To the Editor:
In a prospective, randomized, open-label, parallel-arm study, Yang et al.1 showed that retrograde light-guided laryngoscopy (RLGL) enabled beginners to intubate patients more successfully and quickly than conventional direct laryngoscopy. Many things of this study were done correctly. They chose well-validated endpoints: the Cormack and Lehane grades and duration and success rate of tracheal intubation. They have a large number of subjects (200) and attempt to control most of the factors that can significantly affect the laryngeal visualization and subsequent tracheal intubation, such as patient’s upper airway anatomy and position, experience of the intubator, uses of anesthetics, and neuromuscular blocking drugs, and many more.2,3 All of these are strengths in the study design. However, in this study, other important factors seemed not to be well addressed, such as blade size, type of tracheal tube, use of stylet, and external laryngeal manipulation with the two techniques.
The authors reported that the incidence of Cormack and Lehane grades 3 and 4 laryngeal views was 20% in the RLGL group and 43% in the direct laryngoscopy group, respectively. We would like to know whether a consistent method of Macintosh blade selection was used in the two groups. The proper function of a Macintosh blade is dependent on using an appropriate length of blade. In order to lift the epiglottis out of the line of sight, the Macintosh blade must be long enough to put tension on the glossoepiglottic ligament. Thus, selecting a right blade based on patient’s characteristics is necessary for adequate laryngeal visualization. Moreover, in some patients, it may be appropriate to change the length of the blade one time in order to obtain proper blade function.2
Likewise, in method section, it was unclear whether use of optimal external laryngeal manipulation to improve laryngeal views was allowed in the direct laryngoscopy group. According to figure 1 in the article,1 a large flashlight with weight of approximately 200 g was placed on the caudal edge of the thyroid cartilage for RLGL in the RLGL group. We are concerned that weight of flashlight and backwards force produced by holding the flashlight in place would have resulted in an analogous optimal external laryngeal manipulation. Benumof and Cooper4 demonstrated that optimal external laryngeal manipulation can improve the laryngoscopic view by at least one whole grade in adults. Thus, we cannot exclude the possibility that such an analogous optimal external laryngeal manipulation would have biased overall study results into the RLGL group. This may also be an explanation of retrograde transtracheal light transmission to improve laryngoscopy and subsequent tracheal intubation.
In addition, it has been shown that types of tracheal tubes may significantly affect ease, duration, and success rate of tracheal intubation.5,6 When a styletted tracheal tube is used, moreover, stylet bend angles have significant influences on ease of tracheal tube passage and success rate of tracheal intubation.7 Thus, we argue that a clear description for types of tracheal tubes and adjuvant use of stylet in method section would further improve the transparency of this study.
Finally, this study excludes the patients with a body mass index of greater than 30 kg/m2. Because thickness of the soft tissues of the neck can affect transtracheal light transmission,8 an important question that remains unanswered is whether the RLGL surpasses conventional direct laryngoscopy for tracheal intubation in grossly obese subjects.