The present study shows that 1) the use of the C-MAC videolaryngoscope provides comparable or better glottic views than direct laryngoscopy, and 2) in patients with impeded glottic view (C/L≥2a), C/L class may be improved and subsequently patients may be intubated with the C-MAC4 blade combined with the use of Miller's "straight blade technique".
The C-MAC videolaryngoscope is a relatively new device with the unique advantage that it provides the possibility to obtain both a direct laryngoscopic view and a camera view that is displayed on the video screen, in contrast to many previous videolaryngoscopes.a On the one hand, this may be very helpful for educational purposes, since the student is enabled to follow an ideal intubation process on the video screen, and thereafter, the instructor may directly observe the student's intubation attempts. On the other hand, this may have important ramifications, if the video view is worse than the direct view, as observed with the C-MAC in six patients of the present study, or the intubation itself is difficult due to a high blade angulation, as shown with the GlideScope . The user therefore is in the comfortable situation to decide with a single device whether to intubate by direct laryngoscopic or videolaryngoscopic view, depending on the better view provided, which has been addressed previously . Lower angulations of the blade, times for laryngoscopy and intubation comparable to direct laryngoscopy as well as good handling conditions in the present study suggest that the C-MAC size 3 may be the standard device to use in daily practice; analogous to the standard Macintosh laryngoscope, the C-MAC size 4 may be used in larger patients. In our opinion, in all cases with easy intubation conditions the anaesthesiologist should prefer the direct laryngoscopic view of the C-MAC 3 over the videolaryngoscopic view. However, this issue may be debatable, since increased forces on the maxillary incisors with conventional laryngoscopy compared to videolaryngoscopy have been observed during difficult intubation , but more importantly, videolaryngoscopy-guided intubation has the potential risk of increasing the number of intubation attempts and time, and the use of a tube-guide, respectively, as shown in the present study.
Similar to experiences from previous Storz videolaryngoscopes [18, 19], an endotracheal tube stylet or semi-flexible tube-guide is not mandatory due to the original Macintosh shape of the blade: only 12 of 150 patients in the present study, including 6 of 8 patients with highly limited direct laryngoscopic view, were intubated with the help of a tube-guide. This finding is even more significant because we did not exclude patients with morbid obesity. Even if the safety of using or not using a stylet or tube-guide may be debatable, there have been reports of complications such as oropharyngeal perforations with the use of a previous videolaryngoscope (GlideScope) [20–24]; in that device, a highly angulated blade caused difficulty in advancing the tracheal tube to the glottic entrance, because both pharynx and the glottis were not under direct view, resulting in a partly blind oropharyngeal passage of the styletted tube. For avoidance of such complications, insertion and oropharyngeal passage of the endotracheal tube should be directly visualised as long as possible and training on the device combined with a good technique is mandatory. Further, this may result into prolonged intubation time . In contrast, due to the lower angle of the C-MAC blade, the tip of the blade may always be seen on the video screen; the association between blade angulations and both visualisation and intubation success has been addressed by a recent article of Levitan et al. . Compared to highly-angulated blades that provide optimal glottic visualisation (C/L 1) in most cases, but sometimes at the expense of more difficult tube advancement and subsequent intubation success, glottic exposure with the C-MAC may be incomplete (C/L 2a+b) in a higher proportion of cases but may allow easier intubation conditions anyway. All participating medical personnel are enabled to follow both visualisation of the glottis and intubation process on the monitor, and may help optimising glottic view by external laryngeal manipulations, since manoeuvres such as BURP may improve glottic visualisation both with conventional and videolaryngoscopy, as shown in the present study.
Videolaryngoscopy is not a technique to make endotracheal intubation faster, as shown in the present study: Time to successful intubation was quite comparable between direct laryngoscopy and videolaryngoscopy. However, it may help to make intubation safer. First, as shown previously, video-assisted laryngoscopes reduce the applied forces to the maxillary incisors as an objective measurement of intubation difficulty over standard blades . Second, compared to conventional Macintosh laryngoscopy, videolaryngoscopy, particularly with the C-MAC, has been shown in a manikin model to result in better visualisation, easier use, and faster intubation time . In the present clinical study, we were able to show that if one encounters unexpected difficulty of direct laryngoscopy, the use of the C-MAC size 4 may be advantageous if it is combined with the "straight blade technique", directly elevating the visualised epiglottis with the tip of the blade. Using this technique, we were able to improve the C/L class and subsequently intubation in 15 of 24 patients with suboptimal glottic visualisation.
As expected, subjective handling in patients with good or acceptable glottic view was best with the conventional Macintosh laryngoscope, which may result from the greater familiarity with this device (handle, grip, etc.); however, there were no differences between devices, or even a slight advantage for C-MAC3 and C-MAC4, if glottic visualisation was poor.
Some limitations of this study should be noted. First, we have included seven fasting patients with morbid obesity. It may be criticised that these patients may have a higher risk of regurgitation and aspiration of gastric content; however, standard institutional anaesthetic management for obese patients was applied, and all patients were successfully managed without complications. Second, fogging of the optical lens was transiently observed in 11 of 112 patients. As a result, the manufacturer has optimised the pre-heating system of the lens; thereafter, we did not observe any case of fogging in the remaining 38 patients. However, since fogging occurred transiently, it had no impact on intubation success. In two cases, dazzling (e.g. reflexions or inadequate luminance in comparison to bright surrounding light) of the monitor occurred, which is a common problem with videolaryngoscopy; however, both fogging and dazzling only may be deleterious in an emergency airway situation, if direct laryngoscopic view would not be possible. Third, our intraoperative data collection was performed by a non-blinded observer, which is possible source of bias. Finally, both data of ease or difficulty of intubation and handling the airway devices were subjective.
Footnote: a In the meanwhile, both GlideScope and McGrath videolaryngoscopes have been presented with Macintosh blade shapes.