The incidence of traumatic cervical spine injuries in blunt trauma is known to be relatively low at 2.4%.1 However, these injuries are often coupled with significant morbidity and mortality, largely as a result of associated spinal cord injuries.2 Although little can be done about the initial injury to the spinal cord, the utmost care should be taken to prevent secondary insults, particularly with regard to appropriate spinal immobilisation during airway management.2,3 During any airway intervention, the head should be maintained in a midline neutral position to avoid potentially devastating neurological sequelae.2,4 Current evidence suggests that manual in-line stabilisation (MILS) is superior to other forms of immobilisation, such as rigid collars.5 However, the use of MILS is not without risks; one concern is that the manoeuvre is independently associated with an increased frequency of difficult laryngoscopy.5–8 MILS, in combination with a number of other confounding variables found in multisystem trauma, results in a higher incidence of difficult or failed intubations,9 which has significant implications for anaesthetic-related morbidity and mortality.10
These issues are in part responsible for the increasing demand for secondary intubating devices, such as the intubating laryngeal mask airway (iLMA),11,12 McCoy laryngoscope13,14 and a variety of video laryngoscopes,15–20 many of which have shown promise in the management of the trauma-related difficult airways. However, there are a number of potential limitations, particularly with video laryngoscopes.16,19,20 Portability and ease of use is vital in our own trauma centre, where the bulk of major trauma patients are anaesthetised outside of theatres in the prehospital setting or the Emergency Department. Studies tend to concentrate on nonportable and expensive video laryngoscopes, often not suitable in the prehospital setting.15–19,21,22 Only one other study to date has compared the Macintosh with the McCoy, Airtraq and iLMA;23 however, only novice medical students were studied and no quantitative assessment of the force of intubation was made. The force of intubation has been studied by a number of authors.20,24–26 In most cases, the force of intubation was used as part of the evaluation of a new device against the Macintosh, and not as a discriminating factor across multiple devices.24–26
The purpose of this study was to compare the effectiveness of the Macintosh laryngoscope with cheap and portable secondary intubating devices in a high fidelity simulated difficult airway with MILS. Along with time to intubation, we evaluated the rate of successful intubation, the Cormack and Lehane grades at laryngoscopy, and the force of intubation.
Materials and methods
This study did not involve any human or animal participants and formal ethical approval was not necessary (MsC Bargery, Research and Development Governance Operations Manager, Queen Mary Innovation Centre, University of London). The study was registered with the Trust's Clinical Effectiveness Unit. We studied medical doctors (emergency physicians, anaesthetists and intensive care physicians) who would be expected to perform tracheal intubation in severely traumatised patients. The Royal London Hospital is the UK's busiest trauma centre and the base of London's Helicopter Emergency Medical service (HEMS). The hospital receives over 1800 trauma calls a year, 25% of which fall into the ‘severe’ trauma category (Injury Severity Score >15). Written informed consent was obtained from all participants. The inclusion criteria were a minimum of 5-year postgraduate training, more than 1000 anaesthesia inductions and previous experience with all the devices.
A SimMan manikin (Laerdal, Kent, UK) was placed on a new set of Salter Brecknell WS 60 Electronic Bench Scales (0 to 60 ± 0.02 kg; Salter Brecknell, Smethwick, UK), which was in turn placed on a solid surface on a standard hospital trolley. The scales were used to measure the change in manikin mass during laryngoscopy. Resources available to the participant during the study were a size 3 Macintosh laryngoscope and handle (Optima, Timesco Ltd, London, UK); a size 3 McCoy laryngoscope and handle (Optima, Timesco Ltd); a regular size (blue) Airtraq (Fannin UK Ltd, Reading, UK); a disposable size 4 Fastrach iLMA, size 7.0 mm tracheal tube and 50 ml syringe (Laryngeal Mask Company, Henley-on-Thames, UK); a size 7.0 mm tracheal tube (Mallinckrodt, Chesterfield, UK); and 15 F x 60 cm bougie (Cook UK Ltd, Letchworth, UK), a 2 l adult self-inflating bag (Intersugical, Wokingham, UK) and a competent assistant.
Participants were given a standardised demonstration of each device on the SimMan manikin by one of the investigators. Each participant was then allowed up to 10 min to practice tracheal intubation on the manikin to allow them time to familiarise themselves with the manikin and simulation, as opposed to gaining further experience with individual devices. The participants were told that adequate preoxygenation had been undertaken, the manikin was ‘anaesthetised’ and neuromuscular blockade had been induced. The high fidelity Simman manikin was configured for both a normal and difficult airway scenario. The difficult airway scenario (with simulated MILS) was achieved by activating the cervical spine immobility and tongue inflation modes, and tape was applied across the manikin's forehead to stop any optimisation manoeuvres.
The design was a cross-over, simulation-based study. Participants were asked to intubate the trachea in both scenarios with each of the four intubating devices. The sequence in which the devices were used was randomised (using a random sequence generator). The duration of intubation was measured from when the intubating device passed the between the teeth to the confirmation of cuff inflation and correct tracheal tube placement (primary end-point). In each case, the final tracheal tube position was verified by the investigators. Participants were allowed 120 s for each intubation attempt, after which the attempt was deemed to have failed. Along with success rates, time to intubation, the Cormack and Lehane grade at laryngoscopy, and the maximum change in mass of the manikin were also measured. The change in mass was used as a surrogate measure for the force of laryngoscopy and intubation in a vertical vector.
In a previous manikin study, in which clinicians used a Macintosh laryngoscope in an easy airway scenario, the mean (SD) time for tracheal intubation was approximately 16 (5) s. Malik et al.22 considered that a significant change in the duration of tracheal intubation would be 25%. Thus, using an α value equal to 0.05 and β value equal to 0.2, we estimated that 35 participants would be needed.
Comparisons were only undertaken between devices for each simulated scenario and not between scenarios. Discrete data from intubation success rates were compared using Fisher's exact test. Continuous data, including the time for the successful intubation and the change in weight, were analysed using analysis of variance (ANOVA) testing. Data for Cormack and Lehane grade at laryngoscopy were analysed using Kruskal–Wallis ANOVA on ranks. A P value of less than 0.05 was deemed significant.
Thirty-five doctors from The Royal London Hospital participated in the study; 10 were consultants and 25 were trainees. Two prehospital emergency medicine physicians completed the study, while the remaining participants were from anaesthesia or intensive care medicine. Each of the 35 doctors utilised all of the four intubating devices in both scenarios. A total of 280 intubations were attempted. The results of the study are summarised in Table 1.
All participants were successful in intubating the manikin's trachea with all four devices with a normal airway, and there was no difference in the time taken. There was a difference in the amount of force applied in the vertical vector between the devices during intubation. A significantly better Cormack and Lehane grade was obtained with the Airtraq, followed by the McCoy and then the Macintosh (Fig. 1). Use of the Macintosh, McCoy and Airtraq laryngoscopes was associated with a lifting force off the bed, whereas with the iLMA the direction of force was into the bed. The magnitude of the force applied was significantly greater with the Macintosh laryngoscope, followed by the McCoy and iLMA, and the lowest force of laryngoscopy was seen with the Airtraq.
Difficult airway with manual in-line stabilisation
Participants were successful in intubating the manikin's trachea with all devices, except for one participant whilst utilising the Airtraq. The time taken to intubate the manikin's trachea was longer with the Airtraq than with the other devices. A significantly better Cormack and Lehane grade at laryngoscopy was obtained with the Airtraq, followed by the McCoy and then the Macintosh (Fig. 2). The force applied on the manikin during intubation attempts was different between devices (P < 0.0001), with the Airtraq performing best followed by the iLMA, McCoy and the Macintosh. With the Airtraq, participants applied only 32% of the force associated with the use of the Macintosh, a difference of more than 32 N.
In a simulated difficult airway with MILS, there was no difference in success rates between laryngoscopes, although intubation was fastest with the iLMA. The Airtraq, despite taking longer to intubate the manikin's trachea, performed better than the Macintosh, McCoy and iLMA with regard to the Cormack and Lehane grade at laryngoscopy and force of intubation. The importance of limiting cervical spine motion during laryngoscopy cannot be emphasised enough in undifferentiated trauma and potentially unstable spinal injuries.20 It would be logical in this setting to limit the force exerted during intubation to obtain an adequate but not necessarily optimal glottic view. This could be achieved by accepting inferior glottic views and utilising an intubation adjunct such as a bougie.6 However, repeated attempts at bougie insertion is not without risk, and to avoid such trauma, while still limiting the force of laryngoscopy, it may be appropriate to use an Airtraq in selected patients.27,28 McElwain and Laffey21 suggested that in selected patients with MILS, the Airtraq outperforms the Macintosh. Despite these advantageous characteristics of the Airtraq, it was the only device in our study that resulted in a failed intubation and was also associated with longest time to intubation. One failed intubation, while not statistically significant, may have important clinical implications. As a result, it is not possible to unreservedly recommend the use of the Airtraq.
Participants encountered difficulties inserting the Airtraq into the mouth with the fixed neck and also manipulating the tracheal tube whilst viewing the screen, a well reported problem with video laryngoscopes.16,19 Along with MILS, one of the main sources of failed intubations with video laryngoscopes in trauma patients is the higher incidence of vomitus and blood in the airway.29 The presence of airway debris in trauma patients may limit the effectiveness of flexible fibreoptic intubations, and to a lesser extent, some video laryngoscopes.29
The use of the McCoy during this study was associated with a significant improvement in glottic views and reduction in force of intubation when compared with the Macintosh. Given that no difference could be found in the success rates and time to intubation, an argument for the early use of the McCoy over the Macintosh in patients with MILS could be made. The McCoy has been available since 1993 and offers many advantageous qualities in the trauma patient.13,14
Interest in the use of iLMAs in trauma patients in both the prehospital and hospital setting seems to be expanding.11,12 With the iLMA, the manikin's trachea was intubated by all participants in the fastest time while exerting a minimal force of intubation. The difference with the force exerted by the iLMA was that it was downwards into the trolley, as opposed to a lifting force with all the other laryngoscopes studied. From our study design, it is not possible to delineate the clinical relevance of this downwards force. Keller et al.30 suggested that the insertion of the iLMA was associated with high pressures against the cervical vertebrae and posterior displacement of the cervical spine. This posterior displacement, combined with the fact that the iLMA constitutes a blind technique, should be considered when using the iLMA in trauma patients.
High fidelity simulation plays a vital role in education and improvements in clinical performance.17 Over the years, it has been used in a number of studies from cardiopulmonary resuscitation,31,32 to new airway equipment.17,18,22,28 When assessing new airway equipment, the use of simulation and manikin studies can be very helpful, as long as the results can be translated into clinical practice.17 Despite this, simulation will never quite replicate real clinical scenarios. The authors appreciate that the devices studied represent a small proportion of those available, and may not capture the best current solutions to airway management in trauma patients. In clinical practice, it is vital to not rely on a single device, but instead to use a multimodal approach that often involves a combination of techniques and planning for failure.
During our simulation, no cricoid pressure was applied, which may have influenced the ease of insertion of some of the devices. The use of cricoid pressure is still controversial in patients with cervical spine injuries.33–35 In the interest of reproducibility of scenarios and because of its controversial nature, the authors felt that the choice to not include cricoid pressure was justified. The use of bimanual external laryngeal manipulation (ELM) is of limited benefit in the simulation setting, given the poor elasticity of the manikin's soft tissues. In reality, ELM may be hugely beneficial and has been shown to improve the percentage of glottic opening during laryngoscopy.36
Measurement of the force of intubation in this study oversimplifies a very complex manoeuvre. In our study, the force of intubation was only measured in one vector and as a result will not reflect compressive or distractive forces. The use of bench scales to measure the change in mass of the manikin as a surrogate measure for force of laryngoscopy has rarely been studied. However, the results returned for the Macintosh laryngoscope in the normal and difficult airway are comparable with previously published manikin studies.37,38 For the purpose of this study, a simulated MILS scenario was used instead of actual MILS. Although a fixed cervical spine and tape provided a reproducible and realistic setup for comparing the devices, it did not replicate the dynamic process in true MILS. During such a cross-over study, there is a potential for bias, as it is impossible to blind the operator to the device being used. The subjective nature of Cormack and Lehane grades at laryngoscopy along with the validity of grading video laryngoscopes are also controversial.
The McCoy demonstrated many advantages over the Macintosh and may have a role in the early airway management of trauma patients. The Airtraq delivered the lowest vertical lifting force but was associated with the only failed intubation in this study. The Airtraq may prove useful when managing select patients at a high risk of spinal cord injuries. The iLMA is a simple and readily portable device that was associated with the fastest time to intubation and minimal force of insertion. Such characteristics make the iLMA a valuable device to have available when managing trauma patients with a difficult airway and MILS. Caution should be exercised when attempting to extrapolate these simulation-based study findings, and further human studies should be considered.
Assistance with the study: none declared.
Financial support or sponsorship: none declared.
Conflicts of interest: none declared.
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