The C-MAC (Karl Storz Endoscopy, Tuttlingen, Germany) is a videolaryngoscope that has a modified Macintosh blade design allowing it to be used as an indirect or direct laryngoscope.1 The C-MAC D-blade is a new highly angulated videolaryngoscope blade, designed specifically for patients with difficult airways for use with the current C-MAC modular system.2 The device incorporates a distal lens two thirds of the way along the blade that allows an image to be transmitted via a camera head to a video unit (Fig. 1). The blade is designed to be used as an indirect laryngoscope because of its increased curvature. The problem with indirect laryngoscopes is that despite a good view of the laryngeal inlet, directing the tube into the trachea can be difficult. This was a point highlighted in the Royal College of Anaesthetists National Audit Project Number 4 (NAP4) that examined the major complications of airway management in the United Kingdom.3
In a manikin study of the standard C-MAC, the use of a stylet alone did not confer benefit in the setting of easy laryngoscopy; however, in more difficult laryngoscopy settings, the C-MAC performed best when used with a stylet that angulated the distal tracheal tube.4 The use of an introducer has been shown to aid intubation with nonchannelled indirect laryngoscopes such as the Glidescope (Verathon, Bothell, West Virginia, USA).5 The time to intubation was decreased with the Glidescope when a malleable stylet was inserted into the tracheal tube and the tracheal tube then bent into a ‘hockey stick’ curvature.6 The metallic Gliderite stylet (Verathon) has been developed for this purpose. The manufacturer of the D-blade encourages the use of an introducer and a semiflexible tube guide has been used in a clinical study.2 The benefit of stylets over bougies, however, remains unclear. Preloading the tracheal tube onto a tracheal tube introducer has been advocated by some anaesthetists, especially those in a prehospital environment, as it may reduce the duration of the intubation attempt. We were, therefore, interested in looking at both stylets and bougies when intubating with the C-MAC D-blade. The use of a curved introducer to match the curvature of the D-blade was also deemed an important feature of the introducers being studied.
Our aim was to compare the efficacy of the C-MAC D-blade videolaryngoscope when used with a tracheal tube versus five different tracheal tube introducer strategies in a simulated easy and difficult airway setting using a SimMan 3G manikin (Laerdal, Kent, UK). The expected view obtained with the D-blade in the difficult airway setting of the manikin is shown in Fig. 2.
The study (R&D ref: 500/12) was approved by Aintree University Hospital NHS Foundation Trust Research and Development committee (August 2012). Following discussion with the National Research Ethics Service (ref 04/50), it was not considered to require review by an NHS Research Ethics Committee in keeping with the current policy of the UK health department.
Twenty-four anaesthetists of at least 12 months of experience consented to participate in the study. The participants were given a standardised tutorial and demonstration on how to use the C-MAC D-blade videolaryngoscope. They were then allowed to practise endotracheal intubation on the high fidelity manikin with each of the tracheal tube introducer strategies. This was undertaken in both the easy and the difficult airway settings. For all intubation attempts, the C-MAC D-blade laryngoscope was used as an indirect laryngoscope. All intubations were performed with a 7.0 mm internal diameter cuffed tracheal tube.
The six tracheal tube introducer strategies were as follows: tracheal tube with no introducer (TX); hockey stick stylet (SH); Gliderite stylet (SG); bougie with tube loaded distally, near its curved tip (BD); bougie with tube loaded proximally (BP) and bougie unloaded until tracheal placement (BU) (Fig. 3). The hockey stick shape of the stylet had an angle of 100° created 8 cm from the distal tip of the 7.0 cuffed tracheal tube. The tracheal tube introducer also had a standardised curvature. An appropriate template was used throughout the study to reform and check the shape of this introducer.
Participants were asked to intubate the trachea of the manikin in two different settings using each of the introducer-tracheal tube configurations. The participants had to obtain an adequate indirect view of the laryngeal inlet with the videolaryngoscope before being handed the tracheal tube/introducer. This was assessed by the researcher using a percentage of glottic opening (POGO) score of at least 50%. The easy setting was the normal airway state of the manikin, whereas the difficult setting involved cervical spine immobilisation with a hard collar and maximum tongue oedema.
The primary outcome was the duration of the successful tracheal intubation attempt. This was defined as the time taken from insertion of the blade between the teeth of the manikin until the tracheal tube was seen passing through the vocal cords. The rate of successful tube placement in the trachea was a secondary endpoint. The participants were allowed two attempts at intubation. A failed intubation was defined as taking more than 60 s to intubate or the withdrawal of the videolaryngoscope blade out of the mouth. Failure to intubate by time overrun or giving up was counted as an intubation time of 60 s. When a second attempt was made, the result was recorded as if it was the first and no time penalty was added. An additional secondary outcome was a visual analogue score (VAS) of overall ease of use for the introducer strategy (0 being extremely easy, 10 being extremely difficult). One unblinded investigator recorded all data and also assessed all the POGO scores to ensure consistency in deciding whether laryngoscopy was adequate so that the intubating device could be passed to the anaesthetist.
A crossover design was employed with 12 experimental conditions (six introducer strategies × two manikin settings). The run order was devised using two 12-by-12 Latin squares necessitating 24 anaesthetists to complete two full repetitions of the experiment. The use of Latin squares to devise the run order ensured that each anaesthetist undertook all 12 experimental conditions once and did so in a unique run order. Furthermore, the Latin squares ensured that each experimental condition occurred at each position in the run sequence twice (once per Latin square), for example, endotracheal tube alone in the normal manikin was undertaken as the 10th condition by only anaesthetists 1 and 22. This last property underlies the choice of Latin squares to design the run order, as if a practice effect was present, no experimental condition would be treated preferentially. The use of Latin squares to devise the experimental run order here is not to be confused with the analysis methods associated with Latin square designs used elsewhere.
For the statistical analysis, the results were partitioned into the two manikin settings. Within each manikin setting, Friedman's test was used; no period effect was assumed. Given that there were two tests (one for each manikin setting) to be conducted for the primary endpoint, the F-statistic for the Friedman test was evaluated against a Bonferroni corrected α level of 2.5%. In the cases in which Friedman's test was found to be statistically significant, all post-hoc comparisons were undertaken (from six experimental conditions, there are 15 unique pairs/combinations of size 2). The P values from the post-hoc tests were corrected using the Bonferroni–Holm method and were again evaluated against an α level of 2.5%. Although not strictly necessary, this α level was retained for the secondary endpoint, which was subject to the same analysis strategy.
All statistical analyses were undertaken using the Agricolae package in R software (R: A Language and Environment for Statistical Computing; R Foundation for Statistical Computing, Vienna, Austria).
Twenty-four anaesthetists with at least 12 months of anaesthetic experience were recruited. These were as follows: 12 consultant anaesthetists; six senior trainee anaesthetists (specialist trainee years 5 to 7) and six junior trainee anaesthetists (core trainee year 2). In each laryngoscopy setting, there was no difference between the different introducer-tracheal tube strategies with regard to the laryngeal view obtained with the C-MAC D-blade, as assessed by the POGO score. Any differences seen in the outcomes assessed were, therefore, because of differences in the efficacies of the introducer-tracheal tube strategy tested.
In the easy setting, overall all intubations were successful; however, three (13%) anaesthetists required a second attempt using the tracheal tube with no introducer. Times to intubation [median (interquartile range)] in seconds were as follows: SH 8.5 (7 to 11); SG 10 (8 to 11.5); BD 11 (10 to 12.5); TX 11 (7 to 31.5); BP 12 (11 to 13.5) and BU 13 (11 to 14.5) (Fig. 4a). We found evidence of an overall difference in introducer strategies for time to intubate (P < 0.025) with SG and SH found to be favourable when compared with BU, and SH was also favourable when compared with BP.
The corresponding VASs [median (interquartile range)] were as follows: SH 1 (0 to 2); SG 1 (0.5 to 2); BD 1 (0 to 2); TX 3 (1 to 6.5); BP 2 (1 to 3) and BU 1 (0 to 2) (Fig. 4b). We found evidence of an overall difference in introducer strategies for visual analogue scores (P < 0.025) with BU and SH found to be favourable when compared with the tube alone (TX).
In the difficult setting, 13 (54%) anaesthetists failed to intubate with the tracheal tube with no introducer and one (4%) with the tracheal tube with hockey stick-shaped stylet. A second attempt at intubation was allowed if there had been failure on the first attempt. This occurred in the hockey stick-shaped stylet (SH) and BD.
Times to intubation were as follows: SG 11.5 (10 to 17.5); SH 14 (12 to 22); BD 15.5 (12 to 23.5); BU 16.5 (14 to 21); BP 16.5 (15.5 to 20.5) and TX 60 (26.5 to 60) (Fig. 5a). We found evidence of an overall difference in introducer strategies for time to intubate (P < 0.025) with all introducers found to be favourable compared to TX. SG was found to be favourable when compared with BU and BP.
The equivalent VASs [median (interquartile range)] were as follows: SG 2 (1 to 3); SH 2.5 (1.5 to 3); BD 3 (1.5 to 5); BU 2 (1 to 3); BP 3 (1.5 to 4) and TX 9 (6.5 to 10) (Fig. 5b). We found evidence of an overall difference in introducer studies for visual analogue score (P < 0.025) with all introducers found to be favourable when compared with TX. SG was found to be favourable when compared with BD. We found evidence of an overall difference in introducer studies for visual analogue score (P < 0.025) with all introducers found to be favourable when compared with TX. SG was found to be favourable when compared with BD.
The differences in time to intubation in both the easy and difficult airway settings were small and not clinically significant for the different introducer strategies studied. The stylet introducers tended to take less time because of the decreased manoeuvring required to advance the tracheal tube, but even this did not result in important statistical differences.
The use of the C-MAC D-blade without an introducer (tube alone strategy) resulted in failed intubation attempts and should be avoided. This finding replicated studies with similar nonchannelled indirect laryngoscopes and was seen in the difficult airway setting in which the angulation provided by an introducer was required. It is noteworthy, however, that even in the easy setting, three out of 24 anaesthetists failed to intubate the trachea on their first attempt.
The VASs generally reflected the intubation times and this was more pronounced in the difficult airway setting. There was no clear preference for one particular introducer among the anaesthetists studied.
The comparable results for stylets and bougies when using the C-MAC D-blade is a new finding. The use of bougies to aid intubation is commonplace and is a technique that many anaesthetists are familiar with and may be preferred by some over the use of a stylet. It is reassuring to know that both techniques were similar in performance.
The design of a specific introducer for use with this video laryngoscope may not be required as, despite many claims to the contrary, the alternative introducing strategies used here did not appear to produce any worthwhile advantage relative to one another.
There are limitations to this study. This is a manikin study, although the use of anatomically correct manikins to evaluate airway equipment is well established in the literature and they do have the major advantage of testing alternative devices in precise stable settings.7–11 There is also evidence to suggest good correlation with subsequently performed clinical studies.12,13 The SimMan airway has also been evaluated and has been shown to be generally acceptable as realistic.14 The use of a single endotracheal tube size (7.0 mm internal diameter) was consistent with a previous study of stylets using the SimMan manikin4 and this size was deemed most appropriate for the manikin's airway. It is not possible to comment on whether the findings would be the same for different endotracheal tube sizes.
The researcher observing the intubation was not blinded as this would have been difficult to undertake and so there is a risk of reporting bias. The time limitation was similar to that in previous manikin studies comparing intubation with different devices,4,15 in which a failed tracheal attempt was taken as 60 s. This was because some anaesthetists ceased attempting intubation sooner than 60 s when they felt intubation was impossible (i.e. the observation was right-censored), so if the attempt duration had been used, this would give a falsely low reading in the face of a failed intubation.
Another limitation is the possibility of a practice effect. The initial practice session was put in place to minimise the impact of this effect and the Latin square design of this experiment should ensure that if any practice effect existed, it would bring the results for the various methods closer together (i.e. the comparisons will be conservative). In addition, an informal assessment of the results over the periods demonstrated no obvious practice effect. The use of trainees could be seen as a potential limitation. We felt this was representative of clinical practice and there appeared to be no clear difference between the different grades of anaesthetists.
The introduction of videolaryngoscopes has the potential to greatly improve airway management. The use of these devices for the management of difficult airways has become routine in many institutions. The indirect view of the laryngeal inlet, however, brings with it different challenges. It has become clear that improvement in laryngeal inlet visualisation does not necessarily make the process of intubating the trachea any easier. In fact, the improved view may actually be detrimental to the uninitiated, because with a direct laryngoscope, the problem with tracheal tube delivery can be immediately recognised. In this study, we wanted to determine the most suitable introducer for use with the C-MAC D-blade videolaryngoscope. In conclusion, it would appear that all the introducers studied were comparable and either a stylet or bougie could be used for intubation with this new device.
Acknowledgements relating to this article
Assistance with the study: none.
Financial support and sponsorship: the C-MAC D-blade videolaryngoscope was loaned to the research team by Storz for the duration of the study. There was no funding for the study.
Conflicts of interest: none.
Presentation: presented at the Anaesthetic Research Society Meeting, Royal College of Anaesthetists, London, UK, December 13–14, 2012.
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