Airway Management

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CHAPTER 12 Airway Management

Airway management is the most important skill of the pediatric anesthesiologist, but what exactly is the pediatric airway? Examined through a broad lens, the pediatric airway is a composite of the anatomic development of the head, face, aerodigestive tract, and neck, structures contiguous as well as integral to the airway. It includes the differentiation of the primitive foregut into the trachea and esophagus and the subsequent development and differentiation of the upper and lower airways. It begins where air enters, normally at the nose and mouth, and is continuous through the upper (extrathoracic) and lower (intrathoracic) conducting airways. The dividing line between the upper and lower airways, the thoracic inlet, is bordered by T1 posteriorly, the first pair of ribs laterally, and the superior border of the manubrium anteriorly. Moreover, consideration of the pediatric airway would be incomplete without including its neurophysiology and gas-flow physics. Beyond that, the very practical details of the equipment needed to care for the pediatric airway are critical for the practitioner. In addition, medical conditions with specific airway challenges beget primary as well as secondary effects that must be accounted for as part of the anesthetic plan. Finally, an exit strategy must be established for patients with normal, and in particular, abnormal airways. The anesthesiologist, in collaboration with the surgeon and perioperative physicians and staff, is a critical stakeholder in that plan. Without elaborating on specific procedures covered elsewhere in this text and others, this chapter explores the developmental perspective on the anatomy and physiology of the upper airways and discusses gas-flow characteristics that change with age and abnormal airway conditions.

Developmental anatomy *

The Upper Airway

Formation of the Cranial Vault and Base

The skull is a critical factor in the development of the face and therefore the upper airway. The skull develops from a membranous and cartilaginous neurocranium (Fig. 12-1). The membranous neurocranium gives rise to the flat bones of the cranial vault, and the cartilaginous neurocranium (chondrocranium) forms the skull base. The flat bones of the neurocranium, which form sutures from edge to edge, also form fontanels where more than two bones meet. The base of the skull is formed from the cartilaginous neurocranium, which then becomes the base of the occipital bone, the sphenoid, the ethmoid and petrous bones, and portions of the temporal bone.

FIGURE 12-1 Development of the skull from neurocranium and viscerocranium contributions at 20 weeks’ gestation (lateral view). The skull base, the occipital bone, the body of the sphenoid, and the ethmoid are formed from cartilaginous neurocranium. Cell-signaling as well as geometric influences contribute to further growth of cranial and anterior facial structures.

The cranial base provides a floor for the calvarium and a roof for the face. The shaping of the skull base and contiguous structures is a dynamic process involving reciprocal influences between the cranial base, the pharynx, the face, and the primary and secondary palates. During fetal life and early childhood, neural influences predominate because of the rapid growth of the brain. During postnatal development of the airway, nasal influences play a major role, and because of speech and nutritional requirements, the pharynx also influences the development of the skull base. The anterior portion of the skull base is the roof of the nasomaxillary complex, whereas the posterior portion of the cranial base is the roof of the nasopharynx. During development, the depth of the nasopharynx increases as a result of remodeling of the palate, as well as changes in the angulation of the skull base, providing an enlarged nasal airway for the adult.

The Face

Just as the neurocranium forms the cranial vault and base, the viscerocranium forms the face and is derived mainly from cartilage of the first two branchial arches (Fig. 12-3). Ectodermally-derived neural crest cells of the developing 3- to 4-week-old embryo migrate to branchial arch mesoderm, and the face develops as a result of these massive cell migrations and their interactions. Those cells forming the frontonasal process are derived from the forebrain fold and migrate a relatively short distance as they pass into the nasal region. Those cells that form the mesenchyme of the maxillary and mandibular processes have a considerably longer distance to migrate, because they must move into the branchial arches. At 28 days postconception, the face barely shows its eventual relation to the five primordia from which it is derived: the frontonasal prominence, which is the cranial boundary of the primitive mouth (stomodeum); the paired maxillary prominences (the first branchial arch); and the paired mandibular prominences (also the first branchial arch).

FIGURE 12-3 Development of the face. Neural crest cells form around the anterior neuropore and migrate to give rise to a variety of connective and nervous tissues of the skull, face, and branchial arches. With the frontonasal prominence developing for a shorter distance than the maxillary processes, there is a longer time for clefting of the lip and palate to occur. Severe rhinencephalic malformations may be accompanied by central nervous system abnormalities as well.

The paranasal sinuses begin developing at approximately 40 weeks of gestational age. The completion of turbinate development signals the beginning of sinus development, which continues until early adult life (Fig. 12-4). Although the exact function of the paranasal sinuses is not well understood, inflammatory, infectious, and neoplastic diseases of the sinuses are of major significance to the anesthesiologist, particularly if there is functional impairment before anesthesia and surgery. Sinus disorders are often comorbidities of asthma, immunoglobulin deficiencies, cystic fibrosis, or Kartagener’s syndrome.

FIGURE 12-4 The paranasal sinuses begin their development at the end of fetal life from the mesenchyme of the lateral wall of the nasal cavity, developing into the turbinates. The completion of turbinate development signals the beginning of sinus development, which continues until early adult life.

The oral cavity—a structure without structures—where much of the anesthesiologist’s attention and skills are focused, has a complex developmental heritage. The mouth (stomodeum) appears as a slight depression in the surface ectoderm, separated from the oral cavity by the oropharyngeal membrane. This membrane ruptures at about 24 to 26 days’ gestation and the primitive foregut then communicates with the amniotic cavity. The involved germ layers are the endoderm internally and the ectoderm externally. The tongue surface arises primarily from first arch mesenchyme, with significant contributions from the third and fourth arches, hence its complex innervation by the facial nerve in the anterior two thirds and the hypoglossal nerve in the posterior one third (Fig. 12-5). The muscle bulk of the tongue arises primarily from occipital somites, explaining the hypoglossal nerve (XII) innervation and its susceptibility to injury from errant placement of dental rolls and pressure injury from overinflated LMA cuffs. The upper lip is formed by the merging of the maxillary prominences with the medial nasal prominences, with the lateral basal prominences forming the alae. The intermaxillary segment in the central portion of the upper lip area consists of a labial component (forming the philtrum), a maxillary component (associated with the four incisor teeth), and a palatal component (which becomes the primary palate).

FIGURE 12-5 The tongue is a complex structure, derived as it is from elements of the first four branchial arches. This results in the first branchial arch contributing to the anterior two thirds of the tongue (lingual nerve, a branch of the mandibular division of the fifth cranial nerve), as well as the chorda tympani from the seventh cranial nerve. The third branchial arch contributes to the posterior third of the tongue, resulting in its innervation by the ninth cranial nerve. Tongue muscles, derived from the occipital myotomes in the region of the developing hindbrain, are innervated by the twelfth cranial nerve. The foramen cecum is the pharyngeal floor diverticulum in continuity with the developing thyroid.

The palate divides the nasomaxillary complex from the oral cavity (Fig. 12-6). The palatal processes advance in a medial direction from the maxillary processes of the first branchial arch, fusing in the midline in an anterior-to-posterior sequence and uniting with the premaxilla and the developing nasal septum. The soft palate forms from continued growth of the posterior edges of these palatal processes, ending with the formation and fusion of the two halves of the uvula.

FIGURE 12-6 Formation of the primary and secondary palates, ultimately forming the floor of the nose and the roof of the mouth. The process of midline fusion occurs from posterior to anterior; clefting of the primary or secondary palate may occur in various combinations.

Clinical Correlation

Cleft lip and palate are among the most common of congenital anomalies. They may occur alone, as part of a syndrome (there are over 300 syndromes associated with facial clefting), or as a component of a sequence, (e.g., as with Pierre-Robin syndrome). Clefts of the palate may occur by the same mechanism as cleft lip or be secondary to an anatomic obstruction preventing the medial fusion of the maxillary processes. For example, the cleft palate associated with Pierre-Robin syndrome occurs when the tongue, being displaced superiorly and posteriorly as a result of mandibular hypoplasia, interferes with palatal fusion.

Closure of the cleft palate may result in insufficient tissue for development of normal length or function of the soft palate and require a posterior pharyngeal flap. Velopharyngeal insufficiency is the cause of the hypernasal speech, nasal emission, and nasal turbulence (Sidman and Muntz, 2000).

The nose originates in the cranial ectoderm, which subsequently develops into the frontonasal prominence. The superior portion of the nose is formed from the lateral nasal processes, whereas the inferior portion of the nasal cavity is incomplete until the paired maxillary processes of the first branchial arch grow anteriorly and medially to fuse with the median nasal processes. The nasal cavities extend posteriorly during development, influenced by the posteriorly directed fusion of the palatal processes, thinning out the membrane that separates them from the oral cavity. By the thirty-eighth day of development, the two-layer membrane consisting of nasal and oral epithelia ruptures and forms the choanae (posterior nares). Failure of such rupture results in choanal atresia, although these choanae are not in the same location as the definitive choanae, which will eventually be located more posteriorly. Because the normal nasomaxillary complex grows both downward and forward, however, it does explain the unexpectedly anterior location in choanal atresia given the eventual normal development of the choanae.

Clinical Correlation

Choanal stenosis is not a blockage but rather a narrowing of less than 6 mm. Choanal atresia is also associated with various syndromes in up to 70% of patients, of which colobomas, heart disease, atresia of the choanae, retarded growth, genital anomalies, ear anomalies (the CHARGE association) is the major one. Repair of this form of choanal atresia may be more complicated, requiring a tracheostomy in infancy as an initial stage and more definitive repair later.

Bilateral choanal atresia is apparent immediately in the neonatal period because infants are obligate nasal breathers. These infants have respiratory distress, paradoxical cyanosis (crying relieves the cyanosis), and failure to thrive. The natural position of the tongue promotes obstruction that is relieved by an oral airway or a McGovern nipple (a standard nipple with an enlarged hole). Unilateral atresia often appears later (2 to 5 years of age) with rhinorrhea and chronic mucoid discharge and is often misdiagnosed. Because the intraoral airway is foreshortened as a result of the abnormally anterior choanal aperture, upper airway obstruction while breathing is common, and visualization of the laryngeal structures may be more difficult. Long-standing choanal atresia may also lead to obstructive sleep apnea.

The face, specifically the maxilla and mandible, grows in a dynamic fashion throughout childhood under the influence of bony deposition and resorption, soft-tissue contouring, and hormonal influences. In a reciprocal fashion, the skull base or neurocranium influences midfacial development via the growth of the sinuses, which in turn influence the skull base. Displacement of these structures of the nasomaxillary complex occurs in horizontal, vertical, and anteroposterior axes. These changes ultimately affect the proportions of the face and the morphology of all of the facial structures, including the upper airway.

The Branchial Apparatus

Branchial Arches

The branchial apparatus consists of four branchial arches visible on the surface of the embryo, as well as fifth and sixth arches that cannot be seen on the surface. Branchial pouches and clefts are likewise numbered craniocaudally (Fig. 12-7). The first branchial arch (Meckel’s) cartilage is the position of the future mandible, as well as the eventual malleus and incus. The second branchial arch cartilage produces the stapes, the styloid process, the stylohyoid ligament, and the superior portion of the body of the hyoid. The other branchial arch cartilages contribute to the inferior portion of the hyoid as well as the thyroid cartilage.

FIGURE 12-7 The branchial arches, pouches, and clefts. The musculature of each arch is supplied by the (cranial) nerve of its origin; the first arch by the mandibular division of the fifth cranial nerve, as well as the chorda tympani division of the seventh cranial nerve. The seventh cranial nerve supplies the second arch, the ninth cranial nerve the third arch, and the tenth and eleventh cranial nerves the remaining arches. Branchial clefts, when they fail to obliterate during normal development, may result in sinuses or cysts that communicate with the oropharynx through various pathways in the neck.

Striated muscles are also formed in the respective branchial arch mesenchyme. Myoblasts differentiate and migrate to various parts of the head and neck, where they form the muscles of mastication and facial expression, each retaining their original nerve supply. Although muscular actions in the head and neck are thought of as far removed from the origin and course of the cranial nerves, fetal nerves that supply the branchial arch derivatives only have a short distance to travel from the brain. The trigeminal (V) nerve supplies the skin covering the parts of the face derived from the first branchial arch maxillary and mandibular divisions (the ophthalmic division does not make a contribution). The facial (VII) nerve supplies the muscles derived from the first arch. The nerve of the third branchial arch is the glossopharyngeal (IX) nerve. Two branches of the vagus (X) nerve supply the remaining branchial arches. The superior laryngeal nerve innervates derivatives of the sixth branchial arch.

Branchial Pouches

The first branchial pouch develops into the tubotympanic recess, becoming the auditory tube and the middle ear cavity (Fig. 12-8). The cavity of the second branchial pouch is largely obliterated as the palatine tonsil develops, but part of it remains as the tonsillar fossa. The endoderm of the second branchial pouch becomes the surface epithelium of the tonsil and the lining of its crypts, with the mesenchyme around the pouch differentiating into lymphoid tissue. The endoderm of the dorsal part of the third branchial pouches differentiates into the inferior parathyroids, and the ventral parts unite to become the thymus. The paired parathyroid glands develop from separate pouches, with one pair derived from the third and the other from the fourth branchial pouch. At the seventh week of development, the parathyroid glands migrate caudally from their respective branchial pouches, with the third pouch parathyroids moving more caudally than the parathyroids of the fourth pouch. Accessory parathyroid tissue may be left along the line of migration. The endoderm of the fourth branchial pouches differentiates into the superior parathyroid glands, and the ventral parts develop into the ultimobranchial bodies, the calcitonin-secreting portion of the thyroid.

FIGURE 12-8 The tympanic cavity develops from the tubotympanic recess, an outgrowth of the first pharyngeal pouch, which extends laterally, contiguous with the external auditory canal having formed from the first pharyngeal cleft.

Branchial Clefts

Between the first and second arches, the first branchial cleft forms the external ear, which is the only normal structure to arise from a branchial cleft. However, if the second arch does not grow caudally over the third and fourth arches, as it normally should, then the second, third, and fourth clefts can remain as branchial fistulas, in contact with the skin surface between the clavicle and the mandible. Incomplete or arrested development of the branchial clefts may result in residual cysts, sinuses, and fistulas that may eventually become infected and require excision. The clinical significance of the specific embryology is that characteristic locations are of important surgical significance.

Clinical Correlation

The second branchial arch may occasionally fail to bury the second, third, and fourth branchial clefts completely, resulting in a cervical sinus communicating with the surface of the skin, and forming a branchial sinus, which may also contain a branchial cyst. These sinuses may be accompanied by a draining infection along the anterior border of the sternocleidomastoid muscle, requiring complete surgical dissection and removal. It is easy to understand embryologically how these tracts can open into the pharynx just posterior to the tonsils and may even pass between the external and internal carotid arteries.

The thyroid begins as a thickening of the endoderm of the floor of the pharynx, in the midline between the first and second pouches, at the foramen cecum. A thin connection, the thyroglossal duct, remains attached to the oral cavity, and its point of attachment marks the origin of the thyroid gland. The thyroid descends along the thyroglossal duct and reaches the level of the first tracheal ring at about the seventh week of gestation (Fig. 12-9). The thyroglossal duct is then normally obliterated. Accessory thyroid tissue may be deposited anywhere along this path; on the other hand, failure of the thyroid to descend may result in a lingual thyroid.

FIGURE 12-9 Descent of the thyroid along the thyroglossal duct. A lingual thyroid is the most common form of incomplete descent and is found inferior to the foramen cecum, where it may interfere with swallowing as well as breathing, particularly during anesthetic induction in infants.

Anomalies associated with abnormal development of the branchial arches are varied, with a range of complexity and airway implications. Whereas too much variation among anomalies exists, an appreciation of the developmental anatomy explains (and helps the anesthesiologist anticipate) the following situations:

1. Unilateral or bilateral hypoplasia of upper airway structures, which may result in a small and hypomobile mandible, choanal stenosis or atresia, and hypoplastic or nonexistent sinuses.

2. Fusions or other abnormalities of various cervical vertebrae.

3. Cleft palate may be the result of failure to fuse, or it may be associated with hypoplastic mandibles and glossoptosis (Pierre-Robin syndrome) or hemifacial microsomia (Goldenhar’s syndrome).

4. Obstructive sleep apnea, which may result from chronic upper airway obstruction as a result of decreased available cross-sectional diameter of the airway with a relatively large volume occupied by the tongue and pharyngeal musculature, as well as an abnormally positioned palate.

The Larynx

Development of the larynx begins at approximately 3 weeks of gestational age with the formation of the laryngotracheal tube from the ventral wall of the foregut. The laryngotracheal tube then grows caudally into the splanchnic mesoderm on the ventral surface of the foregut, dividing into the right and left lung buds. The epiglottis begins to form from the hypobranchial eminence of the third and fourth arches at approximately 30 to 32 days’ gestation. The aryepiglottic folds develop from the lateral boundaries of the fourth arch along a line from the hypobranchial eminence (epiglottis) to the arytenoid eminence of the sixth arch. Incomplete development at this stage may produce varying degrees of persistent laryngeal cleft (Fig. 12-10). A definite larynx may be seen by 41 days’ gestation. (Fig. 12-11).

FIGURE 12-10 Laryngeal cleft, in various grades, results from a persistent communication between the trachea and esophagus, most commonly at the cranial end. Type I lesions are localized to the interarytenoid space superior to the vocal cords; type II lesions involve a partial cricoid cleft. Types III and IV lesions traverse the cricoid completely.

FIGURE 12-11 Development of the larynx. The epiglottis begins to form from elements of the third and fourth branchial arches at approximately 30 to 32 days of gestation, and early arytenoids cartilages can be identified on both sides of the laryngeal slit at this time as well. Incomplete development at this early stage may produce variable degrees of laryngeal cleft.

The cricoid and thyroid cartilages begin to develop before the arytenoid cartilages, with chondrification starting at about 7 weeks of gestation. As the thyroid cartilage develops, the glottis deepens, and the true vocal cords align within the thyroid laminae. Failure of the true vocal cords to split to form the primitive glottis at 10 weeks of gestation results in congenital atresia of the larynx or more often, a complete or partial congenital laryngeal web. Although webs may be supraglottic or subglottic, most occur at the level of the glottis. Congenital cysts of the supraglottic region are possibly remnants of the third branchial pouch and lie superior to the derivatives of the fourth arch. By the tenth to eleventh weeks of gestation, the major structures of the larynx have developed and the cartilages are chondrifying. If the separation of the esophagus and trachea is slightly delayed and the margins of the laryngotracheal groove fail to fuse adequately, the rapidly growing trachea separates the proximal and distal esophagus, resulting in the most common form of proximal esophageal atresia and distal tracheoesophageal fistula.

Eckenhoff’s (1951) review of the anatomy of the pediatric larynx has influenced more than a generation of pediatric anesthesiologists with regard to the selection of endotracheal tube size based on the concept of the narrowest (fixed) portion of the upper airway being the level of the cricoid cartilage, or the “funnel-morphing-into-cylinder” concept. Litman et al. (2003) examined laryngeal shape in sedated children undergoing MRI and determined that under sedated conditions with tonic maintenance of laryngeal shape, it is more cylindric than funnel-shaped, as it is in adults, and that the narrowest portion of the airway is at the level of the vocal cords. Furthermore, they concluded that there is no change in this relationship from childhood to adulthood. This finding has been confirmed by video-bronchoscopic imaging in anesthetized and paralyzed children as well and provides an intriguing challenge to traditional teaching (see Chapter 3, Respiratory Physiology) (Dalal et al., 2008, 2009). The adjudication of these disparate concepts, as well as the confirmation in both reports that the cricoid opening is elliptic rather than circular in shape, with the narrowest transverse diameter, help support the transition over the last 10 to 15 years to the common use of cuffed endotracheal tubes with a smaller diameter rather than tightly (or “appropriately”) fitted, uncuffed endotracheal tubes. Moreover, advances in materials and design have allowed the development of thin-walled tubes with shorter, polyurethane cuffs, which provide an equivalent seal at a lower mucosal pressure (Dullenkopf et al., 2005). At this point, the increasingly routine use of cuffed endotracheal tubes, with appropriate care, appears to result in fewer repeated laryngoscopies and reintubations, less postintubation croup and less contamination in the operating room, and lower total fresh-gas flows.

Developmental physiology

The Upper Airway

It is almost axiomatic in pediatric training to be told that the infant is an obligate nasal breather, and the reasons are best understood by developmental anatomy. The epiglottis is located in a more cephalad position relative to the other mobile soft-tissue components of the oral cavity, the tongue and soft palate, and their close approximation even during normal respiration may impair transoral breathing. Nasal resistance may provide up to 50% of total airway resistance and varies depending on alar orientation, the pyriform aperture, the nasal cavity, and the choanae. This is of practical importance to the pediatric anesthesiologist particularly when anatomic or even therapeutic (e.g., nasogastric tube) obstructions are present, because airway resistance is significantly increased and therefore respiration may be compromised.

The tongue, palate, pharynx, epiglottis, and larynx all have to work together to accomplish feeding and breathing. With postnatal growth, the mandible enlarges, descends, and protrudes, and the oral cavity enlarges vertically. The tongue occupies a more anterior position as the oral cavity and pharynx grow, and the larynx descends from its C2 position in the neck to C4 after one year of age. The clinical usefulness of the high-lying larynx is that it places the epiglottis into contact with the soft palate, allowing the infant to be a nasal breather while sucking and swallowing. With the growth of the oral cavity and the rearrangement of intraoral architecture during the first few months of life, infants convert from obligate nasal breathing to nasal and oral breathing.

Isono (2006) has reviewed the physiology of airway maintenance at the pharyngeal level and described the pharynx as a collapsible air-filled tube surrounded by soft tissues enclosed in a rigid box of bony structures, the mandible and vertebrae, with the lumen of the tube determined by the balance of the soft-tissue mass and the size of the surrounding rigid box. This simple approach explains the success of routine clinical airway maneuvers—continuous positive airway pressure (CPAP) serving as a pneumatic stent for the collapsed air-filled tube and surrounding soft tissue, and anterior displacement of the jaw serving to enlarge the rigid box. An oral airway displaces the base of the tongue anteriorly, thereby increasing the transpharyngeal luminal space and moving the base of the tongue away from the prevertebral bodies. Anesthetics, of course, depress the integrity of pharyngeal muscle tone through a variety of mechanisms, including the attenuation of neural input caused by the loss of consciousness and the decrease in tone of the diaphragm and intercostal musculature. The tongue and its muscular attachments such as the genioglossus, geniohyoid, sternohyoid, sternothyroid, and thyrohyoid are affected as well. All of this conspires to narrow the pharyngeal airway, more so in infants and small children than adults, especially during the first year of life (Isono et al., 2000). Adults are more equipped than children to defend against these effects, because they possess a more competent negative pressure reflex serving to augment pharyngeal tone, although the negative pressure reflex has been shown in infants younger than 1 year old as well (Thach et al., 1989; Horner et al., 1991). In addition, the progressive increase in pharyngeal cross-sectional area during the first year of life is positively influenced by the growth of the mandible and maxilla.

Pharyngeal airway obstruction has long been recognized as a significant component of the clinical entity called laryngospasm, and the application of CPAP, whether applied transnasally or transorally, depresses pharyngeal muscle tension and provides a mechanical pneumatic stent for the upper airway (Fink, 1956; Alex et al., 1987). This is a subtle component of the routine technique used by experienced pediatric anesthesiologists in applying gentle amounts of positive pressure during the induction phase, especially with infants and small children.

The fetus is an experienced swallower, well practiced from the age of 10 to 11 weeks’ gestation, with suckling following at 18 to 24 weeks’ gestation. This is fortunate, because coordination of this highly complex task is dependent on practice in utero and learning in pre- and postnatal life. However, breathing, even in utero, is a relatively late event, occurring at about 32 to 37 weeks’ gestational age, and premature infants show their lack of experience with significant discoordination between swallowing and breathing, as do children who are neurologically impaired (Goldson, 1987; Arvedson et al., 1994).

Protective airway reflexes are initiated via adduction of the true and false vocal cords, closing the laryngeal vestibule. The epiglottis deflects posteriorly to cover the laryngeal inlet, diverting food into the pyriform sinuses. Finally, the larynx elevates through the effort of the suprahyoid musculature, contracting prior to the entrance of the food bolus into the hypopharynx, which aids in opening the cricopharyngeal sphincter. Moreover, activation of sensory receptors in the pharynx and anterior tonsillar pillars inhibits respiration during swallowing. Finally, the esophageal phase, from the upper esophageal sphincter to the lower esophageal sphincter (LES), is mediated by contractions that occur throughout the length of the esophagus via a coordinated peristaltic wave. The cricopharyngeal sphincter returns to a tonic state (+5 mm Hg) from its relaxed state (−15 mm Hg), as do the diaphragmatic crura, the angle of the gastroesophageal junction, and the diaphragm, thereby preventing reflux into the hypopharynx. Reflex protection of the airway occurs via two pathways: anterograde protection accomplished during normal swallowing, and retrograde protection accomplished by antireflux mechanisms. Laryngeal and nasopharyngeal closure occur via the influence of the superior laryngeal nerve on laryngeal mucosal receptors. Clinically, the adverse effects of laryngeal secretions manifest themselves as reflex closure of the larynx and sustained apnea (Perkett and Vaughan, 1982; Bartlett, 1985). With maturity, coughing replaces apnea as a protective mechanism (Miller et al., 1952; Leith, 1985). As far as gastrointestinal reflux is concerned, increasing attention has been devoted to its contribution to recurrent pneumonia, reactive airways disease, apnea, and respiratory failure. The upper and lower esophageal sphincters maintain a resting tone of 10 to 30 mm Hg over intragastric pressure, and in combination with crural support and the acute gastroesophageal angle, act to prevent regurgitation of stomach contents. However, premature infants have a lower LES pressure and a less acute gastroesophageal angle; central nervous system (CNS) immaturity is related to abnormalities of the LES relaxation pattern (Hillemeier, 1996).

Developmental airway physics

Because of the variable diameter and length of the trachea, as well as the common (mainly in toddlers) problems of foreign body aspiration and anatomic or functional airway obstruction, understanding the physics of gas flow is even more crucial in pediatric anesthesia than in any other anesthesiology subspecialty. Noisy breathing is often an ominous sign in infants and small children. Stridor is noisy breathing coupled with increased inspiratory efforts, such as nasal and rib-cage flaring and suprasternal and sternal retraction. Severe airway obstruction may result in cyanosis, respiratory distress and fatigue, pneumothorax, pneumomediastinum, and death. Purely inspiratory stridor usually indicates lesions in the upper part of the airway. Lesions distal to the vocal cords usually produce expiratory stridor. Biphasic stridor is most characteristic of obstruction at the level of the subglottic space. Physical laws not only help explain the basis of these signs but provide guidance about effective clinical intervention (see Chapter 3, Respiratory Physiology in Infants and Children).

Laminar and Turbulent Flow

Laminar flow of a fluid (whether the fluid is a liquid or a relatively slowly-moving gas) through a tube is proportional to the pressure gradient (total pressure drop/length of tube), the fourth power of the radius of the tube and inversely proportional to the viscosity of the fluid. This relationship is expressed mathematically in the Hagen-Poiseuille Law:

where equals the flow of the gas, P equals the total pressure drop, r equals the radius of the tube, η equals viscosity, and l equals length.

The most important implication of the Hagen-Poiseuille Law is that halving the radius, for example, decreases flow rate by the fourth power, or 16-fold. Likewise, doubling the radius increases the flow rate 16-fold; this is critically important, especially for infants and small children, and it explains the improvement in respiratory distress with appropriate treatment for conditions such as croup. In normal lungs, laminar flow occurs only in the periphery of the lung (small peripheral bronchi and bronchioli) where the flow is relatively slow in relation to the diameter of the airways. For the rest of the airway system, the flow pattern is predominantly turbulent because of uneven airway diameters and an average of 24 (19 to 27) airway branchings.

For turbulent conditions, the change in the expression from flow rate to the square of the flow rate illustrates that a much greater driving pressure is required to maintain gas flow. Fanning’s equation expresses this relationship mathematically, where the radius of the tube to the fifth power now determines that flow.

where equals the flow of the gas, P equals the total pressure drop, r equals the radius of the tube, μ equals density, l equals length, and f equals Fanning’s friction factor.

Upper and lower conducting-airway air flow is turbulent, whereas the peripheral airways are characterized by laminar air flow, enhancing alveolar ventilation. The conducting airways bifurcate progressively (but not symmetrically). Cartilaginous support is characteristic of the main, lobar, and segmental bronchi, and more distal conducting airways have bronchial muscle forming a geodesic network. The total cross-sectional area of the respiratory tract is minimal at the third generation (Fig. 12-12). Small bronchi extend through about seven generations, and the total cross-sectional area increases by about sevenfold by generation 11. After the eleventh generation, cartilage disappears from the airway wall. With these noncartilaginous walls embedded within the lung parenchyma, contiguous elastic tissue serves to hold the airways open, and therefore lung volume plays a substantial role in the maintenance of conducting airway patency. In the terminal bronchioles, the total cross-sectional area is about 30 times the area at the level of the large bronchi. Flow resistance in these small airways is about 10% of lower airway resistance (Macklem and Mead, 1967).

FIGURE 12-12 Cross-sectional area of the respiratory tract in relation to airway generations. With growth of the airways, compliance increases and resistance to gas flow falls according to the Hagen-Poiseuille Law, where resistance falls according to the fourth power of the radius.

(From Weibel ER: Morphometry of the human lung, New York, 1963, Academic Press.)

Flow in laminar profiles enhances diffusivity, because flow at the center of the shear increases the rate of spread in that direction (Fig. 12-13). Fluids or gas molecules moving through a tube under constant pressure separate into a gradient of velocity profiles, with the fastest component in the center and the slowest components near the wall. When the cross-sectional area is reduced, the velocity must increase (and the pressure decrease) in order to maintain the same volume flow rate; resulting gas flow is much more turbulent. With abrupt narrowing, complete separation of the stream profiles occurs, the center stream constricts to a minimum value, and laminar flow may not be able to resume because of the turbulence. Pragmatically, when difficulty with positive pressure ventilation occurs, the natural response of the anesthesiologist is to apply more pressure to the rebreathing bag. For the patient whose airway is compromised, this results in more turbulence in the upper airways and less effective air flow downstream. Passive exhalation may also be impaired, and air trapping may result. However, if the pressure gradient (P) across the obstruction is reduced (e.g., changing to spontaneous breathing from controlled ventilation in order to reduce the high pressure proximal to the obstruction), then air flow may be enhanced distally. This applies to airway narrowing, regardless of intrinsic or extrinsic causes.

FIGURE 12-13 Taylor dispersion: fluids (or gases) moving through a tube under constant pressure separate into a gradient of velocity profiles with the fastest component in the center and the slowest components near the wall. With a reduction in cross-sectional area, the velocity must increase (and the pressure decrease) in order to maintain the same volume-flow rate. Turbulence results. Clinically, when resistance to air flow occurs, the natural response is to apply more pressure to the breathing bag; this will result in more turbulence in the upper airways and less effective air flow downstream.

Clinical Correlation

Causes of stridor include congenital, inflammatory, traumatic, and foreign bodies; a congenital cause is found in 85% of children younger than 2.5 years of age (Holzman, 2000). Severe airway obstruction may result in cyanosis, respiratory distress and fatigue, pneumothorax, pneumomediastinum, and death.

Bubble Stability and Breathing: Laplace’s Law

The pressure inside a vessel is always higher than the surrounding gas pressure, because the surface of the vessel is in a state of tension. In this respect, alveoli resemble bubbles, or spherical vessels. Laplace’s Law describes the relationship between the radius of a vessel and the wall tension required to withstand a given internal fluid pressure. This is different for cylinders and spheres; a sphere has half the wall tension of a cylinder. Wall tension increases as the sphere becomes larger, and the radius of curvature decreases in order to balance the downward component of tension. A rough analogy is hanging heavy clothing on a clothesline; in order for the clothesline to sag less, its tension must be increased. The exact amount of pressure inside the sphere depends on the surface tension of the liquid and the radius of curvature. Laplace’s equation expresses this mathematically:

where P equals pressure within the sphere, T equals surface tension of the liquid, and R equals the radius of the sphere. In addition to the amazing ability of pulmonary surfactant to dramatically reduce the surface tension at the alveolar surface (gas-liquid interface), the surfactant stabilizes the airspace by its ability to change the surface tension as the surface expands and contracts during the respiratory cycle; it decreases surface tension as the surface expands and increases the surface tension as the surface expands, affecting Laplace’s Law so as to maintain and stabilize air space pressure during the respiratory cycle and prevent alveolar collapse (see Chapter 3, Respiratory Physiology in Infants and Children).

Understanding this relationship also helps explain the improvement in ventilation/perfusion matching with spontaneous breathing when there are two functional lung units of unequal compliance and resistance; an unusually high surface tension, along with interstitial fluid accumulation (edema) and fibrosis, would decrease compliance of the segment. Two functional units (e.g., lungs) of equal compliance and resistance increase their volumes equally when mouth pressure is increased to a constant level; the time course of wash-in is identical for both units. If the compliances of the two units are identical but the resistance of one is twice that of the other, however, the time constant is also twice normal and the unit with the higher resistance fills more slowly, although the volume increase in both units is the same if inflation is prolonged indefinitely. If inspiration ceases after 2 seconds, the pressure is higher in the unit with the lower resistance. Although it is true that paralysis and artificial ventilation do not greatly alter gas exchange ventilation/perfusion (V/Q) matching, dead space to tidal volume ratio (V_D/V_T), and shunt in young, healthy patients in comparison with spontaneous ventilation. This is not true for patients with inequalities in the distribution of ventilation because of asymmetric resistances between lung units (Fig. 12-14). Slow or sustained inflation permits increased distribution of gas to slow alveoli and so tends to distribute gas in accordance with the compliance of the different functional units. Such a situation is the principle underlying frequency-dependent compliance.

FIGURE 12-14 A, An idealized state showing the reciprocal relationship between resistance and compliance; gas flow is preferentially delivered to the most compliant regions, regardless of the rate of inflation. Static and dynamic compliance are equal. B, A typical state of respiratory disease, where alveoli exist in fast and slow groups. The relationship between compliance and resistance results in preferential delivery of inspired gas to the stiff alveoli if the rate of inflation is rapid; an end-inspiratory pause serves to redistribute gas from the fast alveoli to the slow alveoli.

Clinical Correlation

Infantile (or idiopathic) respiratory distress syndrome (IRDS), or hyaline membrane disease, is a major cause of respiratory distress in preterm infants because of surfactant deficiency, as well as ongoing respiratory embarrassment in bronchopulmonary dysplasia. Surfactant is a complex mixture of lipids and proteins secreted in type II respiratory epithelial cells that lowers alveolar surface tension and therefore decreases the pressure needed to keep alveoli open and inflated. Surfactant deficiency and the consequent alteration in the ability of alveoli to remain optimally inflated results in reduced compliance, reduced functional residual capacity, unevenly increased airway resistance, and decreased alveolar ventilation. These anatomic changes therefore result in an increased physiologic dead space, ventilation/perfusion imbalance, hypoxemia, hypercarbia, and mixed respiratory and metabolic acidosis. In addition, for children with peripheral airways obstruction who require assisted ventilation, the work of breathing during spontaneous breaths is decreased by the application of positive end-expiratory pressure (Graham et al., 2007).

Bernoulli’s Principle and Venturi’s Effect

In 1738, Bernoulli demonstrated that the pressure of a fluid is least where its speed is greatest. This decrease in pressure when a fluid passes through a region of constriction is the basis of Venturi’s effect, demonstrated almost 60 years later when Venturi showed that as a liquid passes beyond a constriction to a wider part of a tube, its speed gradually decreases, accompanied by a gradual increase in pressure. Furthermore, Venturi showed that in order for a streaming fluid to regain a pressure much higher than that at the constriction, the tube immediately distal to the constriction has to open out very gradually, usually not more than a cone angle of about 15 degrees. This is really a conservation of energy principle; in a steady flow the sum of all forms of mechanical energy is the same at all points (i.e., the sum of the kinetic and potential energies are constant).

Clinical Correlation

In Venturi jet ventilation, actually based on Bernoulli’s principle and initially described by Sanders in 1967 for clinical use, a jet replaces the constriction of a tube and the oropharynx or trachea acts as the diffuser. An activating toggle switch attached to an oxygen source at 50 psig via a variable pressure-reducing valve controls the stream of gas aimed at the trachea. This technique is often used for laser surgery of the vocal cords. Because the pressure is subatmospheric at the distal end of the jet, air is entrained to swell the stream of gas. This principle is also used during supplemental oxygen therapy (the “Venturi” mask with a variable-orifice air-entrainment port); it is also the principle behind the function of an atomizer.

Airway equipment

Routine Pediatric Airway Equipment

Face Masks

Because the cushion-seal face masks used for anesthetizing children have dead spaces from 50 to 80 mL, they add substantially to rebreathing. For this reason, Rendell-Baker and Soucek (1962) introduced a molded, low–dead-space face mask specifically for use with spontaneously breathing infants and children at a time when the administration of halothane anesthesia by mask was popular. However, it is easier to seal to the patient’s face with a cushioned face mask than a Rendell-Baker model, and a good mask seal is particularly useful for the application of CPAP during the induction or emergence from general anesthesia.

The use of the proper technique by a clinician when employing a face mask is more important than the type of mask selected. The clinician’s fingers must not depress the submental soft tissues of small children, because the tongue will be forced against the hard and soft palates and an upper airway obstruction may result. Children who have craniofacial anomalies may present a special challenge when attempting to obtain a good mask seal. This includes children with midfacial dysmorphism such as occurs in Apert’s syndrome, Crouzon’s disease, choanal atresia, or more severe forms of frontonasal dysplasia. Obtaining an adequate mask fit in patients with craniofacial anomalies may be considerably more difficult than the laryngoscopy or tracheal intubation. A useful technique for the ventilation of the lungs of children with midface hypoplasia is to support their cheeks with dental rolls that are placed between the gingiva and buccal mucosa; this often results in a better mask fit.

Airway Adjuncts

Oropharyngeal airway devices should be available in the full range of sizes at each anesthetizing location. The required airway size can be estimated by a careful external examination of the child and by measuring the distance from the teeth to the base of the tongue. An oropharyngeal airway device that is too small can displace the base of the patient’s tongue inferiorly toward the pharynx, thereby increasing the degree of obstruction, which may worsen with the application of CPAP in an effort to improve the airway obstruction. An airway that is too large may reach the laryngeal inlet and result in trauma or laryngeal hyperactivity and laryngospasm. It is common practice by some clinicians to insert an oropharyngeal airway device upside down, or convex to the natural curvature of the tongue and then to rotate the airway 180 degrees. However, this maneuver may abrade the hard palate and it is therefore not recommended. A less traumatic technique for the insertion of an oropharyngeal airway device is to use a tongue depressor to displace the tongue to the floor of the mouth and to insert the device concave to the tongue’s surface.

Oropharyngeal airway devices are often used as “bite blocks” after a patient’s trachea has been intubated, in order to prevent the clenching of the teeth on the endotracheal tube. This maneuver may, however, be hazardous in children between 5 and 10 years of age with loose deciduous teeth. Oropharyngeal airway devices are responsible for up to 55% of anesthesia-related dental complications (Clokie et al., 1989). Furthermore, when an oropharyngeal airway device is used as a bite block during long cases, it may cause necrosis of the tongue, uvular edema, or lip damage (Moore and Rauscher, 1977; Shulman, 1981). A gauze pad that has been rolled up and placed between the patient’s upper and lower molar teeth is a better method of preventing the teeth from clenching on an endotracheal tube and minimizing dental trauma. Caution must be exercised, however, that the roll not slip and place undue pressure on the lateral aspect of the tongue (paraglossal sulcus), where the hypoglossal nerve runs.

Nasopharyngeal airway devices are generally constructed from red rubber or polyvinyl chloride and are available in various sizes. A nasal airway should be lubricated and gently inserted transnasally. Nasopharyngeal airway devices may traumatize the turbinates or adenoids of young children. Moreover, care must be exercised when using a nasopharyngeal airway device in children who have a bleeding diathesis or a congenital abnormality of the midface such as choanal atresia or frontonasal dysplasia. The proper length for the nasopharyngeal airway may be estimated by measuring the distance between the patient’s auditory meatus and the tip of the nose. The insertion of a nasopharyngeal airway device that is too long may cause laryngospasm. Furthermore, if the airway is too short, the upper airway obstruction may not be relieved.

The LMA is used successfully for routine pediatric anesthetics and even for adenotonsillectomies (Webster et al., 1993; Williams and Bailey, 1993). LMAs are currently manufactured in several sizes, for patients ranging from neonates to large adults. With minimal inflation of the mask’s cushion and thorough lubrication of the nonlaryngeal surface, an LMA should be seated at the laryngeal inlet and cause minimal discomfort to the patient postoperatively. Following its blind passage through the oral cavity, the proper seating of an LMA is generally heralded by a slight rise of the device when the mask’s cushion is inflated with air. Care should be taken to use the minimal effective inflation pressure for the cuff, typically up to 60 cm H₂O. The routine use of a manometer is advocated. In some patients, the cushion of the LMA overrides the proximal portion of the esophagus, thereby exposing the patient to the risk of the aspiration of gastric contents, with the LMA serving as a conduit to the lungs (Nanji and Maltby, 1992). Nevertheless, although ideal positioning of an LMA appears to be achieved in only 50% of cases, the vast majority of patients fare very well (Rowbottom et al., 1991; Goudsouzian et al., 1992; Mizushima et al., 1992).

Although it was originally thought that pediatric-sized LMAs might not function adequately because of differences in airway anatomy, this fear proved to be unfounded. The #1 LMA, a miniature version of the adult LMA, was designed to fit infants who weigh less than 6.5 kg. It has worked satisfactorily even in premature infants as small as 1 kg, in newborn resuscitation, and in airway maintenance for infants with upper-airway congenital anomalies (e.g., Pierre-Robin, Goldenhar’s, Treacher-Collins, and Schwartz-Jampel syndromes). In difficult intubating conditions, it has been used throughout the whole procedure or as a conduit for endotracheal intubation. The endotracheal tube can often be easily directed through an LMA without fiberoptic laryngoscopy.

Flexible LMAs in sizes 2, 2.5, and 3 are also available for pediatric use. The cuff is similar to a standard LMA, but the airway tube is wire-reinforced, longer, and more flexible, allowing it to be positioned away from the surgical field. Although the flexibility of the tube is advantageous for positioning, it is more difficult to insert, it may dislodge more easily, and biting can occlude it. Moreover, the flexibility of this LMA does not allow the rotation technique for insertion. Adenoidectomy or even tonsillectomy can be performed with the flexible LMA, because the cuff prevents soiling of the glottis and the trachea by blood and secretions from the surgical site. If tracheal intubation is planned via LMA, a standard LMA is a more logical choice in children, because it is shorter and has a larger diameter; in adolescents or adults, the intubating LMA (Fastrach) can be used.

The more cephalad and anterior position of the larynx of a child as compared with an adult has prompted the use of an alternate insertion technique in children. In this case, the LMA is inserted with its cushion placed against the hard palate. The device is then rotated through 180 degrees until the cushion is seated at the laryngeal inlet (McNicol, 1991). This method for the insertion of an LMA appears to be especially useful in preschool and young school-age children.

Endotracheal Tubes

The most definitive method for airway management in children remains intubation of the trachea. Polyvinylchloride is still the most popular material employed for the production of endotracheal tubes, although other materials continue to be used and newer technologies are evolving. For example, Weiss and Dullenkof (2007) have the reexamined design requirements of pediatric endotracheal tubes, including cuff placement below the cricoid cartilage, which requires a smaller, more distally placed cuff. Among other features, with polyurethane replacing the polyvinylchloride cuff, the sealing pressures for children are in the range of 6 to 14 cm H₂O (Weiss and Dullenkof, 2007). Recommended performance specifications, as well as detailed standards, for endotracheal tubes have been published (Carroll et al., 1973; Shupak and Deas, 1981).

Table 12-1 lists estimated values for the appropriate endotracheal tube sizes and lengths as well as those for pediatric LMAs (Cole, 1957; Penlington, 1974; Morgan and Steward, 1982; Steven and Cohen, 1990). In general, the size of the tube is more related to the age rather than size of the patient. When selecting an endotracheal tube for a child, it is important to remember that the presence of a deflated cuff adds about 0.5 mm to the tube’s external diameter. However, the external diameters of endotracheal tubes differ widely among the manufacturers of the tubes. Cuffs also differ in their shapes and positions along the endotracheal tube. In addition, because nitrous oxide diffuses into the closed airspace of an endotracheal tube’s cuff when the inflation valve is closed, this valve should be rendered incompetent during long procedures. Otherwise, the intracuff pressure should be monitored and maintained at a level below 25 cm H₂O (18.4 mm Hg). A variety of endotracheal tubes are available for special needs, including preformed oral or nasotracheal tubes (Ring-Adair-Elwyn [RAE] tubes) for oral or dental surgery and wire-reinforced (anode) endotracheal tubes for head and neck surgery or laryngotracheal reconstruction, when the tube may be inserted through the tracheal stoma and sutured to the anterior chest wall. The special precautions that must be taken with endotracheal tubes during laser surgery of the aerodigestive tract have been described (Sosis 1989, 1992; Sosis and Dillon, 1990, 1991, 1993). The majority of laser airway surgery in pediatrics is for the treatment of juvenile laryngeal papillomatosis or other laryngeal anomalies, and these are cases that lend themselves well to Venturi jet ventilation of the lungs. However, wrapped endotracheal tubes may be required for pediatric laser airway surgery when the laryngeal inlet is too narrow to permit a sufficient entrainment of pharyngeal gas for the Venturi technique (Holzman, 1991, 1992).

TABLE 12-1 Airway Device Details

Since the seminal report by Koka et al. (1977), an air leak around an endotracheal tube has been strongly advocated for the prevention of postintubation croup in infants and children younger than 10 years of age. Surprisingly, a great deal of variability exists in the ability of a clinician to recognize an air leak around an uncuffed endotracheal tube (Schwartz et al., 1993). However, cuffed endotracheal tubes are advantageous for certain abdominal or thoracic procedures. Also, patients with pulmonary pathologic conditions with poor lung compliance may require high peak inspiratory pressures to assure adequate ventilation of the lungs, and a cuffed endotracheal tube may be necessary for these cases. Because of an increasingly sophisticated understanding of developing laryngeal anatomy and vastly improved materials science in the manufacture of endotracheal tubes, there is a currently a greatly expanded use of cuffed endotracheal tubes in all ages (see The Larynx, p. 350) Accordingly, the use of a cuffed endotracheal tube should be individualized (Table 12-2). Likewise, children who are susceptible to croup during a viral illness of the respiratory tract may benefit from tracheal intubation with a smaller endotracheal tube than is normally used, with or without a cuff, as long as the lungs can be ventilated adequately. A throat pack may help make a seal when an uncuffed endotracheal tube is employed.

TABLE 12-2 Advantages of Cuffed vs. Uncuffed Endotracheal Tubes

	Cuffed	Uncuffed
Advantages	Not important for subglottic stenosis	Larger tube, therefore less resistance for spontaneous breathing and mechanical ventilation
	Fewer repeat laryngoscopies and reintubations	Lower risk of occlusion
	Less contamination	No cuff = no ridges
	Lower fresh-gas flow	No concern about tip-to-cuff border distance
	Better protection against aspiration	No requirement for pressure monitoring
Indications	High risk of aspiration	Minus the concerns on the left, most likely does not matter much
	Preexisting or impending impaired pulmonary compliance
	Patient with poor lung compliance undergoing minimally invasive abdominal or chest surgery
	Cardiopulmonary bypass
	Requirement for precisely controlled mechanical ventilation

Endotracheal tubes are available with or without a special opening known as a Murphy eye in the wall opposite the tube’s distal bevel. Endotracheal tubes fabricated without the Murphy eye are known as Magill tubes, whereas those that have this opening are called Murphy tubes. The Murphy eye was designed to provide an alternate pathway for the flow of ventilatory gases if the distal opening of the endotracheal tube was occluded—a common situation, especially in infants (Murphy, 1941). However, there are potential disadvantages to the presence of a Murphy eye on an endotracheal tube, including a tendency for accumulation of secretions and the possibility that a stylet, catheter, or bronchoscope may get stuck, requiring the removal of the entire assembly. Cuffed endotracheal tubes manufactured without a Murphy eye can have the cuff located closer to the tube’s tip. Nevertheless, pediatric anesthesiologists generally favor the use of an endotracheal tube that is equipped with a Murphy eye because of the obstruction risk. Recently, Weiss and Dullenkof (2007) have reexamined the design requirements of a more anatomically suitable pediatric endotracheal tube, including cuff placement below the cricoid cartilage, thus requiring a smaller, more distally placed cuff and removal of the Murphy eye. Among other features, with a polyurethane as opposed to a polyvinylchloride cuff, the sealing pressures for children are in the range of 6 to 14 cm H₂O, well under the 20 to 25 cm H₂O for the leak test.

Stylets are often inserted into endotracheal tubes used for children to provide rigidity and bend the tube to a desired shape. However, the increased rigidity provided by a stylet can result in greater trauma. Moreover, because of metal fatigue, old wire stylets may break during their removal from an endotracheal tube, placing the patient at risk for the aspiration of a foreign body. Stylets should be of the proper size for the endotracheal tube, and they should be well lubricated so they can be removed easily. The tip of the stylet should be properly recessed (1 to 2 cm) from the tip of the endotracheal tube to avoid its extrusion from the tube and direct trauma to the mucosa. Each stylet should be carefully inspected for any potential weak points.

The concerns about anatomic differences between children and adults were typically partisan enough that while an emerging acceptance of endotracheal intubation was obvious beginning in the 1950s and 1960s, it remained a relatively rare technique in pediatric anesthesia. Clinicians were warned about the narrowest part of the pediatric airway located at the cricoid cartilage. Outcome studies began to suggest the safety and efficacy of endotracheal intubation in infants and children, and pediatric anesthesia clinicians were becoming more vocal in defense of this emerging technique. See related video online at www.expertconsult.com

Laryngoscopes

The ability to use standard straight and curved laryngoscope blades is mandatory in pediatric anesthesia. The laryngeal structures of infants are encountered at a more cephalad and anterior position, approximately the level of C2 to C3, depending on age, until the child reaches school age, when the laryngeal structures descend to a level opposite the body of C4. It is common practice to use a straight laryngoscope blade such as the Miller 0 or 1, or the Wis-Hippel 1.5, to intubate the trachea of an infant or a child up to school age. A curved laryngoscope blade with a wider spatula, such as the Macintosh blade may be advantageous to control the tongue of an older child. Whether to use the straight or the curved laryngoscope blades can be determined according to this approximate age distribution; however, the individual preferences of the clinician may vary, as can individual circumstances. The larger tongue of a patient with Down’s syndrome, at any age, for example, may be easier to manipulate with the wider spatula of a Macintosh blade. A short laryngoscope handle is most useful for the intubation of the trachea of an infant, a small child, or an obese patient.

Equipment for the Management of a Difficult Airway in a Child

Line-of-sight devices have been the mainstay of direct laryngoscopy techniques, yet difficulties with laryngoscopy and intubation are not rare. Although it is typically acknowledged as a rare event, there is emerging literature that difficulty with laryngoscopy may be encountered in 5% to 20% of adult patients; although this may be substantially less of an issue in children and it may be age dependent (0.57% in newborns and toddlers, 0.12% in preschool, and 0.05% in school-age patients) (Rose and Cohen, 1994, 1996; Ezri et al., 2003; Schmidt and Koch, 2008).

Anterior commissure laryngoscopes, because of their tubular construction, are particularly useful with a difficult-to-visualize “anterior” (cephalad) larynx in the patient with a small oral cavity. They require a fiberoptic external light source. In addition, long bronchoscopy forceps must be available. The 15-mm connector is removed from the endotracheal tube, and the laryngoscope is inserted. With the epiglottis lifted directly from the laryngeal surface, the tip of the laryngoscope should be placed at the glottic opening. The endotracheal tube is inserted through the tubular laryngoscope blade. Although direct visualization of the glottis is usually lost at this point, correct positioning and the tubular structure of the blade guide the tube into the trachea. Using the forceps, the tube is held in place while the laryngoscope is removed over the tube and forceps (Fig. 12-15). The endotracheal tube connector is then replaced, and the tube position is verified.

FIGURE 12-15 Use of an anterior commissure laryngoscope for endotracheal intubation; when an endotracheal tube with its 15-mm connector removed is inserted through the tubular blade after the epiglottis is lifted from the laryngeal surface and the tip of the blade is placed at the glottic opening, the bronchoscopy forceps can grasp the proximal end of the tube while the laryngoscope is removed over the tube and forceps.

Newer technologies, such as fiberoptic bronchoscopy and the availability of video machines, are making it much easier to develop ways of viewing the larynx that are not directly line-of-sight dependent. The skills required for effective use of these devices are different, and although pediatric anesthesiologists must keep abreast of new technologic skills, conventional rigid laryngoscopy is used the majority of the time.

With fiberoptic and video modifications, flexible, malleable, and rigid devices have been developed that have added to the laryngoscopy tool shed in recent years. Optical stylets may be rigid, flexible, or malleable; resemble a conventional laryngoscope; or be more like a flexible fiberoptic bronchoscope (Liem et al., 2003). The basic design consists of a fiberoptic rod with an eyepiece at the user end that allows the anesthesiologist to view the advancement of the endotracheal tube, which is threaded over the device. Examples of these devices include the Bonfils Retromolar Intubation Fiberscope (Karl Storz Endoscopy; Culver City, California), the Shikani Optic Stylet (Clarus Medical LLC; Minneapolis, Minnesota), the StyletScope (Nihon Kohden Corp.; Tokyo, Japan), the Video Optical Intubation Stylet (VOIS, Acutronic Medical Systems, AG; Baar, Switzerland), and the Levitan FPS Scope (Clarus Medical LLC; Minneapolis, Minnesota) (see Chapter 10, Equipment, Fig. 10-24). The Bonfils laryngoscope is a nonmalleable scope, whereas the Shikani laryngoscope has a distally malleable stylet. It can accommodate a size 2.5 endotracheal tube in its pediatric version. All of these stylets can be used alone or in conjunction with rigid direct laryngoscopy. In the absence of direct laryngoscopy, a jaw lift or jaw thrust should be used to increase the anteroposterior distance of the hypopharyngeal space and move the base of the tongue away from the laryngeal inlet.

Rigid fiberoptic laryngoscopes have a fiberoptic bundle encased in a closed structure. They serve the same function as conventional rigid laryngoscopes, in that they are intended for viewing the larynx and confirming the insertion of the endotracheal tube. They cannot view the subglottic space or be inserted by any route other than transorally. Examples include the Bullard laryngoscope, UpsherScope, and WuScope. Only the Bullard laryngoscope is available in pediatric versions (see Chapter 10, Equipment, Fig. 10-30). The Acutronic Video Intubating laryngoscope (Acutronic Medical Systems, AG; Baar, Switzerland) was developed by Weiss and resembles a conventional laryngoscope and blade with a fiberoptic bundle that can be coupled to a video camera.

Rigid video laryngoscopes allow the image to be displayed to an eyepiece or a video screen. The video camera may be located within the handle, mounted on the handle, or displayed remotely. Other rigid video laryngoscopes include the Glidescope (Verathon, Inc.; Bothell, Washington) (see Chapter 10, Equipment, Fig. 10-31; see related video online at www.expertconsult.com). It is available in pediatric and neonatal versions. Because the larynx is not in the line of sight, the Glidescope should be used with a stylet. Exposure of the vocal cords on the image does not necessarily occur from exposure of the larynx. Antifogging solutions are not required, because a charge-coupled video chip covered by a heated glass window is incorporated into the blade, making it fog resistant.

The flexible fiberoptic laryngoscopes that are used for children are generally smaller than adult scopes, and not all small fiberscopes are designed with a channel for suctioning or oxygen insufflation. Also, pediatric fiberscopes may be limited in the extent of their tip excursion in comparison with larger fiberoptic laryngoscopes. A pediatric fiberoptic laryngoscope is often used to intubate the trachea of a child who is breathing spontaneously with a volatile anesthetic. In such cases, an airway intubation (Frei) mask or one with a similar design, which incorporates a grommet through which a fiberscope may be passed, is very useful (Frei, 1995) (see related video online at www.expertconsult.com).

It may seem counterintuitive to advocate blind intubation techniques amidst these advances in technology; however, in skilled and thoughtful hands, these methods are worthy of consideration because they have stood the test of time, and simply put, they work! Devices for blind intubation include the gum elastic bougie (e.g., Eschmann Stylet, Portex Limited; Hythe, United Kingdom) and the light wand (e.g., Trachlight, Laerdal Medical; Wappingers Falls, New York). In one study, children whose tracheas were found to be extremely difficult to intubate by conventional means were intubated without difficulty with the use of a light wand (Holzman et al., 1988). Finally, tactile intubation can be accomplished by using the index and middle fingers of the nondominant hand intraorally to palpate the epiglottis while the endotracheal tube or stylet is advanced along the palmar surface of the index finger to the larynx, laryngeal inlet, and intratracheally.

It is important to emphasize that these newer and developing technologies, as well as the older ones, require familiarization on the part of the anesthesiologist; no one can be expected to use these devices or techniques in the heat of battle with a difficult airway. Preparation and practice under controlled circumstances are the best preparation for emergency situations, and as always, chance favors the prepared mind. A well-stocked cart for use with difficult airways should be immediately available in every operating room suite.

Breathing Systems

The valved-circle anesthesia system or variations of Ayre’s valveless T piece are the most common breathing circuits for pediatric patients (Mapleson, 1954). For years, the T piece was recommended for children weighing less than 10 kg because of the decreased resistance to spontaneous breathing, the better “feel” of the rebreathing bag in the hand of the anesthetist, and the faster anesthesia induction and emergence times using higher nonrebreathing fresh-gas flows. Improvements in machine components have resulted in low-resistance valves, reduced dead space at connections, improved CO₂-absorbent canister design, and the availability of capnography, as well as changes in philosophy that favor controlled pulmonary ventilation in small children; such improvements have rendered these arguments less durable. Likewise, modern, less-soluble inhalation agents make the influence of the breathing circuit less of an issue.

The compression volume of the breathing circuit is an important influence on alveolar ventilation in children, because their tidal volumes are so much smaller. Breathing circuits with large compression volumes can be “ventilated” far in excess of the volume delivered to the patient. Many circle systems modified for pediatric use are not only shorter, but they also have a smaller radius of curvature of the tubing, which according to Laplace’s Law renders them less distensible and thus further decreases compression volume. Compliance of the tubing can also be decreased with fine wire or reinforced plastic.

Another important consideration for small children is the dead space of the breathing system. Dead space only exists when fresh and exhaled gases are mixed (i.e., at the Y piece); dead space no longer exists when fresh and exhaled gases are completely separated. Accordingly, the dead space of the Y piece in a circle system can be decreased by the addition of a median septum. Similarly, the dead space of an elbow in a Mapleson D system can be decreased by the addition of a fresh gas delivery port within the elbow (the Norman elbow).

Clinical airway care

Expert airway care is the most important skill in pediatric anesthesiology, and skill in understanding and controlling anesthetic depth is arguably the most important aspect of providing safe and expert airway care. It is critical to the success of spontaneous breathing techniques for diagnostic examination of the native airway, dynamic changes such as extrinsic compression or tracheomalacia, and for interventions such as the removal of a foreign body from the airway (see videoclip of infant bronchoscopy online at www.expertconsult.com). Aside from the assessment of heart rate, blood pressure, and patient movement, depth of anesthesia can also be evaluated by continuous monitoring of the heart sounds, the “classic” eye signs, and repeated examination of muscle tone. The arms and legs should feel completely relaxed, and the abdominal muscles should be soft and easily compressible, like bread dough, distinct from the boardlike rigidity of the excitement phase or the mild to moderate muscle tone of the awake or sedated state. Disconjugate gaze and pupillary dilation shortly after inhalation induction support the assessment of light anesthesia, whereas midsize pupils in midposition, along with other physical findings of acceptable anesthetic depth, add to the clinician’s assessment. All signs together should be used to form a composite, so that ideally the anesthesiologist should be able to ensure lack of patient movement in response to stimulation. Facility in determining anesthetic depth is also important for successful “deep” extubations—that is, the removal of the endotracheal tube while the patient is deeply anesthetized and breathing spontaneously. The presumption is that the patient will have less coughing, bucking, and airway irritability during the emergence process because of the lack of an endotracheal tube. In experienced hands, this technique is at least as safe as the more typical “awake” extubation (Patel et al., 1991; Pounder et al., 1991; Karam et al., 1995).

Profound surface analgesia of the oropharynx and laryngeal apparatus can be achieved by a combination of topical anesthesia of the tongue and posterior pharyngeal wall, glossopharyngeal nerve block, and superior laryngeal nerve block, as well as a transtracheal injection of local anesthetic (Cooper and Watson, 1975). Because most children will not tolerate this strategy well when they are awake, it is usually accomplished with sedation.

• Nebulized or gargled lidocaine can be used in older (i.e., school age) children to obtain surface analgesia of the tongue and pharyngeal surfaces.

• Local spray anesthesia can be used regardless of age by using age-appropriate concentrations of lidocaine (1% for infants, 2% for young school-age children, and up to 4% for older children and adolescents). Cooperative patients can gargle and swallow the accumulated spray; spray and suctioning sequences may have to be used for more deeply sedated patients.

• The internal branch of the superior laryngeal nerves can be blocked bilaterally as they pierce the thyrohyoid membrane just superior to the greater cornua of the thyroid cartilage (Fig. 12-16). Only a small amount of local anesthetic (e.g., lidocaine 1%) is required; after a sterile preparation of the skin, a fine needle is entered perpendicular to the skin at a point above the greater cornua of the thyroid cartilage, midway vertically between the thyroid and hyoid cartilages; the thyrohyoid membrane is pierced, and local anesthetic is deposited after careful aspiration just below. Then the needle is withdrawn slightly, just above the membrane. The onset of this block takes a few minutes, so it should be done before the glossopharyngeal nerve block.

• The glossopharyngeal nerves can be blocked bilaterally throughout their submucosal course in the posterior tonsillar (palatopharyngeal) pillar with the use of a tonsil block needle (Fig. 12-17). Only a small amount of local anesthetic volume of lidocaine 1% is required; careful aspiration before injection is a must. Typical injection locations are at the 2-, 4-, 8-, and 10-o’clock positions as the anesthesiologist faces the patient directly. The block is established quickly.

• A transtracheal block to anesthetize the recurrent laryngeal nerve can be accomplished with the use of a fine needle or small IV catheter piercing the cricothyroid membrane after the above blocks, using an age-appropriate concentration of lidocaine 1% to 4%. Accurate intratracheal location of the needle or catheter is confirmed by aspiration of air before injection, which is often accompanied by vigorous coughing; therefore, the needle or catheter should be withdrawn as soon as possible (Fig. 12-18).

FIGURE 12-16 The internal branch of the superior laryngeal nerve pierces the thyrohyoid membrane at a vertical midpoint between the thyroid and hyoid cartilage, just superior to the greater cornua of the thyroid. It is here that the nerve can be percutaneously accessed to block sensation to the mucous membranes of the pharynx and supraglottic larynx.

FIGURE 12-17 Intraoral approach to the glossopharyngeal nerve as it runs submucosally posterior to the posterior tonsillar pillar (palatopharyngeal fold). Blocking the nerve as it courses through the palatopharyngeal fold provides sensory analgesia to the mucosa over the palatine tonsil, the adjacent soft palate, the pharynx, and the posterior third of the tongue.

FIGURE 12-18 Transtracheal approach to the subglottic space, providing sensory blockade to the mucosa of the infraglottic trachea via the recurrent laryngeal nerve.

Laryngospasm occurs more often in conjunction with airway-related procedures, and multiple strategies for the prevention and treatment of laryngospasm have been sought. CPAP is routinely used in preventing or treating laryngospasm and soft tissue upper airway obstruction. Usually, pressure at 5 to 10 cm H₂O as measured on the breathing circuit pressure gauge is effective; higher pressures are occasionally required. Care should be exercised, because opening pressure of the lower esophageal sphincter may be as low as 10 to 12 cm H₂O, and therefore the risk of gastric insufflation exists. Also, while CPAP is effective for soft-tissue obstruction, it is not necessarily effective for tumors of the airway or scar tissue.

Care of the child with a difficult airway

General Issues

The care of a child with a difficult airway must begin with an assessment of resources; the most important part of that assessment is a self-assessment. Questions to ask include:

1. Do I routinely take care of children? If not, then the pediatric airway may, for me, be relatively unique.

2. Do I routinely take care of children younger than 2 years of age? This population has consistently different anatomic and physiologic characteristics from those of older children, such as a much higher oxygen consumption and CO₂ production, a smaller functional residual capacity, and a greater reduction in intrathoracic volume when supine, anesthetized, and relaxed; therefore, this age range has substantially less oxygen reserve during airway misadventures.

3. Do I routinely take care of the subset of children with difficult airways? If not, then the pediatric difficult airway is definitely a special instance.

There is some evidence that the toll of providing care for children, especially resuscitation, may occasionally impose a substantially larger psychological burden on the pediatric specialist. For this reason, a color-coded resuscitation aid such as the Broselow-Luten system may decrease practitioner anxiety and afford more time for critical thinking during crises (Luten and Broselow, 1999, Luten et al., 2002). Although the evaluation of difficult airway anatomy in a child is, in principle, very similar to that of an adult, children often do not cooperate. With regard to the difficult airway, patients with a difficult airway as a “syndrome” are not usually the challenge; they have been previously identified, are often well categorized, and may even have a tracheostomy; the challenge is the patient who may look somewhat challenging to the less experienced evaluator but indeed has a difficult airway.

Specific Issues Unique to Children with Difficult Airways

Traditional clinical teaching is that the sniffing position is the optimal position for aligning the axes of the trachea, pharynx, and oral cavity. Because the anteroposterior dimension of the head in children is so much larger than that of the chest, a pillow or ring under the head may not be necessary to achieve this sniffing position; it may simply involve placing the patient on the flat mattress pad of the operating-room table, or even placing a small roll under the shoulders. This presumes normal mobility of adjacent structures of the larynx. With poor mobility of laryngeal structures, such as in the anteriorly placed larynx, direct laryngoscopy in the extended head position actually swings the larynx anteriorly as the anesthesiologist lifts the laryngoscope blade, making visualization more difficult. It is often better to flex the head onto the chest, as the glottic aperture then moves posteriorly and may be visualized directly. Indirect visualization may now be accomplished with many available devices, as described above. In order to develop comfort and expertise and retain familiarity, these devices must be used routinely.

Decision algorithms about the difficult airway are available, and most of the issues that are specific to the anticipated difficult pediatric airway are not that different from those for adults, including a broad range of appropriate equipment, occasional imaging studies, and preparation with sedation and topical local-anesthetic techniques (Fig. 12-19). If a spontaneous-breathing anesthetic technique is chosen, pediatric anesthesiologists are already comfortable with the assessment of anesthetic depth in children. Patient cooperation is a key crucial difference in the recognized difficult airway and in children is influenced by the chronologic, developmental, emotional, and intellectual components of the preoperative assessment. Surgical airway access, whether by needle cricothyroidotomy, open cricothyrotomy, or tracheostomy, is hazardous, especially in inexperienced hands, yet it may be the only option available. If this is a possible part of the airway plan, collaboration with a surgeon is essential.