Development of the Respiratory System

After the fertilization of an oocyte by a spermatocyte, the developing human, similar to all other animals, undergoes a remarkable transformation from a single cell to an individual with a nearly complete set of organ systems. The developmental phases between fertilization and birth are generally divided into the embryonic and fetal periods. The embryonic period of human development occurs during the first 8 weeks and is traditionally organized into 23 stages, known as the Carnegie stages. During the embryonic period, all major organs begin their development. The fetal period occurs during the remaining 32 weeks of gestation. During this period, the organs continue to develop and refine their structure and function.

The respiratory system develops during these periods as a fluid-filled structure that plays no role in gas exchange yet must be developed sufficiently to assume this crucial activity at the time of birth. Its development is a continuous process that begins in the early stages of the embryonic period and extends for years after birth. A mass of cells forms between the yolk sac and amniotic cavity 17 days after fertilization (Carnegie stage 6). This mass is composed of three embryologically distinct germinal tissue layers that ultimately form all tissues and organs: endoderm, mesoderm, and ectoderm. The epithelium lining layer, which forms the mucous and gas exchange membranes, of the entire respiratory system arises from the endoderm, whereas the supporting structures of the tracheobronchial tree, including muscle and connective tissues, develop from the mesoderm that surrounds the developing lung bud. The nervous system of the respiratory tract forms from the cells of the ectoderm that grow within the mesoderm layer.

Based on cellular differentiation and tissue architecture, development of the respiratory system has been categorized into various stages.¹ Figure 8-1 shows the various stages of lung development, and Table 8-1 summarizes the major developmental events in each phase. Respiratory development begins in the embryonic period on or about day 22 after fertilization, when a small mass of cells, the respiratory primordium, begins to develop near the ventral region of the fourth pharyngeal arch of the primitive pharynx. This mass of cells forms a pouchlike bud, the respiratory diverticulum, on about day 26 (Carnegie stage 9) that continues to grow to form a laryngotracheal tube (Figure 8-2). The laryngotracheal tube forms from a groove in the fourth pharyngeal pouch. From the laryngotracheal tube, a tracheal bud forms by the end of the fourth week of life. The dorsal portion of the primitive foregut develops into the primordial esophagus and is separated from the tracheal bud by the formation of a tracheoesophageal septum. During week 5 of development, the tracheal bud continues to develop and bifurcates into left and right primary bronchial buds. The laryngeal structures develop at the superior end of the laryngotracheal bud.

The tracheal bud soon bifurcates into two main stem bronchial buds. The bronchial buds continue to grow and branch into secondary bronchi that form lobar, segmental, and subsegmental bronchi. As the bronchi form, plates of cartilage develop from the surrounding mesoderm to support these airways. During this same period, the vascular components of the respiratory system begin their development from the mesoderm. The pulmonary circulation and nervous system develop in parallel as the airways form. The pulmonary arteries form as buds off of the sixth pair of aortic arches, and primitive pulmonary veins emerge from the developing heart. Injury to the embryo or genetic dysregulation during this crucial phase of development can lead to many congenital anomalies, including tracheoesophageal fistulas, esophageal atresia, choanal atresia, pulmonary hypoplasia, and complex heart and vascular anomalies.

The developmental branching process of the airways and blood vessels of the lung is highly regulated by the timely activation of various genes in different locations. Of the approximate 22,000 genes in the human genome, about 40 are required for normal respiratory development.2–4 Table 8-2 lists many of these genes and the process in which they play an important role. The initial step in the development of the respiratory system is the localized expression of the NKX2-1 gene (also known as thyroid-specific transcription factor, TTF-1) in the anterior wall of the foregut, which stimulates the primary lung bud formation. Failure or mutation of the NKX2-1 gene can lead to failure of lung bud formation and various tracheoesophageal malformations.⁵ Lung bud elongation and the repetitive airway branching process is stimulated and directed by the highly choreographed expression of other key genes, including FGF10, FGFR2IIB, GATA-6, HNF-3, SPROUTY2, SHH, BMP-4, and NOGGIN as well as numerous other genes (see Table 8-2). Mutations of the FGF10 gene can result in tracheal development but fatal failure of further lung formation.⁶

At approximately 6 weeks of development, lung and airway growth has the appearance of a glandular structure—hence the name of the second phase of development, the pseudoglandular stage (Figure 8-3). For the next 10 weeks, the growth and branching of the tracheobronchial tree and pulmonary vasculature continue, under the direction of the various genes described earlier, and culminate with formation of the terminal and respiratory bronchioles. The distinction between these two types of bronchioles is important. Terminal bronchioles, similar to bronchi and the trachea, are conducting airways only and do not participate in gas exchange with blood. Respiratory bronchioles have much more superficial capillaries and are capable of gas exchange with blood and become more elaborate as development continues.

Branching and dividing of the tracheobronchial tree occur in several ways as the result of differential gene expression. A single bud that develops off of an existing structure is termed a monopodial bud. Airways that divide into two or more airways do so through dichotomous branching. Most of the divisions of airways occur in a nonsymmetric fashion termed irregular dichotomous branching.7,8 The epithelial lining of the airways begins to differentiate into columnar epithelia in the proximal airways and differentiates into cuboidal epithelium in the more distal bronchioles (Figure 8-4, A). Development of cilia, mucous glands, and goblet cells occurs at this time, and these are found lining most of the conducting airways.

Below the basement membrane of the epithelia, growth of smooth muscle cells, connective tissue, and blood vessels continues as the airways continue to branch. Mesoderm-derived cartilage provides rigidity, especially for the trachea and main stem bronchi. Beginning with the trachea and moving distally, the amount of cartilage supporting the airway decreases as smooth muscle cells, in the middle layer of the airway, increase in number. Altered development of smooth muscle, cartilage, and vascular structures can lead to other congenital pulmonary disorders, such as tracheomalacia, anomalous pulmonary arteries, and vascular rings that can grow around and pinch the airway.

The third phase of development is termed the canalicular stage (see Figure 8-4, B). It begins at week 16 and continues until week 26. The canalicular stage overlaps with the pseudoglandular stage because the superior regions of the lung are developing slightly faster than the inferior regions. During this phase, primary changes include the development of two to four more generations of respiratory bronchioles from each terminal bronchiole, the formation of blind tubular alveolar ducts from each respiratory bronchiole, and greater blood vessel development. In the last several weeks of this stage, the region beyond each terminal bronchiole forms the functional structure called the acinus, the basic gas-exchanging unit of the lung. At this time, the two principal epithelial cell types that cover the gas exchange surface begin to appear, type I and type II pneumocytes. At the end of the canalicular period (24 to 26 weeks of gestation), the fetus, if born, is capable of sufficient gas exchange and is viable if supported with supplemental O₂, ventilatory support, and surfactant administration.

During the fourth phase, the terminal saccular stage (see Figure 8-4, C), more terminal bronchioles and their associated acini form, and their structure continues to develop from 26 weeks to birth. The formation of the total number of terminal bronchioles is complete at the end of this phase.⁸ The cuboidal epithelia that line the blind tubules of the acinus continue to differentiate into rounded secretory cells (type II pneumocytes) and flatter squamous epithelial cells (type I pneumocytes). Mounting evidence shows that an important source of type I pneumocytes during both development and after lung injury are type II cells that can proliferate and differentiate.^9,10 Capillaries continue to form near and bulge from the surface of the acinus. Although some type II pneumocytes form by 20 weeks’ gestation, they are in such small numbers and of such primitive function that their impact on lung function is marginal. From this point until birth, there is rapid proliferation of alveolar ducts and sacs, formed from the respiratory bronchioles. The type I pneumocytes of the saccule walls thin and elongate to cover the walls of this region. Type I cells become the primary gas-exchange cells in the lung with close approximation to developing pulmonary capillaries. Type II pneumocytes form and secrete the vital pulmonary surfactants that are necessary to alter surface tension and help keep the lungs inflated.

The development of mature alveoli, accompanied by capillary proliferation within the walls, marks the final phase of lung development and is known as the alveolar period (see Figure 8-4, D). This phase begins at about week 32 of gestation and continues for years after birth. During this phase, the terminal saccules develop pouchlike regions called alveoli in their walls that are hexagonal in shape. The process of alveolarization occurs through the formation of crests along the immature airway wall, which develop further into septa that lengthen into the terminal saccule lumen; this effectively divides up the terminal airspace and results in greater numbers of alveoli that enlarge to a mature state with time.

A full-term newborn infant has about 50 million alveoli, and the number continues to increase for about 2 to 3 years after birth.11,12 The alveoli are lined with type I and II pneumocytes covering the pulmonary capillaries that have formed just below the basement membrane.

Human pulmonary surfactant, which promotes lung inflation and protects the alveolar surface, begins to be produced around 24 to 25 weeks of development by type II pneumocytes. It is composed primarily of phospholipids, a small amount of protein (types SP-A, SP-B, and SP-C), and a trace of carbohydrates.¹³ Early research in pulmonary surfactants centered on the phospholipid components, mainly phosphatidylcholine (lecithin [L] and sphingomyelin [S]) and phosphatidylglycerol (PG). Quantification of these phospholipids (the L/S ratio and PG concentration) provides a predictive index of the lung maturity in a fetus before birth and the risks of the development of respiratory distress.¹⁴ An L/S ratio of 2 or more indicates a relatively low risk for the development of respiratory distress syndrome, whereas an L/S ratio of less than 1.5 is associated with a high risk.

Surfactant synthesis is regulated by numerous hormones and factors, including glucocorticoids, prolactin, insulin, estrogens, androgens, thyroid hormones, and catecholamines.¹⁵ Glucorticosteroid production increases at the end of gestation and stimulates receptors in type II pneumocytes to increase surfactant production and improve the L/S ratio. Various key genes are also associated with normal surfactant production (surfactant protein genes A, B, C, and D; surfactant protein A, B, C, and D; and an adenosine triphosphate (ATP)–binding cassette transporter, ABCA3), and their failure, owing to mutation, is linked with the development of respiratory distress syndrome and other pulmonary disorders.¹⁶

A distinctive function of the developing lung is the formation of relatively large amounts of fetal lung fluid that is passed into amniotic fluid. Fetal lung fluid is a unique combination of plasma ultrafiltrate from the fetal pulmonary microcirculation, components of pulmonary surfactant, and other fluids from pulmonary epithelial cells.⁷ This fluid is constantly produced and keeps the fetal lung inflated at a slight positive pressure with respect to amniotic fluid pressure; it is important in stimulating normal lung development.¹⁷ At term, the fetal lung is filled with about 40 ml, and fluid is produced at a rate that results in replacing it multiple times per day. Conditions that lead to reduced fetal breathing and amniotic fluid formation (oligohydramnios) are linked to incomplete inflation of the lung with fluid and poorly developed (hypoplastic) lungs.

A developing fetus begins to make respiratory efforts midgestation and continues these efforts until birth. During these efforts, the fetus moves little or no fluid in and out of the lungs. The rhythm and depth of fetal breathing are periodic and irregular and reflect the development of the respiratory center in the brain and respiratory muscles.

Throughout the developmental period, lung growth is similar in male and female fetuses. There are differences, however. At birth, the lungs of male infants are, on average, larger and have a greater number of respiratory bronchioles than the lungs of female infants when adjusted for gestational age.¹⁸ When evaluating breathing efforts and surfactant production at 26 to 36 weeks of gestation, female fetuses have better developed lung function and are slightly less susceptible to the development of respiratory distress syndrome.^19,20

Transition from Uterine to Extrauterine Life

At birth, the lungs undergo a rapid and remarkable transition from being a liquid-filled organ that possesses very little circulation and is incapable of sufficient gas exchange to an air-filled organ that receives the entire cardiac output from the right heart and carries out all of the necessary gas exchange to sustain life.

Placental Structure and Function

Survival of the embryo and then fetus requires an effective circulatory interface with the circulation of the mother, which is provided by the placenta.²¹ Within 1 week of uterine implantation, vascular projections called chorionic villi arise from the aorta of the embryo and penetrate the uterine endometrium. As gestation proceeds, the villi increase in number and complexity; erode the endometrium; and create irregular pockets called intervillous spaces in the placenta, which fill with maternal blood. The maternal blood flowing through the intervillous spaces bathes the embryonic villi and creates an O₂-rich and nutrient-rich blood environment. As gestation progresses, the villi decrease in size but increase in number and complexity, resulting in an increased surface area that is essential for adequate maternal-fetal gas, nutrient, and waste exchange.

The maternal uterine tissues and blood vessels of the fetal chorionic villi make up the bulk of the placenta. Figure 8-5 shows a cross section of a well-developed placenta. Maternal blood flows into the intervillous space through the spiral arteries, whereas fetal blood is supplied to the villi from two umbilical arteries. Maternal and fetal blood come into close proximity but remain separated by an embryonic membrane that permits the exchange of O₂, CO₂, water, ions, various metabolic molecules, and hormones. Some maternal cells do move into fetal blood, and some fetal cells move into maternal blood and have been found in various maternal organs.

Various chemicals, hormones, bacteria, and viruses can also cross the intervillous space and cause a variety of fetal developmental problems. After exchange occurs with maternal blood, maternal blood exits through venous channels and returns to the maternal circulation. Oxygenated fetal blood leaves the chorionic villi capillaries through placental venules and returns to the fetus through a single umbilical vein. Abnormal implantation of the placenta, tearing of the placenta from the uterine wall, or decreased placental blood flow can retard intrauterine growth and in severe cases can cause fetal asphyxia and increases the risk for brain damage and respiratory distress in the immediate postnatal period.

Various factors enhance the delivery of O₂ to fetal tissues. The partial pressure gradient for O₂ between maternal blood and fetal blood drives the diffusion of O₂ into fetal blood within the chorionic villi capillaries.22,23 The maternal arterial blood has a partial pressure of O₂ (PaO₂) of approximately 100 mm Hg, which mixes with the blood in the intervillous space to produce a mean PO₂ of approximately 50 mm Hg. Fetal blood that enters the villi has a PO₂ of approximately 19 mm Hg, and the pressure gradient between maternal and fetal blood PO₂ (50 − 19 = 31 mm Hg) causes O₂ to diffuse into fetal blood. Blood leaving the villi and entering the umbilical vein has a PO₂ of approximately 30 mm Hg. Table 8-3 summarizes the normal gas and acid-base values in normal fetal umbilical arteries and veins and maternal intervillous blood. Assessment of umbilical vein blood gas data shortly after birth is a method of determining the degree of fetal asphyxiation during the birth process.

TABLE 8-3

Approximate Normal Values of Blood Gases and Acid-Base in Fetal and Maternal Blood

Value	Maternal Intervillous Blood	Fetal Umbilical Artery Blood	Fetal Umbilical Venous Blood
pH	7.38	7.36	7.39
PCO2 (mm Hg)	42	47	43
PO2 (mm Hg)	50	19	30

The O₂ content and delivery by fetal blood are almost the same as adult blood despite the much lower PO₂; this is due to several factors, including relatively higher content of hemoglobin (18 g/dl) and hematocrit (54%) in fetal blood and the presence of fetal hemoglobin (HbF), which has an increased affinity for O₂ and a more pronounced Bohr effect (reduced oxyhemoglobin affinity with acidosis) to enhance O₂ release.²³ Figure 8-6 illustrates how the increased O₂ affinity is manifested by a leftward shift of the fetal oxyhemoglobin dissociation curve. The P₅₀ (PO₂ that saturates 50% of the hemoglobin) is 6 to 8 mm Hg less than the P₅₀ for adult hemoglobin (HbA), which indicates the degree of the shift toward higher affinity. At birth, approximately 70% of circulating hemoglobin is HbF. HbA gradually replaces HbF during the first 6 months of extrauterine life as HbA genes in bone marrow switch on and HbF genes in the liver (major site of fetal erythrocyte development) are switched off.

Fetal Circulation

Fetal circulation is different than the circulation of the neonate after birth.²⁴ Three important bypass pathways function in the developing fetus to enhance the flow of blood to the developing organs: ductus venosus, ductus arteriosus, and foramen ovale. Oxygenated blood from the placenta is carried in the umbilical vein back to the fetal circulation via the hepatic circulatory system (Figure 8-7). Approximately one-third of this blood flows to the lower trunk and extremities. The other two-thirds flows through the ductus venosus, which bypasses the liver’s circulation and flows to the inferior vena cava. This better oxygenated blood in the inferior vena cava mixes with the venous blood returning from the lower trunk and extremities and enters the right atrium. Approximately 50% of this blood is shunted from the right atrium into the left atrium through an opening in the interatrial septum called the foramen ovale. Left atrial blood flows to the left ventricle and then to the ascending aorta, where it continues on to the brain, brachiocephalic trunk, and descending aorta. Venous blood from the superior vena cava is directed downward through the right atrium into the right ventricle and then into the main pulmonary artery.

The relatively low PO₂ and various prostaglandins in fetal blood cause the ductus arteriosus, a muscular vessel attached to the trunk of the pulmonary artery and the aorta, to dilate and the pulmonary arteries to constrict; this leads to increased pulmonary vascular resistance and higher pulmonary artery pressure than aortic blood pressure. As a result, 90% of the blood flow entering the pulmonary artery takes the path of least resistance by shunting through the ductus arteriosus and flows to the aorta. Only 10% flows into the lungs. Blood flowing through the ductus arteriosus mixes with the blood flowing through the aorta routed into the systemic circulation. Some of this blood flows to the gut, lower extremities, and placenta. Two umbilical arteries carry blood from the fetal aorta to the placenta to carry out fetal-maternal gas and nutrient exchange.

Cardiopulmonary Events at Birth

Various mechanisms work together to reduce and clear the amount of lung fluid at birth in preparation for air inflation.²⁵ Days before birth, the epithelia of the lung stop the production of lung fluid. The lung fluid is actively absorbed back into the fetal circulation. Most of the active lung water absorption is facilitated by active sodium channel activity that is stimulated by fetal and maternal thyroid hormones, glucocorticoids, and epinephrine and increasing fetal lung and blood O₂ content. In addition, some evidence suggests that the water channel aquaporin is also active in this process.²⁶ During normal vaginal delivery, approximately one-third of the lung fluid is cleared through compression of the thorax in the birth canal. The pulmonary capillaries and lymphatics clear the remaining fluid.

A newborn must develop very high transpulmonary pressure gradients during the first few breaths to open and replace the remaining lung fluid with air and establish a stable lung volume for gas exchange. These large pressure gradients overcome the opposing forces of fluid viscosity in the airways and surface tension in the alveoli. The stimulus for these initial respiratory efforts is apparently sent via peripheral and central chemoreceptors and augmented further by skin thermoreceptors.

The newborn infant is stimulated by new tactile and thermal stimuli, all of which stimulate breathing. In addition, as placental gas transfer is suddenly interrupted, the newborn quickly becomes hypoxemic, hypercapnic, and acidotic. This situation triggers strong inspiratory efforts (Figure 8-8). At first, no air enters the newborn lung until the transpulmonary pressure gradient exceeds 40 cm H₂O. As lung volume increases in a stepwise fashion with each breath, increasingly less pressure is needed to overcome the opposing forces. The volume trapped in the lung stabilizes quickly and is crucial to adequate gas exchange.

Figure 8-9 summarizes the major cardiopulmonary changes that occur during the transition from a fluid-filled lung to an air-filled lung. As the lung expands with air, and gas exchange starts within the lung, pulmonary blood PO₂ increases, PCO₂ decreases, and pH increases; this results in pulmonary vasodilation, lower pulmonary vascular resistance, and constriction of the ductus arteriosus, which facilitates greater blood flow through the pulmonary circulation. Ductus arteriosus closure is stimulated further by the loss of maternal prostaglandins. The combination of increasing alveolar air content and constriction of the ductus arteriosus promotes progressive improvement in the matching of ventilation and blood flow, which increases the PO₂ and decreases the PCO₂ of blood leaving the lungs. After the clamping of the umbilical cord, cessation of umbilical and placental blood flow causes closure of the ductus venosus and a rapid increase in systemic vascular resistance. As systemic vascular resistance increases, left-sided heart pressures increase. Left atrial pressures also increase as a result of increased pulmonary blood flow that returns from the lungs. With left-sided heart pressures now higher than right-sided pressures, the foramen ovale closes.

When this last right-to-left shunt closes, the transition between fetal and extrauterine circulations is functionally complete. Full transition occurs later as the ductus arteriosus and foramen ovale close anatomically through the formation of fibrosis. Anatomic closure of the ductus normally occurs within 3 weeks of birth. Permanent closure of the tissue flap covering the foramen ovale may take several months.

All of these changes normally occur during the first few minutes after birth and allow the newborn to achieve normal gas exchange. Many abnormal conditions can interfere with these transition events and can lead to persistence of the fetal circulation and cardiorespiratory failure.

The infant lung is a unique structure and not a mere miniaturization of the adult lung. The airways, distal lung tissue, and pulmonary capillary bed all continue to grow and develop after birth. Although the general pattern is well developed at birth, both the upper and the lower airways continue to change and are relatively unique in each person.

Figure 8-10 shows the relative differences of the upper airway in relation to body size in an infant and an adult. The greater relative weight of the head can cause acute flexion of the cervical spine in infants with poor muscle tone. Infant neck flexion causes acute airway obstruction. Although the head is larger, an infant’s nasal passages are proportionately smaller than those of an adult. In addition, the infant’s jaw is much rounder, and the tongue is much larger relative to the size of the oral cavity.²⁷ These anatomic differences increase the likelihood of airway obstruction when an infant becomes unconscious and loses muscle tone.

Most infants breathe through the nose. However, most term newborn infants shift to oral breathing in response to nasal occlusion and hypoxia.²⁸ As normal infants mature, they begin to use the oral breathing route more and are more capable of shifting to oral breathing when nasal obstruction is present.²⁹ At approximately 4 to 5 months of age, most infants are capable of full oral ventilation.

A newborn’s larynx lies higher in the neck compared with the larynx of an adult, with the glottis located between C3 and C4, and is more funnel-shaped than that of an adult. In a child, the narrowest region of the upper airway is through the cricoid cartilage, rather than the glottis, as it is in adults. The epiglottis of an infant is longer and less flexible than the epiglottis of an adult and lies higher and in a more horizontal position. During swallowing, the infant’s larynx provides a direct connection to the nasopharynx. This connection creates two nearly separate pathways, one for breathing and one for swallowing, allowing infants to breathe and suckle at the same time. Anatomic descent of the epiglottis begins at to 3 months of age. Mechanical and chemical irritant laryngeal reflexes develop at birth and can initiate protective laryngeal closure; these reflexes can trigger prolonged apnea in some and may be a cause of sudden infant death syndrome.³⁰ In addition, infections in this area or repeated attempts at intubation or suctioning can easily cause swelling and obstruction of this area.

The large conducting airways of infants are shorter and narrower than the airways of adults. The normal newborn trachea is approximately 5 to 6 cm long and 4 mm in diameter, whereas in small preterm infants, it may be only 2 cm long and 2 to 3 mm wide. Because of the smaller airways, a newborn’s anatomic dead space is proportionately smaller than the anatomic dead space of an adult, being approximately 1.5 ml/kg of body weight. Figure 8-11 compares the tracheal anatomy in an adult and a newborn. The main stem bronchi branch off from the trachea in the infant at less acute angles than in the adult. However, similar to adults, the right main stem bronchus of the infant is still more in line with the trachea, which promotes right main stem intubation when airways or suction catheters are inserted to deeply. Mean airway diameter, from main bronchi to respiratory bronchioles, increases about two to three times from birth to adulthood.³¹

Smooth muscle is present in the airways of a neonate down to the level of the respiratory bronchioles and continues to increase until the infant is approximately 8 months old. After this age, smooth muscle proliferation occurs primarily in the proximal airways, whereas chondrocytes producing cartilage predominate in the proximal airways. Distinct C-shaped rings of cartilage are found in the trachea and main stem bronchi of the neonate. The amount of cartilage progressively decreases in the more distal bronchi and eventually disappears in airways smaller than 2 mm in diameter.

Despite the presence of cartilage in the central airways of an infant, the trachea and larger bronchi of a neonate lack the rigidity of adult central airways. The compliant nature of these airways makes them prone to compression and collapse.

Chest Wall Development, Diaphragm, and Lung Volume

The thoracic wall in infants is more compliant, and their muscles are less developed than the muscles of adults and provide little structural support. The infant thoracic cage is also more boxlike, with the ribs being horizontally oriented or elevated (Figure 8-12). In addition, the diaphragm inserts into the thoracic cage in a horizontal plane, which decreases the effective ability to enlarge the thorax.

As an infant inhales, the diaphragm moves down, but the flexible chest wall moves very little in the anteroposterior dimension as the chest wall muscles attempt to pull it upward and outward. Compounding this situation is a proportionately larger abdominal visceral content that restricts the vertical motion of the diaphragm. The ribs take on a progressively downward slope as a child grows, and by 10 years of age, the rib cage has the configuration seen in adults. Ossification of the ribs and sternum is normally complete by 25 years of age, and this, combined with muscular development, results in a stiffer chest wall that moves more in the anteroposterior dimension with inspiratory effort.

With a more compliant thorax, the resultant balance of these static forces in an infant favors a reduced lung volume. Proportionately lower lung volumes in an infant can lead to early airway closure, widespread alveolar collapse (atelectasis), ventilation/perfusion () mismatch, and resultant hypoxemia. The combination of a reduced lung volume and high O₂ consumption in an infant renders the infant more susceptible to profound hypoxemia in situations that disturb ventilation, lung volume, or matching further. Infants possess a remarkable ability to elevate their lung volume dynamically. Infants, especially infants in distress, can actively increase lung volume by trapping gas, which improves matching and gas exchange. Infants accomplish gas trapping actively by using the diaphragm during exhalation to slow expiration and to adduct (close) the vocal cords and narrow the glottis. The combination of these two maneuvers effectively regulates volume in the lung and dynamically elevates lung volume. The narrowing of the glottis or larynx during exhalation is referred to as “laryngeal braking.” Infants in respiratory distress commonly grunt, a manifestation of laryngeal braking. A more compliant chest wall contributes to suprasternal, substernal, intercostal, and subcostal retractions in distressed infants and young children (see Mini Clini).

Mini Clini

Significance of Thoracic Soft Tissue Retractions

Supraclavicular and intercostal retractions are inward movements of the soft tissues above the clavicle and between the ribs of the chest wall during inspiration. This inward movement causes the clavicle and ribs to stand out prominently during inspiratory efforts.

 Problem

Why do infants and adults in respiratory distress with severe airway obstruction or reduced compliant (“stiff”) lungs exhibit thoracic soft tissue retractions?

Answer

The pressure within the intrapleural space is normally slightly negative (e.g., −5 cm H₂O) as a result of the tendency of the lung to recoil inward and the rib cage to recoil outward. The intrapleural space pressure becomes more negative (e.g., −8 cm H₂O) during inspiration as the respiratory muscles enlarge the chest, the diaphragm descends, and the intrathoracic volume increases. During conditions that cause severely obstructed airways (e.g., partial upper airway obstruction from epiglottitis) or reduced lung compliance and stiff lungs (e.g., viral pneumonia and pulmonary edema), much greater inspiratory effort is required because of the high resistance to airflow or stiffer lung condition. This increased effort translates into a much greater decrease in intrathoracic and pleural pressures (e.g., −40 cm H₂O). This greater decrease in intrathoracic and pleural pressure “sucks” the soft tissues inward and causes soft tissue retractions. Thoracic soft tissue retractions signal greatly increased work of breathing.

Thoracic shape and dimension vary from individual to individual and are linked to age, gender, and race. At birth, the thorax has a smaller transverse dimension, which widens with the onset of walking. Thoracic size and volume continue to increase throughout childhood and especially during the adolescent growth spurt. However, development of the thorax and lung volume is not equal in both sexes. When evaluating lung size and volume throughout puberty and into adulthood, boys and men are consistently found to have larger lungs than age-matched and height-matched girls and women.⁴⁰ Some races have a proportionately larger thorax-to-height ratio than others. In females, the location of the nipple varies with the size and shape of the breast. In males, the nipple is usually located in the midclavicular line at the level of the fourth intercostal space.

Imaginary lines are commonly used to establish reference points and identify landmarks on the thorax. These lines and points help identify the location of underlying structures and the location of abnormal findings. On the anterior chest, the midsternal line divides the thorax into equal halves. The left and right midclavicular lines are parallel to the midsternal line. These are drawn through the midpoints of the left and right clavicles (Figure 8-13). The midaxillary line divides the lateral chest into equal halves. The anterior axillary line is parallel to the midaxillary line. It is situated along the anterolateral chest. The posterior axillary line is also parallel to the midaxillary line. It is located on the posterolateral chest wall (Figure 8-14). Three imaginary vertical lines are located on the posterior thorax. The midspinal line divides the posterior chest into two equal halves. The left and right midscapular lines are parallel to the midspinal line. They pass through the inferior angles of the scapulae in a relaxed upright subject (Figure 8-15).

Components of the Thoracic Wall

The thoracic cavity is formed by the tissues of the chest, upper back, and diaphragm.⁴¹ It is a cone-shaped cavity that houses the lungs and the contents of the mediastinum (Figure 8-16). It functions to protect the vital organs within and is capable of changing shape to enable air to be moved into and out of the lungs. The thoracic cavity is formed from epithelial, connective, and muscle tissues.

The various parts of the thoracic wall are shown in Figure 8-17. The outer covering of the thorax is formed by the integumentary system, which includes skin, hair, subcutaneous fat, and breast tissues. Skin is a composite of an outer epidermis and an inner connective tissue layer called the dermis. Below the dermis is a layer of subcutaneous fat. Skeletal muscle, encased in a layer of connective tissue called fascia, is found under the subcutaneous fat. Skeletal muscle tissue forms the various muscles of the chest and back and lies over and between the ribs. The ribs of the rib cage lie in the inner portion of the thoracic wall. The inner layer of the thoracic wall is lined with a serous membrane called the parietal pleura. It is apposed by another serous membrane called the visceral pleura, which covers the lung. A thin, fluid-filled pleural space forms between the parietal and visceral pleural membranes.

The rigidity of the thorax is provided by the bone tissue of the rib cage. The bony parts of the rib cage include the sternum, ribs, thoracic vertebral bones, scapula, and clavicle (Figure 8-18). The sternum is a long, vertical flat bone found on the anterior side that is composed of three bones: the manubrium, the body (or gladiolus), and the xiphoid process. The superior edge of the manubrium forms a shallow depression that is known as the suprasternal (or jugular) notch. The fused connection between the manubrium and the body is known as the sternal angle; it is also known as the angle of Louis. The sternal angle is an external marker of the point where the trachea divides into the left and right main stem bronchi. A cartilaginous joint called the costal cartilage is on the lateral edges of the manubrium and sternal body and forms the attachment between the ribs and sternum. This joint allows the rib cage to bend and permits the thorax to increase and decrease in size.

 Rule of Thumb

Where the manubrium and body of the sternum meet, the anterior chest wall shows a slight depression that forms an oblique angle (when viewed from the side). This depression is referred to as the angle of Louis. Beneath this important landmark, the trachea divides into the right and left main stem bronchi.

The rib cage is formed by 12 pairs of ribs.⁴¹ Rib pairs 1 through 7 are known as the true ribs because they are attached directly to the sternum. The first ribs and the upper sternum form the opening into the thorax that is called the thoracic inlet, or operculum. Ribs 8 through 12 are called false ribs because they are either indirectly attached to the sternum or not attached at all. The vertebrochondral ribs include rib pairs 8, 9, and 10, which are indirectly attached to the sternum through a common cartilaginous strap. Rib pairs 11 and 12 are called floating ribs because they are not attached to the sternum. Each rib has a sternal end; a long, curved, and relatively flat body; and a head that articulates with the thoracic vertebrae (Figure 8-19). Intercostal muscles lie between the ribs and hold them together. Just below each rib is a thoracic artery, vein, and nerve that supply blood flow and nerve communications to that region of the chest wall (see Figure 8-17).

 Rule of Thumb

Numerous procedures require entry into the pleural cavity, such as thoracentesis to drain fluid or pus and placement of chest tubes to treat pneumothorax. Incisions or punctures made during such procedures are always done immediately above a selected rib. The intercostal nerves, veins, and arteries all lie in a groove below each rib. To avoid these structures, needles and tubes are placed above a rib.

The upper and lateral regions of the thorax house the bones of the pectoral girdles. The pectoral girdle on each side is formed by the clavicle and scapula.⁴¹ The scapula forms the socket for the shoulder joint and is stabilized or moved by skeletal muscles of the upper back. The clavicle supports and stabilizes the shoulder joint through a flexible attachment to the manubrium of the sternum.

Rib Movement

The various ribs move in different ways, and some may move more than others at different times. The first rib moves slightly, raising and lowering the sternum. Its slight motion increases the anteroposterior diameter of the chest. This action is not used during quiet breathing and becomes active only under conditions that require increased ventilation or deep breathing. Ribs 2 through 7 move simultaneously about two axes (Figure 8-20). As each rib rotates about the axis of its neck, its sternal end rises and falls. This movement increases the anteroposterior thoracic diameter in what is commonly referred to as a “pump handle”–like motion. At the same time, the rib moves about its long axis from its angle at the sternum. This motion causes the middle part of the rib to move up and down in what is commonly described as a “bucket handle.” The compound action of ribs 2 through 7 changes both the anteroposterior and the transverse dimensions in an upward and outward motion. Ribs 8 through 10 rotate in a pattern similar to that of ribs 2 through 7. However, elevation of the anterior ends of these ribs produces a small backward movement of the lower sternum that slightly reduces the thoracic anteroposterior diameter. Outward rotation of the middle section of these ribs increases the transverse diameter of the thorax. Ribs 11 and 12 participate in changing the contour of the chest in a minor way as they are pulled upward and outward in a “caliper”-like motion.

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