Audit of a systematic approach to laryngoscopy for predicted difficult intubation under general anaesthesia

Editor, In Canada, the GlideScope is frequently used to manage difficult airways during general anaesthesia [1,2]. For example, this video laryngoscope is likely to serve as both the primary laryngoscope for predicted difficulty and the alternative laryngoscope for unpredicted difficulty. This practice is in contrast with the UK, where the McCoy is more likely to be used as the primary or alternative laryngoscope for difficult airways or both. Although no study has compared the McCoy laryngoscope with the GlideScope, neither has a GlideScope study specifically selected patients with difficult airways, even though this video laryngoscope was designed primarily for difficult intubations. Should direct laryngoscopy be performed first with a Macintosh or McCoy blade before video laryngoscopy to at least confirm the predicted difficulty? The McCoy blade has a Macintosh profile when the tip is in neutral position. If the direct laryngeal view then confirms the difficulty, one can lift the McCoy tip during the same laryngoscopy and potentially reduce the use of the GlideScope. By systematically using the McCoy as the first laryngoscope, we therefore aimed to provide a diagnostic and systematic airway management strategy that is missed when only the GlideScope is used. After obtaining ethical approval from our hospital and university departments, we undertook a 2-month quality assurance audit to document the GlideScope usage and need during predicted difficult direct laryngoscopy under general anaesthesia. The attending anaesthesiologist evaluated the airway history and examination and selected patients with predicted laryngoscopic difficulty but easy facemask ventilation for intubation under general anaesthesia. The following systematic approach to laryngoscopy and intubation was then audited: McCoy laryngoscopy Start with the McCoy tip in the neutral position (i.e. a Macintosh profile) to confirm the predicted difficulty (laryngeal grade 3 or 4) but intubate if McCoy tip-neutral view is laryngeal grade of less than 3 (modified Cormack and Lehane score, i.e. original grade 2 modified to 2a and 2b). If predicted difficulty is confirmed, lift the McCoy tip and intubate if laryngeal grade is less than 3. GlideScope laryngoscopy if the McCoy tip-lift view is laryngeal grade 3 or 4. Using an anonymous quality assurance form, the attending anaesthesiologist recorded the patient's age, sex, documentation of previous difficult airway, Mallampati score, laryngoscopy grade(s) and intubation outcome. Sixty-one patients were audited. The mean age was 53 years, and the gender distribution was almost even. Ninety-three per cent of the patients were ASA grades 2 and 3, reflecting the higher proportion of older patients with multiple comorbidities operated upon at Vancouver General Hospital. Forty-two of the 61 patients had a Mallampati score of at least 3, demonstrating the most common reason for suspicion of difficult intubation. Using the above systematic approach, direct laryngoscopy with the McCoy neutral tip confirmed the predicted difficulty in 42 patients (69%; Table 1). The McCoy tip-lift then converted: four out of four Macintosh grade 4 views to grade 3 and 27 out of 38 (70%) Macintosh grade 3 views to grade of less than 3. In the 15 patients who were not intubated with the McCoy, the GlideScope converted all 15 out of 15 to laryngeal grade of less than 3. Therefore, 19 out of 61 (31%) patients were intubated with the McCoy neutral tip, 27 (44%) with the McCoy tip-lift and all the remaining patients (those with the most difficult laryngoscopies), 15 (25%), were intubated with the GlideScope. There were no failed intubations in this audit.Table 1: Systematic laryngoscopy and intubation in 61 patients with predicted difficultyIn summary, the McCoy blade is a simple and portable device that successfully converted difficult grade 3 direct Macintosh views into manageable views in 70% of patients. We found, however, that it was only able to convert the grade 4 Macintosh views to grade 3 views. This result mirrors the results of a study on the McCoy laryngoscope published by Chisholm and Calder [3] in 1997. The GlideScope appears to be a superior tool in that it converted all grade 3 and 4 direct views into intubatable views and gave a far better laryngoscopic exposure than the McCoy. It must be remembered, however, that a good GlideScope view does not necessarily mean an easy intubation. By first using the McCoy blade, we were able to reduce GlideScope usage by 75%. Importantly, valuable diagnostic airway information was provided for these patients from this audit for subsequent use by anaesthesiologists. If this systematic approach to airway diagnosis and management is used in a practice such as ours, we predict that the associated reduction in GlideScope use would decrease the GlideScope's overall cost and downtime because of cleaning and maintenance. Finally, consideration should be given by both the ASA and the Difficult Airway Society to make reference in their difficult airway guidelines to the availability of video laryngoscopy.

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