To the Editor:
I read with interest the article by Aziz et al. on the comparative effectiveness of the C-MAC®video laryngoscope versus direct laryngoscope in the setting of the predicted difficult airway.1I congratulate them on a well designed and executed prospective comparison study, providing more information to clinicians regarding the performance of this device.
Aziz et al. examined the clinical entity of patients who are best described as being at increased risk of difficulty during laryngoscopy because of abnormal preoperative airway testing, but not truly difficult at laryngoscopy. By examining their figure 2, it can be seen that the majority of the study population had an easy view at laryngoscopy (Cormack–Lehane view grades I or II) irrespective of whether the C-MAC®(Karl Storz, Tuttlingen, Germany) or standard direct laryngoscopy was used.
Unfortunately, their findings are incorrectly extended to conclude a performance benefit when using the C-MAC®in true difficult airways compared with standard direct laryngoscopy. The low incidence of true difficulty at intubation in their study population is unsurprising given the weakness of our current preoperative airway tests to predict true difficulty at laryngoscopy.2Prediction is further weakened by the accepted definition of true difficulty at laryngoscopy (Cormack–Lehane view grades III or IV during direct laryngoscopy), as in clinical practice many patients with a grade III view are relatively easily intubated with or without the use of a bougie. The use of a bougie during standard direct laryngoscopy was not considered in their study design, limiting its overall clinical relevance.
Success at first laryngoscopic attempt was chosen to be the outcome of interest. The authors reasoned that this was because of safety concerns regarding multiple intubation attempts in patients with true difficult airways. As previously discussed, the great majority of their study population consisted of easy laryngoscopy, making the point less valid. This outcome of first-attempt success is particularly affected by unblinding, because the clinician randomized to the direct laryngoscopy group may be inclined to abandon the direct laryngoscopy attempt early and move on to another device managing these patients thought to be at risk of laryngoscopic difficulty. In contrast, the unblinded providers may have been more inclined to persevere with the C-MAC®, with the idea that this device has particular utility in the difficult airway. After examining their figure 1, it can be seen that the actual overall success rate for C-MAC®versus direct laryngoscopy use was 96%versus 92%, respectively. This difference is not statistically significant. The emphasis on first-attempt success is certainly of interest, but does reduce the clinical relevance of their findings. The term “overall success rate” is used misleadingly throughout the manuscript when describing the first-attempt success rate.
The efficacy of a new laryngoscopic device in patients with a known difficult airway is very hard to study. The use of such a device in an anesthetized, paralyzed patient with a known difficult airway is ethically dubious given the accepted guidance that a technique that retains spontaneous ventilation should be considered when difficulty is anticipated at laryngoscopy.3The most common method used by North American anesthesiologists in this situation is awake fiberoptic intubation.4For this reason, prospective studies of novel airway devices in elective intubations, where laryngoscopy is known to be truly difficult, are rare and likely unethical. The use of good retrospective data to study such a rare occurrence should not be discounted, particularly when ethics, blinding, and the requirement of adequate statistical power preclude a prospective study design.