Clinically Preferred Videolaryngoscopes in Airway Management: An Updated Systematic Review

Videolaryngoscopes (VLs) have emerged as a safety net offering several advantages over direct laryngoscopy (DL). The aim of this study is to expand on our previous study conducted in 2016, to deduce which VL is most preferred by clinicians and to highlight any changes that may have occurred over the past 7 years. An extensive systematic literature review was performed on Medline, Embase, Web of Science, and Cochrane Central Database of Controlled Studies for articles published between September 2016 and January 2023. This review highlighted similar results to our study in 2016, with the CMAC being the most preferred for non-channelled laryngoscopes, closely followed by the GlideScope. For channelled videolaryngoscopes, the Pentax AWS was the most clinically preferred. This review also highlighted that there are minimal studies that compare the most-used VLs, and thus we suggest that future studies directly compare the most-used and -preferred VLs as well as the specific nature of blades to attain more useful results.


Introduction
Airway management is one of the most critical tasks an anaesthetist will encounter in their practice. Facing an unanticipated difficult airway is a complex and stressful task for any anaesthetist and requires great skill to overcome [1]. The primary goal of Endotracheal Intubation (ETI) is to establish a patent and safe airway whilst avoiding complications such as dental injuries or trauma to the anatomy surrounding the trachea. Over the years, there has been an evolution in the shape, size and material of laryngoscopes to optimise the effectiveness and, therefore, the safety of ETI [2]. Currently, direct laryngoscopy (DL) is standard practice for ETI, despite not always yielding a positive outcome [3]. A negative outcome can be referred to as a 'can't intubate, can't ventilate" scenario, potentially requiring front-of-neck access, possibly leading to complications such as hypoxia and neurological injury [4]. Although significant advancements in DL have taken place over the past few decades, predicting a difficult airway remains a challenging task for anaesthetists.
Despite extensive pre-anaesthetic assessments and precautions being undertaken, no single parameter can accurately predict an unanticipated difficult airway [5]. Hence, it is vital that clinicians are always prepared to encounter an unanticipated difficult airway, with the literature suggesting that approximately 90% of difficult intubations are unanticipated by clinicians [6].
Therefore, in problematic situations, a safety net for anaesthesiologists is essential to provide the utmost care for the patient. Contemporarily, videolaryngoscopes (VLs) have provided a safeguard for anaesthetists due to the advantages they provide over DLs. Videolaryngoscopy (VL) is a modern technique that uses a camera and a light source mounted on a laryngoscope blade to enable a magnified and improved view of the larynx 1.
Higher first-pass success: VLs increased the chance of first-pass success (successful intubation on first attempt) when compared to DLs. 2.
Fewer failed intubations: VLs resulted in fewer failed intubations when compared to DLs, even in anticipated difficult airways. 3.
Fewer hypoxemic events: VLs showed a reduction in hypoxemic events when compared to DLs. 4.
Increased glottic views: certain VLs provided an improved visualisation of the glottis in accordance with the Cormack-Lehane grade, which can reduce the likelihood of adverse events such as failed intubation or airway trauma. 5.
Less sore throat: there was a lower incidence of patient-reported sore throat postintubation using VLs, hence possibly reducing patient anxiety with intubation.
With an increase in VL use over the past decade, it is important to juxtapose the safety and efficacy of the different types of VLs. The ideal VL should deliver a high chance of first-pass successful intubation with good glottic views in a short amount of time and be readily available and cost-effective. Our previous study published in 2016 identified that the most clinically preferred VLs were the Pentax-AWS VL for channelled VLs and the Karl Storz C-MAC for non-channelled VLs [7].
The aim of this paper is to expand on our previous study to establish which VL is most preferred by clinicians, to further aid decision-making for safer and more effective ETI, and to ascertain whether any changes have occurred over the past 7 years in clinician preference.

Materials and Methods
Eligibility and Search Strategy: An extensive systematic literature review was performed on Medline, Embase, Web of Science, and Cochrane Central Database of Controlled Studies for articles published between September 2016 and January 2023. The search terms included, but were not limited to: laryngoscope, laryngoscop*, videolaryngoscope, video-laryngoscope, "video laryngoscope", videolaryngoscop*, video-laryngoscop*, "video laryngoscop*", glidescope, macintosh, cmac, mcgrath, airtaq, "king vision", "pentax aws". Results were further refined to cohort studies and randomised controlled trials. A manual review of references was also performed to ensure an elaborate search. We restricted our search to English-only articles. A PRISMA checklist was used to ensure an optimal strategy, with a summary provided in Figure 1. This study was registered in PROSPERO.
Study Selection and Data Extraction: We included studies that compared either of the following between two or more videolaryngoscopes: (a) improved glottis view (either Cormack-Lehane grade or percentage of glottic opening); (b) time to successful intubation; (c) first-pass intubation success rate; (d) use of corrective manoeuvres or adjuncts; e) final outcomes judged by authors as "preferred" and/or "best" VL. Based on the factors 'a-e', we determined which VL was clinically preferred for each study. Data were manually extracted and cross-reviewed by the authors to optimise interrater reliability. These data were further collated to evaluate the favoured percentage of each VL. The studies chosen were grouped into either clinical patient studies or simulation studies. Table 1 demonstrates the VLs identified through our comprehensive study selection and data extraction phase. Study Selection and Data Extraction: We included studies that compared either of the following between two or more videolaryngoscopes: (a) improved glottis view (either Cormack-Lehane grade or percentage of glottic opening); (b) time to successful intubation; (c) first-pass intubation success rate; (d) use of corrective manoeuvres or adjuncts; e) final outcomes judged by authors as "preferred" and/or "best" VL. Based on the factors 'a-e', we determined which VL was clinically preferred for each study. Data were manually extracted and cross-reviewed by the authors to optimise interrater reliability. These data were further collated to evaluate the favoured percentage of each VL. The studies chosen were grouped into either clinical patient studies or simulation studies. Table 1 demonstrates the VLs identified through our comprehensive study selection and data extraction phase.

Results
Our search yielded 3841 studies (Medline-794 results, Embase-792 results, Cochrane Library-1543 results, Web of Science-712 results). After the removal of duplicates, studies that had undergone erratum, trials registered but not yet published and meeting abstracts not yet published, 1405 articles remained. Publications that contained unrelated context or did not meet the inclusion criteria were excluded from this study.
This review included 81 studies with a total of 50 surgical studies conducted on 6274 patients, and 31 simulation studies conducted with 1353 participants. Table 2 highlights the key elements of each surgical study, while Table 3 highlights the key elements of the simulation studies. Importantly, we identified the clinically preferred VL and blade type in each of these study groups, as demonstrated in the respective tables.                    In studies evaluating surgical patients, the CMAC was the most-used VL, with 25 out of 50 studies investigating it, closely followed by the McGrath VL (19 out of 50 studies). Of note, in surgical patients, the CMAC Macintosh blade was used 14 times, the CMAC angulated D blade was used 8 times, and the CMAC Miller blade was used 3 times. In the studies that investigated the McGrath VL, 13 used the Macintosh-style blade and 6 used the angulated blade. For studies evaluating manikins, the GlideScope was the most commonly used VL, with 14 out of 31 studies investigating it, all of which used an acute-angulated blade. Table 4 summarises the review, with less commonly used VLs excluded. The CMAC was found to be the most-preferred non-channelled VL overall (preferred in 70% of studies that investigated CMAC), closely followed by the GlideScope (preferred in 67% of studies that investigated GlideScope). It is interesting to note that the most frequently used nonchannelled VLs all scored > 50% preference in surgical studies, yet only the CMAC and GlideScope scored > 50% in simulation studies.  For channelled laryngoscopes, the AWS was the most-preferred overall, with 69% of studies that evaluated this VL preferring it; however, only 50% of surgical studies preferred this VL.
When analysing the data, it was established that the most-used VLs were not often directly compared. Out of the 50 studies reviewing clinical scenarios, the CMAC and GlideScope were only directly compared three times, and when reviewing simulation studies, they were only compared two times. Out of the clinical studies, the CMAC was preferred in two out of the three studies, whilst in the simulation studies the CMAC was preferred in one study, and the other found the CMAC and GlideScope to be similar. When comparing the CMAC to Pentax AWS, no clinical study directly compared the two VLs, and the one simulation study that compared them showed a preference for the Pentax AWS. Figure 2 demonstrates the most commonly preferred VLs.

Discussion
This review aimed to establish which VL is most clinically preferred, with the goal of expanding on our previous review published in 2016. Similar to previously, the Pentax AWS was the most preferred overall for channelled VLs, and the CMAC was the most preferred overall for non-channelled VLs.
However, our review discovered that the more recent literature suggests that other VLs are also gaining popularity among clinicians. For instance, our previous review suggested that the GlideScope acute-angled blade was preferred in only 41% of clinical studies and 7% of simulation studies, whereas our updated review highlights that it was preferred in 69% of clinical studies and 64% of simulation studies. Likewise, the efficacy of the McGrath VL appears to have increased drastically from the previous review, increasing from a preference rate of 25% to 63% in clinical studies, and 17% to 44% in simulation studies. We hypothesise that this increase is due to the ever-expanding use, availability and familiarity with VLs. Further to this, McGrath Blades have released Macintosh-style blades, which clinicians have historically been more familiar with. With regards to our study, it is noted that a McGrath Macintosh-style blade was used in 13 of 19 clinical studies and 7 of 9 simulation studies. Again, this change may be a reason for the growing popularity and preference for the McGrath VL. Similarly, GlideScope has also released Macintosh-style blades; however, the studies in this review all appear to use the acute-angle blade. Ultimately, clinicians are more likely to perform better with and prefer a VL that they are familiar with and use more often.
The literature suggests that acute-angle blades should be reserved for predicted or known difficult airway situations, especially in patients with an anterior larynx [89]. Thus, the use of acute-angle VLs may be detrimental, in comparison to standard Macintosh-style blades, for the intubation of normal airways. One such reason is that acute-angle VLs only provide an indirect view and present with a sharp angle, resulting in the ETT needing to be introduced with a device such as a stylet to ensure it is able to be manipulated around the steep angle [89]. Thus, one limitation of the papers studied in this review is the comparison of acute-angle blades to Macintosh-style blades, as the clinical indication for each is different.
In addition to this, there were a minimal number of studies that directly compared the most-preferred VLs. For instance, the two most-preferred non-channelled VLs, the CMAC and GlideScope, were only directly compared three times in clinical studies and two times in simulation studies. Similarly, when attempting to discuss the CMAC and Pentax AWS, no clinical study directly compared these two VLs and only one simulation study directly compared them. This limits the generalisability of the current literature, as a direct comparison and evaluation of the most-preferred VLs are not able to be conducted

Discussion
This review aimed to establish which VL is most clinically preferred, with the goal of expanding on our previous review published in 2016. Similar to previously, the Pentax AWS was the most preferred overall for channelled VLs, and the CMAC was the most preferred overall for non-channelled VLs.
However, our review discovered that the more recent literature suggests that other VLs are also gaining popularity among clinicians. For instance, our previous review suggested that the GlideScope acute-angled blade was preferred in only 41% of clinical studies and 7% of simulation studies, whereas our updated review highlights that it was preferred in 69% of clinical studies and 64% of simulation studies. Likewise, the efficacy of the McGrath VL appears to have increased drastically from the previous review, increasing from a preference rate of 25% to 63% in clinical studies, and 17% to 44% in simulation studies. We hypothesise that this increase is due to the ever-expanding use, availability and familiarity with VLs. Further to this, McGrath Blades have released Macintosh-style blades, which clinicians have historically been more familiar with. With regards to our study, it is noted that a McGrath Macintosh-style blade was used in 13 of 19 clinical studies and 7 of 9 simulation studies. Again, this change may be a reason for the growing popularity and preference for the McGrath VL. Similarly, GlideScope has also released Macintosh-style blades; however, the studies in this review all appear to use the acute-angle blade. Ultimately, clinicians are more likely to perform better with and prefer a VL that they are familiar with and use more often.
The literature suggests that acute-angle blades should be reserved for predicted or known difficult airway situations, especially in patients with an anterior larynx [89]. Thus, the use of acute-angle VLs may be detrimental, in comparison to standard Macintosh-style blades, for the intubation of normal airways. One such reason is that acute-angle VLs only provide an indirect view and present with a sharp angle, resulting in the ETT needing to be introduced with a device such as a stylet to ensure it is able to be manipulated around the steep angle [89]. Thus, one limitation of the papers studied in this review is the comparison of acute-angle blades to Macintosh-style blades, as the clinical indication for each is different.
In addition to this, there were a minimal number of studies that directly compared the most-preferred VLs. For instance, the two most-preferred non-channelled VLs, the CMAC and GlideScope, were only directly compared three times in clinical studies and two times in simulation studies. Similarly, when attempting to discuss the CMAC and Pentax AWS, no clinical study directly compared these two VLs and only one simulation study directly compared them. This limits the generalisability of the current literature, as a direct comparison and evaluation of the most-preferred VLs are not able to be conducted based on the current literature. One potential reason hypothesised is the cost involved with comparing the more expensive and most-preferred VLs. Further to this, the financial implications of acquiring, using, and maintaining VLs may limit certain departments' ability to use VLs, and we suggest they check what is available and suitable for their needs. We also propose that the current literature favours the most commonly used VLs, and true clinician preference would be better ascertained in future studies evaluating more head-to-head comparisons and thus, more direct parameters for clinician preference.
Furthermore, as discussed in our previous review, it must be acknowledged that clinical studies conducted on patients will differ rather significantly compared to simulation studies performed on manikins. This is highlighted in the stark difference in the performance of some VLs in clinical studies versus simulation studies. If we base our results purely on clinical studies, it would highlight that the King Vision non-channelled and Airtraq VLs may be the most suitable to use. However, both these VLs performed quite poorly in manikin models and had a <35% preference in their respective studies. Furthermore, with a minimal direct comparison between these VLs and the aforementioned most-preferred VLs, this would not be generalisable. The CMAC, GlideScope and Pentax AWS were the only VLs preferred in ≥50% of both clinical and simulation studies.
For future use and teaching prospects, it is important to recognise extra components that may render particular VLs more preferable. For instance, the Macintosh-style blades used in the CMAC VLs can be used as both a direct and indirect laryngoscope as the video monitor is mounted separately to the laryngoscope. Therefore, in a scenario where a clinician wants to teach with this VL, the trainee can use a direct view for education, whilst the teacher can observe through the monitor to ensure the adequate placement of the endotracheal tube. Similarly, if a difficulty is encountered, a supervisor can take over, or the anaesthesia assistant can anticipate what to do next, such as preparing a bougie. Anecdotally, this method of laryngoscopy ensures safer management and education. Similarly, the McGrath VL can serve the same teaching process; however, a key difference is that the monitor is attached to the top of the VL itself, rather than mounted separately. Furthermore, different manufacturers offer different specifications in their VLs. Some provide the option for multiple types of blades or attachments such as fibreoptic scopes to be used with one VL handle, others offer longer battery life, whereas some offer the option for single-use vs reusable blades. These additional features make particular VLs more appealing, depending on the clinicians' preferences and requirements.
A limitation to this article is that VLs were purely chosen based on their functionality in intubating a patient or manikin, without considering the resources/equipment used. For future studies, it is important to compare the cost burden for hospitals alongside clinician preference to ensure a more complete approach towards selecting the most ideal VL. Studies in the future should also be more transparent with the specific details of the VLs being used, including the type of blade, size, reusability, and/or single-use nature of the VLs. We also advocate for future studies to more thoroughly identify patient and institutional factors that may lead to the use of, or preference for, a particular VL, with the aim to provide transparency to readers regarding scenarios where particular VLs may be more favourable.
Furthermore, as aforementioned, a limitation in the current literature is the lack of comparison between the most-used and -preferred laryngoscopes. In addition to this, studies in the future should compare VLs with Macintosh-style blades to other VLs with Macintosh-style blades, and likewise acute-angle blades with acute-angle blades. This will allow for more streamlined and consistent results. Essentially, we advocate for standardisation criteria to ensure the appropriate comparison of devices. The studies reviewed also did not consistently describe the experience level of the operators performing intubation, and suggestions for future studies would be to highlight this to ensure transparency, as it is well-known that clinicians will prefer and perform better with equipment that they are more familiar with. To achieve optimal clinical outcomes, we advocate that clinicians choose the VL that they are most comfortable with depending on the clinical situation, as each VL offers its own set of advantages and disadvantages. We believe that VLs should become the gold standard for ETI, with several advantages highlighted over DLs. Further-more, VLs can also allow us to transition from blind insertion techniques for procedures including temperature probes and nasogastric tubes to 'vision-guided' insertion, ultimately limiting the potential for wrong space insertion, with future studies potentially exploring this field.

Conclusions
The purpose of this review is to critically assess and compare the effectiveness of VLs in modern airway management, both in real patients and simulation scenarios involving manikins. This study was performed as an extension of our 2016 analysis, to deduce whether there were any changes in clinician preference over the past 6 years. This review highlighted similar results to our study in 2016, with the CMAC still being the most preferred for non-channelled laryngoscopes, closely followed by the GlideScope, and the Pentax AWS being the most preferred for channelled laryngoscopes based on the current literature. We hope this audit increases the awareness of both individual practitioners and departments of anaesthesia, to highlight the importance of VL use and to lay a platform for future studies to expand knowledge in this field.