An increasing number of new indirect laryngoscopes have become available as a result of improved optical technologies. A quantitative review and meta-analysis of the performance of these devices concluded that ‘there is very limited and inadequate comparative data between devices and compared to the standard Macintosh laryngoscope. A new approach to this area of research is needed’.1 In addition to optical changes, many of the new devices appear different in shape from the Macintosh blade, which implies that they may also be functionally different. One of the main limitations of Macintosh laryngoscopy is failure to provide an adequate view when the mandibular space available for tongue displacement is reduced.2–7 Current mannequins are designed to deal with this feature of Macintosh laryngoscopy in that they have a compressible and displaceable tongue. To understand whether the new blades perform differently, we considered that a graded system of relative space reduction (easy, intermediate, difficult) would be most likely to demonstrate any functional differences. To this end, we decided to use the ‘easy’ (normal tongue/normal space) and ‘tongue oedema’ (large tongue/normal space) standard settings for the Laerdal mannequin. For the ‘intermediate setting’, we designed a purpose-built ‘tongue restrictor’ (normal tongue/restricted space) based on a non-deformable, easily removed and inserted prosthesis that was positioned inside the mandible.
We decided to undertake a study with two objectives: to validate our model of graded difficulty and suggest functional comparisons between devices. Indirect laryngoscopes were chosen on the basis of different shapes relative to Macintosh. Glidescope Video Laryngoscope (GVL) has a 60° angulation in the blade, Truview EVO2 a 35° angulation, and Airtraq a gentle distal curve (Fig. 1).
Following Local Research Ethics Committee approval and written informed consent, 20 anaesthetists consented to participate in this study. Power analysis for the number of participants required was derived from the results of our previous similar study.8 Using time to tracheal intubation, we calculated that 20 study participants would be needed for 90% power to find a 15 s difference in intubation time for the difficult (tongue oedema) setting and a 5 s difference in the easy (normal) setting. All the participants were briefed about the study. The instruments we studied were: Truview EVO2 (Truphatek Int. Ltd, Netanya, Israel), Glidescope (GVL size 4; Verathon Medical, Buckinghamshire, UK) and Airtraq (regular size 3; Prodol Meditec S.A., Vizcaya, Spain), with a standard size 4 Macintosh blade (Optima; Timesco Limited, London, UK). Apart from the Macintosh blade, none of the anaesthetists participating in this study had any prior experience with any of these laryngoscopes. As an introduction to these devices, all the anaesthetists taking part were given an explanation as to the recommended use of the laryngoscopes and each was demonstrated in the normal setting with SimMan. They were then given 30 min to practise intubations using the mannequin. Intubations with Glidescope were performed using the GlideRite rigid stylet specifically designed by the manufacturer (angle of stylet complements the angle within the Glidescope blade), and instructions for its use were followed as per the manufacturer recommendations. For intubations with Truview, the Optishape stylet recommended by the manufacturer was used. A Frova single-use introducer was used for intubations with the Macintosh laryngoscope. For Airtraq no introducer was used. Once they felt suitably familiar with all the instruments, anaesthetists went on to perform the study protocol.
The simulator used was a Laerdal SimMan. Three settings were used: easy (resting mannequin setting), difficult setting (tongue oedema setting) and intermediate setting (insert setting). The intermediate setting was designed to restrict the tongue compression and fashioned from an elastomer silicone kneadable material, Finopaste (FINO, Kissengen, Germany). The size and shape for this prosthesis was determined in a preliminary study using the Macintosh blade. The size used was chosen for the view obtained and time for laryngoscopy so that each parameter was approximately midway between the easy and difficult settings with the Macintosh blade. The suitability of the material was confirmed by its easy insertion, removal and ability to withstand repeated laryngoscopy (Fig. 2).
Participating anaesthetists were not told the order or nature of the test settings (i.e. easy, intermediate (mandibular insert) and difficult (tongue oedema)), although these could obviously not be blinded. We used block randomisation with the order determined by blind drawing of previously marked cards. All four blades were used in each of the three settings so that each participant performed 12 intubations. Successful tracheal intubation was confirmed by opening the flap at the front of the mannequin neck.
The primary outcome measures were view obtained and time to intubate. Each participant was asked to grade the view according to both Cormack–Lehane grade9 and the percentage of glottic opening (POGO) score.10 Time to intubate was the time in seconds from the time the anaesthetist picked up the laryngoscope to the time the tracheal tube cuff was inflated.
Secondary outcome measures were success or otherwise, number of intubation attempts and degree of difficulty with laryngoscopy and intubation separately, and user-rated feeling of tongue compression. Failure was when intubation was not achieved or the time exceeded 120 s. An attempt was defined as forward thrust made with the tracheal tube or introducer with the intention of advancing it into the trachea. At the end of each laryngoscopy, each participant was asked to score the degree of ease with laryngoscopy and intubation separately on a visual analogue scale (VAS) (0–100; 0 being extremely easy and 100 extremely difficult) and their subjective tongue compression score (1–5, 1 being the lowest and 5 the highest).
The study data were analysed using SPSS statistical package (version 13). Time to intubate was analysed by Kaplan–Meier survival analysis (time to intubate as the time variable, successful intubation as the event). The difference between these curves was analysed using the log-rank test. Success rates were compared with Friedman's test and this was also used to compare POGO, Cormack–Lehane grade, number of attempts and force used. Pairwise analysis was performed within the settings with McNemar's or Wilcoxon test as appropriate. P values of 0.05 or less were considered to be significant.
All the 20 anaesthetists completed the study protocol. The anaesthetists who participated in this study were a combination of trainees with varied level of experience and consultant anaesthetists with many years of experience. The median (range) anaesthetic experience of the participants was 6.7 (0.5 to 20) years. None of them had any prior clinical experience with any of the indirect devices tested in this study.
Table 1 shows the comparisons between easy vs. intermediate settings and intermediate vs. difficult settings for individual blades. For Macintosh, there is a consistent gradation which is true for almost all outcome measures (apart from the number of successful intubations and the number of attempts needed between easy and intermediate settings). Similarly, Glidescope shows a parallel pattern of worsening of measures with increasing difficulty (except from the number of successful intubations between each pair of settings). On the contrary, Truview shows little change between easy and intermediate settings (apart from tongue compression and ease of laryngoscopy) but a worsening between intermediate and difficult (apart from the number of successful intubations and the number of attempts). Airtraq is different again in that it demonstrates Macintosh-like worsening between easy and intermediate settings, but it is the only blade to show no worsening from intermediate to difficult.
Table 2 shows that in the easy setting Macintosh was superior in terms of intubation times. Truview scored worse for both ease of laryngoscopy and intubation. Both Glidescope and Airtraq had worse ease of intubation, but Airtraq provided a better view and with less tongue compression. In the intermediate setting, Macintosh was again superior in terms of time required to intubate and number of attempts (except for Glidescope). Ease of intubation was better than for all the other blades. However, both Airtraq and Truview were better in terms of the view obtained. Airtraq was again better in terms of the tongue compression score and this time also better for ease of laryngoscopy. For the difficult setting, Macintosh had poor scores in all measured outcomes. Airtraq was superior to Macintosh in all respects apart from the number of attempts. Glidescope was more successful than Macintosh with better views and ease of laryngoscopy scores. Truview provided better views and ease of laryngoscopy with less tongue compression.
Additional post-hoc comparisons were made between the indirect laryngoscopes in the difficult setting wherein Airtraq proved to be superior to both Glidescope and Truview in providing a significantly better view, ease of laryngoscopy and less tongue compression. However, ease of intubation was not significantly better than either Glidescope or Truview.
Certain design constraints in this sort of study are inevitable. Anaesthetists are familiar with Macintosh laryngoscopes and these devices cannot be blinded in use. Equally it was not considered possible to blind the settings in our model of progressive difficulty. However, all the indirect laryngoscopes were treated equally in that none of the participating anaesthetists had prior exposure to them. Simulation studies of difficult laryngoscopy, although popular in recent years,11–19 have been criticised as to whether the results represent the real world.1 On the contrary, because patients are heterogeneous, clinical trials will always have their own limitations (i.e. whether the sample is truly representative). In our view, there are two important uses for simulation: first, in trying out devices before using them in humans (e.g. how easy they are to use) and second, to test various mechanical or functional hypotheses because with a mannequin we can impose specific reproducible test conditions. The native mannequin settings (resting and tongue oedema) for SimMan are well validated in previous studies.20–24 In designing this model we wanted to develop an intermediate stage of difficulty based on some of the most important limitations for Macintosh laryngoscopy, that is, mandibular space reduction and tongue compression. The insert we designed to this end was chosen as a result of a pilot study comparing similar prostheses of different size. It is easily manufactured and reproduced. It successfully withstood repeated insertions into and out of the mannequin and multiple laryngoscopies. For the Macintosh laryngoscope, our model provided a consistent gradation of difficulty for all measures between the three settings. In that sense, the insert was validated as a useful intermediate setting.
In deciding which laryngoscope blades to use for this study, we were interested in comparing some of the newer indirect laryngoscopes with Macintosh. First, we were interested in shape/angulation characteristics and second, that indirect laryngoscopes have been suggested to fall into two broad classes – either ‘steering’ or ‘channel’ devices.15 Airtraq was our choice of channel device and Glidescope was our choice for steering device. We have been interested for some time in angled blades and reported previously on a comparison between Truview and the original Belscope angled blade, which could also be used with a prism and, as such, was one of the first indirect viewing laryngoscopes.25–26 (Truview also happens to be steering in type.)
From our insert pilot studies, we had anticipated some failures with Macintosh in the intermediate setting due to reduced view. However, user familiarity and ability to cope with reduced view using a Macintosh laryngoscope meant that there were no failures. A positive feature of Macintosh direct laryngoscopy is that it provides an all-round view making it particularly amenable to use with a bougie. Indirect laryngoscopes, on the contrary, did not show 100% success or were not faster in the intermediate setting, despite a significant improvement in the view. This finding is similar to previous studies wherein a better view was not equivalent to an easier intubation (i.e. ease of laryngoscopy does not equate with ease of intubation).17–19 This should be considered the main negative feature of indirect laryngoscopy.
This feature has been suggested to vary according to whether an indirect laryngoscope is a channel device (e.g. Airtraq or Pentax AWS) or a steering device (e.g. Glidescope or Truview EVO2).15,21,27 In our study, Glidescope (our typical steering device) showed no advantage in the easy and intermediate settings; in fact, it scored worse for ease of intubation in the intermediate setting, despite the ease of laryngoscopy being the same as Macintosh. This is similar to the findings reported in the study by Savoldelli et al.28 in which pharyngeal obstruction and cervical spine rigidity scenarios were used to compare Glidescope with Macintosh. In order to overcome the difficulty with intubation, various tube directing manoeuvres have been suggested, including the use of different types of stylet and altering their angulation.24,29,30 For our study, participants were asked to use the rigid GlideRite stylet only and no modification of the stylet was allowed. (The tube was loaded onto the stylet and used as per the manufacturer's instructions.) It is worth noting, however, that other studies have shown that a standard malleable stylet performed equally well compared with GlideRite31 and better than the Flex-It stylet.32
Channel devices may have a natural advantage over steering devices because problems directing the tube towards the larynx are facilitated by the channel itself.21,27,28 In our study, Airtraq was the most successful device in the difficult setting; however, it did require longer intubation times in the easy and intermediate settings. This is suggested to be due to emergence characteristics of the tracheal tube from the channel and relevance of the distance of the tube tip from the inlet.33,34 Alternative insertion techniques and manipulations of the Airtaq have been described to improve intubation times.34,35
As far as testing the blades in the model was concerned, the patterns of change in the study parameters with worsening laryngoscopy conditions were interesting. The model was designed to produce worsening of all parameters for Macintosh laryngoscopy and it did. However, Glidescope was the only indirect laryngoscope to show similar worsening to Macintosh. Truview showed lesser worsening in the intermediate setting but similar worsening for the difficult setting. Airtaq was unique in the sense that after initial worsening in the intermediate setting, it showed no further worsening in the difficult setting. We suggest that this implies similar functionality for Glidescope compared with Macintosh and totally different functionality for Airtraq. From the point of view of individual blade comparisons, Truview gave a better view than Macintosh in the intermediate setting even though this was at the cost of increased number of attempts. Equally in the difficult setting, it gave better views and an easier laryngoscopy than Macintosh but not an easier intubation. On the contrary, Airtraq gave better views than Macintosh in the intermediate setting and was the most successful laryngoscope and with faster intubation times in the difficult setting. This was achieved with significantly less tongue compression in all the settings. It was also noteworthy that the mean POGO score for Airtraq in the difficult setting was almost identical to that for Macintosh in the easy setting.
Our idea was to compare the chosen devices in the context of their functionality which the intermediate setting allowed. As a result, we can state that Glidescope should be considered as similar to Macintosh because of the sequence of change between the easy, intermediate and difficult settings. Using similar arguments, comparing Airtraq with Macintosh allowed us to conclude that Airtraq is quite different. Airtraq may, therefore, represent an important step in the right direction in dealing with the main limitations of the Macintosh blade. Even with reduced mandibular space, it appeared to reduce the need for tongue compression while offering improved views. Macintosh should still be seen as having advantages in our intermediate setting but it remains to be seen whether this can be explained entirely on the basis of familiarity with the device in question. Our model also suggests that Glidescope and to some extent Truview are functionally similar to Macintosh. Although others have had the same results as ours, for the difficult (tongue oedema) setting, it is the intermediate setting which allows a convincing argument as to the basis for these differences.
We suggest that this new insight will change the perception as to how indirect laryngoscopes should be analysed. The new optical systems should be expected to have an obvious advantage even if there is some cost in terms of directing the tube into the larynx. However, many different shapes of blades have appeared with no justification as to why the particular shape was chosen. It is, therefore, reasonable to question whether a better optical system is actually matched by any improvement in the blade design. Indeed, it is essential to question whether optical improvements could even compensate for and overshadow a change in the blade shape that was functionally worse than that of Macintosh.
We would like to thank our anaesthetic colleagues who agreed to be involved in this study and the staff at the Aintree Simulator Centre.
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