Yu et al4) conducted a meta-analysis of 14 clinical studies of infant intubations, and concluded that video laryngoscope improved visibility of vocal cords but increased IT and incidence of intubation failure compared with direct viewing laryngoscope using ML. In the current study, no significant difference was observed in IT between the AWS and ML groups. Therefore, these data indicate that AWS may be a viable substitution to ML for intubation of infants with CLP.
4.1 Factors affecting IT
(1) Visibility of vocal cords
Previous studies have attributed the difficulty of laryngoscopy in infants with CLP to young age2), degree of cleft, and micrognathia3). In the current study, the Cormack and Lehane class of the ML group was statistically greater than that of the AWS group. However, quality of view was not statistically different between the groups. This result may be obtained when the Cormack and Lehane grade is less than III, which is an index of difficult intubation; this was not observed in any of the groups. Accordingly, the quality of view was appropriate for intubation in both AWS and ML; hence, the quality of view was not related to IT.
(2) Difficulty of blade insertion
It has previously been reported that the tip of AWS blade may inadvertently access the esophagus rather than the trachea when inserted in the infant airway5). In the current study, the esophagus was seen first on screen following insertion into the pharynx in the AWS cases; therefore, the number of cases in which the blade was moved backward or forward was greater in the AWS group than in the ML group. Because of this, re-insertion was favorable to moving forward and backward to avoid injuring the pharynx in the AWS group; hence, the number of attempts of blade insertions was greater than that in the ML group. Moreover, the length of the AWS blade is 65 mm, which is longer than that of infant upper airway; therefore, it may have necessitated the increased instances of moving forward and backward in this group. Conversely, the attached documents for the Halyard micro cuff® recommends a Φ3.0 mm tracheal tube for infants aged <8 months whose weight is >3 kg, and thus the size of blade for neonates may be suitable for most of the patients in this study. In our hospital, a Φ3.5 mm tracheal tube of Halyard micro cuff® is generally the first choice for infants with CLP. Therefore, we chose the blade for pediatrics that was fit for the Φ3.5 mm tracheal tube. IT might be shortened using a Φ3.0 mm tracheal tube of Halyard micro cuff® and neonate blade of AWS (length is 12 mm shorter than that for pediatrics) because of reduction in the time required to determine the location of the esophagus and detect the vocal cord by moving backward.
(3) Difficulty of tracheal tube insertion
Unless the AWS target mark coincides with the vocal cords at the monitor screen before progression of the tube, it is easy for the tip of the tube to inadvertently hit the right Rima glottides as it is progressed to the vocal cords after removing from the blade groove in infants5). Therefore, the tube needs to be turned left following removal of the tube from the blade groove prior to progressing to the vocal cords. “Difficulty of tracheal tube insertion” was not statistically different between the two groups in this study; however, this maneuver might be administered as needed with the progression of the tube; hence, average of IT of AWS may be longer than that of ML.
(4) Correlation between patient characteristics and IT
In bilateral CLP, the premaxilla and cleft palate edge protrude to the oral cavity and narrow the space. This formation may limit controllability of the AWS blade, explaining the correlation of degree of cleft and IT in the AWS group.
The AWS method has an increased risk of pharynx damage owing to the limited space between the blade and pharynx wall5). However, bleeding from upper airway was observed only in the ML group. The bleeding observed in the ML group was detected after fixing the tracheal tube with tapes on the lip. Nevertheless, anesthesiologists could watch the blade directly from the beginning of insertion to complete intubation when using ML. Therefore, the upper airway damage by ML may have been caused when the blade was removed from the pharynx or oral cavity. The number of cases with bleeding was not statistically different between the two groups; however, upper airway damage should be avoided, particularly in CLP surgery. More attention to upper airway mucosa is required not only at blade insertion but also at blade removal in ML, which might increase the IT time.