Elizabeth Flasch, Nicole Brueck, Justin Lynn, and Jennifer Henningfeld
DEVELOPMENTAL ANATOMY OF THE PULMONARY SYSTEM
A. Embryology of the Lung
In humans, there are five well-recognized stages of lung development: embryonic, pseudoglandular, canalicular, saccular, and alveolar (Figure 2.1).
1. Embryonic Stage. Day 26 to 52—In primitive development, the foregut forms a lung bud from the pharynx 26 days after conception. This lung bud elongates and forms the trachea and two bronchial buds, which then separate from the esophagus. By the end of the embryonic period, the larger airways, including the trachea, main stem, and segmental and subsegmental bronchi are developed. During this phase, the respiratory endothelium and diaphragm develop (Schnapf & Kirley, 2010).
2. Pseudoglandular Stage. Day 52 to week 16—During this stage, branching morphogenesis continues, leading to the formation of subsegmental bronchi, bronchioles, and primitive acinar tubules. Cilia appear on the surface of the epithelium of the trachea and the main stem bronchi at 10 weeks of gestation and are present on peripheral airways by 13 weeks gestation. Lymphatics appear in the lung itself by week 10. Goblet cells appear in the bronchial epithelium at 13 to 14 weeks gestation (Schnapf & Kirley, 2010).
3. Canalicular Stage. Week 17 to 26—This stage is so named because of the appearance of vascular channels, or capillaries, which begin to grow by forming a capillary network around the air passages. By the end of this period, pulmonary acinar units are formed, consisting of a respiratory bronchiole, alveolar ducts, and alveolar sacs. By 20 to 22 weeks of gestation, type I and type II epithelial cells can be differentiated. By the end of this period, the development of the air–blood barrier is thin enough to support gas exchange. These developments, along with increasing synthesis of surfactant, are critical to the extrauterine survival of the fetus. At 22 to 24 weeks gestation, the survival of the fetus becomes possible during this stage (Schnapf & Kirley, 2010).
4. Saccular Stage. Week 26 to 36—During this period, there is a marked increase in the potential gas-exchanging surface area due to thinning of the epithelium and mesenchyme and further formation of the capillary net. The terminal structures are referred to as saccules and are relatively smooth-walled cylindrical structures. They then become subdivided by ridges known as secondary crests. As the crests protrude into the saccules, part of the capillary net is drawn with them, forming a double capillary layer. Further septation between the crests results in smaller spaces termed subsaccules (Schnapf & Kirley, 2010).
5. Alveolar Stage. Week 36 and beyond—Alveolarization is rapidly progressing during this period. Alveologenesis is characterized by a complex interaction of epithelial, fibroblast, and vascular growth factors with extracellular matrix components (Schnapf & Kirley, 2010).
6. The First Breath. The thorax is compressed as it passes through the birth canal, forcing out some of the fetal lung fluid. Chest recoil after the thorax is delivered results in air entry into the lungs. The first inspiratory effort must be large enough to overcome the viscous resistance to movement of the intrapulmonary liquid and overcome the tissue and surface tension. For the first several minutes to 2 hours, expiration is often incomplete, resulting in progressively increasing functional residual capacity (FRC). For infants born by cesarean section, it takes longer to establish FRC (Schnapf & Kirley, 2010).
B. Postnatal Lung Development
Normal lung growth is a continuous process that begins early in gestation and extends through infancy and childhood. Estimates of alveolar number at birth vary greatly but the general accepted number is 50 million. Eventually 300 million alveolar will form after birth. Lung volume will increase 23-fold, alveolar number will increase sixfold, alveolar surface area will increase 21-fold, and lung weight will increase 20-fold. Alveolar development is thought to continue through early childhood with implications for recovery of lung function after certain childhood insults (Schnapf & Kirley, 2010).
C. Upper Airway Development
The upper airway is responsible for warming, humidifying, and filtering air before it reaches the trachea. There are notable differences between pediatric and adult airways (Table 2.1).
a. Embryology. Nasal cavities begin as widely separated pits on the face of the 4-week-old embryo. The ethmoid and the maxillary sinuses form in the third to fourth gestational month and, accordingly, are present at birth. The sphenoid sinuses are generally pneumatized by 5 years of age; the frontal sinuses appear at age 7 to 8 years but are not completely developed until late adolescence. The ethmoid cells are present and increase in size throughout life. Frontal and sphenoid sinuses do not begin to invade the frontal or sphenoid bones until several years after birth (Walsh & Vehse, 2010).
b. Until the age of 6 months, infants are obligatory nose breathers because the elongated epiglottis, positioned high in the pharynx, almost meets the soft palate. However, they are still able to mouth breathe because blocked nares do not lead to complete upper airway obstruction. By the sixth month, growth and descent of the larynx reduce the amount of obstruction. Nasal breathing doubles the resistance to airflow and proportionately increases the work of breathing (WOB; Walsh & Vehse, 2010).
a. Embryology. The oropharyngeal membrane between the foregut and the stomodeum begins 37to disintegrate to establish continuity between the oral cavity and the pharynx in the 4-week-old embryo (Walsh & Vehse, 2010).
Pediatric Anatomic Difference
Proportionally larger head
Increases neck flexion and obstruction
Increases airway resistance
Increases airway resistance
Decreased muscle tone
Increases airway obstruction
Longer and more horizontal epiglottis
Increases airway obstruction
More anterior larynx
Difficult to perform blind intubation
Cricoid ring is the narrowest portion
Inflated cuffed tubes not recommended for routine intubation in children younger than 8 years of age
Increases risk of right main stem intubation
Increases airway resistance
b. The pharynx is a musculomembranous tube that extends from the base of the skull to the esophageal and laryngeal inlets. The pharynx is the conduit for inhalation and exhalation and is vital to the production of speech. The nasopharynx is located above the soft palate. The oropharynx extends from the soft palate to the level of the hyoid bone. The hypopharynx extends from the level of the hyoid bone to the esophageal inlet (Walsh & Vehse, 2010).
a. Embryology. During the fourth week of embryologic life, the laryngotracheal groove begins as a ridge on the ventral portion of the pharynx. Vocal cords begin to appear in the eighth week. In the newborn infant, the larynx is approximately at the level of the second cervical vertebra. In the adult, the larynx is opposite the fifth and sixth cervical vertebrae (Walsh & Vehse, 2010).
b. The larynx is a funnel-shaped structure that connects the pharynx and trachea. It includes the thyroid cartilage, vocal cords, epiglottis, and the cricoid cartilage. It is important in the production of the cough and protects the airway from aspiration of food during deglutition (Walsh & Vehse, 2010).
c. Compared with the adult epiglottis, the child’s epiglottis is longer and more flaccid. The epiglottis in a newborn extends over the larynx at approximately a 45-degree angle. This more anterior and cephalad epiglottis may make intubation of the airway more difficult in the small infant (Walsh & Vehse, 2010).
d. The cricoid cartilage ring is the only point in the larynx in which the walls are completely enclosed in a circumferential ring of cartilage. In the rest of the trachea, the incomplete C-shaped cartilaginous rings are located anteriorly and laterally and the posterior wall is membranous. Resistance to airflow is inversely proportional to the fourth power of the radius. Thus, swelling from trauma or infection can produce large increases in airway resistance (Walsh & Vehse, 2010).
e. Vocal cords must abduct to allow inhalation and exhalation and adduct to prevent aspiration (Walsh & Vehse, 2010).
a. Embryology. The trachea begins to develop in the 24-day-old embryo. At 26 to 28 days, a series of asymmetric branchings of the primitive lung bud initiate the development of the bronchial tree (Walsh & Vehse, 2010).
b. The trachea is a thin-walled rigid tube flattened posteriorly that is characterized by a framework of 16 to 20 cartilages that encircle the trachea, except in its posterior aspect, which is membranous and contains smooth muscle.
c. The trachea’s nervous, vascular, and lymphatic supplies are independent of those in the lungs.
a. At birth, the lungs weigh about 40 g and double in weight by 6 months. By age 2 years, they weigh about 170 g. Normal adult lungs weigh approximately 1,000 g.
b. The right lung has three lobes, and the left has two lobes separated by divisions called fissures, each further subdivided into bronchopulmonary segments. The surface of the lung is covered with the visceral pleura.
2. Conducting Zone
a. Intrapulmonary airways are divided into three major groups: cartilaginous bronchi, membranous bronchioles, and gas exchange units.
i. Cartilaginous bronchi are the large airways that include nine to 12 divisions terminating in bronchi having a diameter of approximately 1 mm.
ii. Membranous bronchioles comprise an additional 12 divisions before ending as terminal bronchioles, the last conducting structure in the lung.
b. The airways are lined with an epithelial membrane, which gradually changes from ciliated pseudostratified columnar epithelium in the bronchi to a ciliated cuboidal epithelium near the gas exchange units.
c. In the largest airways, a smooth muscle bundle connects the two ends of the C-shaped cartilage. As the amount of cartilage decreases, the smooth muscle assumes a helical orientation and gradually becomes thinner.
3. Gas Exchange Units (Alveoli)
a. Alveoli are complex networks in which gas exchange takes place. Alveoli are lined with two epithelial cell types. Type I cells cover about 90% of the total alveolar surface. These cells are adapted to allow for the rapid exchange of gases. Type II cells constitute the other 10% and secrete surfactant material that lowers surface tension and maintains the patency of alveoli during respiration. Alveoli patency refers to the alveoli’s ability to maintain a spherical shape, preventing collapse, which is necessary for efficient gas exchange.
b. Two types of intercommunicating channels provide collateral ventilation for the gas exchange units. Alveolar pores of Kohn are holes in the alveolar wall that provide channels for gas movement between alveoli. These pores are not present until 6 to 8 years of age. While the pores of Kohn connect alveoli to adjacent alveoli, the canals of Lambert are accessory channels in the lungs connecting some terminal bronchioles to adjacent alveoli.
c. Gas exchange involves the movement of gas between the atmosphere and the alveoli and the pulmonary capillary blood. This movement occurs via simple passive diffusion whereby the gases travel from an area of high pressure to an area of low partial pressure.
E. Thoracic Cavity
a. The diaphragm is the principal muscle of inspiration. If the chest wall is stiff, contraction of the diaphragm during inspiration decreases the pressure within the thoracic cavity and increases thoracic volume. This negative intrathoracic pressure in the chest cavity is discussed in detail later in this chapter. The diaphragm is innervated by the phrenic nerve (third, fourth, and fifth cervical spinal nerves).
b. The diaphragm sits more horizontally in the infant than in the older child or adult.
2. The Chest Wall. The infant’s chest wall is very compliant compared with the rigid chest wall of the older child and adult. In the presence of lung disease, contraction of the diaphragm results in intercostal and sternal retractions rather than in inflation of the lungs. These retractions occur because the intercostal muscles are not strong enough to stabilize the chest against the stronger diaphragm contraction.
F. Pulmonary Circulation
The development of pulmonary circulation closely follows the development of the airways and alveoli.
1. Embryology. Preacinar arteries, which branch along the airways, develop in utero. Muscular arteries end at the level of the terminal bronchiole in the fetus and 39newborn but gradually extend to the alveolar level during childhood. Prematurely born infants have less well-developed vascular smooth muscle.
2. Pulmonary Blood Volume. The lungs receive the entire cardiac output from the right ventricle (RV) if no intracardiac right-to-left shunts are present.
3. Pulmonary Lymphatics. The lymphatic system is composed of a superficial network found in the pleura and the deep network around the bronchi and pulmonary arteries and veins. An increase in the hydrostatic pressure of the pulmonary and systemic circulation can result in effusions by decreasing the rate of pleural fluid absorption. Lymphatics may be disrupted by thoracic surgery or trauma, leading to lymphatic effusions.
DEVELOPMENTAL PHYSIOLOGY OF THE PULMONARY SYSTEM
A. Physiologic Function
The primary function of the lung is gas exchange. Its prime function is to allow oxygen to move from the air into the venous blood and for carbon dioxide to move out. Other functions include metabolizing certain compounds, filtering the blood, and acting as a reservoir for blood. During inspiration, the diaphragm contracts, the chest wall expands, and the volume of the lungs increases. Gas flows from the atmosphere into the lungs and oxygen diffuses into the blood at the alveolar–capillary interface. During expiration, the diaphragm and the chest wall relax, thoracic volume decreases, intrathoracic pressure increases, and gas flows out of the lungs. This process is affected by pulmonary compliance and resistance and by pulmonary vascular pressures and resistance (West, 2012).
1. Pulmonary Compliance and Resistance
i. Volume change is produced by a transpulmonary pressure change (CL = ΔV/ΔP). For example, if the volume change produced by a given pressure change is small, the lungs must be stiff or have decreased compliance (West, 2012).
ii. Compliance of the infant chest wall (especially in premature infants) is considerably greater than that of the adult. There is less opposition to lung collapse. Compliance is decreased by pulmonary edema, pneumothorax, pulmonary fibrosis, and atelectasis. Compliance is increased by asthma, lobar emphysema, and in the normal aging lung (West, 2012).
b. Airway resistance is the driving pressure of air divided by the airflow rate determined by airway diameter. It is directly proportional to flow rate, the length of the airway, and the viscosity of the gas and is inversely proportional to the fourth power of the airway radius (Poiseuille’s law; West, 2012).
i. The upper airway contributes 70% of the total airway resistance in adults and 50% of total airway resistance in infants. In infants, the small peripheral airways may contribute as much as 50% of the total airway resistance compared with less than 20% airway resistance for the adult (West, 2012).
ii. Resistance is increased by asthma, cystic fibrosis (CF), bronchopulmonary dysplasia (BPD), bronchiolitis, tracheal stenosis, and increased respiratory secretions. High resistance increases the WOB and creates respiratory distress.
2. Pulmonary Vascular Pressures and Resistance
a. Changes in pulmonary circulation at birth. The fetus has low pulmonary blood flow related to high pulmonary vascular resistance (PVR). 40This high PVR is due to hypoxic vasoconstriction, thicker pulmonary musculature, relatively low lung volumes, and smaller surface area. At birth, there is a decrease in PVR associated with ventilation and the effect of oxygen. This drop in PVR after birth increases blood flow to the lungs, thus facilitating the transition from fetal circulation. In the 6 to 8 weeks following birth, there is a further progressive fall in resistance associated with thinning of the smooth muscle layer (West, 2012).
b. Intravascular pulmonary pressure is measured by placing a catheter in the pulmonary artery and measuring systolic, diastolic, and mean arterial pressures (MAPs). The pressures in the pulmonary circulation are remarkably low. The mean pressure in the main pulmonary artery is about 15 mmHg; the systolic and diastolic pressures are about 25 and 8 mmHg, respectively (West, 2012).
c. Ventilation–perfusion matching. Regional differences in lung perfusion exist. Blood flow is lightest at the apex and increases at the base of the lung in an upright position (Figure 2.5; West, 2012).
i. Zone I is located in the apexes of the lungs of an upright adult. Mean pulmonary arterial pressure is less than alveolar pressure (West, 2012).
ii. Zone II is located in the midlung field. Here pulmonary artery pressure is greater than alveolar pressure, which is greater than pulmonary venous pressure (West, 2012).
iii. Zone III is found in the base of the lung of an upright adult. Here the pulmonary artery and venous pressures are greater than alveolar pressure (West, 2012).
d. PVR can be calculated by dividing the pressure across the lungs by the pulmonary blood flow. A decrease in resistance to blood flow can occur only through an increase in the blood vessels’ diameters or an increase in the number of perfused vessels, that is, an increase in the cross-sectional diameter of the pulmonary vascular bed. An increase in cardiac output decreases the calculated PVR. The interrelationship between lung volume and PVR is complex and is influenced by pulmonary blood volume, cardiac output, and initial lung volume. Active changes in PVR can be mediated by neurogenic stimuli, vasoactive compounds, or chemical mediators (West, 2012). The pulmonary vascular resistance can be calculated as
This equation is conceptually equivalent to the following:
R = ΔP/Q
where R is the PVR, ΔP is the pressure difference across the pulmonary circuit, and Q is the rate of blood flow through it.
3. Control of Breathing
a. Central respiratory centers. The medullary respiratory center is essential for the generation of the respiratory rhythm. In addition, the medulla is associated with inspiration and expiration. The pons contains the apneustic center and pneumotaxic center responsible for “fine tuning” the respiratory rhythm.
b. Cortex. Within limits, the cortex can override the function of the brainstem.
c. Other parts of the brain. The limbic system and hypothalamus can alter the pattern of breathing during emotional states (including rage and fear).
d. Peripheral neural reflexes. Respiratory mechanoreceptors that can affect respiration are located within the upper airways, trachea, and lungs. Impulses are transmitted to the brainstem respiratory centers via the vagus nerve.
e. Chemical control of respiration (Figure 2.6)
i. Central chemoreceptors: The central chemoreceptors are surrounded by brain extracellular fluid and respond to changes in its H+ ion concentration. An increase in H+ concentration stimulates ventilation, whereas a decrease inhibits it. The composition of the extracellular fluid around the receptors is governed by the cerebrospinal fluid (CSF), local blood flow, and local metabolism. Of these, the CSF is the most important.
ii. Peripheral chemoreceptors: These are located in the carotid bodies at the bifurcation of the common carotid arteries and in the aortic bodies above and below the aortic arch. The carotid bodies are the most important in humans.
4. Mechanics of Breathing. Elastic properties of the lung come from the elastic tissue and collagen that support the lungs’ internal structures. Lung compliance changes with age. The thorax of the infant is much more compliant than that of an adult.
a. Total lung capacity is the total volume of the gas contained in the lung at maximum inspiration.
b. Vital capacity (VC) is the maximum volume expired from the total lung capacity with maximal expiration.
c. FRC is the volume of gas remaining in the lung after a normal expiration.
d. Residual volume is the volume of gas remaining in the lung following a maximal respiratory effort.
B. Gas Exchange and Transport
Respiratory gas exchange involves the movement of gas from the atmosphere to the alveoli to the pulmonary capillary blood. The alveolar capillary membrane permits the transfer of oxygen and carbon dioxide while restricting the movement of fluid from the pulmonary vasculature to the alveoli.
a. Oxygen diffuses from the alveolus through the alveolar epithelial lining, basement membrane, capillary endothelial lining, plasma, and red blood cell. Blood passing through the lung resides in a pulmonary capillary for only 0.75 second. Diffusion of oxygen depends on the difference (gradient) between alveolar and oxygen tension (West, 2012).
b. Carbon dioxide diffuses from the red blood cell to the plasma, through the capillary endothelial lining, basement membrane, and alveolar epithelial lining. The pulmonary capillary mean alveolar carbon dioxide gradient is smaller than that of oxygen (i.e., CO2 diffuses more readily than O2; West, 2012).
2. Oxygen Transport
a. Oxygen is carried in the blood in two forms: in combination with hemoglobin (Hgb) and dissolved in plasma. The arterial oxygen content (CaO2) describes the total amount of oxygen carried by arterial blood. Parameters are defined and other oxygen values are calculated in a similar fashion as in Table 2.2. CaO2 is represented in the following equation (West, 2013):
(Hgb × 1.34 × SaO2) + (0.003 × PaO2)
b. Oxyhemoglobin dissociation curve (Figure 2.8)
i. The oxyhemoglobin dissociation curve is an S-shaped curve with Hgb on the y-axis and PaO2 on the x-axis. The release of oxygen to the tissues is directly related to Hgb concentration and the affinity of oxygen for Hgb. Thus, on the steep portion of the dissociation curve, relatively small changes in PaO2 cause large changes in oxygen saturation of Hgb (West, 2012).
ii. A shift to the right, which facilitates the unloading of oxygen from Hgb, is caused by a decrease in pH, an increase in PaCO2, elevated temperature, or an increase in 2,3-diphosphoglycerate (2,3-DPG). 2,3-DPG decreases the affinity of Hgb for oxygen. During hypoxia or anemia, oxygen availability is increased within a matter of hours by an increase in 2,3-DPG (West, 2012).
iii. A shift to the left, which increases the binding of oxygen to Hgb, is caused by an increase in pH, a decrease in PaCO2, a decrease in temperature, or a decrease in 2,3-DPG. Fetal Hgb has decreased 2,3-DPG, shifting the curve to the left of the adult Hgb curve. Thus, at a given PaO2 and hematocrit, fetal Hgb is more readily saturated than adult blood is. Fetal Hgb also releases oxygen less readily to the tissues than adult Hgb. Fetal Hgb is replaced by adult Hgb within 4 to 6 weeks after birth (West, 2012).
3. Cellular Respiration. All cells depend on a continuous supply of oxygen to support aerobic metabolism. Oxygen delivery data do not provide information about the adequacy of tissue oxygenation. In septic shock, tissue oxygen extraction is altered. To assess tissue oxygen extraction, a true mixed venous blood sample should be obtained from the pulmonary artery.
CaO2 = (Hgb × 3.4 × SaO2) + (PaO2 × 0.003)
CvO2 = (Hgb × 3.4 × SvO2) + (PvO2 × 0.003)
CaO2 = CvO2
DO2 = CaO2 × Cl × 10
620 ± 50 mL/min/m2
VO2 = (CaO2 – CvO2) × Cl × 10
(CaO2 − CvO2)/CaO2 × 100
25% ± 2%
a-vDO2, arteriovenous oxygen difference; CaO2, arterial oxygen content; CI, cardiac index; CvO2, venous oxygen content; DO2, oxygen delivery; Hgb, hemoglobin; O2ER, oxygen extraction ratio; PaO2, arterial oxygen partial pressure; PvO2, venous partial pressure of oxygen; SaO2, arterial oxygen saturation; SvO2, venous oxygen saturation; VO2, oxygen consumption.
CLINICAL ASSESSMENT OF PULMONARY FUNCTION
1. Prenatal and Delivery
a. Birth weight
b. Gestational age
c. Apgar scores
d. Respiratory distress in the neonatal period, including oxygen requirements and ventilatory assistance
e. Length of hospital stay
a. Verify immunization history and tuberculosis (TB) tests
b. Family history of asthma, allergies, eczema, respiratory illnesses
c. Episodes of wheezing with previous illness
d. Frequency of colds and upper respiratory tract infections
e. Has the child every been intubated
f. Environmental exposure to smoke, molds, or pets
g. Supervise child, especially younger than 5 years, to evaluate risk of foreign-body aspiration
h. Recent illnesses of child or family members
i. Recent emergency room visits and/or hospitalizations
j. Use of medications, medications to treat respiratory symptoms, and nonprescription and alternative medications
k. Recent international travel
3. Other Significant Information
a. Chest pain (rarely of cardiac origin) is described in quality, timing with respiratory phase, duration (continuous or intermittent), and precipitating factors.
44b. Growth and development. Failure to gain weight is often the first sign of chronic pulmonary disease. Activity level or milestones may be delayed with chronic pulmonary dysfunction.
c. Gastrointestinal (GI) symptoms
i. Pneumonic processes frequently manifest as generalized abdominal pain in young children.
ii. Acute or chronic infection may cause anorexia and occasional vomiting.
iii. Upper airway mucus may impede swallowing and cause gagging, vomiting, or diarrhea in infants and toddlers.
iv. Bulky, foul-smelling stools may indicate CF.
v. Gastroesophageal reflux may cause chronic pulmonary aspiration.
d. Sleeping habits
i. Evaluate duration of sleep at night and causes of interruptions.
ii. Nighttime coughing is a frequent symptom of asthma or other lower respiratory tract diseases.
iii. Note positioning for sleep, such as head flat or requiring head elevation.
iv. If a humidifier is used, note care and maintenance of the humidifier.
v. Note signs of obstructive sleep apnea, such as sleeping propped up on pillows, apnea, orthopnea, and snoring.
B. Physical Examination
Anatomic landmarks of the thorax provide a method for describing physical examination findings (Figure 2.9).
a. Thoracic inspection
i. Note thoracic contour: A neonate’s chest is round with the anteroposterior diameter equal to the transverse diameter. Chest contour is more oval by 2 to 3 years of age. Disproportionate size may be detected by comparing head circumference (occipitofrontal circumference [OFC]) to chest circumference. From birth to 2 years, the head and chest circumferences are generally equal. During childhood, chest size 45is 5 to 7 cm greater than the OFC. Chronic disease may cause enlarged anteroposterior diameter or “barreled chest” which is similar to the neonate’s chest contour (Figure 2.10).
ii. Note any skeletal deformities: Anomalies, such as sternal depression (pectus excavatum) or protrusion (pectus carinatum), may cause or be associated with respiratory abnormalities by altering pulmonary mechanics. Inspect posterior thoracic structures and the spine. Kyphosis and scoliosis can impair pulmonary mechanics.
iii. Note symmetry of excursion (depth of respiration).
b. Respiratory effort
i. Rate and rhythm are age related: Respiratory rate is about one fourth of the pulse rate in the normal infant at rest.
ii. Evaluate the adequacy of thoracic excursion.
iii. Note the effort of breathing: Infants and young children breathe principally with the diaphragm because of immature intercostal muscles. Infants in respiratory distress may exhibit nasal flaring, head bobbing, expiratory grunting, or head extension.
iv. Note the use of accessory muscles: Signs of respiratory insufficiency include suprasternal, substernal, intercostal, and subcostal retractions.
v. Variations in respiratory patterns are also observed with various neurologic abnormalities (Figure 2.11).
vi. Note the quality of the voice and breathing; pay particular attention to a muffled voice; stridor; expiratory/inspiratory wheezing; and a barky, loose, congested, or paroxysmal cough.
c. Skin color and appearance
i. Cyanosis is observed when 3 to 5 g of Hgb becomes desaturated. Conditions that can mask or mimic cyanosis include hypothermia, anemia, or polycythemia. If cyanosis persists with oxygen administration, this may indicate the presence of an intracardiac shunt.
ii. Clubbing of fingertips is an indication of chronic hypoxia. The distal phalanx is flat and broad, causing a “club” appearance.
iii. Cyanosis is not related to increased PaCO2 (hypercarbia). A patient may be well oxygenated with a supplemental flow of oxygen diffusing through the airway but still be significantly hypercarbic and without a cyanotic appearance.
a. This is a limited technique in infants, but can be useful in older children.
b. Expand the hands bilaterally and symmetrically across the anterior chest wall and then across the posterior chest wall to evaluate the following:
i. What is the expansion of the thoracic cage?
ii. Fremitus is conduction of the child’s voice while the child says, for example, “99.” Vibrations should be noted at the trachea and upper airway. Decreased sensation is normally observed centrally near large airways, otherwise it is associated with occlusions. Increased sensation is associated with solid masses (consolidations). In infants, palpation during crying allows a similar evaluation.
c. Palpation of fine vibrations may indicate underlying pleural friction rub.
d. Palpate the entire thoracic cage for crepitus, a coarse, crackly feeling (and sound) of air in the subcutaneous tissue.
e. With a history of trauma, evaluate the skeletal structure, especially the clavicles.
f. Palpate the tracheal position (midline); if it is shifted, locate the position of maximal impulse of the heart. A shift in either or both may indicate fluid or air collection or a collapsed lung.
a. Technique used to determine the presence of air, fluid, or masses in the underlying lung and to determine anatomic landmarks (such as the upper margin of the liver; Figure 2.12).
b. Percuss using the middle finger of one hand flush against the chest wall in interspaces, noting the quality of sound produced by striking this finger with the middle finger of the other hand.
i. Right side of the anterior chest: Percussion sounds should be resonant in each intercostal space down to the fifth to sixth intercostal space, where the superior liver margin begins. There the sound changes and has a dull quality. Farther down where the lung field ends and the liver border continues, the percussion note goes flat.
ii. Left side of the anterior chest: Percussion of the heart borders can be determined. The superior border is often percussed between the second and third intercostal spaces. The inferior border is at the fourth to sixth intercostal spaces, and the left border is just lateral to the midclavicular line. At the sixth intercostal space and below, tympany may be observed because of an air-filled stomach.
iii. Posterior chest: Percuss side to side to identify abnormal densities.
c. Variations in sounds define the density of structures:
i. Flat: Short, soft; heard over bone
ii. Dull: Medium pitch; duration heard over the liver, spleen, and mass densities
iii. Resonance: Low, loud, and long; heard over an air-filled lung
iv. Hyperresonance: Deep pitched, loud, and prolonged; heard over an overinflated lung or air collections such as pneumothorax
v. Tympany: High, musical quality, loud; heard over gas-filled organs such as the stomach
a. Evaluate the pitch, intensity, quality, and duration of each phase using the diaphragm of the stethoscope. For small infants, using either a small diaphragm or the bell of a stethoscope may enable localization of sounds.
b. Compare side to side, starting at the apex and proceeding methodically to the bases. The thin-walled chests of infants create transmitted breath sounds throughout the lung fields. Listen for discreet changes from one location to the next. For emergent situations, a quick check under each axilla (rather than the upper lobes of the lungs) allows gross determination of the presence of bilateral aeration.
c. Quality of pitch. Vesicular breath sounds (inspiratory [I] >expiratory [E]) are of long inspiration, low pitch, and soft intensity and are heard over most of the lungs. Bronchial breath sounds (I <E) have an equal or longer expiratory phase; are high pitched, loud, and blowing; and are heard over the large airways. Bronchial breath sounds are abnormal when heard over the peripheral lung tissue. Bronchovesicular breath sounds (I = E) are high-pitched tubular sounds.
d. Adventitious sounds are abnormal sounds superimposed on normal breath sounds. Abnormal sounds have classically been defined as rales, rhonchi, and wheezes. However, confusion over definitions has prompted a focus on describing the quality and location of these sounds to associate them with common causes. Attention should be given to pitch, timing (I or E), location, and whether they clear with cough.
e. Fine, high-pitched crackling noises (similar to the sound of rolling hair between fingers) may be heard at end inspiration over peripheral lung fields in pneumonia and pulmonary edema. Medium-pitched crackles are heard in early to midinspiration with pulmonary edema and 48diffuse secretions in the bronchioles. These may partially clear with coughing. Upper airway secretions may cause coarse, bubbling (rhonchi) sounds that clear with cough.
f. Inflamed pleural surfaces may result in a very fine, low-pitched crackle over the focal areas of the chest during both I and E phases. With cessation of breathing, the crackles are not heard.
g. Wheezing results from narrowed airway lumina. Inspiratory wheezing usually results from high obstruction, such as laryngeal edema or foreign bodies. Expiratory wheezing often results from lower obstruction, such as with bronchiolitis, severe asthma, or chronic obstructive lung disease.
h. Diminished or absent breath sounds are noted as the focal absence of sounds with occasional crackling, or as an abnormal quality or abnormal location of normal sounds. This may occur with severe asthma, atelectasis, pneumothorax, or pleural fluid accumulation. This finding is usually an ominous sign.
C. Abnormal Physical Examination Findings
1. Stridor (Table 2.3)
a. Description. Stridor is noisy breathing caused by increased turbulence of airflow through a lumen.
i. Inspiratory stridor is related to the inward collapse of structures during inspiration. It is most common with supraglottic or glottic lesions because of the negative pressure generated during inspiration. Inspiratory stridor is common in laryngotracheomalacia and viral croup. Postextubation endotracheal tube (ETT) trauma is another possible source of stridor.
49ii. Expiratory stridor is most commonly observed with subglottic lesions.
iii. Fixed lesions (e.g., subglottic stenosis) may cause both inspiratory and expiratory stridor.
b. Evaluation of the child with stridor includes checking nasal patency, the size of tongue and mandible, the quantity of oropharyngeal secretions, the presence of drooling, the quality of phonation, head and neck range of motion, evidence of tooth evulsion or oral trauma, presence of fever or infectious symptoms, neurologic status, and the rate of progression of symptoms.
i. Note the position of preference. Infants with laryngomalacia, micrognathia, or macroglossia often have less distress when placed in a prone position. Children with epiglottitis or croup (laryngotracheobronchitis) often position themselves upright; children with moderate obstruction may exhibit forward extension of the head.
a. Description. Central or peripheral blue discoloration of skin tissue caused by desaturated Hgb. Cyanosis is usually not appreciated until 3 to 5 g Hgb per deciliter of serum is desaturated, corresponding to an SaO2 of 80% to 85% in a normal child. Cyanosis is not caused by an elevated PaCO2. Cyanosis can be caused by other nonpulmonary conditions (Figure 2.13).
b. Cyanosis may be masked by anemia, causing a more pallid color; or polycythemia, which may create a more “ruddy” color.
c. Evaluation of the cyanotic child
i. Oxygen should be administered before proceeding with the evaluation. Note the response to oxygen and the general degree of distress. If distress is moderate or severe, emergency management should be given before proceeding with further evaluation.
ii. Note whether cyanosis is peripheral (nail beds), central (lip and tongue color), or both.
iii. Assess pulmonary function to identify upper or lower airway causes, including the presence or absence of stridor, phonation, use of accessory muscles, and general state of alertness and activity level.
iv. Obtain historical information such as the evolution of the cyanosis (sudden or gradual onset) and associated factors such as illness or decreased environmental temperature.
a. Description. A cough is an attempt to clear the airway of particulate matter or may result from general tissue irritation. It is produced by a reflex response in cough receptors, found in ciliated epithelium, or it may be initiated in higher cortical centers.
i. Many causes of cough are age specific (Table 2.4).
ii. Note historical information such as the presence or absence of infectious disease or exposures.
iii. Characteristics of the cough may suggest the cause:
1) Loose and productive. CF, bronchiectasis, asthma
2) Croupy. Viral laryngotracheobronchitis
3) Paroxysmal. Pertussis, mycoplasma, foreign body, chlamydia
4) Brassy. Tracheitis, upper airway drainage, psychogenic
5) Nocturnal. Asthma, sinusitis, gastroesophageal reflux, upper respiratory tract disease
6) During exercise. Asthma, CF, bronchiectasis
7) Loud honking that disappears with sleep: Psychogenic
Viral: cytomegalovirus, rubella, pertussis
Causes mentioned for neonates plus
Diffuse interstitial pneumonia
Infections in suppurative disease (e.g., CF)
Viral infections with or without reactive airway disease
Foreign body aspiration
Reactive airway disease
Reactive airway disease
Mycoplasma pneumoniae infection
Psychogenic cough tic
CF, cystic fibrosis; TEF, tracheoesophageal fistula.
iv. Other associated symptoms. Examine sputum samples for white blood cells (WBCs) or eosinophils. Note hemoptysis. Poor weight gain, steatorrhea, and cough are strongly suggestive of CF.
v. A cough that persists longer than 2 weeks or a cough that causes immediate respiratory distress warrants investigation.
INVASIVE AND NONINVASIVE DIAGNOSTIC STUDIES
A. Diagnostic Approach
1. Individualization of Evaluation. All sick children are at higher risk for respiratory insufficiency or failure than adults because of the age-related anatomic differences described previously. Monitoring of pulmonary function can be individualized according to the acuity of illness and the age of the child. For a patient with the lowest acuity, clinical examination and serial observations are adequate, but with increasing severity of illness, other monitoring devices should be used. The options for monitoring include continuous observation and clinical examination, oxygen monitoring, CO2 monitoring, monitoring of pulmonary function, and laboratory and roentgenographic studies.
2. Immediate Assessment and Care. A brief estimation of the severity of distress should be made to determine whether oxygen or airway assistance is needed.
B. Baseline Respiratory Monitoring
1. Physical Examination. All children suspected of having respiratory distress should have their clothing removed for maximum observation, ensuring that ambient temperature is controlled. Refer to the clinical assessment section in this chapter for details on physical examination techniques. A quick-look observation should be done with all patients for early recognition of respiratory distress requiring emergency management. This 20-second appraisal should include level of consciousness, color, and respiratory effort.
2. Diagnostic Studies for Children in Respiratory Distress From Any Cause
a. Complete history and physical examination
b. Chest x-ray examination
51c. Sinus x-ray examinations (depending on age)
d. Simple oxygen saturation monitoring
e. Nasal cannula end-tidal CO2 monitoring
f. Complete blood count (CBC)
g. Tuberculin skin test (TST)
h. Nasopharyngeal swab for respiratory syncytial virus (RSV) and viral panel and pertussis, if applicable
i. Diagnostic studies to consider for children in respiratory distress according to symptoms:
i. Bronchoscopy with alveolar lavage (BAL)
ii. Laboratory testing as indicated
iii. pH probe testing for suspected gastroesophageal reflux disease (GERD)
iv. Pulmonary function testing (PFT)
C. Laboratory Studies (Table 2.5)
D. Blood Gas Analysis
1. Arterial, venous, or capillary blood gas (CBG) analysis can assist in the respiratory assessment (Table 2.6). Typically, interpretation of blood gas values involves acid–base interpretation to evaluate the pH and PCO2 values and evaluation of oxygenation, or PO2, separately. An arterial blood gas (ABG) analysis is the traditional method of estimating the systemic carbon dioxide tension and pH, usually for the purpose of assessing ventilation and/or acid–base status. However, an ABG analysis requires a sample of arterial blood, which can be difficult to obtain. A venous blood gas (VBG) analysis is an alternative method of estimating systemic carbon dioxide and pH that does not require arterial blood sampling. A VBG analysis can be performed using a peripheral venous sample (obtained by venipuncture), central venous sample (obtained from a central venous catheter), or mixed venous sample (obtained from the distal port of a pulmonary artery catheter). CBG samples may be used in place of samples from arterial punctures or indwelling arterial catheters to estimate acid–base balance (pH) and adequacy of ventilation (PaCO2). Capillary PO2 measurements are of little value in estimating arterial oxygenation.
2. PaCO2 directly reflects the adequacy of alveolar ventilation (VA). Hypercarbia is PaCO2 greater than 55 mmHg.
3. Oxygenation is evaluated using the PaO2 value. Hypoxemia is an arterial PaO2 less than 60 mmHg.
Significant Associations With Altered Levels
Norms are age-related assays.
Decreased levels of any immunoglobulin are usually associated with congenital deficiencies and patterns of infections beginning early in life. Altered levels are associated with specific causes as follows
IgA: Deficiency is associated with an increased incidence of mucosal bacterial infections
IgG: Found in blood, lymph, CSF, pleural fluid, peritoneal fluid, and breast milk; slow response (appears 1 wk after stimulus)
IgM: Intravascular; predominant first response to bacterial or viral infection; activates the complement system
Appears early in infectious course but may persist with chronic infection
IgD: Predominant activity on the surface of B cells (involving antibody formation)
Increased with chronic infections
IgE: Found in the serum and triggers release of histamine
Increased with allogenic stimulation (e.g., asthma, associated with allergenic stimulus)
Differential WBC Count
<1 y: maximum = 20,000
1–12 y: maximum = 15,000
Infections may cause an elevated or remarkably low (<4,000/mm3) WBC count
Segmented neutrophils (PMNs)
<12 y = 25%–40%
≥12 y = >50%
Band neutrophils <10%
Increase in bands associated with bacterial infections
<12 y = >50%
≥12 y = <40%
Increased with specific infections such as pertussis, Epstein–Barr virus, hepatitis
Pilocarpine Lontophoresis (Sweat Chloride Test)
Sodium <70 mEq/L
Chloride <60 mEq/L
Potassium <60 mEq/L
Higher levels suggest CF
Sputum or Tracheal Aspirate Cultures
Normally should have few if any PMNs and mixed flora
PMNs: 3–4+ with dominant organism is more likely to be valid indicator of infection than one with <2+ PMNs and multiple organisms
Deep tracheal secretions preferred; protected brush specimen technique
Evaluate Gram stain for presence of PMNs
Endotracheal tubes and tracheostomy tubes become quickly colonized with existing flora, which may be misleading if microbiology results are interpreted independent of other clinical indicators
CF, cystic fibrosis; CSF, cerebrospinal fluid; IgA, immunoglobulin A; IgD, immunoglobulin D; IgE, immunoglobulin E, IgG, immunoglobulin G; IgM, immunoglobulin M; PMNs, polymorphonuclear neutrophils; WBC, white blood cell.
534. Acid–base balance is indicated by the pH. Acidosis is an arterial pH less than 7.35, and alkalosis is an arterial pH greater than 7.45.
E. Radiologic Procedures for Pulmonary Evaluation
A variety of imaging techniques allow visualization of anatomy, motion dynamics, and identification of abnormalities. Frequently, a patient may require more than one imaging procedure to detail a specific anatomic site.
1. Chest roentgenography permits visualization of lung parenchyma (tissue), pulmonary vascular markings, heart silhouette, and bone densities.
2. Fluoroscopy provides evaluation of thoracic motion, particularly diaphragm movement, which is essential to the infant, and is especially useful in the evaluation of a paralyzed diaphragm.
3. CT scan is the visualization of very thin slices of tissue in a predetermined plane of dimension, enabling identification of masses, fluid accumulation, and anatomic definition. A spiral CT may provide better definition. A spiral CT is especially useful in the evaluation of a pulmonary embolus.
4. MRI uses an external magnetic field around the patient to cause rotation of cell nuclei. Imaging provides well-defined visualization of soft tissues. No radiation is involved in this procedure.
5. Ventilation–perfusion scan (V/Q scan) is obtained by injecting a radioisotope into a peripheral vein and imaging its flow through the pulmonary vessels. The V/Q scan is made following the inhalation of a radioactive gas, which distributes to aerated alveoli. Comparison between ventilated areas with perfused areas of the lung can be made, looking for “matched” segments. Although many disorders may cause a ventilation–perfusion mismatch, a complete segmental mismatch is a useful clue to the diagnosis of pulmonary embolism (PE).
6. Pulmonary angiography involves the introduction of a catheter into a peripheral vein; the catheter is guided to the right side of the heart and to the pulmonary artery trunk. Injection of contrast media with serial radiographic examinations of the pulmonary vascular bed allows definitive recognition of vascular obstruction.
7. Echocardiography utilizes ultrasound waves to detect the structures and function of the heart and its surrounding vasculature. This method serves as a standard noninvasive diagnostic screening tool for pulmonary hypertension (PH). Abnormal findings obtained through this exam may be confirmed via a more invasive heart catheterization.
A. Oxygen Monitoring
1. Pulse Oximetry
a. Principles of operation. The device consists of a probe that contains an infrared light source and a photodetector. Both are housed in a wraparound strip so that the light source is aligned to emit light through tissue (such as a nail bed) to the photodetector. Saturated Hgb absorbs little light, whereas desaturated Hgb absorbs a large amount of light. The photodetector measures the amount of light that crosses the tissue and with a microprocessor computes the percentage of saturated Hgb.
b. Uses. The pulse oximeter requires pulsatile tissue to provide accurate measurements. Fingers, palmar wrap (in small infants), toes, and ear clips are used. The microprocessor unit measures pulse rate and most units have a mechanism to provide a “poor signal” alert or perfusion indicator if the tissue has an inadequate pulse. It provides a stable, accurate measurement over a wide variety of physiologic conditions.
i. Poor perfusion (low cardiac output states) may interfere with a stable signal.
ii. A major source of error in children is motion artifact, which causes poor tracking of the pulse. New oximeters feature an algorithm to identify both motion artifact and poor perfusion (Fouzas, Prifitis, & Anthracopoulos, 2011).
iii. With elevated carboxyhemoglobin or methemoglobin levels, the true oxygenated Hgb values are inflated because the pulse oximeter does not distinguish between Hgb saturated with oxygen and other bound molecules.
iv. When retinopathy is a concern in the neonatal population, SaO2 may not provide any margin of safety because the PaO2 can vary widely when oxygen saturations are greater than 90% as the result of factors that alter the oxygen dissociation curve.
v. Newborns and children with uncorrected congenital heart disease may have variance in measurements depending on the placement 54of the oximeter. In cardiac lesions with right-to-left shunting, the right hand or foot is used for preductal measurement and the left hand or foot is used for postductal measurement.
2. Transcutaneous Oxygen Monitoring
a. Principles of operation. A small, heated probe is placed on the skin surface. Localized heating increases capillary blood flow to the site. Oxygen diffuses across the skin and is measured by a thermistor in the probe.
b. Uses. Transcutaneous oxygen monitoring (PtcO2) functions well in infants and has shown good correlation with PaO2 measurements. It can reduce the quantity of invasive blood gas measurements and provide continuous data for patients who are either being weaned or are otherwise labile. In the past, PtcO2 demonstrated utility in detecting hyperoxia in neonates; however, this has been demonstrated to be more readily monitored through maintaining SpO2 between 85% and 93% (Kacmarek, Stoller, Chatburn, & Kallet, 2017).
c. Limitations. With increasing skin thickness, the accuracy of measurements diminishes. Poor perfusion and local skin hypoxia can interfere with measurements. The probe requires site change every 4 to 6 hours with a 10- to 15-minute warm-up time (and a blood gas analysis to verify measurement accuracy). Note that the heated probe may cause blisters in some infants.
3. Mixed Venous Oxygen Saturation Monitoring
a. On occasion, pediatric patients are monitored with pulmonary artery catheters for hemodynamic measurements. One type of catheter includes a fiberoptic tip that measures reflected wavelengths of light from saturated Hgb in the local (pulmonary artery) blood flow (Kacmarek et al., 2017).
b. The mixed venous oxygen saturation (SvO2) measurement, which is usually between 65% and 80%, is a global indicator of total oxygen consumption (VO2).
c. Either a change in oxygen supply (arterial oxygen saturation) or in oxygen demand (VO2) will alter this measurement (see Chapter 3 for further information).
B. Carbon Dioxide Monitoring
1. Transcutaneous Carbon Dioxide Monitoring
a. Principles. A CO2 sensor, housed in a small probe, is mounted on the skin surface to measure diffused carbon dioxide with an electrode similar to that found in standard blood gas machines. Although in the past PtcO2 electrodes used local heat to “arterialize” the site, recent studies showed that a predictable linear relationship can be obtained with nonheated electrodes. Typically, the PtcO2 is higher than the PaCO2, but the gradient should remain stable.
b. Uses. In the past, consistent correlations have been documented in neonates particularly; however, recent advances in technology have expanded this technology for use in older children and adults (Kacmarek et al., 2017). After the probe is positioned and the warm-up period has passed, the monitor measurement should be compared with a simultaneous blood gas measurement to establish the gradient. After this point, blood gas measurements can be reduced until the next site rotation (usually every 4 hours).
i. The sensors appear to be somewhat fragile and prone to discreet alterations, such as inadequate fluid in the sensor. If the gradient between PtcO2 and PaCO2 is unstable, a site change is warranted.