460 SECTION V Pediatric Critical Care Pulmonary • Fig 40 14 Necrotizingepiglottitis Larynx The larynx is a complex structure of cartilage, ligaments, and muscles involved in respiration, protection[.]
460 S E C T I O N V Pediatric Critical Care: Pulmonary • Fig 40.14 Necrotizing epiglottitis Larynx The larynx is a complex structure of cartilage, ligaments, and muscles involved in respiration, protection of the lower airways, and phonation The larynx begins at the level of the true vocal folds and extends to the inferior border of the cricoid cartilage Proper airflow requires that the larynx have functioning vocal folds so that during inspiration, there is abduction of the true vocal folds, allowing the least restrictive inflow of air During expiration, slight adduction occurs and the false vocal folds (also known as the ventricular folds) modulate the expiratory airflow Protection of the lower airways is key to survival Prevention of aspiration of secretions or ingested food requires multilevel coordination of several sphincters The most superior level is the epiglottis, which has a flattened lingual surface directing secretions laterally and posteriorly as it folds into the larynx during deglutition The paired arytenoid cartilages sit at the posterior aspect of the larynx and provide the next level of protection These cartilages, along with the coordination of the aryepiglottic folds, contract medially to close the glottis Simultaneously, the larynx is elevated and pulled forward by hyolaryngeal excursion, which provides the airway with further protection during swallowing Finally, the paired true and false vocal folds adduct and provide yet another level of protection During quiet respiration, the highly innervated larynx repels unwanted secretions by the highly sensitive cough reflex, mediated primarily through cranial nerves IX and X In the intubated, often sedated, or paralyzed child, these protective mechanisms are absent, and the dangers of aspiration of secretions become of great concern Frequent suctioning of oral cavity secretions may reduce this risk but is unlikely to eliminate aspiration completely Innervation of the larynx for its protective and respiratory functions is located centrally in the brainstem, making it reflexive The sensory and motor innervations to this area are critical to proper function Thus, following prolonged intubation, immediate return of function is unrealistic A cautionary approach to the reestablishment of oral feeds is prudent Another issue to consider is that intubation may cause recurrent laryngeal nerve injury with vocal fold paralysis or paresis The mechanism for this injury is believed to result from pressure on the cricoarytenoid joint close to where the nerve enters the larynx Children presenting with bilateral vocal fold paralysis should be evaluated for airway obstruction and intracranial or thoracic causes of their obstruction, such as an Arnold-Chiari malformation Unilateral vocal fold paralysis is a known complication following pediatric heart surgery, especially patent ductus arteriosus ligation Spontaneous recovery may occur in 3% to 45% of cases in to years In a study of 404 children with unilateral or bilateral vocal cord paralysis, 25% needed a tracheotomy, and 40% needed a gastrostomy tube for nutrition If feeding, voice, and breathing are appropriate, no intervention is required Laryngomalacia is a prolapsing of the posterior arytenoid mucosa and foreshortened aryepiglottic folds that cause a spectrum of airway obstruction Laryngomalacia is characterized by inspiratory stridor, associated airway obstruction, and dysphagia Most children (.80%) outgrow this condition, but children with sleep apnea, failure to thrive, and pectus excavatum many need further surgical management Nearly 20% of these children will also have a synchronous airway lesion in the lower airway Anatomic differences between the upper aerodigestive tracts of infants and children are listed in Table 40.1 TABLE Anatomic Airway Differences Between Infants and Children 40.1 Anatomic Location Infant Older Child Oral cavity • • • • • • • • • • • • Pharynx • No definite/distinct oropharynx • Obtuse angle at skull base in nasopharynx • Elongated pharynx with distinct oropharynx • 90-degree angle at skull base Larynx • • • • • • • • Tongue fills mouth Edentulous Tongue rests between lips and sits against palate Cheeks have sucking pads (fatty tissue within buccinators) Relatively smaller mandible Sulci important in sucking One-third adult size Half true vocal fold is cartilage Narrow, vertical epiglottis High in neck Mouth is larger Dentulous Tongue rests on floor of mouth Tongue rests behind teeth, not against palate Mandibular-maxillary relationship relatively normal Sulci have little functional benefit Near adult size Less than one-third true vocal fold is cartilage Flat, wide epiglottis Adult position approximated by years of age Modified from Arvedson JC, Brodsky L, Lefton-Greif MA Pediatric Swallowing and Feeding: Assessment and Management 3rd ed San Diego: Plural Publishing; 2020 CHAPTER 40 Structure and Development of the Upper Respiratory System 461 Trachea and Bronchi Key References Inferior to the larynx is the trachea It conducts air to the bronchi, which branch to ever-smaller lumen tubes that eventually become alveoli, the anatomic location of gas exchange The trachea is made of cartilaginous rings that are essentially the same diameter until they reach the carina, where the trachea splits into left and right mainstem bronchi The right mainstem bronchus is shorter, wider, and takes off at a less acute angle than does the left mainstem bronchus Bronchial foreign body aspiration is seen more often in the right mainstem bronchus than in the left mainstem bronchus Bronchial intubation is more often encountered on the right The right mainstem bronchus leads to the right upper, middle, and lower lobes of the lungs The left main stem bronchus is longer, narrower, and more acutely angled than the right mainstem bronchus Its bronchi lead to the left lower and upper lobes As mentioned previously, these airways are lined by pseudostratified, ciliated columnar epithelium This epithelium is readily injured through suctioning Prolonged intubation, particularly after tracheotomy, results in diffuse squamous metaplasia Without functioning cilia, airway secretions remain in the airway and can be the source of irritation, inflammation, and atelectasis, all complicating factors in the management of the upper airway during a critical illness Arvedson J, Lefton-Greif M Anatomy, embryology, physiology, and normal development In: Arvedson J, Brodsky L, Lefton-Greif M, eds Pediatric Swallowing and Feeding: Assessment and Management 3rd ed San Diego: Plural Publishing; 2020:11-73 Jabbour J, Martin T, Beste D, Robey T Pediatric vocal fold immobility: natural history and the need for long-term follow-up JAMA Otolaryngol Head Neck Surg 2014;140(5):428-433 Laitman J, Reidenberg J Specializations of the human upper respiratory and upper digestive systems as seen through comparative and developmental anatomy Dysphagia 1993;8:318-325 Marcus C, Smith R, Mankarious L, et al Developmental aspects of the upper airway: report for the NHLBI Workshop, March 5-6, 2009 Proc Am Thorac Soc 2009;6:513-520 Minocchieri S, Burren J, Bachmann M, et al Development of the premature infant nose throat-model (PrINT-model)—an upper airway replica of a premature neonate for the study of aerosol delivery Pediatr Res 2008;64:141-146 Randall D, et al Development of the respiratory system and respiration In: Chamley C, Carson P, Duncan R, eds Developmental Anatomy and Physiology of Children Philadelphia: Elsevier; 2005:165-185 Acknowledgment The authors dedicate this chapter to the memory of Dr Linda Brodsky The full reference list for this chapter is available at ExpertConsult.com e1 Abstract: The structures of the upper airway undergo extensive changes from infancy through young adulthood An understanding of the numerous variations, congenital anomalies, and resulting special vulnerabilities of the developing airway and underlying illness should result in improved health outcomes and a lower morbidity rate in children with airway disease This chapter reviews relevant anatomy for the critical care provider as well as possible acquired and congenital abnormalities that may affect the critical care of a pediatric patient Key words: airway obstruction, stenosis, vocal cord paralysis, laryngomalacia 41 Structure and Development of the Lower Respiratory System JOHN E BAATZ AND RITA M RYAN PEARLS • Overview of the Lungs While the upper airway of the respiratory system is involved in uptake of air for delivery to the lungs and in the removal of large particulates from the air, the primary purpose of the lower respiratory tract is to efficiently deliver oxygen to the blood and remove expired carbon dioxide The main components of the lower respiratory tract are the trachea, bronchial tubes, and lungs The lungs are dense, spongy structures composed of smaller branches of the bronchi called bronchiolar tubes After several branching generations, the bronchiolar tubes terminate at alveolar sacs, where oxygen and carbon dioxide are exchanged with the blood of alveolar sac–associated capillaries The trachea, bronchial tubes, and bronchioles provide rapid delivery of large volumes of air; the alveolar sacs provide the large surface area required for sufficient gas diffusion 462 • • • Lower Respiratory System • • • • • • • Lungs increase in volume from about 250 mL at birth to 6000 mL in the adult Each lung lobe is subdivided into 19 bronchopulmonary segments, which receive a primary segmental bronchus and a tertiary pulmonary artery branch and are drained by pulmonary veins The airway branching pattern in the lung undergoes multiple generations, yielding a total of 27 or 28 divisions when counting begins from the primary bronchus The aggregate length of the airways in the adult lung spans approximately 1500 miles (2400 km) The bronchial mucosa contains several epithelial cell types, with the ciliated cell comprising more than 90% of the epithelial cell population in the conducting airways, but the proportion and number of cilia per cell decrease from the proximal to distal airways The acinus, which is approximately spherical in shape and has a diameter of about mm and a length of 0.5 to cm, is the gasexchange portion of the lung At the alveolar level, many changes occur in the postnatal period Although there is disparity concerning the time in which • • alveolarization is completed, alveoli in a normal adult number from 300 to 500 million and have a diameter of 150 to 200 mm The two epithelial cells of the alveolus are the gas-exchanging type I cell and type II cell, which are responsible for the production of pulmonary surfactant and have a central role in repair The alveolar-capillary unit is composed of three major constituents: the epithelial lining of the alveolus, capillary endothelial cells, and a mixture of cellular and extracellular interstitial components Following birth, the pulmonary vasculature undergoes extensive remodeling When fully matured, the thickness of the pulmonary artery is only about 60% that of the aorta The large pulmonary arteries traverse the lung with the cartilaginous airways and extend from the hilum nearly halfway down the bronchial tree Smaller pulmonary arteries measure between 100 and 1000 mm in diameter, branch with the bronchial tree, and lie close to bronchi and bronchioles Pulmonary veins not course with the bronchial tree; instead, they are seen within the interlobular septae Externally, the lungs are paired structures that, with the mediastinum, fill the thoracic cavity Normally, the right lung is composed of three lobes and the left lung consists of two lobes and the lingula, arising from the left upper lobe The lobes are separated by fissures and have hili that receive a primary lobar bronchus, pulmonary artery and veins, bronchial arteries and veins, lymphatics, and nerves.1,2 The lobes are further subdivided into 19 bronchopulmonary segments that receive primary segmental bronchi and a tertiary pulmonary artery branch and are drained by pulmonary veins Pulmonary veins not course with the airway and pulmonary artery; instead, they course midway between the dyads and can be readily identified in the intersegmental septa The connective tissue septa that demarcate each bronchopulmonary segment define the smallest surgically resectable portions of the lung The lung bud develops in the first month of the embryonic period, after which extensive branching progresses (20 generations) 463 CHAPTER 41 Structure and Development of the Lower Respiratory System forming the bronchial tree by the middle of the second trimester.3 The most rapid period of human lung development occurs between 22 weeks’ gestational age and term (40 weeks) This is during the saccular period, when alveolarization is initiated and accelerates, yielding approximately 20,000,000 to 50,000,000 alveoli at birth— only 6% to 16% of that in the full-grown adult lung.4 The beginning of the third trimester (22–27 weeks) is not only critical with respect to premature births but also because early development of fetal human lungs primarily occurs in the presence of fetal hemoglobin (a2g2) Later stages (,27 to ,38 weeks) of human fetal lung development (which include initiation of both pulmonary surfactant expression and accelerated alveolarization) occur during the main g to b globin transition—that is, from fetal to adult hemoglobin (a2b2).5–7 Fetal hemoglobin binds oxygen more efficiently than adult hemoglobin, yielding higher oxygen tensions in the lung tissue of developing human fetuses Therefore, treatment of premature infants with regard to oxygen delivery should consider potential risks associated with the oxygen-binding properties of the predominant hemoglobin form At birth, the lungs weigh about 40 g and double in weight by months Lung volume increases from about 250 mL at birth to 6000 mL in the adult.8 Mature respiratory alveoli appear at approximately 36 weeks of gestation and continue to develop until about years of age, with an approximate total surface area of 2.8 m2 By age years, most of the alveolarization process is complete, but newly formed alveolar septa still contain a double capillary network rather than the single one observed in adult lungs Therefore, over the next several years the capillary bed will reorganize well after alveolar formation After years of age, the lung enters the phase of natural growth.9 Airways In the lung, the airway undergoes multiple generations of branching, yielding a total of 23 to 28 divisions when counting begins from the primary bronchus The bronchi are the larger intrinsic cartilaginous airways, comprising to 12 generations, starting with the primary bronchus and terminating in bronchi with a diameter of approximately mm Bronchioles, sometimes called membranous bronchioles or distal noncartilaginous airways, are conducting airways They comprise an additional 12 generations before ending as terminal bronchioles, the last purely conducting structure in the lung Horsfield and Cumming showed that the course from the trachea to the alveolar level may comprise as few as or as many as 24 airway branch points.10 The first 16 generations of branching are genetically determined, while more distal branching and alveolarization are more plastic and are much more likely to be influenced by maternal nutrition and other extrinsic factors.11 For this reason, a particular airway diameter may be found at various points along the course of the airway Determination of the total cross-section of airways is important in understanding the distribution of airway resistance Weibel12 showed that as the peripheral generations of the airways are approached, the total cross-sectional area of the lung is markedly increased, suggesting that peripheral airways account for only a small proportion of total airway resistance In the adult, an asymmetric dichotomous branching pattern is seen, each daughter branch having a cross-sectional area about 75% of its parent branch This results in an increase of combined cross-sectional area of the two daughter branches It is well known, however, that peripheral airway resistance in children’s lungs is disproportionately high The size of the conducting airways is related to stature; thus, the airways’ cross-sectional area in children increases at a slow rate with growth and aging Because the peripheral airways make up a significant portion of the total respiratory resistance in children, disease in the bronchioles can be serious The bronchi maintain the histologic appearance of the trachea in that mucosa, submucosa, muscularis, adventitia, and cartilaginous support are present As the bronchi branch deeper into the lung parenchyma, the cartilage rings become plates and less regular, and the muscularis becomes continuous, being located between the submucosa and cartilage plates Also contained within the bronchial submucosa are mucus-secreting submucosal glands, nerves, ganglia, and bronchial arterial branches (Fig 41.1) As the bronchi decrease in diameter, the pseudostratified columnar epithelium becomes lower and the mucoserous glands become fewer in number Although the glands decrease in number in the more distal parts of the lung, mucous cells persist and can be found in very small bronchi and some membranous bronchioles The bronchial mucosa contains several epithelial cell types: ciliated, mucus-producing (goblet cells), basal, brush, and neuroendocrine.13,14 The ciliated cell constitutes more than 90% of the epithelial cell population in the conducting airways, but the proportion and number of cilia per cell decrease from the proximal to distal airways The microtubular structure within the cilia has been shown to be altered in the primary ciliary syndromes (Fig 41.2) In addition to its ciliary beating movement, the ciliated columnar cells regulate the depth of the composition of the periciliary fluid and transport ions across the epithelium The basal cell has a progenitor cell role and functions to maintain adherence of columnar cells to the basement membrane The brush cell, thought to have a role in fluid absorption and/or chemoreceptor function, is found rarely in the tracheobronchial and alveolar epithelia The mucociliary apparatus is the primary defense mechanism in the respiratory system Although mucous goblet cells secrete mucin, it is the submucosal glands that produce more than 90% of the mucus needed for mucociliary function The glandular unit of the bronchial submucosa comprises mucous, serous, myoepithelial cells, collecting duct cells, and, occasionally, neuroendocrine (Kulchitsky) cells.14,15 The physical characteristics of the mucous layer reveal that the superficial layer is more viscous than B M C • Fig 41.1 Bronchus (B) with surrounding smooth muscle (M) and cartilage (C) The airway mucosa (inset) is composed of ciliated epithelial cells and vacuolated goblet cells (arrowheads) (Gomori trichrome, 340 and 3400.) ... Elsevier; 2005:165-185 Acknowledgment The authors dedicate this chapter to the memory of Dr Linda Brodsky The full reference list for this chapter is available at ExpertConsult.com e1 Abstract:... mentioned previously, these airways are lined by pseudostratified, ciliated columnar epithelium This epithelium is readily injured through suctioning Prolonged intubation, particularly after tracheotomy,... should result in improved health outcomes and a lower morbidity rate in children with airway disease This chapter reviews relevant anatomy for the critical care provider as well as possible acquired