To the Editor:—
We commend Adnet et al. 1for daring to question a traditional, though not scientifically validated, aspect of anesthesia practice—the sniffing position for intubation. However, several aspects of their study warrant comment.
Although the authors acknowledge that the unblinded nature of the study was a limitation, this deserves more emphasis. Bias may have been quite easily introduced during assessment of the view at laryngoscopy (the main outcome variable) and/or during the force exerted on the laryngoscope blade at laryngoscopy. The potential for bias in this particular study is amplified by the fact that four of the investigators were coauthors of two previous articles refuting the ability of the sniffing position to produce alignment of the three axes considered important for optimal laryngoscopic view. 2,3
Another problem with this study is the lack of use of neuromuscular blocking agents. As the majority of adult intubations during surgery in this country are facilitated by neuromuscular blocking agents, their elimination creates a situation which may be difficult to extrapolate to usual clinical practice. Neuromuscular blocking agents may both improve and obscure the view at laryngoscopy. Eliminating patient movement and facilitating mouth opening contribute to the former, while a decrease in muscle tone may precipitate collapse of the pharyngeal walls, thereby hindering visualization. The net effects of these opposing forces is unpredictable and may vary from individual to individual and even time to time.
We very much agree with the authors’ attempts to standardize the type of laryngoscope blade used in their study. Nonetheless, not all MacIntosh No. 3 blades are created equal. For example, the eMAC Welch Allen® and the Propper® blades have much less curvature than many others. It would be important to know exactly which type of MacIntosh blade was used in this study, and to account for the differences during data analysis if more than one type was utilized.
Finally, the conclusion that the sniffing position does not aid laryngoscopy except during two specific circumstances, obesity and decreased neck movement, is misleading. The authors present four other potential predictors of difficult intubation which were not associated with an improvement of laryngoscopic view with the sniffing position. However, for three of these (limited mouth opening, short thyromental distance, and “other anatomic factors”), the number of patients with each predictive factor was between zero and four—numbers much too small for statistical analysis. Even for the remaining factor, Mallampati classes III and IV, the trend was toward improvement in laryngoscopic grade with the sniffing position, but the sample size was too small to achieve statistical significance. Thus, the authors have merely demonstrated that the sniffing position was useful for the two most frequent predictors for difficult intubation studied. It is possible that the sniffing position is beneficial for all potentially difficult intubations but unnecessary for easier intubations. This is consistent with our clinical impression that when an airway is “easy,” it does not really matter what one does, but when an airway is more difficult, one must resort to a variety of intubation “tricks,” such as the sniffing position.