Discussion
Although many studies have shown that videolaryngoscopes could provide better views of the glottis when compared with conventional direct laryngoscopy, this was the first study comparing the McGrath videolaryngoscope with the C-MAC videolaryngoscope in patients with one clinical marker of a potentially difficult airway. In this study, we showed that the McGrath videolaryngoscope provided significantly more grade 1 laryngoscopic views than the C-MAC videolaryngoscope in patients with poor Mallampati scores. However, the McGrath required much longer intubation time and more attempts before successful intubation could be achieved. The C-MAC videolaryngoscope was regarded by most anaesthetists as a much easier device to use when compared with the McGrath videolaryngoscope. There was, however, no significant difference in the proportion of successful intubation, the number of complications, and the haemodynamic changes between the two groups.
The difference of 17 s in the time of intubation between the two laryngoscopes is consistent with the result found in one previous study, which compared the earlier model of the C-MAC videolaryngoscope, the V-MAC videolaryngoscope with the McGrath videolaryngoscope in morbidly obese patients. This difference may reflect the fact that the McGrath videolaryngoscope blade has a more significant anterior bend and hence requires a modified intubation technique when compared with the standard Macintosh blade design of the C-MAC videolaryngoscope. The McGrath videolaryngoscope requires a styletted tracheal tube and the time taken to remove the stylet could be an additional factor for the slower average time to intubation. Although the use of end-tidal CO2 as the endpoint for time to intubate may have disadvantaged the McGrath videolaryngoscope in this study, it is the most clinically relevant measure as it reflects the time when the patient is without ventilation.
Although the McGrath videolaryngoscope provided significantly more grade 1 laryngeal views than the C-MAC videolaryngoscope, it required more intubation attempts. This is in line with existing studies which suggest that the McGrath videolaryngoscope consistently provides optimal laryngeal inlet views in manikins with simulated difficult airways and patients with difficult airways. However, a good laryngeal view does not necessarily translate to easy intubation.
Interestingly, all of the five failed intubations in the McGrath group had grade 1 views at videolaryngoscopy; yet, the anaesthetists were not able to pass the tracheal tube through the vocal cords. They were subsequently successfully intubated with a Macintosh laryngoscope with three of these intubations requiring the aid of a bougie. This might support the findings of a previous study that the McGrath videolaryngoscope may prolong the intubation time of patients with uncomplicated airways. The C-MAC videolaryngoscope resulted in one failed intubation where a grade 2 view was obtained at laryngoscopy. However, the large epiglottis was obscuring the vocal cords and the anaesthetist was unable to pass the tracheal tube or a bougie behind the epiglottis. The McGrath videolaryngoscope was chosen by the intubating anaesthetist as the alternative airway device and this resulted in a grade 1 view with an easy pass of the bougie. This highlighted the fact that in some patients, it might be beneficial to use a videolaryngoscope with a sharp anterior bend on the blade design.
Both devices had been available in the department for over a year before the commencement of the study. Most anaesthetists were familiar and competent in the use of both devices. However, the C-MAC videolaryngoscope was consistently rated by anaesthetists as an easier device to use than the McGrath videolaryngoscope. This is in agreement with the tightly clustered inter-quartile range for the time to intubation in the C-MAC group. This may reflect clinicians' familiarity with the shape of the blade as the C-MAC matches the shape and the curve of the Macintosh and can be used with the same technique. The McGrath videolaryngoscope could potentially be made easier to use if there is a stylet or bougie available, of which the distal tip could be freely manoeuvred to couple the exaggerated curve of the McGrath blade. In conventional laryngoscopy, the Cormack and Lehane view correlates well with overall intubation difficulty. This was not replicated in our study. In this study, we found that the time to intubate and the number of intubation attempts were more closely related to the ease of intubation than the laryngoscopic view. Therefore, it is important to have a composite of outcomes in studies involving videolaryngoscopes.
There are a few limitations to this study. First, we were not able to blind the intubating anaesthetists or the independent observer from the randomization of the videolaryngoscope. This could have led to bias if the anaesthetist already had a preference towards one device. However, the primary outcome, time to intubation, and most of the secondary outcomes were well defined and objective. Secondly, we chose the Mallampati score as the only predictor of difficult airway. A previous study has shown that 2.1% of patients with the Mallampati grade of ≥ 3 would fall into Cormack and Lehane grade 3 or 4 under videolaryngoscopy. We understand that this may limit the clinical application of our findings from this study. However, we believed that this test was one of the most routinely performed preoperative airway assessment test among anaesthetists. It was feasible and clinically relevant.
To conclude, the C-MAC videolaryngoscope allows a significantly faster intubation time, fewer intubation attempts, and is considered an easier device to use in patients with poor Mallampati scores when compared with the McGrath videolaryngoscope. On the other hand, the McGrath videolaryngoscope provides more grade 1 laryngeal views, possibly due to the exaggerated angulation of the blade. Videolaryngoscopes in general may offer promise for intubation of the difficult airway. Further studies to compare different videolaryngoscope blade designs used in patients with different types of airway problems will be useful to assist anaesthetists in selecting the most appropriate device in each individual clinical scenario.