In the very interesting article of Darshane et al. 1 that validated a new model of graded difficulty (based on mandibular space reduction) in the Laerdal ‘SimMan’ mannequin and suggested functional comparisons between Macintosh direct laryngoscope (MDL), Glidescope videolaryngoscope (GVL), Truview EVO2 laryngoscope (TEL) and Airtraq, some issues in the study design may make the interpretation of their conclusions questionable.
First, to establish the intermediate setting that restricts the tongue compression by reduced mandibular space, an elastomer silicone insert was placed at the anterior part of the mandibular space. However, this may not be an accurate representation of a real mandibular space reduction in clinical practice, such as a patient with micrognathia. Anatomically, the base of the tongue located at the posterior part of the mandibular space resides close to the glottic aperture. When the mandibular space is reduced, the relatively large base of the tongue will take up excessive space in the oropharynx and will interfere with the line of sight into the glottis during direct laryngoscopy. Moreover, the base of the tongue resides so close to the larynx that inability to displace it adequately anteriorly creates another problem. As the base of the tongue hangs down over the larynx, the glottis is hidden from view. Under such circumstances, the glottis is anatomically anterior to the base of the tongue and cannot be seen. In fact, indirect laryngoscopes including the GVL, TEL and Airtraq are designed to offer the advantage of being able to ‘look around the corner’ at the base of the tongue; hence, inadequate laryngeal view in a direct laryngoscopy can be improved with an indirect laryngoscope.2 In contrast, when an insert is placed at the anterior part of the mandibular space, the position of a direct or indirect blade in the upper airway will be moved posteriorly relative to the larynx, resulting in an inadequate laryngeal view. That is, in their intermediate setting, an inadequate laryngeal view contributed to an abnormal performance of a direct or indirect laryngoscope, rather than the anatomic changes by the reduced mandibular space. This problem may be more significant when a blade with the greater curvature is used, such as the GVL blade.
Second, the authors did not describe the details of direct or indirect laryngoscopy, for example, mannequin head position and application of external laryngeal pressure. It is generally recommended that for studies of difficult laryngoscopy to be reliable and for the preceding laryngoscopic grading system to be helpful, the reported grades of laryngeal views must describe the best views that are obtained, which, in turn, depend on the best possible performance of laryngoscopy.3 An optimal head position may be unimportant for the laryngeal exposure using the indirect laryngoscope, without having to align the oral, pharyngeal, and laryngeal axes or distort the airway structures.2 However, a ‘sniffing’ position, obtained by placing a pillow under the patient's head, helps to achieve a better laryngeal view with MDL, because it can align the oral, pharyngeal and laryngeal axis into more of a straight line.4 Like the MDL, moreover, field of view of both the GVL and TEL cannot cover the distal tip of the blade, resulting in a blind area just below the blade tip. By examining measurements and the data from available reviews,5 we find that this blind area is about 6–10 mm in size 4 TEL, MDL and GVL. In contrast, Airtraq, airway scope, and C-MAC videolaryngoscope do not have the blind area below the blade tip. Evidently, a blind area below the blade tip may interfere with laryngoscopy, especially when the position of the glottis is moved anteriorly relative to that of the blade. This means that the external laryngeal pressure manoeuvre may be required to obtain a better exposure of the glottis. According to the management of difficult airway algorithm of the American Society of Anesthesiologists3 the external laryngeal pressure manoeuvre should be an inherent part of laryngoscopy and an instinctive reflex response to a poor laryngeal view by the MDL, because it can frequently improve the laryngoscopic view by at least one entire grade.6 We are concerned that lack of a requirement for an optimal best attempt at laryngoscopy may have underestimated the performance using the different blades.
Third, in this study, the GVL and TEL were used as representatives of ‘steering’ indirect laryngoscopes to compare with the MDL. In the available ‘steering’ indirect laryngoscopes, the C-MAC videolaryngoscope has really the same curvature as the MDL. Also, the laryngoscopy and intubation procedures with the C-MAC videolaryngoscope are identical to the traditional one with the MDL.2 Therefore, if this study had included a C-MAC videolaryngoscope group, it would have probably provided more useful information.
There are no conflicts of interest to declare.
1. Darshane S, Ali M, Dhandapani S, Charters P. Validation of a model of graded difficulty in Laerdal SimMan: functional comparisons between Macintosh, Truview EVO2
, Glidescope Video Laryngoscope and Airtraq. Eur J Anaesthesiol
2. Niforopoulou P, Pantazopoulos I, Demestiha T, et al. Video-laryngoscopes in the adult airway management: a topical review of the literature. Acta Anaesthesiol Scand
3. Hagberg CA, Benumof JL. The American Society of Anesthesiologists’ management of difficult airway algorithm and explanation-analysis of the algorithm. In: Hagberg CA, editor. Benumof’ Airway Management
. 2nd ed. St. Louis, MO: Mosby-Year Book Inc; 2007. pp. 241–246.
4. Benumof JL. Difficult laryngoscopy: obtaining the best view. Can J Anaesth
5. Hirabayashi Y, Otsuka Y. Apparent blind spot with the GlideScope video laryngoscope. Br J Anaesth
6. Benumof JL, Cooper SD. Quantitative improvement in laryngoscopic view by optimal external laryngeal manipulation. J Clin Anesth