Where is the narrowest segment in the upper airway?

Your correspondents (Whyte et al. Anaesthesia 2003; 58: 196) stated that, as in children, the narrowest segment of the upper airway in adults is not the glottis, but at the cricoid cartilage, by referring to a paper of Koufman and colleagues [1]. In fact, this reference states that ‘the subglottic cricoid is the smallest, fixed cross section in the upper airway’. Nevertheless, they only measured the diameter of the cricoid cartilage of cadavers [1], and compared it with the anteroposterior diameter of the glottis and trachea that were reported previously by different researchers. The anteroposterior distance is unlikely to be narrowest in the glottis, and thus it cannot be concluded whether the narrowest diameter of the glottis is greater or smaller than the narrowest length of the cricoid lumen. As far as we know, there has been only one study that measured the length of both the glottis and the cricoid lumen [2]: Seymour & Prakash decided to measure the glottic and cricoid lumen in 134 cadavers because they sometimes had difficulties in advancing a double-lumen tube beyond the subglottis in adults. By inserting a cylindrical nylon sounds into the glottis and cricoid lumen, they measured the maximum effective size, and found that the diameter of the glottis was always either similar to, or greater than, the diameter of the cricoid lumen. Therefore, it was just last year (2002) when we finally obtained clear evidence rejecting the orthodox opinion that the narrowest segment in the upper airway is the glottis in adults.