Some in vivo studies question the traditional “funnel-shaped” infant larynx; further anatomic examinations were warranted. Examination of fixative free fresh autopsy laryngeal and upper tracheal specimens and multiple measurements was needed to determine consistency between current tracheal tube designs and anatomic observations.
Larynges from 19 males and 11 females (Caucasian term newborn to 126 months) were examined by the same forensic pathologist. Measurements included anterior/posterior (A/P) and transverse (T) diameters of the cricoid outlet (CO), interarytenoid diameter (IAD), cricothyroid membrane (CTM), distance from the vocal cords (VC) to CO (VC-CO), and calibration of the larynx lumen with uncuffed tracheal tubes as measuring rods. Assessment of “safe tracheal tube placement” was assessed using manufacturer recommended cuffed Microcuff (Kimberly-Clark, Koblenz, Germany) tubes.
In 77% (95% confidence interval [CI], 58-90) of specimens, the proximal end of the cuff was within the CO and in 23% even with or close to the CO. The VC-CO varied from 9.1 to 13.17 mm in infants, 11.55 to 15.17 mm in toddlers, and 13.19 to 18.34 mm in children. The AP/T ratio of the CO was nearly 0.99 in most larynges; the IAD was greater than CO in all specimens. The CTM could be minimally distended in all specimens.
First, despite being marketed as a safer tracheal tube design, the proximal end of the Microcuff cuff rested within or close to the cricoid cartilage theoretically increasing potential cuff-induced injury when using the VC markings for positioning. Our data suggest that the optimal cuff free distance (VC-CO) would be ~13.5 mm for a Microcuff internal diameter (ID) size 3.0, ~15 mm for size 3.5, and ~16 to 19 mm for greater sizes.
Second, the CO was virtually circular in all specimens, suggesting that appropriately sized uncuffed tubes should provide an adequate seal in most neonates and toddlers, thus avoiding the potential for cuff-related necrosis injury.
Third, the IAD was always greater than CO confirming that the narrowest point of the infant larynx is the nondistensible cricoid cartilage and not the easily distended glottis.
Fourth, appropriately sized Microcuff tubes with the cuff deflated completely filled the lumen of the CO and proximal trachea in all specimens. Our data suggest the need for all manufacturers to further evaluate tracheal tube cuff locations and lengths in relation to the VC safe insertion markings, particularly for neonates and toddlers.
Fifth, the CTM is minimally distensible, thus having important implications for emergency surgical airway access with most currently available emergency airway devices.