A possible explanation for cricoid pressure to improve the laryngeal view with the Truview Evo2™ laryngoscope

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To the Editor, In a randomized clinical trial, Kumar et al. showed that the application of cricoid pressure (CP) improved the laryngeal view with a Truview Evo2 laryngoscope (TEL) without altering the ease of tracheal intubation. However, the authors did not specify the size of blade used in their study. The TEL blade is available in two adult sizes with lengths of 129 mm and 143 mm, and proper functioning of the TEL may depend on using a blade of an appropriate length. Similarly, the authors made no mention of an attempt to standardize the lifting force of the laryngoscope. Increased lifting force can alter the resultant laryngeal view, and this increase can occur in response to a poor view without the laryngoscopist even being aware. We suggest an alternative explanation why CP can improve the laryngeal view with the TEL. In patients with normal and difficult airways, the TEL has been shown to provide a better laryngeal view than the Macintosh laryngoscope due to its tubular optical system that provides an unmagnified laryngeal view at a 42 anterior refracted angle. However, the manufacturer’s description specifies that the upper refracted line of sight of the device’s optical system passes just below the blade tip (www.truphatek.com) (Figure, Panel A). That is, during tracheal intubation with the TEL, the blade tip can always be seen. Actually, the visual field obtained by the tubular optical system of the device cannot cover the blade tip, resulting in a blind area below the blade tip. Measurements, as shown in the Figure 1 (Panel B), show that the blind area below the blade tip of the device’s tubular optical system is 2, 4, 7, 9, and 11 mm in size 0, 1, 2, 3, and 4 blades, respectively. This makes it difficult to visualize what is happening at the blade tip during tracheal intubation, perhaps because there is a relatively large anterior curve between the distal portion and the tip of the blade. Furthermore, a similar flaw has been found in the GlideScope videolaryngoscope, which also has an accentuated curve on its blade. The visual field obtained by the tubular optical system of the TEL includes only the glottis without the surroundings. The length of the adult glottis is about 23 mm in males and 17 mm in females. Thus, when an adult blade (size 3 or 4) is used, a 9-11 mm blind spot below the blade tip can undoubtedly interfere with the laryngeal exposure with the device (Figure 1, Panel C). Moreover, this issue may be especially significant in patients with their larynx in a more anterior position, as it requires a visual field in a more anterior direction. Nevertheless, when the external force of the CP results in a downward movement of the larynx, the upper part of the glottis, which cannot be seen, will be moved out of the blind spot, and the TEL will provide an improved laryngeal view. With a gentle lifting force of the TEL, perhaps the favourable effect of the CP on the laryngeal view will be more apparent. In practice, both external laryngeal pressure and CP have previously been used with this device as optimization maneuvers to aid laryngeal exposure and tracheal intubation in adult patients. In summary, the TEL appears to have a blind spot below its blade tip, and external laryngeal pressure or CP may be used with this device to improve the laryngeal view during laryngoscopy. F. S. Xue, MD (&) Y. J. Yuan, MD J. H. Liu, MD Plastic Surgery Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, People’s Republic of China e-mail: fruitxue@yahoo.com.cn