Lit of the Week — Airway Assessment Score and Predictors of Difficult BVM

Lit of the Week – 2/2/16

Reed MJ, Dunn MJ, McKeown DW. Can an airway assessment score predict difficulty at intubation in the emergency department? Emerg Med J. 2005 Feb;22(2):99-102.

Clinical question / background:

• Can an airway assessment score based on the LEMON method detect difficulty at intubation (Cormack and Lehane Grade 2-4) in the ED?

Design:

• Prospective observational study
• 156 patients in ER population, 1 UK teaching hospital
• Inclusion: Adult patients requiring intubation
• Exclusion: None
• Patients’ airways evaluated using LEMON criteria and given points (0-10)
  • Look (4 criteria used, 1 point each) – max 4 points
    • Signs of airway trauma, large incisors, large tongue, presence of beard
  • Evaluate 3-3-2 rule – max 3 points
    • Interincisor distance (3 finger breadths), mentum to hyoid, (3 finger breadths) floor of mouth to thyroid notch (2 finger breadths)
    • 1 point for each
  • Mallampatti class – max 1 point
    • 1 point for either class III / IV
  • Obstruction – max 1 point
    • ex epiglottitis, PTA, trauma, blood, FB
  • Neck Mobility – max 1 point
    • Chin to chest, extension so as to look at ceiling
  • At time of intubation, Cormack and Lehane Laryngoscopy Grade Score was recorded to objectively determine difficulty of intubation
    • A difficult intubation was a Cormack and Lehane grade 2,3, or 4

Results:

• Of the above LEMON criteria, patients with large incisors (p <0.001), reduced inter-incisor distance (P<0.05), or a reduced thyroid to floor of mouth distance (P<0.05) were each significantly associated with more difficulty laryngoscopic view
• Patients with high airway score (median score 2) more likely to have poor laryngoscopic view compared to lower (median score 1)
• 114 Patients with C/L Grade 1, 42 C/L Grades 2-4

Take-home:

• Airway assessment score based on LEMON method can stratify risk of increased intubation difficulty
  • Major criteria were incisor distance, floor of mouth to thyroid distance, and presence of large incisors

Strengths:

• Simple, designed to target ER patient population

Weaknesses / Critiques

• Same practitioner performed LEMON assessment and intubation which may have led to bias
• Interobserver variability in determining LEMON scores
• Subjective terms such as ‘large incisors’
• Difference between airway score for the two groups was only 1 point

Follow-up / Real World Application

• Cormack and Lehane Views
  • Grade 1 – complete glottis visible
  • Grade 2 – posterior glottis and arytenoid cartilage only
  • Grade 3 – epiglottis only
  • Grade 4 – no epiglottis or laryngeal structures visible

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Paper #2

Kheterpal S, Han R, Tremper KK, Shanks A, Tait AR, O’Reilly M, Ludwig TA. Incidence and predictors of difficult and impossible mask ventilation. Anesthesiology. 2006 Nov;105(5):885-91.

Clinical Question / Background

• What are the predictors of difficult and impossible bag-mask ventilation in patients undergoing intubation during general anesthesia?

Design

• Prospective observational study
• 22,660 BVM attempts at a large academic surgical center
• Inclusion: all adult patients undergoing general anesthesia
• Exclusion: none
• Primary outcome: ease or difficulty of BVM defined as below
  • Four-point scale ranging from grade 1 to 4 used to objectify difficulty of BVM with Grades 3 and 4 defined as difficult and impossible, respectively
    • Grade 1 – ventilated by mask
    • Grade 2 – ventilated by mask with oral airway/adjuvant with or without muscle relaxer
    • Grade 3 – difficult mask ventilation defined as BVM that is inadequate to maintain oxygenation, unstable BVM, or BVM requiring two providers
    • Grade 4—impossible mask ventilation noted by absence of end tidal CO2 measurement and lack of perceptible chest wall movement during positive pressure ventilation attempts despite airway adjuvants and additional personnel
  • Secondary outcomes
    • Cormack and Lehane DL view, subjective assessment of difficulty of intubation defined as 3+ attempts, and ability to perform DL

Results

• Incidence of grade 3 BVM – 1.4% (n=313)
• Factors statistically significantly associated with Grade 3 BVM
  • BMI > 30, presence of a beard, Mallampati III or IV, age 57 or older, severely limited jaw protrusion, snoring

• Incidence of grade 4 BVM – 0.16% (n=1.4%)
• Factors statistically significantly associated with Grade 4 BVM
  • Snoring, thyromental distance < 6 cm

• Incidence of grade 3 or 4 BVM + difficult intubation – 0.37% (n=84)
• Factors statistically significantly associated with Grade 3 or 4 BVM + difficult intubation
  • Limited jaw protrusion, abnormal neck anatomy, sleep apnea, snoring, BMI > 30

Take Home

• Advanced age, increased BMI, presence of beard, history of snoring associated with difficult BVM
• Limited jaw protrusion associated with difficult intubation

Strengths

• Large study, powered to detect statistically significant associations between airway exam and procedural difficulty

Weaknesses

• Surgical patients optimized for intubation so does not apply as firmly to ER population
• Multiple subjective analyses
• Presence of beard is only modifiable risk factor that was found
• Lack of control or uniform conditions applied to all BVM attempts