The laryngotracheal groove arises from the ventral surface of the foregut during the third and fourth weeks of gestation (3). Cells lining the celomic cavity also divide during this time, becoming a proliferating primitive mesenchyme and eventually pulmonary muscle, cartilage, and connective tissue. During the fifth week of gestation, caudal progression of the embryonic trachea is followed by bifurcation and the appearance of lung buds. During the sixth week, the lobar bronchi lengthen and abut the esophagus. Asymmetric buds (the left bud being shorter and more nearly horizontal than the right) rapidly divide into lobar bronchi and tertiary bronchi by the seventh week. At this stage, pseudostratified epithelium lines the larger airways. Differentiation and branching of the epithelial bronchial tree depends on the presence of the mediastinal mesenchyme (3,4). The formation of new smaller bronchi continues through the sixteenth week. Bronchial subdivisions continue until the seventeenth order is established between the sixth and seventh month. At that time, alveolar differentiation occurs at the site of the distal terminal bronchi and continues until well after birth.

The initial vascular supply of the differentiating bronchial buds develops from the splanchnic plexus that originates from the dorsal aorta and drains into the plexus of cardinal veins. The bronchial arterial blood supply develops from gradual interconnection with the sixth aortic arch, and the bronchial vasculature is left as a remnant of the embryonic vascular system. Tracheal bifurcation is
initially high in the cervical region, but descends to the level of the first thoracic vertebra by 8 weeks of gestation and to the fourth thoracic vertebra at birth. Cartilage appears in the trachea in approximately 10 weeks and in the segmental bronchi at 16 weeks (4). After this time, fetal lung development primarily consists of the successive generation of terminal airways and alveoli.

Congenital anomalies of the respiratory system are relatively uncommon and can occur along the entire tracheobronchopulmonary axis. Associated extrapulmonary anomalies are frequently present and may be part of a syndrome or association. Approximately 45% of children with congenital airway obstruction have associated congenital anomalies (1,2,5), some of which have a significant impact on prognosis. Although the cause of such anomalies is usually unknown, they are believed to result from causal mechanisms that differ in time, mode of action, and the embryonic region affected. As such, they are failures of normal growth and differentiation of different parts of the embryonic respiratory system. More specifically, defective mesodermal development in early embryogenesis may be responsible for the VACTERL association. Primary abnormalities of the developing lung bud itself may lead to a variety of abnormalities, including congenital lobar emphysema, bronchogenic cysts, sequestrations, and cystic adenomatoid malformations, which are discussed elsewhere. Gestational teratogens may have selected effects on specific organ development. Experimental evidence suggests that vitamin A may play an important role in the development of the airway and lungs. In animal embryos, its deficiency causes a keratizing metaplasia of the tracheobronchial tree and pulmonary agenesis (6). Retinoic acid appears to be important for embryonic cell differentiation in many organ systems and has been studied intensely (3). Investigators have described a relationship between the maternal use of valproic acid and both tracheomalacia and laryngeal hypoplasia in offspring (7). Also, a constellation of abnormalities, including severe congenital tracheal stenosis, has been reported in infants of diabetic mothers (8).

Several anatomic features of the airway in infants differ from those of older children or adults (5,9) and are important to keep in mind because of their impact on symptom progression, diagnosis, and disease management. The caliber of the airway is small and can be readily obstructed by secretions or mucosal swelling. The larynx is more cephalad and anterior, making visualization more difficult, especially for the inexperienced clinician. The cricoid, which is the only completely circumferential laryngeal cartilage, is the narrowest portion of the child’s upper airway. Last, the length of the trachea is very short (approximately 4 to 5 cm), thus increasing the risk of unplanned extubation or right mainstem intubation during flexion or extension of the infant’s head and neck.

Because of this unique subglottic anatomy, care must be taken to minimize trauma to this region during endotracheal intubation and airway procedures. Tightly fitting endotracheal tubes may cause mucosal or submucosal injury, thus resulting in stenosis at the level of the cricoid cartilage. A 50% reduction in cross-sectional area results for every millimeter of airway edema or narrowing of the lumen of an infant’s airway. The correct fit of the child’s uncuffed endotracheal tube can be estimated as follows (10):

4 + (years of age/4) = tube size (mm)

When positioning the endotracheal tube, there must be an air leak at less than 18 to 25 cm of water pressure to ensure an appropriate fit (10).

Mapping the respiratory tract into three distinct regions helps identify possible pathologic correlates of stridor (10). The first area comprises a supraglottic and supralaryngeal region, which includes the pharynx; the second is an extrathoracic tracheal region, which includes the glottis and the subglottis; and the third is made up of an intrathoracic tracheal region, which includes primary and secondary bronchi. In certain regions, stridor occurs more frequently during inspiration, whereas in other regions, it occurs more frequently during expiration. These patterns of stridor are of utmost importance in that they indicate possible etiologies that warrant further investigation or signal imminent emergency. In the first region, stridor generally occurs during inspiration. A patient showing this pattern should undergo careful investigation for upper airway lesions (e.g., choanal atresia), expanding lesions in the tongue (e.g., a dermoid cyst or an internal thyroglossal duct cyst), or lack of structural airway support from mandibular hypoplasia, as seen in Pierre-Robin sequence or from macroglossia, as seen in Beckwith-Wiedeman syndrome. In the second region, stridor is heard during both inspiration and expiration, and is referred to as biphasic. When this pattern is heard, the glottic and subglottic lumina have reached a critically small size and tremendous effort is required to move air through a pinpoint opening. Biphasic stridor often signals impending respiratory collapse and is thus a medical emergency likely to require intubation or tracheotomy. In the intrathoracic bronchial region, the relative positive pressures of expiratory forces within the chest wall narrow the bronchial lumen in
normal children. As air moves during expiration, the Venturi principle adds a constricting force. In the presence of a bronchial foreign body or a lesion, these forces act jointly to close the airway lumen. The expiratory phase sound can be heard as either stridor or wheezing.

Airway endoscopy, also referred to as laryngotracheobronchoscopy, is the instrument-aided visual examination of the airway. It can reveal airway structure and dynamics as well as airway contents, and also provide access to foreign bodies, secretions, and washings from the lower airways. As such, it has increasingly been used in infants and children for both diagnostic and therapeutic purposes (11,12,13). For a child with signs of respiratory distress, chest wall retractions, severe stridor, or tripod posture with drooling, airway endoscopy is urgent and imperative (10). Early endoscopic intervention in the setting of worsening stridor may avoid more extreme measures such as endotracheal intubation or a tracheotomy. Both of these procedures may interfere with establishing an accurate diagnosis. In cases where some degree of airway obstruction coexists with feeding difficulties and insufficient weight gain, airway endoscopy is warranted to determine the underlying cause. These circumstances are more likely to be found in chronic conditions such as laryngomalacia and bronchotracheomalacia. When diagnostic imaging techniques suggest an abnormality such as a vascular ring, endoscopy may be required to confirm the diagnosis. Because a significant percentage of children with stridor have more than one airway lesion, the entire upper and lower airway should be examined unless there are contraindications such as critical tracheal stenosis (11).

For airway endoscopy to be carried out safely and effectively, cooperation and collaboration between the surgeon and the anesthesiologist are absolutely essential, and an overall strategy must be discussed and agreed upon. Optimally, the attending pulmonologist should be included in the operative team, particularly when the patient has a history of pulmonary dysfunction. Also, essential resources should be available in the event of the need for urgent airway access by cannula or a tracheotomy.

Although rigid and flexible bronchoscopy offer different advantages and disadvantages, they are best viewed as complementary techniques to assess airway anatomy and function. They are often used concurrently, and of utmost importance, they both require gentle technique. They also both require an appreciation of the unique anatomy of infants and children, and the potential risks involved in attempting to visualize airway structures.

The surgeon treating infants and children with tracheobronchial abnormalities must be competent in performing different techniques of establishing a secure airway and with managing the postoperative care that each technique requires. Essential techniques include cricothyroidotomy, tracheotomy (temporary airway), and tracheostomy (long-term airway). The treatment selected is based on the surgeon’s determination of whether securing the airway is emergent or elective and permanent versus temporary.