To the Editor:
In a prospective, randomized, clinical study including a broad range of patients with predictors of difficult intubation, Aziz et al. 1demonstrated that compared with the direct laryngoscope, the C-MAC® video laryngoscope (Karl Storz, Tuttlingen, Germany) achieved a higher intubation success rate on first attempt, but required a longer intubation time. Other than the limitations described in the discussion, however, there are several issues related to this study that warrant cautious interpretation of the results.
First, the 296 airway management procedures were completed by a total of 91 participants. It is reported that novice anesthesia residents or nonanesthesia trainees require about 47–56 tracheal intubations to achieve a success rate of 90% or more using direct laryngoscope.2,3Thus, we believe that all participants have achieved proficiency with the direct laryngoscope. In Materials and Methods, the authors stated that all participants were given didactic instruction on the proper use of the C-MAC® and were afforded the opportunity to use the device for clinical use in the 3 months preceding the study. However, they did not provided the actual or lowest number of tracheal intubation attempts with the C-MAC® by each participant. In previous studies comparing performance of different video laryngoscopes (including V-MAC®, an older model of C-MAC®) with Macintosh laryngoscope,4,5the experienced anesthesiologists were required to have a minimum of 30–50 uses of each video laryngoscope before the study. More importantly, the authors should explain if they attempted to define proficiency with the uses of the C-MAC® for tracheal intubation. The C-MAC® has a standard Macintosh blade, but the tracheal intubation procedure under video laryngoscopy significantly differs from that under direct laryngoscopy. For example, a challenge for the operator is to become familiar with the view on the monitor, and to coordinate the eyes and hands appropriately.6Therefore, practice is needed to develop the skill needed for advancing the endotracheal tube while viewing the monitor. Recently, Behringer and Kristensen7emphasize that for the results of a comparative study to be valid, the participants must be equally proficient with each airway device to avoid bias. If acceptable proficiency with an airway device is not defined and confirmed before the initiation of a comparative study, conclusions of the study may merely suggest that the peak of the learning curve of the airway device has not been attained by many of the study participants. We consider that addressing this factor would further clarify the transparency of this study with a diverse group of anesthesia providers.
Second, in this study, sample size (141 patients per group) was selected to detect a projected difference of 10% in the incidence of multiple intubation attempts between the two devices, with a power of 80% and P = 0.05. Obviously, the sample size of studied population is insufficient to detect statistically significant differences between the two devices with respect to the intubation success rate achieved by the certified registered nurse anesthetist providers or attending anesthesiologists, and intubation success rate in patients with two or multiple predictors of difficult intubation.
Third, there were a total of 34 failures with the primary intubation approach. Of these 34 cases, 6 of 11 (54%) in the C-MAC® group and 8 of 23 (35%) had an adequate laryngeal view. Although a good laryngeal view with video laryngoscopy does not always guarantee intubation success,6the laryngeal view obtained by direct laryngoscopy is usually an important determinant of successful intubation.8Unfortunately, the authors did not provide the detailed cause of failed primary intubation approach in these patients with a good laryngeal view. In Materials and Methods, they did not describe whether the endotracheal tube with a malleable stylet was used on first intubation attempt. Use of a stylet to preform or stiffen an endotracheal tube can facilitate guidance through the glottis when this is seen under direct laryngoscopy, or can be used as a blind technique with a narrow endotracheal tube.9Furthermore, it has been shown that the Macintosh blade of the V-MAC® can reduce, but does not replace, routine stylet use for tracheal intubation. Without use of a stylet, incidence of failed intubation on first attempt with the V-MAC® is 16% in patients with normal airways5and 24% in morbidly obese patients,4respectively. Therefore, when a successful initial intubation attempt is important for patient safety – for example, in managing a known or predicted difficult airway – mounting the endotracheal tube onto a stylet and angling the distal tip upward is very helpful for bringing the tube tip up to the glottis under direct or indirect laryngoscopy.6,8,9In addition, Levitan et al. 10suggest that if a stylet is used with the C-MAC®, a tube shape similar to that of direct laryngoscopy (straight-to-cuff, with a 35-degree “hockey-stick” bend11) should be used, because excessive tube shaping can create tube advancement problems. This is significantly different from the McGrath and GlideScope video laryngoscopes with angulated blade, in which much greater tube bend angles (60–90 degrees) are often required to navigate a tube around the curve of the tongue and to the glottis.6,10We deduce that a prolonged intubation time and six failed cases of primary intubation approach under a good laryngeal view with the C-MAC® may be contributed to no use of a stylet.