All Singaporean male doctors are required to undergo at least 3 months of emergency medicine or anesthesia clinical rotations in order to improve their airway management skills prior to re-enlistment as military medical officers. Given that our scenario featured simulated difficult airways in trauma patients, we concluded that airway management proficiency of these junior doctors compared favorably to other large studies which reported success rates ranging between 71.2 to 85% for emergent intubations [18,19,20].
Chew et al. did a similar study on a cohort of military medical officers and found the intubation success rates were higher with channeled King Vision and McGrath as compared to the King Vision non-channeled laryngoscope [21]. This corroborated with our study findings.
In addition, our study showed that KVC and MG VLs were superior to DL in terms of intubation success rates, but did not significantly reduce the time to intubate in successful intubations. Several other studies also compared KVC/MG to DL [22,23,24,25,26,27]. Mehmet et al. demonstrated that MG produced a better view of the glottis, but the time to successful intubation was not significantly different from the DL [25]. Piepho et al. studied 30 paramedics using DL and MG on normal and difficult airway simulators, which demonstrated that the use of MG resulted in a better view of the glottis though success rates between MG and DL were similar [27]. Interestingly, Piepho’s study participants took longer time to intubation when they used MG, compared to DL [27]. Our team hypothesized that the superiority of VL over DL became more apparent with difficult airways. On the other hand, the familiarity of DL would be more advantageous when dealing with normal (easy) airways. For difficult airway scenarios, the first attempt at intubation tends to be the best attempt. This is because repeated attempts may result in laryngeal trauma and make intubation even more difficult. Hence, our study team recommends VL (KVC and MG) to be the first line laryngoscope for intubating anticipated difficult airways, especially in out-of-hospital settings.
The channelled conduit for ETT was designed to tackle the often-criticized problem of a ‘can see, but cannot intubate’ situation when trying to pass the ETT based on indirect visualisation of vocal cords when using VL [28, 29]. Our study echoed the findings of Akihisa et al., who demonstrated higher intubation success rate for KVC at 86.6%, compared to KVNC at 47.3% [29]. The same study also demonstrated an intubation success rate of 91.4% for DL, which proved to be better than for both KVC and KVNC [29].
Despite being a non-channeled VL, the MG compared very favourably compared to KVC, the channeled variant for the KingVision Laryngoscope. It also proved superior to the KVNC in terms of intubation success rate in our study. Most junior doctors in Singapore are familiar with the use of standard Macintosh laryngoscope, which is part of the standard equipment for securing the airway. We postulated that since the MG has a similar blade curvature and shape as the Macintosh DL, the MG VL was more intuitive and thus the success rates were higher. The KVNC utilised an acute-angle blade, which allowed for easy visualisation of the manikin vocal cords whilst the manikin was positioned on the floor. However, guiding the ETT through this acute angle was a tricky manouvre that most candidates failed to achieve, which is a similar problem seen in other studies on acute angled laryngoscopes [30, 31].
Limitations
While the SimMan® 3G manikins used are high-fidelity advanced patient simulators, intubating a manikin remains different from intubating real patients. While the patient simulator can produce cervical immobilization and tongue edema, other difficult airway scenarios such as blood and secretions in the oral cavity, anatomic variations or mandibular injuries could not be simulated.
One possible bias in the study design was the ‘learning effect’ from successive intubations. With successive intubations, the study participant would be more familiar with the characteristics of the mannikin. This could possibly lead to higher success rates with subsequent intubations. Our study protocol required all participants to intubate with DL as their 1st attempt, before they were allowed to use video laryngoscopes. The ‘learning effect’ could possibly give VL an unintended advantage over DL.
Lastly, observation bias (aka Hawthorne Effect [32]) might lead to unexpectedly worse intubation success rates and timings for participants with performance anxiety, or improved intubation timings for participants who viewed this as a ‘time challenge’ and intubated much faster than they would have in real life situations.