Indirect laryngoscopes were developed to facilitate intubation in case of difficult direct laryngoscopy. In the past few years, multiple different devices have been commercialized. These tools have been mainly developed to facilitate intubation in cases of difficult direct laryngoscopy. Visualization of the vocal cords with these devices is achieved indirectly through either an optical or a video system [1–4]. At our institution, we have recently introduced three of these devices, which are shown in Fig. 1a–c: the Glidescope (GVL) (Verathon Inc., Bothell, USA), the McGrath (MGL) (Aircraft Medical Ltd, Edinburgh, UK) and the Airtraq laryngoscopes (AOL) (Airtraq, Prodol Meditec S.A., Vizcaya, Spain) [5–10]. The legend to Fig. 1a–c provides additional information pertaining to these devices.
Studies comparing indirect laryngoscopes, either on simulated airways or on humans, are emerging [11,12]. Some have used medical students as subjects, thus limiting the generalization to a more experienced population . Overall, the available data pertaining to the learning curves and usability of these techniques are still scarce.
In this study, we observed anaesthesia providers as they learned how to use the GVL, MGL and AOL laryngoscopes in simulated normal airways. Our purpose was to compare the learning curves, the efficacy, and the ease of use of these devices with the traditional Macintosh blade, in subjects experienced with the latter but inexperienced with using indirect laryngoscopes.
Following ethics approval and informed consent, we recruited sixty anaesthesia providers (20 staff, 20 residents and 20 nurse anaesthetists). Participants were excluded if they had performed fewer than 50 direct laryngoscopies or if they had ever used any of the indirect laryngoscopes.
The study design was a randomized crossover trial. Three different indirect laryngoscopes were compared with direct laryngoscopy using a Macintosh blade. All intubations were performed using a size 7 mm internal diameter endotracheal tube (ETT) on a SimMan manikin (Laerdal Medical Canada Ltd., Toronto, Canada) with normal airway settings. The participants were asked to intubate the manikin swiftly five times in a row with each laryngoscope. The Macintosh blade size 4 was always the first device to be used. Thereafter, the sequence of the use of the GVL (adult large blade), the MGL (mid-length position) and the AOL (regular size) was randomly assigned. Before testing a new device, a computer-based presentation and a live demonstration were given by a single instructor. Constructive feedback was also provided between each attempt.
All laryngoscopes were used according to manufacturers' instructions. A rigid stylet (Glidescope, Verathon Inc., Bothell, WA, USA) was used to help intubation with the GVL and MGL. The AOL was used with its video camera connected to an external screen.
The primary endpoint was the duration of intubation attempt defined as the time taken from insertion of the blade between the teeth until chest inflation with a self-inflating bag. The secondary endpoint included time taken at the fifth attempt to visualize the vocal cords (‘time to view’) and time taken to place the ETT in the trachea (‘time to place ETT’ = ‘duration of intubation attempt’ minus ‘time to view’). Additional endpoints included the modified Cormack grades , difficulty of use of the laryngoscopes (assessed with a visual scale from 0 easy to 100 difficult), and severity of dental trauma visually graded by the pressure on the upper teeth (0 = none, 1 = mild, 2 = moderate, 3 = severe) . After study completion, participants were asked to indicate their preferred laryngoscope.
Data for duration of intubation, time to view, time to place ETT, modified Cormack grades, severity of dental trauma and difficulty of use were analysed using paired nonparametric tests. The Friedman and Wilcoxon signed rank tests were used for multiple and posthoc comparisons, respectively. The choice of the preferred laryngoscope was analysed using a χ2 test. A Kruskal–Wallis analysis was performed to test for differences between participants' positions (staff and resident or nurse).
All participants completed the study and all intubation attempts were successful. Participants' experience in clinical anaesthesia ranged from 0.5 to 35 years (median 7 years).
The durations of the five attempts with each device are graphically displayed in Fig. 2 and comparisons are detailed in Table 1. With all laryngoscopes, the duration of intubation significantly decreased from the first to the fifth attempt. The Macintosh blade always provided the fastest intubation. Among the indirect laryngoscopes, the AOL had the steepest learning curve and, from the second attempt, mirrored the Macintosh. The GVL and MGL had steep and similar learning curves, but, after five attempts, intubation times were longer than with the Macintosh and the AOL.
At the fifth attempt, the time taken to visualize the vocal cords did not differ among the four laryngoscopes (Fig. 3a). However, the time taken to manipulate and to place the ETT was shorter for the Macintosh and AOL than with the GVL and MGL (Fig. 3b). These differences explained the differences observed in the total duration of the intubation attempt (Fig. 3c).
Modified Cormack grades, severity of dental trauma and difficulty of use differed between the devices (Table 1). Post-hoc analysis indicated that indirect laryngoscopes provided better laryngeal exposure and potentially less dental trauma than direct laryngoscopy. Differences also existed among indirect laryngoscopes: the AOL and MGL provided better results than the GVL. Interestingly, the choice of the participants for their preferred laryngoscope did not differ.
With the Macintosh blade, staff anaesthesiologists obtained better laryngeal views (P = 0.013) and shorter intubation times (P = 0.004) than nurses and residents. Laryngeal exposure with indirect laryngoscopes was not influenced by the participants' position; however, intubation times differed for the MGL (P = 0.001, staff and residents faster than nurses) and for the AOL (P = 0.022, staff faster than nurses).
Using a normal airway model, we compared the learning curves, the efficacy, and the ease of use of three indirect laryngoscopes with the Macintosh blade. With all laryngoscopes, intubation time decreased from the first to the fifth attempt reflecting rapid learning and skills acquisition. For the Macintosh, this may appear surprising; however, since it was always the first to be used, this probably reflects the fact that participants were getting used to the manikin. The AOL displayed the most favourable learning curve of the three devices tested.
At the fifth intubation attempt, time taken to visualize the vocal cords did not differ between the three indirect laryngoscopes. However, the time taken to manipulate and to place the ETT was shorter for the AOL than for the GVL and the MGL. These differences between the three devices probably reflect differences in the techniques for ETT placement. The AOL has a channel to guide the ETT; therefore, once an adequate view of the glottis has been obtained the AOL is kept in place with the left hand, whereas the ETT is advanced with the right hand. If the glottis is centred, aligned and not too close to the tip of the AOL then the ETT is self-guided towards the glottis. The GVL and the MGL use a different technique for placing the ETT: once an adequate view of the glottis is obtained, the operator holds the laryngoscope in place with the left hand and must steer the ETT with the right hand. The view on the screen is used to guide and to correct the direction of the ETT. The loss of three-dimensional visual depth on the screen and the fulcrum effect of the stylet may have an influence on psychomotor skills acquisition. These kinds of difficulties are well known in laparoscopic surgery training . Therefore, it is likely that the skills to direct the ETT in the larynx with the GVL and the MGL are more complex and require more time to be performed than with the AOL. Whether such a difference in time to place the ETT exists in clinical practice and is of any clinical relevance is currently unknown and deserves further clinical comparative studies. The problem is certainly more complex in practice. For example, with the AOL it may be difficult to align the view of the glottis. We recently reported the case of a difficult airway patient where the guiding channel of the AOL was a clear imitation and where the steering technique of the MGL was in fact very helpful .
Compared with the Macintosh, indirect laryngoscopes provided better layngeal views and potentially less dental trauma. Regarding these outcomes, the GVL appeared slightly less efficient than the MGL and AOL. These findings suggest that not only do these devices facilitate endotracheal intubation when direct laryngoscopic view is poor, but they may also reduce the risk of dental trauma in such situations. All laryngoscopes were judged easy to use by the participants with minor differences among devices of uncertain clinical relevance.
The main limitation of our study is the use of a normal airway model instead of humans and the uncertainty of skills transfer to the clinical realm. For example, as suggested above the guiding channel facilitates rapid intubation, but steering techniques may provide greater manoeuvrability in case of difficult airway . In addition, although the realism of the SimMan airway is fair, it is not perfect . Therefore, some aspect of the model itself could have worked in favour of a device or could have put it at a disadvantage. In addition, we have used only ‘normal’ airway for this ‘learning curve study’ and transferability of the results to simulated ‘difficult airways’ is an area that we are investigating. Finally and of greater importance, some factors such as the presence of secretions, blood or fog, increases the difficulty in clinical practice and can lead to failure of indirect laryngoscopic intubation. Currently, even the best manikin model cannot reproduce these difficulties. Bearing in mind these caveats, the simulation laboratory remains a safe and useful environment to test new pieces of equipment and to learn how to use them before bringing them into clinical practice and testing them in clinical situations. Another limitation of the study is that the duration of intubation did not reach a plateau after five attempts, suggesting that improvement could continue.
In conclusion, our results indicate that, in a manikin-based study, and for practitioners experienced with direct laryngoscopy but inexperienced with using indirect laryngoscopes, the AOL and, to a lesser extent, the MGL and the GVL are easy to learn and appear to provide advantages over the Macintosh blade. Because this population of practitioners represents a significant proportion of our community, we believe that these results are important. However, further comparative studies are needed in simulated difficult airway scenarios as well as in patients with or without difficult airways to compare the relative advantages and limitations of indirect laryngoscopes.
We would like to thank the Department of Anaesthesia for financial support, the staff, residents and nurses for their participation in the study, and the simulation centre coordinator, José Manuel Garcia, for his priceless logistical support.
This study was supported by departmental funding.
Presented in part at the European Society Anaesthesiology Meeting, Euroanaesthesia 2008, Copenhagen, Denmark, June 2008.
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Keywords:© 2009 European Society of Anaesthesiology
Airtraq laryngoscope; anaesthetic equipement; education; glidescope videolaryngoscope; intratracheal; intubation; laryngoscopes; McGrath videolaryngoscope