Which factors make videolaryngoscopy difficult?

Predictors of Difficult Videolaryngoscopy with GlideScope or C-Mac with D-blade: secondary analysis from a large comparative videolaryngoscopy trial Aziz et al. British Journal of Anaesthesia 117 (1): 118-23 (2016)

“Life is simple, but we insist on making it complicated.” – Confucius.


  • Acute-angle videolaryngoscopes are designed to overcome some predictors of difficult direct laryngoscopy, and are intended to improve the view of the relatively anterior airway or when cervical motion is limited.
  • However, it’s a different technique to direct laryngoscopy with different challenges, and it is unclear what specific factors make acute angle videolaryngoscopy difficult.  This paper aimed to identify potential predictors of difficult videolaryngoscopy.


  • Secondary analysis of comprehensive data set from original study which compared 2 acute angle videolaryngoscopy blades in patients with predicted difficult direct laryngoscopy[i]
    • Original study – Single blinded, two-parallel arm, non-inferiority RCT
    • 1100 patients with predicted difficult airway recruited across three academic institutions in the US; GlideScope n = 552, C-MAC n=548
    • Primary outcome – successful first intubation attempt
    • All patients going under GA, with neuromuscular blockade and single lumen tube with predicted difficult airway defined as 1 or more of the following:
      • MP III/IV
      • Reduced MO < 3cm (but not <2cm)
      • Large neck circumference (>40cm M/>38cm F)
    • Exclusion criteria – known easy intubation previously, history of failed intubation or difficult BMV, unstable c-spine injury, age < 18yrs, emergency surgery, planned awake technique, nasal intubations, MO < 2cm
    • Laryngoscopists – > 6/12 clinical anaesthesia & undefined level of experience with videolaryngoscopy
  • Secondary measures recorded including patient variables, surgical approach, head positioning and provider characteristics. These were subsequently analysed for this paper
    • Primary outcome to determine difficult acute angle videolaryngoscopy – 1st attempt failure or time >60s to secure airway
    • Univariate analysis of potential predictors of difficult vs. non-difficult laryngoscopy carried out
    • Multivariate logistic regression model and stepwise model selection techniques conducted to identify independent predictors of difficult videolaryngoscopy


  • 301/1100 difficult videolaryngoscopy
    • 244 > 60s
    • 27 > 1x intubation attempt
    • 30 > 60s + >1x intubation attempted
  • Four predictors identified as being associated with difficult videolaryngoscopy – limited mouth opening, type of surgical procedure, laryngoscopist training level, and patient positioning
  • When compared to the ‘supine neutral’ position, the ‘supine sniffing’ position was associated with an increased risk of difficult videolaryngoscopy (p=0.023, OR 1.625, CI 1.142 – 2.315)
  • Difficult videolaryngoscopy was associated more with particular surgeries (p=0.0021) when compared with general surgery: ENT/oral surgery (OR 1.891, CI 1.190 – 3.006) and cardiac surgery (OR 6.133, CI 1.847 – 20.367)
  • Resident anaesthetists have a higher likelihood of success when using videolaryngoscopy compared with attending anaesthetists (p<0.0001, OR 0.546, CI 0.342 – 0.873)
  • Difficult videolaryngoscopy is more likely to occur in patients with reduced mouth opening (p=0.0225, OR 0.848, CI 1.190-3.006)


  • Difficulty with supine sniffing position was attributed to a more challenging blade insertion
  • ENT/oral surgery also known to be associated with difficult direct laryngoscopy.  The reasons for an association of difficult videolaryngoscopy with cardiac surgery are poorly explained.  The numbers in the original data set for cardiac surgery are not specified but fall into the category of ‘other’ which totals just 40.  The small data set and wide CI question the authenticity of this association
  • A higher likelihood of attendings encountering difficulty is potentially due to them performing fewer intubations day to day as they are frequently in a supervisory role
  • Small MO may hinder initial scope placement and subsequent tube insertion

Overall paper analysis/review

  • Large database from which to draw information from, with appropriate exclusion criteria and a credible aim in trying to identify predictors of difficult videolaryngoscopy


  • Study design not powered towards secondary analysis of the data set
  • Multiple variables not randomised so the results are associations at best
  • Criteria for predicting a difficult airway too broad (could include neck circumference alone)
  • Unable to assess the predictors of difficult airway in direct laryngoscopy independently
  • Primary outcome of defining difficult videolaryngoscopy as >60s to secure airway arguably too severe in view of identified potential difficulty, broad range of skill across those intubating, and lack of required structured training with both videolaryngoscopes
  • Provider background in academic institutions and in a well controlled theatre environment not necessarily transferable to other institutions or clinical areas e.g. ICU/A&E
  •  Not transferable to other types of videolaryngoscope (eg. Channeled/non acute angle)
  • Other potential factors associated with difficult videolaryngoscopy, not in the data set, were not examined

[i] ‘First-attempt intubation success of video laryngoscopy with anticipated difficult direct laryngoscopy: a multicentre randomised controlled trial comparing the C-Mac D-Blade vs the GlideScope in a Mixed Provider and Diverse Patient Population’ M F Aziz, R O Abrons, D Cattano, E O Bayman, D E Swanson, CA Hagber, M M Todd, A M Brambrink Anesth Analog 2016: 122: 740 – 50

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