tongue allowing for better visualization during laryngoscopy.
• The modified Mallampati scores are as follows:
  • Class 0: Visualization of any part of the epiglottis (rarely seen)
  • Class I: Full view of soft palate, uvula, and tonsillar pillars
  • Class II: Soft palate and uvula visible, pillars are partially obscured
  • Class III: Soft palate and uvula base visible, tonsils are obscured
  • Class IV: Visualization restricted to the hard palate
• A Mallampati score of III to IV indicates a difficult airway.⁶

Craniofacial Anatomic Distances

Multiple anatomic distances should be evaluated before induction, as they can indicate a difficult airway. These include:

• Short thyromental distance (under 6cm), thyroid notch to the mentum.
• Short sternomental distance (under 12cm), suprasternal notch to the mentum.
• Small interincisor distance (under 3.8cm)
• “The 3-3-2 Rule”: Patients should have a minimum of 3 finger breaths between the upper and lower teeth, 3 finger breaths from the chin to the hyoid bone, and 2 finger breaths from the hyoid bone to the thyroid cartilage. These are referred to as the “inter-incisor distance”, the “hyoid-mental distance”, and the “hyoid-thyroid distance”, respectively.⁷

Upper Lip Bite Test

Among bedside predictors, a higher class on the upper lip bite test has the largest odds ratio for a difficult airway.⁷

• Class 1: The lower incisors can bite the upper lip above the vermillion line.
• Class 2: The lower incisors can bite the upper lip, but only below the vermillion line.
• Class 3: Lower incisors cannot bite the upper lip.

Neck Range of Motion

• Limited neck range of motion can suggest a difficult airway, as it may be difficult to properly align the airway for intubation.
• Short and/or thick necks can be concerning for difficult mask ventilation and intubation.

Mouth Opening

A smaller mouth opening makes it difficult to insert the laryngoscope blade.

Intra-Oral Evaluation

• Longer upper incisors can make intubation more difficult.
• A prominent overbite (with the maxillary incisors anterior to the mandibular incisors) is a predictor of a difficult intubation.
• A high arched and/or narrow palate is a potential concern for intubation.

A summary of predictors of difficult airway management is summarized in Figure 1 below.