Efficacy of tracheal tube introducers and stylets for endotracheal intubation in the prehospital setting: a systematic review and meta-analysis

Endotracheal intubation (ETI) can be a life-saving advanced airway management technique for critically ill or injured patients. It remains a technically difficult but vital skill for prehospital care providers, especially for physician-led teams that regularly encounter airway compromise [1, 2]. It is not without risk, as significant morbidity and mortality can result when ETI is performed poorly [3]. Success is unsurprisingly more common amongst experienced personnel [3], highlighting a need for devices that might improve success and reduce adverse events among less experienced personnel, or particularly difficult airways.

Tracheal tube introducers (commonly referred to as bougies) and stylets are two of the most widely used devices that aid practitioners in ETI. Bougies can be passed under full or partial vision or even blindly through the vocal cords, providing a scaffold that the endotracheal tube (ETT) can be easily passed over before the bougie is withdrawn [4]. They vary in size, shape, structure and thickness and may be single-use or reusable, angulated or straight, soft tip, hard tip or even hollow-core [4]. Stylets can be inserted into the ETT before intubation, allowing it to be moulded into a shape that facilitates easier ETI before being removed as soon as the cords are passed [4]. These are also manufactured in various forms similar to bougies [4].

Clinical practice varies between healthcare systems and there is no universal consensus on when and how these two devices should be implemented into airway management protocols. In the emergency department (ED), success rates for ETI can be as high as 99% with first pass rates of 80–90% [5]. Although ETI is used in EDs to secure patient airways, a Cochrane review in 2018 found no difference between ETI and other airway securing strategies for reducing death or injury and indicated better studies were needed to make conclusive statements [6]. Both the Difficult Airway Society and American Society of Anesthesiologists currently recommend that intubating bougies and stylets may form part of a difficult airway intubation strategy [7, 8], while the Resuscitation Council UK believe a bougie should always be available for prehospital ETI [9]. A previous systematic review and meta-analysis was conducted that sought to compare the efficacy of both devices in patients undergoing ETI, though only one prehospital study was included [10]. These results may, therefore, not be generalisable to the vastly different prehospital environment.

The aim of this study was to systematically review the literature relevant to the use of bougies and stylets for ETI in the prehospital setting. This should inform a conclusion regarding their prehospital efficacy and allow practitioners to decide whether and how best to incorporate these devices into their practice. The primary outcomes of this systematic review are first pass ETI and overall ETI success. Secondary outcomes include time to ETI, ease-of-use and ETI complications such as oesophageal intubation.

Our systematic review qualitatively and quantitatively integrates the literature relevant to the use of bougies and stylets for ETI in the prehospital setting. It is clear that both bougies and stylets should have a role in prehospital ETI, though bougies appear to offer a statistically significant increase in first pass ETI success when used alongside video technology. Additionally, there are suggestions that particular devices may convey advantages in certain clinical situations. These devices, as well as prehospital environment considerations, practitioner experience and study design quality all necessitate consideration when choosing between bougies or stylets.

Our results suggest that bougies offer a statistically significant increase in first pass ETI success when used alongside video technology in the prehospital setting. Though, conclusions inferred from this isolated statistically significant result must be considered in the context of high study heterogeneity and the multiplicity of comparisons being made. Nevertheless, some of the highest first pass and overall ETI success rates in both prehospital and simulation studies incorporated some form of video technology. Ångerman et al. [15] achieved this in the prehospital setting by utilising a Frova intubating introducer alongside a C-MAC videolaryngoscope. Alternatively, Cooney et al. [19] achieved their high first pass success in the simulation setting by utilising the Clarus Video System (CVS), a video-assisted semi-rigid fibreoptic stylet, alongside direct laryngoscopy. Improvements in ETI success with videolaryngoscopy are not a novel concept and have been previously demonstrated by another systematic review in the hospital environment [27]. The advantage that video-assistance conveys might be more notable and prove particularly useful with novice intubators [28]. Newer technologies such as the CVS then widen the scope of how we might install video technology into pre-existing devices such as the simple malleable stylet. When working with these devices, however, consideration must be given towards the cost that is inevitably higher than in their technologically sparse counterparts. Still, whether incorporated into the analogous laryngoscope or the stylet/bougie itself, our review highlights that video technology might offer the key to further improving upon ETI success in the prehospital setting. In particular, where video technology is available, our findings suggest that some practitioners may wish to opt for a bougie over a stylet to maximise their first pass ETI success.

Our review of secondary outcomes suggests that stylets offer speed while bougies offer ease-of-use, though neither device appears to lead to a higher rate of complications than the other. Again, conclusions inferred from these results must be considered in the context of high study heterogeneity and the multiplicity of comparisons being made. Still, where video technology is unavailable, bougies may have a role where the intubating practitioner is less experienced, while stylets may have a role in cases where catastrophic airway compromise is imminent. However, it is worth noting the results of Gregory et al. [25] reported that the Portex reusable bougie was both the most difficult to use and led to the most complications while having the lowest overall ETI success across simulation studies. A practitioner’s specific preference of bougie might therefore instead be towards the Portex single-use or Frova intubating introducer. Overall, these aspects must be considered on a case by case basis in the context of a markedly diverse and fast changing prehospital environment. Confronted with a patient with significant inhalation injury who is rapidly losing an airway, the practitioner may opt for the superior speed of a stylet to secure it as soon as possible. However, in a scenario where the practitioner is significantly less experienced, the ease-of-use of a bougie might facilitate success. Thus, where video technology is unavailable, the choice of device might be dictated by a combination of the clinical scenario, practitioner expertise and personal preference. This is in accordance with the results of Sheu et al.’s systematic review and meta-analysis [10].

The issue of practitioner experience has been touched upon and demands further discussion as it is clearly able to significantly influence success with devices assessed in this study. Models of prehospital care and their respective expertise vary across the world, with mainland Europe favouring physician-led services while the UK is predominantly paramedic- and technician-led [29]. Even compared to seemingly similar services such as the USA, there are large differences in personnel competencies, with more comprehensively trained US paramedics recruited at a higher level than in the UK [29]. Therefore, prehospital care systems in different countries may report varying degrees of success with the same array of ETI devices, simply by virtue of their prehospital personnel training and experience with ETI. Participant intubators across our studies varied from physicians, paramedics and emergency medical technicians to even include students. Some of our studies acknowledged and attempted to account for this by recording participants’ previous ETI experience and familiarity with the devices being assessed. In line with this, it is integral that pre-existing participant ETI experience is treated as a potential confounding variable in future studies, particularly when the make-up of prehospital care teams has proven to be so varied.

A mixture of both simulation and real-world prehospital studies were incorporated into our study. We felt that this was necessary to capture a wider range of data due to a lack of real-world prehospital studies, reflecting the distinct challenges involved with research in this setting. A possible solution to this lies with high-quality simulation, perhaps including dedicated advanced prehospital simulation laboratories [30]. By developing high-quality simulation, we can overcome the obstacles of research in the prehospital context (such as lack of control), while still being able to study realistic prehospital scenarios complete with serious game-inspired techniques and methods [30]. This would facilitate the generation of much needed prehospital-oriented randomised controlled trials (RCTs) that can be systematically reviewed to inform guidelines. Without simulation, the power of reviews such as ours becomes handicapped by the methodological quality of the few prehospital studies we can analyse.

It should finally be noted that our study has considered the efficacy of bougies/stylets in the context of their conventional and direct use in facilitating ETI. Two studies were excluded during the selection process that utilised bougies in an exchange manoeuvre, converting from one type of airway to another [31, 32]. One study even used a lighted stylet device for ETI without the aid of a laryngoscope [33]. The efficacy of either of these methods cannot be validated nor revoked by our study. However, readers should have an awareness that the utility of bougies/stylets may not be limited to a singular ETI technique as in this review.

This study is limited in the specific device comparisons that can be made due to the sheer number and variety of bougies/stylets currently in clinical use or trials. Within the realms of video-assisted ETI alone, variations in software can significantly change a device’s performance. Drawing absolute conclusions concerning efficacy has, therefore, proven problematic and general trends and themes have instead been explored. Having such a wide variety of devices available may appear beneficial to prehospital services, though it poses difficulties for synthesis of comprehensive clinical guidelines. This may ultimately lead to ambiguity in clinical decisions regarding what device to use and when. Pre-existing experience of participant intubators also limit the conclusions that can be drawn regarding the efficacy of any one device. Aforementioned variations in prehospital provision across the world mean that efficacy of a device tested in the USA may not be comparable to its efficacy in France and vice versa [29]. Future studies must account for this and an internationally standardised method of assessing a prehospital practitioner’s ETI capabilities should be employed. Lastly, this review is limited by a distinct lack of RCTs, with a heavy reliance on observational studies. Sheu et al.’s systematic review and meta-analysis conversely included RCTs only, yet three of their five trials were in a preoperative setting and one was in an ED [10]. This review aimed to focus exclusively on the prehospital efficacy of these devices to guide the practice of prehospital personnel in their unique setting, thus inclusion of hospital-based RCTs was deemed inappropriate. Despite including studies of higher evidence, Sheu et al. concurs in part with the conclusion of our review for when video technology is unavailable, that clinician expertise and personal preference should determine choice of intubating device, owing to a lack of significant differences in primary efficacy outcomes [10]. Nevertheless, the methodological robustness of some of the studies that were included in this review is questionable. Many of the studies ran with admittedly small samples while one study satisfied just 57% of our quality assessment criteria [23]. This stresses the need for greater quality control amongst prehospital studies, reinstating the importance of high-quality simulation to overcome the inherent lack of controls in a real-world prehospital setting [30].

It can be concluded that both tracheal tube introducers and stylets function as efficacious aids to intubation in the prehospital environment. However, where video technology is available, bougies could offer a statistically significant advantage in terms of first pass ETI success. The overwhelming variety of devices and competency disparities across different countries’ prehospital services make a true assessment of individual efficacy challenging. While acknowledging the potentially higher costs, video-assisted devices may offer distinct advantages and techniques that incorporate this technology could be the answer to further improving ETI success. Where video technology is unavailable, a combination of clinical scenario, practitioner expertise and personal preference might be used to ultimately guide the choice of device. Future studies must account for pre-existing practitioner experience when assessing efficacy of any ETI technique and emphasis should be placed on high-quality simulation with robust quality control.