The incidence of unstable cervical spine injuries in blunt trauma patients is known to be 2.4%.1 Critically injured patients with traumatic brain injury have better outcomes with early intubation and mechanical ventilation.2 Although early airway management is a vital component of trauma care, any cervical spine manipulation during intubation attempts may result in a secondary spinal cord insult in patients with unstable injuries.3,4 Accordingly, it is recommended that cervical spine immobilisation is maintained during tracheal intubation in patients with suspected cervical spine injuries.5 Current evidence suggests that manual in-line stabilisation (MILS) is superior to other forms of immobilisation, such as rigid collars.4
MILS is known to make glottic visualisation more difficult and increase the rate of failed intubation.6 The use of a bougie with MILS is associated with a higher intubation success rate compared with direct laryngoscopy alone.7–9 Nolan and Wilson7 recommended a low threshold for the use of the bougie in patients with suspected cervical spine injuries, particularly when the glottis is not immediately visible. Along with difficult laryngoscopy, the application of MILS increases the maximal pressure applied by the laryngoscope blade.10 Increased pressure could potentially result in cervical spine movement, so it would seem prudent to limit the force of laryngoscopy in patients with potential unstable injuries. Numerous studies have been undertaken comparing the force of laryngoscopy exerted by various laryngoscopes in a variety of settings.11–13 To date, there is little literature comparing the forces generated using a laryngoscope versus a laryngoscope and bougie combination in patients with MILS.
We evaluated the difference in forces exerted by direct laryngoscopy with a Macintosh laryngoscope compared with the addition of a bougie, in a high fidelity manikin with MILS. Our primary outcome was the force exerted during tracheal intubation. Success rate and the time to successful tracheal intubation were also measured.
All participants were anaesthetists who form part of the trauma team at the Queen's Hospital in Romford. All participants had a minimum of 1 year of anaesthetic experience, and all gave informed consent prior to undertaking the study. The study was approved by the Trust's Clinical Effectiveness Unit and further formal ethical approval was deemed unnecessary.
A SimMan manikin (Laerdal, Kent, UK) was placed on a standard hospital trolley, with a new set of scales (Salter Brecknell WS 60 Electronic Bench Scales, Salter Brecknel, Smethwick, UK) placed under the head and shoulders of the SimMan, which was in turn placed on top of a solid surface. The scales measured the change in overall manikin mass during laryngoscopy.
All participants were provided with a size 3 Macintosh laryngoscope and handle (Optima; Timesco Ltd, London, UK), a size 7.0 mm tracheal tube (Mallinckrodt, Chesterfield, UK), a 15F × 60 cm bougie (Cook UK Ltd, Letchworth, UK), a 2 l adult self-inflating bag (Intersugical, Wokingham, UK) and a competent assistant.
The study was a randomised, cross-over simulation. Each participant was asked to obtain an adequate glottic view and intubate the manikin's trachea with and without a bougie. The sequence for each participant was randomised. For the purposes of the study, all participants were told that the manikin was adequately preoxygenated, anaesthetised and paralysed, and the SimMan was set up to simulate a patient with apnoea and normal vital signs. To simulate a difficult airway scenario with MILS, the trismus and cervical spine immobility mode on the SimMan was activated, which allowed a Cormack & Lehane grade 2b view to be achieved during laryngoscopy. Simulated MILS was provided by the same assistant for all intubation attempts without touching the manikin. The primary end point was the maximal force measured during tracheal intubation. Success rate and the time to successful tracheal intubation were also measured. The duration of successful intubation was measured from the point of passing the laryngoscope beyond the teeth to the correct placement of the tracheal tube. Placement of the tube was only deemed successful when the cuff was inflated and ventilation was confirmed by the simulated measurement of end-tidal carbon dioxide. The force exerted was measured by the same observer in all cases by recording the maximal change in mass during tracheal intubation. The force of laryngoscopy was measured in a vertical vector only, utilising the formula force (N) = mass (kg) × acceleration (m s−2). In this case, acceleration due to gravity was taken to be 9.80665 m s−2. Participants were allowed a total of 120 s for each attempt. Failure to intubate the trachea after this time was deemed a failed intubation.
In a previous trial with a similar set up, the mean ± SD force of laryngoscopy was 60.8 ± 13.8 N when intubating patients without a bougie. A 25% reduction in the force of laryngoscopy was considered to be a clinically significant change. On the basis of these figures, and using an α of 0.05 and β of 0.1 (power 90%), a total of 20 participants were required. Data analysis for the measured forces and the time taken for successful intubation was undertaken with a two-tailed, paired t-test. A P value <0.05 was deemed significant.
Twenty anaesthetists participated in the study including nine consultants, five clinical specialists and six specialist trainees. There was significantly less force exerted when utilising a Macintosh laryngoscope in combination with a bougie, compared with a Macintosh laryngoscope alone (24.9 versus 44.5 N; P <0.001). Tracheal intubation utilising a bougie exerted 44% less force than direct laryngoscopy alone. In both scenarios, all participants successfully intubated the trachea within 120 s. The use of a bougie was associated with a nonsignificant reduction in the time to tracheal intubation (30.0 versus 36.5 s).
We found that use of a bougie during tracheal intubation was associated with less lifting force than direct laryngoscopy alone.
The importance of limiting cervical spine motion during laryngoscopy in potentially unstable cervical spine injuries is well known.14 To obtain full glottic exposure during direct laryngoscopy of the unrestrained neck, maximal or supramaximal extension of the intact upper cervical spine is required.15 These extensions are approximately halved by maintaining the head at a neutral position and keeping laryngeal exposure to a minimum.15 Takenaka et al.16 showed that when using an Airway scope (AWS-S100; AWS, Hoya-Pentax, Tokyo, Japan) and bougie, cervical spine extension was reduced by 9.5°, while not significantly affecting the time to intubation. Use of a bougie may prove particularly advantageous in a potential cervical spine injury, as the angled tip can be manually directed towards the glottis even when the glottic opening is not seen or restricted.17,18 These studies highlight the difficulties faced during laryngoscopy with MILS, and also the advantages of a bougie in minimising neck extension to reduce damage to the spinal cord. The use of a bougie allowed all participants in our study to successfully intubate the trachea with no prolongation in the time to intubation. A possible explanation for these results could be that when using a bougie, participants naturally exerted the minimum force required to obtain an adequate (yet inferior) view of the glottis to allow passage of the bougie alone. The extra time taken to load the tracheal tube onto the bougie would seem to be offset by the potential increased difficulty and multiple attempts required when placing a tracheal tube under direct vision alone. Although it would seem logical to limit the force of laryngoscopy and any potential secondary spinal cord injury, neurological complications are rare following tracheal intubation of unstable cervical spine injuries.19 However, should these complications develop, they could be associated with significant morbidity, mortality and long-term economic burden. It would, therefore, appear sensible to exert the minimal force of laryngoscopy to reduce the risk of any potential devastating neurological consequences.3,4
The multimodal approach to airway management should always be considered when delivering any anaesthetic. Consideration of different airway techniques and adjuncts should be tailored to each individual patient and scenario. Indeed, the use of a variety of alternative laryngoscopes and intubation devices has also been shown to limit the force of laryngoscopy and cervical spine movements.20,21 A study involving experienced anaesthetists comparing the Macintosh with a variety of videolaryngoscopes in a simulated immobilised cervical spine, showed that the Macintosh was associated with the fastest time to tracheal intubation.21 Videolaryngoscopes are not universally advantageous in all scenarios and environments, and in trauma patients the higher incidence of blood and soiling in the airway may impact on the performance of such devices.22 A randomised trial comparing the Airtraq to the Macintosh laryngoscope when used by physician in prehospital trauma patients found that there was a higher rate of failed intubations with the Airtraq.22 Given this, and the cost implication of modern videolaryngoscopes, it would seem reasonable to assume that they will not be universally available across all countries, hospitals and departments involved in the provision of emergency airway interventions. Furthermore, the collective experience of these devices is unlikely to rival that of the Macintosh laryngoscope and bougie. Bougies are inexpensive and readily available in most Emergency Departments. This combination may be a desirable first-line technique for trauma patients with MILS and potentially unstable cervical spine injuries.
Use of a bougie is not without risks, and a number of complications have been reported. An observational study by Hodzovic et al.23 found that 5% of bougie intubations resulted in trauma to the airway. Damage occurred from ‘railroading’ the tracheal tube forcefully over the bougie and also by forcing the bougie beyond the ‘hold up’ when the tracheal tube was being advanced.23 Creation of a false passage in the posterior membranous trachea with subsequent entry of a tracheal tube into it has also been described.24 Insufflation of air into a wrongly placed tracheal tube caused ballooning of the posterior membrane of the trachea and created an airway obstruction.
The use of cricoid pressure in rapid sequence induction and intubation is a controversial topic, particularly when a patient has a potentially unstable cervical spine injury.25 As a result, cricoid pressure was not included in the protocol for this simulation study. Using bench scales to measure the change in mass of the manikin and equating it to force of laryngoscopy is an oversimplification of the complex procedure of intubation, in which there are a number of force vectors. Although a fixed cervical spine and trismus provided a reproducible and realistic setup for comparing the devices, it did not replicate the dynamic process that is true MILS. Indeed, a manikin cannot truly reflect the conditions involved in the intubation of real humans. Hesselfeldt et al.26 showed that cervical spine mobility was significantly reduced in the ‘reduce movement’ mode on the SimMan manikin, but this had minimal effect on the difficulty of intubation. However, the SimMan is considered realistic, and Jordan et al.27 illustrated that the Laerdal SimMan performed the best during simulated intubation scenarios when compared with other brands of manikin. The potential for bias is also present due to the fact that the operator cannot be blinded to the device being used. Further human trials would be required to confirm the potential clinical correlation.
There was a significant reduction in the force of laryngoscopy when a bougie was utilised in addition to a Macintosh laryngoscope. We suggest that a bougie be considered in the first instance when attempting to intubate a patient with potentially unstable cervical spine injuries to avoid any neurological sequelae.
Assistance with the study: none declared.
Financial support and sponsorship: none declared.
Conflicts of interest: none declared.
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