A.  Illustrated is a conventional cuffed tracheal tube.  Dotted lines represent the vocal cords.  The tip of the tube lies very close to the carina.  B.  Illustrated is a tracheal tube with a shortened cuff.  The glottis depth mark on the tube lies close to the vocal cords.  The cuff of the tube is close to the inferior edge of the cricoid ring.  (Image source: Anesthesia and Analgesia)
A. Illustrated is a conventional cuffed tracheal tube. Dotted lines represent the vocal cords. The tip of the tube lies very close to the carina. B. Illustrated is a tracheal tube with a shortened cuff. The glottis depth mark on the tube lies close to the vocal cords. The cuff of the tube is close to the inferior edge of the cricoid ring. (Image source: Anesthesia and Analgesia)

How far should an endotracheal tube be inserted into the trachea of a child?  As with adults, the cuff of the tube should be inserted just beyond the vocal cords.  If the tube is inserted too far, the tip of the tube may be inserted beyond the carina and only one lung will be ventilated.  If the tube sits on the carina, the patient may cough and require deeper levels of anesthesia than if the tube was not inserted as far.  If the tube is not inserted far enough, accidental extubation might occur if the head or neck is manipulated.  What is the age-specific length of the subglottic airway, specifically of infants and children?

Most studies of length are based on autopsy specimens.  However, in the study “Novel Measurements of the Length of the Subglottic Airway in Infants and Young Children” published in this month’s issue of Anesthesia and Analgesia, Dr. Karen A. Brown, Montreal Children’s Hospital, Montreal, Quebec, Canada, and colleagues used computed tomography studies to measure the subglottic airway length of children who underwent CT scans of the neck.

The authors analyzed 56 CT scans.  During the scans, each patient’s trachea was not intubated.  Scans were obtained using IV injection of Omnipaque 300TM, an iodine-free contrast.  They defined age (in months, 28 days/month) as the postnatal age at the time of study.  Regression analysis resulted in a subglottic mean airway length (mm) equal to 7.8 + 0.03 times age.  Vocal cord to carina length (cm) was 5.3 + 0.05 times age.  Tracheal length was 4.5 + 0.05 times age.  Tracheal length can vary according to flexion of the neck, and neck flexion here varied from 45° to 89°.  In the neutral position, then, the lengths could be greater.

These measures are in good agreement with autopsy studies.  The authors also illustrated that given their findings and the dimensions of cuffs on endotracheal tubes used for infants, when conventional cuffed endotracheal tubes are placed so that the cuff is at an appropriate distance from the cricothyroid membrane, this may result in the tip of the endotracheal tube lying too close to the carina.   Is coughing, due to the tracheal tube abutting the carina, more likely?  Conversely, in endotracheal tubes with shortened cuffs, using the glottic depth mark on the tube to place it near the vocal cords may result in the cuff abutting the cricoid ring.  Might the likelihood of swelling near the cricoid ring be more likely when these kinds of endotracheal tubes are used?

Clearly more study is needed.  OpenAnesthesia has a discussion on airway management of the child.