Tongue

In the past, it was thought that an infant’s tongue is relatively large in proportion to the rest of the oral cavity and therefore can more easily obstruct the airway, especially in a neonate. However, magnetic resonance imaging (MRI) studies have now demonstrated that there is proportional growth of the tongue and other soft tissues in relation to the bony structures of the oral cavity in children 1 to 11 years of age.⁵ Furthermore, the contribution of the tongue to upper airway obstruction with sedation or induction of anesthesia is relatively minor; much of the obstruction is more likely due to nasopharyngeal and epiglottic collapse, although in a patient of any age the tongue may also contribute to obstruction.^6,7

Position of the Larynx

An infant’s larynx is higher (more cephalad) in the neck, classically described at the level of C3-4, than an adult’s larynx, which is at the level of C4-5 (Fig. 12-1). MRI and computed tomography (CT) have confirmed the higher (more cephalad) position of the larynx in children and demonstrated that the hyoid bone is at the C2-3 level in infants and children up to 2 years of age.⁸ Consequently, the distances between the tongue, hyoid bone, epiglottis, and roof of the mouth are smaller in infants than they are in an older child or adult.

FIGURE 12-1 In a preterm Infant, the larynx is located at the middle of the third cervical vertebra (C3); in a full-term infant, it is at the C3-4 interspace; and in an adult, it is at the C4-5 interspace.

(Adapted from Negus VE. The comparative anatomy and physiology of the larynx. Oxford: Butterworth-Heinemann, 1949.)

The proximity of the tongue to the more superior larynx also makes visualization of laryngeal structures more difficult, because it produces a more acute angle between the plane of the tongue and the plane of the glottic opening. It is for this reason that a straight laryngoscope blade, which lifts the tongue from the field of view during laryngoscopy, facilitates visualization of an infant’s larynx. This anatomic relationship is further complicated in certain conditions such as the Treacher Collins anomaly and other syndromes associated with mandibular and midfacial hypoplasia that make direct visualization of the glottis difficult and sometimes impossible with standard laryngoscopy (Fig. 12-2). The reason for this difficulty is that with mandibular and midfacial hypoplasia, the base of the tongue is positioned more caudally (known as glossoptosis) and in closer proximity to the laryngeal inlet than normal; the result is an even greater acute angle between the plane of the tongue and the plane of the laryngeal inlet (often 90 degrees) (Fig. 12-3). In this situation, conventional rigid laryngoscopy provides excellent visualization of the esophageal inlet rather than the laryngeal inlet, necessitating the use of special equipment or special techniques to intubate the trachea.

FIGURE 12-2 Three-dimensional reconstruction of a child with the Treacher Collins anomaly demonstrates the retrognathic and more posterior position of the mandible, the midfacial hypoplasia, and the closer proximity and exaggerated angle between the base of the tongue and the laryngeal inlet (almost 90 degrees), which makes direct visualization of the larynx difficult.

FIGURE 12-3 The larynx in children with mandibular hypoplasia is located more posteriorly than in children with normal anatomy. A, Lateral radiograph of the upper airway including the base of the skull and cervical spine of a normal 7-year-old child; the arrows denote the posterior border of the ramus of the mandible and the anterior border of the second cervical vertebra. B, Diagrammatic representation of the normal anatomy in A. C, The same radiographic projection in a 6-year-old child with Treacher Collins syndrome; the arrows again denote the posterior border of the ramus of the mandible and the anterior margin of the second cervical vertebra. D, Diagrammatic representation of the anatomy in C. Notice the significantly smaller space between the ramus of the mandible and the second cervical vertebra, compared with the normal anatomy; the anterior margin of the first cervical vertebra overlaps the posterior margin of the mandible. This extreme posterior location of the tongue and larynx makes direct visualization of the laryngeal inlet almost impossible in many children with this anomaly because of the acute angulation between the base of the tongue and the laryngeal inlet.

(Radiographs courtesy Donna J. Seibert, MD, John A. Kirkpatrick, Jr., MD, and Robert H. Cleveland, MD.)

Epiglottis

An adult’s epiglottis is flat and broad, and its axis is parallel to that of the trachea (Fig. 12-4), whereas the infant’s epiglottis is narrower, omega shaped, and angled away from the axis of the trachea (Fig. 12-5). It is therefore more difficult to lift the infant’s epiglottis with the tip of a laryngoscope blade.

FIGURE 12-4 Lateral neck xerogram (A) and schematic diagram (B) of the larynx in an adult. Notice the relatively thin, broad epiglottis, the axis of which is parallel to the trachea. The hyoid bone “hugs” the epiglottis; there is no subglottic narrowing. Also note how the vocal cords are perpendicular to the axis of the trachea.

FIGURE 12-5 Lateral neck xerogram (A) and schematic diagram (B) of an infant’s larynx. Notice the angled epiglottis and the narrow cricoid cartilage. Also note that the vocal cords are angled with a higher attachment anteriorly than posteriorly compared with the perpendicular position of the vocal cords in adults.

Vocal Folds

In contrast to the adult, in whom the axis of the vocal folds is perpendicular to that of the trachea, the vocal folds (cords) of an infant are angled such that the anterior insertion is lower (caudad) compared with the posterior insertion (compare Fig. 12-4, A, with Fig. 12-5, A). This anatomic feature alters the angle at which the tracheal tube approaches the laryngeal inlet and occasionally leads to difficulty with tracheal intubation, especially with the nasal approach. In the latter case, the tip of the endotracheal tube (ETT) may be held up at the anterior commissure of the vocal folds.

Subglottis

Classic teaching holds that the narrowest part of an infant’s larynx is the cricoid cartilage; in an adult, it is the rima glottidis. This teaching was supported by an MRI and CT study in young children (<2 years of age) who were sedated with oral medications and breathing spontaneously.⁸ In contrast, a more recent study in children 2 months to 13 years of age undergoing MRI with propofol sedation and spontaneous respirations reported that the narrowest portions of the pediatric larynx were the glottic opening and the immediate sub–vocal cord level and that this finding did not change relative to the dimensions of the cricoid ring throughout childhood.⁹ Nonetheless, when a relatively large-diameter tube is inserted into the glottic aperture, the tube passes through the cords but may meet resistance immediately below the cords (e.g., in the subglottic or cricoid ring region). Although these studies demonstrate in vivo physiologic relationships, the cricoid cartilage is functionally the narrowest portion of the upper airway.

Growth of the subglottic airway occurs rapidly during the first 2 years of life; thereafter, growth of the airway is linear.¹⁰ At 10 to 12 years of age, the cricoid and thyroid cartilages reach adult proportions, thus eliminating both the angulation of the vocal cords and the narrow subglottic area.

In the adult, the rima glottidis is considered the narrowest part of the airway, and an ETT that traverses the glottis passes into the trachea without resistance. However, in about 70% of adult cadavers, the narrowest portion of the airway was also in the subglottic region.¹¹ The range in diameter for adult females was 10 to 16 mm, and for adult males it was 13 to 19 mm. The likely reason that ETTs pass easily through the glottic opening into the trachea of an adult is that, overall, the narrowest portion of the airway is still larger than the most commonly used ETT sizes. The apparent subglottic narrowing in adults is generally not evident unless there is the need to pass a larger-diameter ETT such as a double-lumen tube. In contrast, in a child, it is common for an ETT to pass easily through the vocal folds (glottic opening) but not through the subglottic region (Fig. 12-6; see Video 12-1). The larynx in both adults and children should be considered funnel shaped, although this configuration is exaggerated and is of greater import in infants and young children.

FIGURE 12-6 Configuration of the larynx of an adult (A) and an infant (B). Notice that both larynxes are somewhat funnel shaped, but this shape is exaggerated in the infant and toddler. The adult laryngeal structures are of such size that most endotracheal tubes pass easily into the trachea. In infants and toddlers, it is common for the endotracheal tube (ETT) to pass easily through the vocal cords but to become snug at the level of the nondistensible cricoid cartilage. Concern for causing edema at this point resulted in the classic teaching that uncuffed ETTs should be used in young children (see text for more details). A, Anterior; P, posterior.

The cricoid is the only complete ring of cartilage in the laryngotracheobronchial tree and is therefore nondistensible. Because the mucosa that lines the upper airway is loose-fitting pseudostratified columnar epithelium, pressure on the mucosa may cause reactive edema that could encroach on the diameter of the lumen. A tight-fitting ETT that compresses the tracheal mucosa at this level may cause inflammation and edema when it is removed, reducing the luminal diameter and increasing the airway resistance at the time of extubation (e.g., postextubation croup). Because the subglottic region in the infant is smaller than in the adult, the same degree of airway edema results in greater resistance in the infant. For example, assuming that the diameter of the cricoid ring in the infant is 4 mm and the diameter of the adult cricoid ring or trachea is 8 mm, 1 mm of edema circumferentially within the airway (i.e., reduction of the diameter of the airway by 2 mm) would decrease the cross-sectional area of the airway in the infant by approximately 75% (to 2 mm), whereas the adult cross-sectional area would decrease by only about 44% (to 6 mm). Physiologically, because the resistance to airflow in the upper airway is turbulent during crying or deep breathing, this reduction in diameter of the upper airway would increase the resistance to flow by the radius to the fifth power, or 32-fold, in the infant, compared with 5-fold in the adult. (Fig. 12-7).²

FIGURE 12-7 Relative effects of airway edema in an infant and an adult. The normal airways of an infant and an adult are presented on the left. Edematous airways display 1 mm of circumferential edema, reducing the diameter of the lumen by 2 mm. Notice that resistance to airflow is inversely proportional to the radius of the lumen to the fourth power for laminar flow (beyond the fifth bronchial division) and to the radius of the lumen to the fifth power for turbulent flow (from the mouth to the fourth bronchial division). The net result in an infant with a 4-mm diameter airway is a 75% reduction in cross-sectional area and a 16-fold increase in resistance to laminar airflow, compared with a 44% reduction in cross-sectional area and a 3-fold increased resistance in an adult with a similar 2-mm reduction in airway diameter. With turbulent airflow (upper airway), the resistance increases 32-fold in the infant but only 5-fold in the adult.

Anatomy

Structure

The larynx is composed of one bone (hyoid) and eleven cartilages (the single thyroid, cricoid, and epiglottic cartilages and the paired arytenoid, corniculate, cuneiform, and triticea cartilages). These cartilages are suspended by ligaments from the base of the skull. The body of the cricoid cartilage articulates posteriorly with the inferior cornu of the thyroid cartilage. The paired triangular arytenoid cartilages rest on top of, and articulate with, the superoposterior aspect of the cricoid cartilage. The arytenoid cartilages are protected by the thyroid cartilage (Fig. 12-8). The triticeal cartilages are rounded nodules of cartilage, approximately the size of a pea in adults, located in the margins of the lateral thyrohyoid ligament.

FIGURE 12-8 Laryngeal cartilages. The natural positions of the laryngeal cartilages are presented on the left, with the individual cartilages separated on the right.

(Reprinted by permission from Fink BR, Demarest RJ. Laryngeal biomechanics. Cambridge, Mass.: Harvard University Press, © 1978 by the President and Fellows of Harvard College.)

Tissue folds and muscles cover these cartilages. In contrast to adults, but comparable to most mammals, the cartilaginous glottis accounts for 60% to 75% of the length of the vocal folds in children younger than 2 years of age.¹⁰ Contraction of the intrinsic laryngeal muscles alters the position and configuration of these tissue folds, thus influencing laryngeal function during respiration, forced voluntary glottic closure (Valsalva maneuver), reflex laryngospasm, swallowing, and phonation (Fig. 12-9).

FIGURE 12-9 Photograph (A) and schematic diagram (B) of the larynx of a premature infant.

The laryngeal tissue folds consist of the following:

Paired aryepiglottic folds extending from the epiglottis posteriorly to the superior surface of the arytenoids (the paired cuneiform and corniculate cartilages lie within for support and reinforcement)

Paired vestibular folds (false vocal cords) extending from the thyroid cartilage posteriorly to the superior surface of the arytenoids

Paired vocal folds (true vocal cords) extending from the posterior surface of the thyroid plate to the anterior projection or vocal process of the arytenoids

A single interarytenoid fold (composed of the interarytenoid muscle covered by tissue) bridging the arytenoid cartilages

A single thyrohyoid fold extending from the hyoid bone to the thyroid cartilage

Function

Obligate Nasal Breathing

Infants are considered to be obligate nasal breathers.^24,25 Obstruction of their anterior or posterior nares (nasal congestion, stenosis, choanal atresia) can cause asphyxia.^26–28 Immaturity of coordination between respiratory efforts and oropharyngeal motor and sensory input accounts in part for obligate nasal breathing.²⁹ Furthermore, because the larynx is higher (more cephalad) in the neck of an infant and oropharyngeal structures are closer together, the tongue rests against the roof of the mouth during quiet respiration, resulting in oral airway obstruction.²⁵ Multiple sites of pharyngeal airway obstruction may also contribute to airway obstruction when the infant attempts to breathe against a partially obstructed upper airway or with relaxation of upper airway muscle tone after sedation or induction of anesthesia.^30–34

As the infant matures, the ability to coordinate respiratory and oral function increases. The larynx enlarges and moves down lower (more caudad) in the neck as the cervical spine lengthens and the infant begins to breathe adequately through the mouth. This maturation occurs by age 3 to 5 months. Studies have shown that the ability to breathe through the mouth when the nares are obstructed is age dependent: 8% of preterm infants of 31 to 32 weeks postconceptional age were able to breathe through the mouth in response to nasal occlusion, compared with 28% of more mature preterm infants of 35 to 36 weeks postconceptional age³⁵; approximately 40% of full-term infants can switch from nasal to oral breathing.³⁶ However, more recent data contradict these earlier data. Slow and fast nasal occlusion applied to 17 healthy preterm infants (gestational age, 32 ± 1 weeks; postnatal age, 12 ± 2 days) led to a switch from nasal to oral breathing. The authors attributed the difference in findings to the more extended observation period in their study (>15 seconds).³⁷ The presence of a nasogastric tube may also significantly affect the infant’s breathing if the “unobstructed” nasal passage has an existing underlying obstruction.

Tracheal and Bronchial Function

Tracheal and bronchial diameters are a function of elasticity and of distending or compressive forces (Fig. 12-10). The larynx, trachea, and bronchi in the infant are quite compliant compared with those in the adult and therefore are more subject to distention and compression forces.^24,38,39 The intrathoracic trachea is subject to stresses that are different from those in the extrathoracic portion.³⁸ During expiration, intrathoracic pressure remains slightly negative, thus maintaining patency of the intrathoracic trachea and bronchi (see Fig. 12-10, B). During inspiration, a greater negative intrathoracic pressure dilates and stretches the intrathoracic trachea and bronchi.⁴⁰ The extrathoracic trachea at the thoracic inlet is slightly narrowed by dynamic compression that results from the differential between intratracheal pressure and atmospheric pressure. However, the cartilages of the trachea, along with the muscles and soft tissues of the neck, maintain patency of the airway (see Fig. 12-10, A).

FIGURE 12-10 A, With descent of the diaphragm and contraction of the intercostal muscles, a greater negative intrathoracic pressure relative to intraluminal and atmospheric pressure is developed. The net result is longitudinal stretching of the larynx and trachea, dilatation of the intrathoracic trachea and bronchi, movement of air into the lungs, and some dynamic collapse of the extrathoracic trachea (arrow). The dynamic collapse is due to the highly compliant trachea and the negative intraluminal pressure in relation to atmospheric pressure. B, The normal sequence of events at end-expiration is a slight negative intrapleural pressure stenting the airways open. In infants, the highly compliant chest does not provide the support required; therefore, airway closure occurs with each breath. Intraluminal pressures are slightly positive in relation to atmospheric pressure, with the result that air is forced out of the lungs. C, Obstructed extrathoracic airway. Notice the severe dynamic collapse of the extrathoracic trachea below the level of obstruction. This collapse is greatest at the thoracic inlet, where the largest pressure gradient exists between negative intratracheal pressure and atmospheric pressure (arrow). (Extrathoracic upper airway obstruction is characterized by inspiratory stridor.) D, Obstructed intrathoracic trachea or airways. Notice that breathing against an obstructed lower airway (e.g., bronchiolitis, asthma) results in greater positive intrathoracic pressures, with dynamic collapse of the intrathoracic airways (prolonged expiration or wheezing [arrows]).

Obstruction of the extrathoracic upper airway that can occur with epiglottitis, laryngotracheobronchitis, or an extrathoracic foreign body alters normal airway dynamics. Inspiration against an obstruction results in more negative intrathoracic pressure, further dilating the intrathoracic airways. Clinically, the net effect is a dynamic collapse of the extrathoracic trachea below the level of the obstruction. This collapse is maximal at the thoracic inlet, where the greatest pressure gradient exists between negative intratracheal and atmospheric pressures. As a result, inspiratory stridor is prominent (see Fig. 12-10, C, and Video 12-1).^38–45 With intrathoracic tracheal obstruction (e.g., foreign body, vascular ring) (see Video 12-1), stridor may occur during both inspiration and expiration.^46–49 In lower airway obstruction (e.g., asthma, bronchiolitis), significant intrathoracic tracheal and bronchial collapse may occur as a result of the prolonged expiratory phase and greatly increased positive extraluminal pressure (see Fig. 12-10, D).⁵⁰ In addition, because the airways in children are very compliant, they may be more susceptible to closure during bronchial smooth muscle contraction (e.g., with reactive airway disease). Preterm and term infants may experience airway closure even during quiet respirations.

Avoiding dynamic airway collapse is particularly important. The very compliant trachea and bronchi of an infant or child are prone to collapse, particularly at the extremes of transluminal pressures that may occur when a child is crying vigorously. The susceptibility of a child to these dynamic forces on the airway is inversely related to age, with preterm infants being most susceptible and adults being least susceptible.⁵¹ For this reason, it is essential that children with airway obstruction remain calm. Skill and understanding are required on the parts of the parents, nursing staff, and physicians. Sedatives and opioids should be used with caution before insertion of an ETT, because they may depress or ablate the life-sustaining voluntary efforts to breathe, resulting in significant morbidity or mortality.

Work of Breathing

Work of breathing (WOB) may be defined as the product of pressure and volume. It may be analyzed by plotting transpulmonary pressure against tidal volume. The WOB per kilogram body weight is similar in infants and adults. However, the oxygen consumption of a full-term neonate (4 to 6 mL/kg/min) is twice that of an adult (2 to 3 mL/kg/min).⁵² This greater oxygen consumption (and greater carbon dioxide production) in infants accounts in part for their increased respiratory frequency compared with older children. In preterm infants, the oxygen consumption related to breathing is three times that in adults.⁵³

The location of airway resistance within the tracheobronchial tree differs between infants and adults. The nasal passages account for 25% of the total resistance to airflow in a neonate, compared with 60% in an adult.^25,54 In infants, most resistance to airflow occurs in the bronchi and small airways. This results from the relatively smaller diameter of the airways and the greater compliance of the supporting structures of the trachea and bronchi.^24,55,56 In particular, the chest wall of a neonate is very compliant; the ribs provide less support to maintain negative intrathoracic pressure. This lack of negative intrathoracic pressure combined with the increased compliance of the bronchi can lead to functional airway closure with every breath.^57–59 In infants and children, therefore, small-airway resistance accounts for most of the WOB, whereas in adults, the nasal passages provide the major proportion of flow resistance.^{25,57,58,60–65}

In the presence of increased airway resistance or decreased lung compliance, an increased transpulmonary pressure is required to produce a given tidal volume, and therefore the WOB is increased. Any change in the airway that increases the WOB may lead to respiratory failure. Recall that the WOB (resistance to air flow) is inversely proportional to the fourth power of the radius of the lumen during laminar flow (beyond the fifth bronchial division) and proportional to the fifth power of the radius during turbulent flow (upper airway to the fifth bronchial division). Because the diameter of the airways in infants is smaller than in adults, pathologic narrowing of the airways in infants exerts a greater adverse effect on the WOB. Increase in the WOB may also occur with a long ETT of small diameter, an obstructed ETT, or a narrowed airway. All of these situations increase oxygen consumption, which in turn increases oxygen demand.⁶⁶ The increased oxygen demand is initially addressed by an increase in respiratory rate, but the increased WOB may not be sustainable. The end result may be exhaustion, which leads to respiratory failure (CO₂ retention and hypoxemia).

The difference in histology of the diaphragm and intercostal muscles of preterm and full-term infants compared with older children also contributes to increased susceptibility of infants to respiratory fatigue or failure. Type I muscle fibers permit prolonged repetitive movement; for example, long-distance runners through repeated exercise increase the proportion of type I muscle fibers in their legs. The percentage of type I muscle fibers in the diaphragm and intercostal muscles changes with age (preterm infants < full-term infants < 2-year-old children) (Fig. 12-11). Any condition that increases the WOB in neonates and infants may fatigue the respiratory muscles and precipitate respiratory failure more readily than in an adult.^67–69

FIGURE 12-11 Muscle fiber composition of the diaphragm and intercostal muscles related to age. Note that a preterm infant’s diaphragm and intercostal muscles have fewer type I fibers compared with term newborns and older children. The data suggest a possible mechanism for early fatigue in preterm and term infants when the work of breathing is increased.

(Data from Keens TG, Bryan AC, Levison H, Ianuzzo CD. Developmental pattern of muscle fiber types in human ventilatory muscles. J Appl Physiol 1978;44:909-13.)

Airway Obstruction during Anesthesia

Airway obstruction during anesthesia or loss of consciousness appears to be most frequently related to loss of muscle tone in the pharyngeal and laryngeal structures rather than apposition of the tongue to the posterior pharyngeal wall.^30,31,70,71 The progressive loss of tone with deepening anesthesia results in progressive airway obstruction primarily at the level of the soft palate and the epiglottis.^{30,31,34,70,72,73} In children, the pharyngeal airway space decreases in a dose-dependent manner with increasing concentrations of both sevoflurane and propofol anesthesia.^74–76 This reduction in pharyngeal space has been observed mainly in the anteroposterior dimension. As the depth of propofol anesthesia in children increases, upper airway narrowing occurs throughout the entire upper airway but is most pronounced in the hypopharynx at the level of the epiglottis. Extension of the head at the atlantooccipital joint with anterior displacement of the cervical spine (sniffing position) improves hypopharyngeal airway patency but does not necessarily change the position of the tongue. This observation supports the concept that upper airway obstruction is not primarily caused by changes in tongue position but rather by collapse of the pharyngeal structures.^32–34

Pharyngeal airway obstruction also occurs during obstructive sleep apnea in infants and adults.^29,77 The sniffing position increases the cross-sectional area and decreases the closing pressure of both the retropalatal and the retroglossal space in anesthetized adults with obstructive sleep apnea.⁷⁸ The application of continuous positive airway pressure (CPAP) is a common method to overcome such airway obstruction (see Figs. 31-6 and 31-7). During propofol anesthesia in children, CPAP works primarily by increasing the transverse dimension of the airway.⁷⁵ This occurs despite the fact that anesthesia obstructs the airway mostly by narrowing the anteroposterior dimension. Chin lift and jaw thrust also improve airway patency in anesthetized children with adenotonsillar hypertrophy.^79–81 Lateral positioning (also known as the “recovery position”) dramatically enhances the effects of these airway maneuvers^80,81; lateral positioning alone improves airway dimensions.⁶ Compared with chin lift and CPAP, the jaw thrust maneuver is known to be the most effective means to improve airway patency and ventilation in children undergoing adenoidectomy.⁷⁹ (See Video 4-3, A and B.)

Clinical Evaluation

The medical history (both present and past) should investigate the following signs and symptoms; a positive history should alert the practitioner to the potential problems that are noted in parentheses.

The physical examination should include the following observations:

Facial expression

Presence or absence of nasal flaring

Presence or absence of mouth breathing

Color of mucous membranes

Presence or absence of retractions (suprasternal, intercostal, subcostal [see Video 12-1])

Respiratory rate

Presence or absence of voice change

Mouth opening (Fig. 12-12, A)

Size of mouth

Size of tongue and its relationship to other pharyngeal structures (Mallampati Score)

Loose or missing teeth (see Fig. 12-12, B)

Size and configuration of palate

Size and configuration of mandible

Location of larynx in relation to the mandible (see Fig. 12-12, C)

Presence of stridor and, if present:

Is stridor predominantly inspiratory, suggesting an upper airway (extrathoracic) lesion (epiglottitis, croup, extrathoracic foreign body)?

Is stridor both inspiratory and expiratory, suggesting an intrathoracic lesion (aspirated foreign body, vascular ring, or large esophageal foreign body)? (see Video 12-1)

Is the expiratory phase prolonged or stridor predominantly expiratory, suggesting lower airway disease?

Baseline oxygen saturation in room air

Microtia: Bilateral but not unilateral microtia is associated with difficulty in visualizing the laryngeal inlet (grade 3 or 4 in the Cormack-Lehane classification, see Fig. 12-22).⁸² Five (42%) of 12 children with bilateral microtia were found to have a difficult laryngeal view, compared with 2 (2.5%) of 81 children with unilateral microtia and 0 of 93 children without microtia.⁸² Microtia may represent a mild form of hemifacial microsomia and its associated mandibular hypoplasia. The advantage of understanding this association is that ear deformity is often a more easily recognized clinical finding than mandibular hypoplasia.

Global appearance: Are there congenital anomalies that may fit a recognizable syndrome? The finding of one anomaly mandates a search for others. If a congenital syndrome is diagnosed, specific anesthetic implications must be considered (see E-Appendix 12-1, which is available online).

FIGURE 12-12 A, How far can a child open his or her mouth? Are there any abnormalities of the mouth, tongue, palate, mandible? B, Are any teeth loose or missing? C, Is the mandible of normal configuration? How much space is there between the genu of the mandible and the thyroid cartilage? This space is an indication of the extent of the superior and posterior displacement of the larynx; there should normally be at least one finger breadth in a newborn and three finger breadths in an adolescent.

Diagnostic Testing

Routine evaluation of the airway usually requires only a careful history and physical examination. In the presence of airway pathology, however, laboratory and radiologic evaluation can be extremely valuable. Radiographs of the upper airway (anteroposterior and lateral films and fluoroscopy) may provide evidence about the site and cause of airway obstruction. When necessary, MRI and CT provide more detailed information.^96–112 Radiologic airway examination in a child with a compromised airway may be undertaken only if there is no immediate threat to the child’s safety and only in the presence of skilled and appropriately equipped personnel able to manage the airway. Securing the airway through tracheal intubation must not be postponed in order to obtain a radiologic diagnosis when the child has severely compromised air exchange. Blood gas analysis is occasionally of value in assessing the degree of physiologic compromise, especially with chronic airway obstruction and compensated respiratory acidosis. Performing an arterial (or venous) puncture for blood gas analysis, although providing helpful information, is often upsetting to the child and may risk aggravation of the underlying airway obstruction through dynamic airway collapse. Candidates for blood gas analysis must be carefully selected and the procedure skillfully performed.

Endoscopic evaluation (flexible fiberoptic endoscopy) of the airway before tracheal intubation can be useful in infants and in cooperative older children if a glottic pathologic process is suspected or if difficulty is anticipated when visualizing the glottis.

Mask Ventilation

Face masks are available in many sizes and shapes. We commonly use the disposable, clear plastic masks with an inflatable cushioned rim. The inflatable rim molds to the contour of the face to provide an atraumatic seal. The use of clear plastic in the cone of the mask allows visualization of humidity (indicating air exchange), secretions, vomitus, or cyanotic lip color. The appropriately sized mask should rest on the bridge of the nose (avoiding the eyes) and extend to the mandible. Although mask anesthesia appears to be easy, it is, in fact, one of the most difficult skills to master. The most common error during mask ventilation is to tightly compress the submental triangle with fingers placed below the mandibular ridge, thereby partially occluding the airway. Minimal pressure is required, and the fingers should rest on the mandible. Another common problem arises when the mouth is completely closed while the face mask is being applied. The upper airway may become completely obstructed, with ventilation becoming impossible both spontaneously and by manual control. In such a circumstance, the fingers should be removed from the mandible and face and a single digit applied to each coronoid process while lifting toward the hairline. This maneuver subluxes the temporomandibular joint, thereby opening the mouth and pulling the tongue and other soft tissues off the posterior pharyngeal wall. A hand should be on the reservoir bag at all times to monitor the effectiveness of ventilation and to provide CPAP if needed to maintain a patent airway. An alternative method is to partially close the adjustable pressure-limiting valve to inflate the reservoir bag and provide CPAP. Insertion of an oral airway may also facilitate gas exchange.

Admonitions against extreme positions of the infant’s head during bag-and-mask ventilation are intended to minimize the risk of stretching and thus narrowing and obstructing the very compliant infant trachea. However, a study of 18 healthy, full-term infants younger than 4 months of age showed that the tracheal dimensions did not change when the head position changed.¹¹³ Therefore, stretching of the trachea may not result in narrowing of the tracheal lumen in otherwise healthy infants (Video 12-4). However, this study did not examine the effects of these head positions on the supraglottic airway or in the preterm infant. It is possible that these maneuvers (head extension) could result in supraglottic airway obstruction in some children.

Oropharyngeal Airways

An infant’s tongue may obstruct the airway during induction of anesthesia or loss of consciousness. An oropharyngeal airway of appropriate size (or a supraglottic airway [SGA] such as the laryngeal mask airway [LMA; LMA North America, San Diego]) may be inserted to relieve the obstruction. By holding the oral airway as shown in Figure 12-13, one can estimate the appropriate size for the child; airways one size larger and one size smaller should be readily available as well. A tongue depressor may be inserted over the tongue to facilitate insertion of the oral airway by preventing downfolding of the tongue, which could impair venous and lymphatic drainage, causing tongue swelling and airway obstruction. If the airway device is too long, it may push the epiglottis into the glottic aperture, creating an additional site of airway obstruction or causing traumatic epiglottitis, or the tip may impinge on the uvula, causing uvular swelling and airway obstruction (see Fig. 12-13, C, D).^114,115 If the airway device is too short, it may rest against the base of the tongue, forcing it posteriorly against the roof of the mouth and further aggravating airway obstruction (Fig. 12-13, E, F). Oral airways should not be considered panaceas for upper airway obstruction. Care must be taken to avoid trauma to the lips and tongue, which may be caught between the teeth and the flange of the airway. An oral airway is also used to protect an ETT from compression by the child’s teeth, and it serves to separate the mandible and maxilla to facilitate oropharyngeal suctioning.

FIGURE 12-13 Correct airway selection. An artificial airway of proper size should relieve airway obstruction caused by the tongue without damaging laryngeal structures. The appropriate size can be estimated by holding the airway against the child’s face: the tip of the airway should end just cephalad to the angle of the mandible (A). Use of the correct size should result in proper alignment with the glottic opening (B). If too large an oral airway is inserted, the tip will line up posterior to the angle of the mandible (C) and obstruct the glottic opening by pushing the epiglottis down (D, arrow). If too small an oral airway is inserted, the tip will line up well above the angle of the mandible (E) and exacerbate airway obstruction by kinking the tongue (F, arrows).

Nasopharyngeal Airways

Nasopharyngeal airways are occasionally used in children to relieve upper airway obstruction; the distance from the naris to the angle of the mandible approximates the proper length. Commercial airways are available in sizes 12F to 36F (Rüsch Inc., Duluth, Ga.). Some have an adjustable flange that enables manipulation of the airway to the appropriate length. Alternatively, for infants and small children, a shortened ETT may be used, although this is not as soft and pliable as a commercially available nasopharyngeal airway and may be more likely to cause trauma on insertion. The nasopharyngeal airway may be better tolerated in the lightly anesthetized child than an oropharyngeal airway. Nasopharyngeal airways are usually avoided to prevent trauma to, and bleeding from, hypertrophied adenoids, and they are more commonly used to relieve residual airway obstruction on emergence from anesthesia.

Tracheal Intubation

Technique

As previously discussed, because of differences in anatomy, there are differences in techniques for intubating the trachea of infants and children compared with adults.1–4,20–22^,101,116,117 Because of the smaller dimensions of the pediatric airway, there is increased risk of obstruction with trauma to the airway structures. A technique to be avoided is that in which the blade is advanced into the esophagus with laryngeal visualization achieved during withdrawal of the blade. This maneuver may result in laryngeal trauma when the tip of the blade scrapes the arytenoids and aryepiglottic folds.

There are several approaches to exposing the glottis in infants with a Miller blade. One approach consists of advancing the laryngoscope blade under constant vision along the surface of the tongue, placing the tip of the blade directly in the vallecula, and then using this location to pivot or rotate the blade to the right to sweep the tongue to the left and adequately lift the tongue to expose the glottic opening. This technique avoids trauma to the arytenoid cartilages. Lifting the base of the tongue lifts the epiglottis, exposing the glottic opening. If this technique is unsuccessful, the epiglottis may be lifted directly with the tip of the blade (see Video 12-1). Another approach is to insert the Miller blade into the mouth at the right commissure over the lateral bicuspids/incisors (paraglossal approach). The blade is advanced down the right gutter of the mouth, aiming the blade tip toward the midline while sweeping the tongue to the left. Once the blade is under the epiglottis, the epiglottis is lifted with the tip, exposing the glottic aperture. By approaching the mouth over the bicuspids/incisors, dental damage is obviated. This is a particularly effective approach for the infant or child with micrognathia. Whichever approach is used, care must be taken to avoid using the laryngoscope blade as a fulcrum through which pressure is applied to the teeth or alveolar ridge. If there is a substantive risk that pressure will be applied to the teeth, a plastic tooth guard may be applied to cover the teeth at risk (the central incisors of the maxilla).

Optimal positioning for laryngoscopy changes with age. The trachea of older children (≥6 years) and adults is most easily exposed when a folded blanket or pillow is placed beneath the occiput of the head (5 to 10 cm elevation), displacing the cervical spine anteriorly.¹¹⁸ Extension of the head at the atlantooccipital joint produces the classic “sniffing” position.^101,119,120 These movements align three axes: those of the mouth or oral (O), pharynx (P), and trachea (T). Once aligned, these three axes permit direct visualization of laryngeal structures. They also result in improved hypopharyngeal patency.^{32,34,70,78,119,120} Figure 12-14 demonstrates maneuvers for positioning the head during airway management. In infants and younger children, it is usually unnecessary to elevate the head because the occiput is large in proportion to the trunk, resulting in adequate anterior displacement of the cervical spine; head extension at the atlantooccipital joint alone aligns the airway axes. If the occiput is displaced excessively, exposure of the glottis may actually be hindered. In neonates, it is helpful for an assistant to hold the patient’s shoulders flat on the operating room table with the head slightly extended. Some practitioners have adopted the practice of placing a rolled towel under the shoulders of neonates to facilitate tracheal intubation. This technique may be disadvantageous if the laryngoscopist stands but may be advantageous if he or she is seated.

FIGURE 12-14 Correct positioning for ventilation and tracheal intubation. When a patient is lying flat on the bed or operating table (A), the oral (O), pharyngeal (P), and tracheal (T) axes pass through three divergent planes (B). A folded sheet or towel placed under the occiput of the head (C) aligns the P and T axes (D). Extension of the atlantooccipital joint (E) results in alignment of all three axes (F).

The validity of the three-axis theory (alignment of the O, P, and T) to describe the optimal intubating position in adults has been challenged.^121–124 Some authors question the notion that elevation of the occiput improves conditions for visualization of the laryngeal inlet based on evidence from both MRI and clinical investigations.^121,123 However, one MRI study in children with an LMA in place found that slight head extension improved the alignment of the glottic and pharyngeal axes but worsened the alignment of the pharyngeal and laryngeal axes.¹²⁵ In a study of adults, neck extension alone was adequate for visualization of the larynx in most patients, but for obese patients and those with limited neck extension, an optimal intubating position was not determined.¹²¹ Others favor the sniffing position but with varying support for the three-axis theory.^126–132 Even if the tracheas of only a few patients are intubated more easily when placed in the sniffing position compared with simple head extension, the current routine application of the sniffing position appears to be the best clinical practice.

Laryngoscopy can be performed while the child is awake, anesthetized and breathing spontaneously, or anesthetized and paralyzed. Most tracheal intubations in children who are awake are performed in neonates, an approach that is not usually feasible or humane in older awake and uncooperative children. Awake intubation in the neonate is generally well tolerated if it is performed smoothly and rapidly; however, an international consensus group and others have cautioned against this practice unless intravenous (IV) access is not available or there is a life-threatening situation.^133–136 Data suggest that preterm and term infants are better managed with sedation and paralysis to minimize adverse hemodynamic responses.^137–141