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 in the military. 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 intubations18-20.
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 laryngoscope21. 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. This result deviated from other studies comparing KVC/MG to DL22-27. For example, Mehmet et al demonstrated that MG produced a better view of the glottis, but time to successful intubation was not significantly different from the DL25. Piepho et al studied 30 paramedics using DL and MG on normal and difficult airway simulators, which demonstrated that use of MG resulted in a better view of the glottis, but success rates between MG and DL were similar27. Interestingly, participants took longer time to intubation when they used MG, compared to DL27. 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 is the best attempt. This is because repeated attempts may result in laryngeal trauma and make intubation even more difficult. Hence, our study team recommended for VL (KVC and MG) to be the first line laryngoscope for intubating anticipated difficult airways, especially in out-of-hospital settings.
The channelled conduit 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 VL28-29. 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 both KVC and KVNC29.
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 as the Macintosh, the MG VL was more intuitive and thus the success rates were higher. The KVNC utilised an acute blade angle, 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 that failed to achieve, which was a similar problem seen in other studies on acute angled laryngoscopes30,31.
LIMITATIONS
While the SimMan® 3G manikins used are high-fidelity advanced patient simulators, intubating a manikin remained different from 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 and more familiar with the characteristics of the mannikin. This could possibly lead to higher success rates with subsequent intubations. We standardised for 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 Effect32) 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.