Differences between the infant and adult airway

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

Print this page

This article have been viewed 12167 times

Differences between the infant and adult airway

Nicole W. Pelly, MD

The infant airway differs from the adult airway in structure and in functionality (Figure 192-1). Understanding these differences is important for airway management and, when necessary, for successful tracheal intubation of neonatal, infant, and pediatric airways.

Figure 192-1 Differences between the pediatric and adult airway. (By permission of Mayo Foundation for Medical Research and Education. All rights reserved.)

Anatomy

Head size

The head of an infant is proportionally larger than that of an adult because of the infant’s large occiput. Elevation of the head to produce an anatomic sniffing position in an infant or child is not needed. An infant or child with a large occiput can sometimes benefit from placing a small folded towel under the shoulders and neck, which slightly elevates the thorax. In addition, the head can be stabilized to prevent side-to-side movement.

Laryngeal position

The infant larynx is more cephalad than the adult larynx. At birth, the larynx is located opposite the first and second cervical vertebrae (C1 and C2), and the cricoid cartilage is opposite C3. This relationship provides a functional separation between breathing and swallowing so that the infant can suck, swallow, and breathe at the same time without aspirating. By the time a child is 2 years old, the larynx and cricoid have descended to C3-C4. The infant and pediatric larynx is in a more anterior position, and distances between the tongue, hyoid bone, epiglottis, and other oral structures are smaller than in an adult. In adults, the laryngeal opening is opposite the C5-C6 interspace, and the lower cricoid cartilage is located opposite the top of C7. The laryngeal position changes throughout childhood and, by age 8, is in the adult position.

Tongue

The infant tongue is proportionally larger than the adult tongue and is a common cause of airway obstruction. When infants or children are being mask ventilated, their airways are often enhanced by using an oral airway to relieve this obstruction.

Epiglottis

The infant epiglottis is proportionally longer, compared with the adult epiglottis. It is often omega-shaped, whereas the adult epiglottis is flatter and more flexible. The hyoid bone in the infant overlaps the superior aspect of the thyroid cartilage, which causes the base of the tongue to depress the epiglottis and leads to its protrusion into the pharyngeal cavity. With age, the hyoid and thyroid cartilage separate, and the epiglottis becomes more flexible.

Vocal folds

The vocal folds are composed of the vocal ligament anteriorly and the cartilaginous vocal process of the arytenoid posteriorly. The anterior vocal ligament in an infant is attached in a more caudal position, which causes the vocal folds to be angled. In an adult, the vocal folds are perpendicular to the trachea. Infantile vocal folds are concave, whereas concavity is minimal in the adult. Forward movement of the thyroid cartilage with growth straightens the folds. The angle of the vocal folds in an infant can sometimes cause difficulty in passage of the tracheal tube because the tip of the tube can be caught by the anterior commissure of the vocal folds. This difficulty can sometimes be relieved by gentle rotation of the tracheal tube.

Subglottis

The subglottic region in an infant is funnel-shaped as a result of the structure of the cricoid cartilage. The cricoid ring is the only complete cartilage in the larynx and is nonexpandable, whereas the trachea has a membranous muscle in its posterior aspect that allows for increased compliance with respiration. Classic teaching is that the narrowest point in an infant airway is at the level of the cricoid cartilage, whereas the vocal folds are the narrowest point in an adult. It is now understood that the funnel shape of the larynx is present in adults as well. Because the diameter of an adult trachea is usually still larger than the diameters of most commonly used adult tracheal tubes, a tracheal tube that passes the vocal folds in an adult usually also passes the cricoid ring. In infants and children, a tube that cannot pass beyond the vocal folds is usually too large to pass through the cricoid ring and should be replaced with a smaller tracheal tube. Failure to do so may result in trauma, airway edema, and postextubation stridor and can lead to the development of subglottic stenosis.

Tracheal tube

Size and type

Choosing the correct size of tracheal tube in infants and children is dependent upon the size of the child. Table 192-1 gives some approximate tracheal tube sizes used in infants and children. The presence of an air leak around the tracheal tube is critically important in preventing postintubation edema of the trachea. An air leak should be present at less than 30 cm H₂O peak inflation pressure (the approximate capillary pressure of the tracheal mucosa). If it is not, then the tube should be replaced with the next half-size smaller. Airway resistance is inversely proportional to the radius of the lumen to the fourth power for laminar flow. Therefore, in an infant airway, 1 mm of edema will obstruct the airway by more than 40%, according to the law of Poiseuille.

Table 192-1

Sizes of Tracheal Tubes

Age	Size
Premature (<2.5 kg)	2.5
Term neonate	3.0
2-8 months	3.5
8-12 months	4.0
18-24 months	4.5
Older than 24 months	(Age in years/4) + 4

A cuffed tracheal tube is acceptable for pediatric use as long as a leak is present at less than 30 cm H₂O. However, it is important to remember that the presence of a cuff contributes a half size to the tracheal tube (4.0 cuffed tracheal tube is equivalent to a 4.5 uncuffed tracheal tube). A cuffed tracheal tube should not be automatically inflated with air because the presence of the cuff itself is sometimes enough to decrease the leak. An air leak should occur between 12 and 25 cm H₂O. If a leak is present at less than 12 cm H₂O and interferes with the ability to ventilate the child, then air can be carefully added to the cuff. However, this air should be titrated to an ideal leak between 15 and 20 cm H₂O to prevent airway edema.

Length

The position of the tracheal tube in the trachea is critically important in infants because a small movement can cause mainstem bronchus intubation or an accidental extubation. Preterm infants have an insertion distance of between 6 and 9 cm, depending on their weight. Term newborns have an insertion distance of approximately 10 cm; a 1-year-old child, 11 cm; and a 2-year-old child, 12 cm. An approximate formula for tube insertion distance is three times the size of the tracheal tube. After the tracheal tube is inserted, it is important to auscultate bilaterally in the axillae and to watch for equal anterior chest wall movement. A small but persistent change in O₂ saturation in an infant should cause a reassessment of the position of the tracheal tube. Although there are many formulas for estimating tube insertion distance, it is important to clinically assess and confirm the position in each individual child.

Related

Fausts Anesthesiology Review Expert Consult 4e