Below about 21 to 23 mm of mouth opening, Macintosh videolaryngoscopy starts to fail

Design

prospective observational cohort with predefined difficulty and failure indicators

Population

192 adults at increased risk of difficult Macintosh videolaryngoscopy undergoing head and neck surgery

Primary outcome

interincisor distance thresholds discriminating difficult videolaryngoscopy and videolaryngoscopy failure

Effect

difficulty in 56.8% and failure in 26.0%; best discrimination for failure at 21-23 mm and for difficulty at 23-35 mm; mouth opening fell after induction in 51.0%, with degenerative cervical joint disease OR 6.43 (95% CI 1.27-32.5)

Videolaryngoscopy is offered as the answer to a limited mouth opening, but a Macintosh-shaped blade still has to fit. This prospective observational study took 192 adults already at raised risk of difficult airway management, having head and neck surgery, and managed all of them with a Macintosh videolaryngoscope as first choice. Interincisor distance was measured before and after induction, and seven predefined indicators of difficulty and of failure were assessed - multiple attempts, conversion to a hyperangulated blade, vocal cords not visible, an alert card issued, and so on.

Difficulty was common in this selected group: 56.8% met a criterion for difficult videolaryngoscopic intubation and 26.0% met one for failure. Failure correlated inversely with mouth opening, and discrimination between low and high risk of failure was best at interincisor distances of 21 to 23 mm; the range that best separated easy from merely difficult was 23 to 35 mm.

The second finding is the one to act on tonight. Mouth opening changed after induction in most patients - it increased in 31.8%, stayed the same in 17.2% and decreased in 51.0%. The largest risk of a reduction was in degenerative cervical joint disease (odds ratio 6.43, 95% CI 1.27-32.5), with facial fractures least likely to worsen (odds ratio 0.27, 0.08-0.92). A patient assessed at 25 mm awake may present 20 mm to the blade. The authors are explicit that this should not be extrapolated to routine intubation in patients without predicted difficulty, and that is a real limitation - more than half of this cohort was difficult, which is not the general theatre list.

• In a patient with anticipated difficulty, treat an interincisor distance under about 21 to 23 mm as a reason to plan an alternative or awake technique from the start
• Re-measure after induction: mouth opening decreased in half of these patients
• Be most cautious in degenerative cervical joint disease, where the odds of a reduction were over six times higher
• Have a hyperangulated blade and a flexible endoscope available before induction where the measurement is borderline
• Do not apply these thresholds to an unselected list - this was a cohort chosen for predicted difficulty

Why it matters

It puts a number on the point at which the device most anaesthetists reach for stops being the answer to a limited mouth opening.

Don't overread it

A single-centre prospective cohort selected for predicted difficulty, with thresholds derived from the same data - not validated, and explicitly not applicable to routine intubations.

The statistics, in plain English

These thresholds come from Youden index optimisation, which finds the cutoff giving the best combined sensitivity and specificity in this dataset - a method that tends to produce thresholds flattering to the data they were derived from, and which needs validating elsewhere before being treated as a rule. The odds ratio of 6.43 for degenerative cervical disease has an interval of 1.27 to 32.5, so the direction is clear and the magnitude is not. And with 56.8% difficulty and 26.0% failure, this is an enriched cohort: the same measurement will perform differently on a routine list.

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