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  • Overall incidence of difficult mask ventilation (DMV) is 1.4% with impossible mask ventilation (IMV) 0.15%
  • Obesity, snoring, and lack of teeth can probably be overcome with simple airway maneuvers, which is why they are not risk factors for IMV:
    • Lack of teeth actually makes DL and intubation easier
  • Neck circumference (> 50 or 60 cm at the level of the cricoid cartilage) is the most predictive factor of IMV
  • Patients with three or more risk factors have a much higher likelihood of being IMV (odds ratio 8.9)
  • Most patients (75%) who are IMV will be easy to intubate

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Risk Factors Associated with Difficult versus Impossible Mask Ventilation
Difficult mask ventilationImpossible mask ventilation
Neck circumferenceNeck circumference
Male sexMale sex
Sleep apneaSleep apnea
Mallampati class 3 or 4Mallampati class 3 or 4
BMI >26
Lack of teeth

  • Overall incidence of difficult intubation (DI) is 5.2% (obstetric: 3.1%, obese: 15.8%) with failed tracheal intubation 0.15%
  • A combination of tests tends to be a better predictor of DI than individual tests

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Reliability of Different Characteristics Associated with Difficult Intubation
Characteristics associated with difficult intubationDefinitionSensitivity (%)Specificity (%)Positive predictive value or positive likelihood (PL)
Interincisor gap<4 cm309728
Range of neck extensionHead extension ≤80°109318
Thyromental distance<6.5 cm48799.4 (<4 cm)
Mentohyoid distance<4.5 cm16916
Sternomental distance<12.5 cm448711
Neck circumference>43 cm928437
Mallampati classificationGrade 3 or 4 view49863 (PL)
Indirect laryngoscopyGrade 3 or 4 view699831
Upper lip bite testClass 3 (lower incisors cannot bite upper lip)8978
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Tests Predicting Difficult Intubation
Combinations of testsSensitivity (%)Specificity (%)
Wilson risk score4689
Arné multivariate risk index9393
Simplified predictive intubation difficulty score6576
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Arné Multivariate Simplified Score
Risk factorPoints
Previous difficult intubation
• No0
• Yes10
Pathologies associated with difficult intubation (facial malformation, acromegaly, cervical spondylosis, occipitoatlantoaxial disease, airway tumors, DM with "stiff joint syndrome")
• No0
• Yes5
Clinical symptoms or airway pathology (dyspnea due to airway compression, dysphonia, dysphagia, sleep apnea syndrome)
• No0
• Yes3
Interincisor gap (IG) and mandible luxation (ML)
• IG ≥5 cm or ML <00
• 3.5 cm < IG < 5 cm and ML = 03
• IG <3.5 cm and ML <013
Thyromental distance
• ≥6.5 cm0
• <6.5 cm4
Maximum range of head and neck movement
• 100°0
• 90 ± 10°2
• <805
• Class ...

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