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Inverse intubation in entrapped trauma casualties: a simulator based, randomised cross-over comparison of direct, indirect and video laryngoscopy
  1. Patrick Schober,
  2. Ralf Krage,
  3. Dick van Groeningen,
  4. Stephan A Loer,
  5. Lothar A Schwarte
  1. Department of Anaesthesiology, VU University Medical Center Amsterdam, Amsterdam, The Netherlands
  1. Correspondence to Dr Patrick Schober, Department of Anaesthesiology, VU University Medical Center Amsterdam, P.O. Box 7057, Amsterdam 1007 MB, The Netherlands; p.schober{at}


Background Airway management in entrapped casualties with restricted access to the head is challenging. If tracheal intubation is required and conventional laryngoscopy is not possible, intubation must be attempted in a face-to-face approach. Traditionally, this is performed with a standard laryngoscope held in the right hand with the blade facing upward. Recently, alternative methods have been developed to facilitate difficult intubations, and we hypothesised that such techniques are also useful for face-to-face intubations.

Methods 24 (trainee) anaesthesiologists attempted tracheal intubation in a patient simulator (SimMan, Laerdal, Norway) using three techniques in random order: (1) direct laryngoscopy (Macintosh blade #3), (2) indirect optical laryngoscopy (Airtraq, Prodol, Spain) and (3) video laryngoscopy (McGrath, Aircraft Medical, UK). The manikin was sitting with the neck immobilised and only accessible from the left anterolateral side. Success rate (percentage (95% CI)) and tube insertion time (median (IQR)) were recorded.

Results Success rate did not differ significantly (Airtraq and McGrath 100% (84% to 100%), direct laryngoscopy 88% (68% to 96%)). Intubation was faster with Airtraq (25 s (22–34), p<0.001) and direct laryngoscopy (34 s (22–48), p<0.05) compared with the McGrath technique (55 s (37–96)).

Conclusions All three techniques have a high success rate, but the usefulness of the video laryngoscope is limited due to longer intubation duration. Inverse direct laryngoscopy showed reasonable intubation times and, given the widespread availability of Macintosh laryngoscopes, seems a useful technique. Intubation was always successful and tended to be fastest with the Airtraq device, suggesting that this technique may be a promising alternative.

  • Airway
  • Prehospital Care
  • Trauma

Statistics from


Securing a compromised airway is one of the first priorities in the management of acutely injured patients.1 ,2 This can especially be challenging with entrapped casualties when access to the patient is restricted. In such situations, prehospital emergency personnel will usually administer oxygen and keep the airway open using basic airway techniques or supraglottic airway devices until the patient has been extricated. However, if rapid extrication is not possible and when life-threatening airway obstruction and severe hypoxia persist, the advanced life support (ALS) provider may—in rare instances—be forced to attempt securing the airway with an endotracheal tube while the patient is still entrapped. Herein, conventional laryngoscopy is often not possible due to limited access to the head, and some ALS providers advocate the use of a primary surgical technique under such circumstances. Alternatively, a laryngoscopic approach is available to perform endotracheal intubation from a ventral position while directly facing the victim. This technique has repeatedly been reported, but systematic data on the usefulness of this approach are scarce. Traditionally, this ‘inverse intubation’ method is performed using a standard laryngoscope, which is held in the right hand with the Macintosh blade facing upward.3 ,4 In recent years, alternative intubation techniques including indirect optical laryngoscopy and video laryngoscopy are increasingly used to facilitate tracheal intubation,5 ,6 and we hypothesise that such techniques may also be useful in the context of face-to-face intubations. Therefore, we compared intubation success rate, endotracheal tube insertion times and user preference using direct laryngoscopy, indirect optical laryngoscopy and video laryngoscopy for inverse intubation in a simulated scenario.

Materials and methods

Twenty-four anaesthesiologists (11 medical specialists, 13 in training, 14 male, 10 female, mean±SD age: 36±7 years) were recruited to participate on a voluntary basis and with informed consent. For this simulator study not involving patients, formal approval of the ethics committee was not required by Dutch law.

The SimMan full-scale patient simulator (Laerdal Medical Corporation, Stavanger, Norway) was used to simulate an entrapped trauma casualty. The manikin was in sitting position and accessible only from the left anterolateral side (figure 1), simulating the driver of a motor vehicle (or passenger of a car with British configuration) who can only be approached through the left front door. According to common practice, the neck of the manikin had been immobilised (Stifneck Select, Laerdal, Stavanger, Norway).

Figure 1

SimMan full-scale patient simulator (Laerdal Medical Corporation, Stavanger, Norway) in sitting position and with the head immobilised. Access was only allowed from the left anterolateral side to simulate an entrapped trauma patient. For direct inverse laryngoscopy, the laryngoscope is held in the right hand with the blade facing upwards like an ‘ice-pick’ (A). The blade is inserted into the mouth from the right side and advanced into the vallecula while the tongue is shifted to the left. Vocal cords are visualised by gentle pulling on the laryngoscope handle, and the tube can then be inserted into the trachea with the left hand (B). After removal of the stylet and inflation of the cuff, the patient can be ventilated (C).

Using a randomised cross-over study design, each participant subsequently used three techniques in random order to attempt inverse intubation: inverse direct laryngoscopy, indirect optical laryngoscopy and video laryngoscopy. Randomisation was performed by drawing three labelled cards from an opaque box, and the order in which the cards were drawn determined the order in which the techniques were performed.

For inverse direct laryngoscopy, a standard Macintosh blade size #3 attached to an adult size laryngoscope handle was used (Heine Optotechnik, Herrsching, Germany, figure 1). Indirect optical laryngoscopy was performed with a size #3 Airtraq device (Prodol Meditec, Las Arenas, Spain, figure 2). A McGrath Series 5 Video Laryngoscope (Aircraft Medical, Edinburgh, UK, figure 2) was used for video laryngoscopy. In a standardised briefing, the handling of these devices as well as the classic inverse direct laryngoscopic technique, in which the laryngoscope is held with the blade facing upwards like an ice-pick in the right hand and the tube with the left hand (figure 1), was explained and demonstrated to all participants.

Figure 2

Airtraq device (left, author's photograph) and McGrath Series 5 Video Laryngoscope (right, kindly provided by Aircraft Medical). The Airtraq device (Prodol Meditec, Las Arenas, Spain) is an indirect optical laryngoscope. A light-emitting diode (LED) at the tip of the device (not visible on the photograph) illuminates laryngeal structures and the image is transmitted via a combination of lenses and mirrors to a proximal viewfinder (A). Adjacent to the housing that contains the optical components, a channel (B) follows the curvature of the device to guide the tracheal tube (C) towards the glottic opening. The McGrath Series 5 Video Laryngoscope (Aircraft Medical, Edinburgh, UK) consists of a camera stick (D), which is mounted with its proximal end to a laryngoscope handle (E). At its distal tip, the camera stick has two LEDs and a small digital camera (not visible on the photograph). A 1.7″ liquid crystal display (LCD) colour monitor (F) is attached to the top of the handle. A disposable transparent blade (G), shaped similarly to a Macintosh blade, covers the camera stick.

A cuffed 7.5 mm Mallinckrodt endotracheal tube (Covidien, Dublin, Ireland) was used with all techniques. For direct laryngoscopy and video laryngoscopy, a lubricated semirigid stylet was inserted into the tube; this was not required for the Airtraq, which has an integrated channel to guide the tube. A 10 mL syringe to inflate the tube's cuff and a ventilation bag were handed to the participant when requested.

We recorded success rate, defined as successful tracheal insertion of the tube, and tube insertion time, defined as the time interval from insertion of the instrument into the simulator's mouth until delivery of first ventilation of the lungs. Moreover, participants were asked to give a subjective ranking on which technique would be their first, second and third choice.

Statistical analysis

Success rate was the primary outcome parameter on which sample size calculation was based. A previous study reported a success rate of 50% for inverse intubation of the SimMan manikin with direct laryngoscopy.7 We consider a 50% success rate clinically inacceptable and presume that a useful alternative technique should have a success rate of more than 85% (ideally approaching 100%). Hence, we powered our study to minimally detect a clinically meaningful difference in success rate of more than 35%. With a targeted power of 80% and a two-tailed α level of 0.05; this requires a sample size of 24 per group.

Results were analysed by GraphPad Software, San Diego, California, USA (Prism V.5.0 statistical package and QuickCalcs calculators). Tube insertion times showed skewed distribution and are presented as median (IQR). Tube insertion times as well as ordinal data were compared between the groups by Friedman test, corrected for multiple comparisons with Dunn's post hoc test. Success rate is presented as percentage (95% CI) and was compared with McNemar's test with continuity correction and Bonferroni’s correction for pairwise comparisons. A p<0.05 was considered statistically significant.


Success rate

Success rate did not differ significantly between the groups. Tracheal intubation was successfully performed in all attempts with the Airtraq technique and with the McGrath technique (both techniques: success rate 100% (84% to 100%)) and in 21 out of 24 attempts with direct laryngoscopy (success rate 88% (68% to 96%), p>0.05).

Endotracheal tube insertion time

Tube insertion time was faster with the Airtraq technique (25 s (22–34 s), p<0.001) and the direct laryngoscopy technique (34 s (22–48 s), p<0.05) as compared with the McGrath video laryngoscopy technique (55 s (37–96 s), figure 3). Airtraq tended to be the fastest technique, but this failed to reach statistical significance versus direct laryngoscopy.

Figure 3

Tube insertion times with the Macintosh, McGrath and Airtraq techniques. Dots are individual values; the horizontal lines with error bars represent median values and IQR.

Subjective rating scale

The Airtraq technique was the preferred technique compared with direct laryngoscopy (p<0.05) or McGrath video laryngoscopy (p<0.001). While participants tended to prefer direct laryngoscopy to video laryngoscopy, this trend was not statistically significant.


We compared three techniques for inverse intubation in a simulated scenario of an entrapped trauma casualty. All techniques showed a high success rate, however the Airtraq technique tended to achieve fastest ventilation time and was the preferred technique among the participants of our study.

Critique of methods

Participants of our study had limited or no prior experience with inverse intubation. Similarly, because this technique is only infrequently performed in the field, most prehospital ALS providers will also have only limited experience. However, all of our participants were proficient in direct laryngoscopy and regularly perform tracheal intubations in trauma patients. Therefore, our results may not necessarily apply to personnel with less experience and training in airway management.

A variety of different video laryngoscopes and optical laryngoscopes are available in the market, several of which have rather large external monitors to provide optimal view in the operating room, emergency department or intensive care unit. Our investigation focuses on use of such devices at an accident scene, and we therefore tested two devices whose properties presumably make them particularly useful for prehospital intubations: the McGrath video laryngoscope and the Airtraq device are battery operated, small, lightweight and do not require an external monitor, which facilitates storage in an ambulance or helicopter, and allows their use in remote and confined spaces. To ensure that all participants theoretically knew how to handle and use the instruments, all participants received a standardised briefing. Practicing conventional intubations on the simulator with the video laryngoscope and the Airtraq device was allowed, however attempting inverse intubations was not allowed prior to the study.

The McGrath video laryngoscope and the Airtraq device are not available for clinical use in our hospital, hence both devices are not used routinely by any of the participants of our study. However, it is possible that some participants did have prior experience with any of these devices, for example, due to training at difficult airway workshops, but we did not systematically address prior experience. A potential bias might occur if one of the devices would have been used more often by the participants. In this context it should be noted that, while the McGrath video laryngoscope is not available in our hospital, we do routinely have another video laryngoscope available, and hence, our participants did have prior experience with video laryngoscopy. Yet, the video laryngoscopy technique was associated with the longest tube insertion times. This does not suggest that prior experience of our participants with video laryngoscopy has biased the results towards better performance with this technique.

We used a simulated scenario with the manikin in sitting position; however it was not actually entrapped. The investigator group meticulously paid attention that participants only approached the manikin from the left anterolateral side and did not allow intubation attempts from any other position.

The full-scale patient simulator SimMan is widely used for airway management training and airway related research.5 ,7–10 A validation study found an excellent realistic character of most anatomical features relevant to tracheal intubation, including ease of laryngoscope insertion, space in the oropharynx and lifting force necessary to obtain glottic view.11 In line with common practice, we immobilised the manikin's neck to simulate this extra difficulty, which healthcare providers routinely encounter during prehospital intubations. Nevertheless, intubating conditions may differ from human trauma patients, for example, due to absence of facial trauma and bleeding. Therefore, we cannot exclude that different results would be observed if blood or vomitus was present in the oral cavity, pharynx or airway. Such a situation could possibly favour direct laryngoscopy, because smearing of the lens of the Airtraq or McGrath device with blood or other secretions can impair visibility. In this context, it should be noted that a recent randomised controlled trial about prehospital use of the Airtraq device reports a success rate of only 47%, while direct laryngoscopy had a success rate of 99%.12 Indeed, the authors identified impaired sight due to blood or vomitus as one of the reasons for the low success rate with the Airtraq device, underlining that care must be taken when extrapolating our data to the clinical setting. However, performing a similar study in actually entrapped patients would not be feasible and use of a realistic simulator offers the best possible approximation to the prehospital setting and allowed for a cross-over randomised study design in which potential confounders could be controlled.

Interpretation of results

Airway obstruction is common in trauma patients with survivable injuries who die before hospital admission,13 and securing the airway is therefore a first priority in the prehospital management of trauma patients.14 Herein, tracheal intubation is the ‘gold standard’ to improve oxygenation, control ventilation and to prevent aspiration. This, however, is challenging when the patient is entrapped and when access to the head is severely restricted. In such cases, it is likely prudent to administer oxygen and to attempt clearing the airway using basic techniques and supraglottic airway devices until the patient has been extricated. However, circumstances such as persisting obstruction, hypoxia and/or apnoea may in rare instances force the ALS provider to urgently perform tracheal intubation while the patient is still entrapped. Limited space above the patient's head often precludes conventional laryngoscopy, and some healthcare providers advocate using primary surgical access to the airway (cricothyrotomy) in such circumstances. While this technique may be a quick and promising option for experienced (otorhinolaryngological) surgeons, it may be quite challenging for the average prehospital ALS provider. The victim’s neck cannot easily be extended to identify landmarks and to access the cricothyroid membrane, and cricothyrotomies are associated with a considerable failure and complication rate.15 Prehospital ALS providers rarely, if at all, perform cricothyrotomies and are more familiar (and likely more comfortable) with laryngoscopic techniques. Hence, a laryngoscopic ‘face-to-face’ approach could be a feasible method to perform endotracheal intubation in entrapped victims with limited access to the head. To our knowledge, this approach has been first described by Gürtner et al3 in 1993, and case reports of its successful and unsuccessful prehospital use have sporadically been published since then.4 ,16 However, systematic evaluation or comparisons of different techniques in humans are lacking, and simulator data of this potentially life-saving intervention are scarce. This prompted us to evaluate the standard inverse laryngoscopic intubation approach in a full-scale patient simulator and to compare it with other techniques that have been developed to facilitate intubation under difficult conditions.

Success rate

All three techniques showed a high success rate. Direct laryngoscopy has been termed ‘particularly difficult to use’ under circumstances as simulated in our study,17 and a previous simulator study reported a success rate of only 50%.7 In contrast, the well-trained participants of our study performed markedly better with a success rate of 88%. This suggests that personnel who are proficient with direct laryngoscopy can often successfully use this technique for face-to-face intubation even if they have never or hardly ever done it before. This is particularly important because a Macintosh laryngoscope is an inherent part of every ALS kit and therefore ubiquitously available, while alternative intubation devices may not readily be available when needed. Nonetheless, even experienced personnel may fail to secure the airway with direct laryngoscopy. The 95% CI of success rate is 68% to 96%, which implies that the true failure rate likely lies somewhere between 4% and 32%. Therefore, an alternative backup technique should be available, and in recent years video laryngoscopy and indirect optical laryngoscopy have been developed to facilitate tracheal intubation in difficult conditions. Our data show that both techniques have a high success rate for inverse intubation, suggesting that they might be useful alternatives to direct laryngoscopy when inverse intubation is required.

To our knowledge, two previous studies have also compared success rate of different face-to-face intubation techniques. In the previously mentioned study that found a success rate of 50% with direct laryngoscopy, Asai compared direct laryngocopy with the Pentax Airway Scope.7 Although this device is technically a video laryngoscope and not an indirect optical device, its design with a channel to guide the tube and its handling characteristics are very similar to the Airtraq technique. Similarly to the 100% success rate that we observed with the Airtraq device, Asai report a 100% success rate with the Pentax Airway Scope. Also in a simulator study, Amathieu et al17 recently compared the Airtraq device with a video laryngoscope (GlideScope). However, they did not compare these techniques with the standard technique (direct laryngoscopy). In line with our own data, these authors report a 100% success rate for the Airtraq device. Amathieu et al report a success rate of only 70% for video laryngoscopy, while our participants were always successful with video laryngoscopy. However, in that study a different type of video laryngoscope was used, precluding a direct comparison with our data. While the McGrath device that we used has a monitor mounted to the laryngoscope handle approximately in the same view axis as direct laryngoscopy, the GlideScope has a separate stand-alone monitor. Due to the space restrictions with entrapment, the monitor usually needs to be placed next to the patient or outside the car wreck. This requires the operator to look sideward, which makes eye-hand coordination more difficult and could thus reduce success rate. Likewise, other differences such as configuration of the laryngoscope blade or resolution of the camera or screen could also contribute to differences in performance of different video laryngoscopes.

In essence, our data show that direct laryngoscopy is, in contrast to previous reports and concerns, a technique with a high success rate for inverse intubation when performed by experienced laryngoscopists. Moreover, our data confirm findings of a high success rate of the Airtraq technique in earlier simulator studies, and show that video laryngoscopy with the McGrath device is also a successful technique.

Tube insertion time

While success rate is a key parameter to evaluate the usefulness of an intubation technique, tube insertion times are also of pivotal clinical importance. Entrapped trauma patients can often not be adequately preoxygenated or may already be hypoxic when the decision to intubate is made, and therefore the fastest technique should be preferred when several techniques with comparably high success rates are available.

Video laryngoscopy with the McGrath laryngoscope was the slowest technique, limiting its clinical usefulness despite the high success rate. We did not systematically evaluate the reasons for the prolonged intubation time; however several participants reported that they were confused by the fact that the video image was upside down when the laryngoscope is inserted into the mouth in an ‘ice-pick’ fashion. Moreover, since the laryngoscope is held in the right hand, the tube must now be directed towards the glottis with the left (usually non-dominant) hand, which, in combination with the upside-down image, impedes eye-hand coordination and obviously prolongs intubation time.

The Airtraq technique was significantly faster than video laryngoscopy, although the viewfinder of this device is also upside down when intubation is attempted from a ventral position. However, in contrast to a video laryngoscope, the Airtraq has an integrated channel to guide the tube towards the glottis, requiring substantially less eye-hand coordination. Similarly to our findings, Amathieu et al17 also observed that the Airtraq technique was significantly faster than video laryngoscopy. In contrast to our study, these authors did not evaluate direct laryngoscopy. We observed a median tube insertion time of 34 s with direct laryngoscopy, which was significantly faster than video laryngoscopy and appears a clinically acceptable duration for intubating a patient under difficult conditions.


Success rate was similarly high with all techniques, however the usefulness of the McGrath video laryngoscope for inverse intubation is limited due to rather long tube insertion times. In contrast with common belief, direct laryngoscopy performs well in the hands of proficient anaesthesiologists, with a high success rate and reasonable tube insertion times. Given that this technique requires no more than a standard laryngoscope and a tube, which are widely and readily available in every advanced airway management kit, this method seems useful and should be further promoted and taught for performing inverse intubations in the field. In this simulator study, the Airtraq technique had a high success rate and fast tube insertion times, and was the most preferred technique by our participants, suggesting that the Airtraq device might be a promising alternative or backup method when inverse intubation is required.



  • Contributors RK, LAS and PS proposed the research idea. LAS, RK, DvG, SAL and PS performed literature search and developed the study protocol. DvG, RK, LAS and PS collected data. SAL monitored the study progress, provided logistical support and coordinated work within the study group. Data analysis was done by PS, SAL, RK and LAS. PS had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the statistical data analysis. PS drafted the first version of the manuscript. All authors critically revised the manuscript for important intellectual content and approved the final version.

  • Funding Departmental funding only.

  • Competing interests None.

  • Ethics approval No ethical approval required.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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