Учебники / Otolaryngology - Basic Science and Clinical Review

Infants and children cannot just be considered small adults. There are differences in metabolism and in the maturation of almost all organ systems, from the cardiovascular and respiratory systems to the central nervous system.This chapter will describe many of these differences as they pertain to the pediatric airway, specifically of the premature infant, the term newborn, and young infants. Finally, physiological changes that accompany tracheotomy will be discussed along with concerns during decannulation.

During fetal life, respiratory effort is part of normal development, providing work for the diaphragm. Fetal pulmonary blood flow is minimal due to pulmonary arterial vasoconstriction. The lung bud appears by the end of the fourth week of gestation. Bronchi appear by the sixth week. Terminal bronchioles are completed by 16 weeks’ gestation. Alveolar formation begins in the late third trimester. A fullterm infant has 20 million to 50 million terminal air sacs, 10% of a normal adult complement. Lung volumes in newborns are disproportionately small compared with body size. The ratio of metabolic rate to body weight in an infant is twice that of an adult, making ventilatory requirements higher for the infant with smaller lungs and giving infants much less respiratory reserve.

RESPIRATION

As lung growth continues for the first decade of life, most alveoli develop postnatally. With the first few breaths at birth, major changes take place in the lungs and the cardiovascular system. During the transition to extrauterine life, the liquid in the air spaces and airways of the lungs is rapidly absorbed and exchanged for air, leading to pulmonary vasodilatation and increased pulmonary blood flow. Cord clamping initiates continuous rhythmic breathing; however, normal neonatal respiratory adaptation and control will take several weeks. Continued and rhythmic breathing is maintained by normal arterial pH. Continuous breathing is independent of the partial pressure exerted by carbon dioxide dissolved in arterial plasma and red blood cell water (PaCO2) and unaffected by carotid body denervation. Unlike hypoxia in older infants and children, newborn hypoxia does depress respiration and cause apnea.

The precise control of fetal breathing is uncertain. During the last 10 weeks of gestation, the fetus “breathes” 30% of the time. The respiratory rate at 30 weeks’ gestation is 58 breaths per minute; at term the rate falls to 47.There are acute increases in respiratory rate a few hours after a maternal meal, correlating with maternal blood sugar. Spontaneous fetal breathing

occurs during rapid eye movement (REM) sleep and is independent of chemical and nonchemical stimuli. The normal fetal PaO2 is 20 to 25 mm Hg, which normally would have an inhibitory effect on breathing and REM sleep. Severe fetal hypoxia (PaO2 10 mm Hg), usually associated with fetal acidosis, results in gasping respirations. Fetal breathing may represent “practice” and is a stimulus to fetal lung growth. Experimental cessation of breathing causes hypoplastic lungs.

Respiratory function in the first months of life is very different from later childhood, adolescence, and adulthood. In young infants, the diaphragm is the most important muscle of respiration. These infants have an extremely compliant chest wall.The intercostal muscles stabilize the chest wall, counterbalancing the force of diaphragmatic contraction, so that the lungs can expand. Any disturbance that changes the relationship of these two forces increases the work of breathing for the infant. Muscle fatigue or weakness leads to chest wall collapse seen clinically as retractions of the chest wall during inhalation. Parenchymal lung diseases such as pneumonia and respiratory distress syndrome decrease the compliance of the lungs, and, in the face of a very compliant chest wall, the chest wall collapses, with a resulting major increase in the work of breathing. An infant, therefore, develops respiratory failure much earlier than an older child or adult, necessitating assisted ventilation. The infant lung is normally relatively noncompliant, tending to collapse as lung disease develops. Low lung compliance, along with a relatively compliant chest wall, results in a low functional residual capacity very close to residual volume. An infant compensates for this by narrowing the laryngeal airway during exhalation, an attempt to maintain positive intrathoracic airway pressure.As lung disease progresses, this becomes a more pronounced audible exhalatory “grunt.” Grunting respirations are an important sign of impending respiratory failure.

Surfactant produced by type II alveolar cells is responsible for the decreased surface tension that is necessary to prevent alveolar collapse at small lung volumes. Inadequate or deficient surfactant is the etiology of respiratory distress syndrome (RDS) in newborns, common in premature infants and infants of diabetic mothers. A lack of surfactant leads to alveolar collapse, atelectasis, and decreased pulmonary compliance. Surfactant replacement therapy provides good clinical results and has been employed as treatment for both meconium aspiration syndrome and adult respiratory distress syndrome.

Control of breathing is maintained by the respiratory center in the brainstem, with input from central and peripheral chemoreceptors, and stretch receptors in the

airways and lung parenchyma. In the brainstem the dorsal respiratory group of neurons is located along the tractus solitarius, which is the principal site for the ninth and tenth cranial nerves, which carry afferent nerves from the lungs, airways, heart, and peripheral arterial chemoreceptors. Ventilation is adjusted to maintain a normal blood pH between 7.35 and 7.45, while maintaining the arterial PaCO2 in a very narrow range. This occurs no matter how oxygen consumption and carbon dioxide production change with metabolism. Inspiration is initiated by diaphragmatic contraction developing subatmospheric intrapleural pressure and resultant lung expansion. Exhalation is passive, resulting from elastic recoil of the lungs and chest wall.

Three afferent neurons have been described. I fibers are inhibited by lung inflation. I fibers react to lung inflation and pulmonary stretch receptors.The third type, called P cells, receive impulses following lung inflation. Excitation of I fibers is associated with shortening of inspiratory time and prolongation of exhalation time. This response is likely responsible for the Hering-Breuer reflex, which inhibits respiration during lung inflation. The ventral respiratory group is responsible for quieting inspiratory drive during exhalation, and the pontine respiratory group plays a secondary role in this response. Rhythmic breathing can occur in the absence of feedback from peripheral receptors.Transection of the pons has little effect on respiratory pattern, implying that the respiratory generator is likely in the medulla.

Peripheral receptors can be found in the upper airways, trachea, bronchi, lungs, and chest wall. Upper airway receptors are responsible for sneezing, apnea, changes in bronchomotor tone, and initiation of the “diving” reflex, which is the profound bradycardia associated with a shift of blood flow to the brain and heart thought to occur during submersion. Epipharyngeal reflexes produce sniffing, moving mucus from the nasal airway to the pharynx. Pharyngeal receptors facilitate swallowing, inhibit breathing, close the larynx, and coordinate contractions of the pharyngeal muscles. Laryngeal receptor activation causes apnea, coughing, and changes in the respiratory pattern.

Slow-adapting or stretch receptors are located in the submucosal muscles in the membranous portion of the trachea and are activated by distention of the airways. The Hering-Breuer reflex inhibits inspiratory activity, modulating respiratory effort. Hypocapnia augments these reflexes, and hypercapnia inhibits them. Apnea provoked by an inflated endotracheal tube cuff may be due to these slow-adapting reflexes.

Rapid-adapting receptors in the mucosa of the carina and large bronchi can be stimulated by irritant gases,

anesthetic or toxic, and are responsible for initiating coughing, bronchoconstriction, and mucus production. C-fiber endings located in capillary walls are stimulated by pulmonary vascular congestion, pulmonary edema, microvascular emboli, and noxious gases, producing apnea and slow, shallow breathing, hypotension and bradycardia, bronchoconstriction and mucus production, and laryngospasm.

Newborn animals are very sensitive to stimulation of receptors innervated by the superior laryngeal nerve resulting in respiratory depression or apnea. This could explain the tendency of small infants to become apneic from any noxious substance in the larynx. Inhalation of anesthetic agents in small infants frequently produces coughing, breath holding, and laryngospasm.

Central receptors on the surface of the ventrolateral medulla are anatomically separate from the respiratory center. These receptors respond to changes in hydronium ion (H ) concentration in the cerebrospinal fluid (CSF) and not to PCO2 or pH. Carbon dioxide rapidly diffuses into the CSF, quickly determining H concentration. Acute respiratory acidosis produces a rapid stimulation of these central receptors, whereas metabolic acidosis or alkalosis does not generate as rapid a response. During chronic respiratory acidosis, as seen in children with bronchopulmonary dysplasia, the CSF pH remains relatively normal. In this situation respiratory stimulation is more dependent on peripheral chemoreceptors that respond to hypoxemia. Oxygen therapy blocks this hypoxic drive, which puts these patients at risk for hypoventilation during oxygen administration.

Peripheral chemoreceptors are located in the carotid bodies and are responsible for the tachypnea that is present during hypotension. They also respond to changes in arterial pH and PaCO2. An arterial PaO2 less than 60 mm Hg results in a significant increase in ventilation, except in premature and term newborns, when hypoxemia results in respiratory depression. Anemia or carbon monoxide poisoning does not result in stimulation of the respiratory center because the arterial PaO2 is usually normal under these conditions. General anesthesia, opioids, and sedatives depress ventilation, variably affecting tidal volume and respiratory rate.

Neonates and young infants develop a transient stimulation of respiration in response to hypoxia. They usually generate one good breath and then become apneic. The transient stimulation is abolished in a cool environment like an operating or delivery room. By 3 weeks of age, the response to hypoxia generally is just respiratory stimulation. Newborns respond to hypercapnia with hyperventilation, but with a somewhat attenuated response compared with older infants.

Periodic breathing and central apnea are respiratory patterns common in young infants. There is a concern that these breathing patterns associated with immaturity may be related to sudden infant death syndrome (SIDS), but this is as yet unproven. Periodic breathing is described as an irregular breathing pattern with periods of breathing interspersed with short periods of apnea from 5 to 10 seconds in length.This irregular breathing is usually associated with REM sleep. Full-term infants spend 50% of sleep time in REM sleep as compared with 20% for adults. Premature infants remain in REM sleep most of the time and breathe irregularly. Periodic breathing may occur during wakefulness, active REM sleep, and quiet sleep.The incidence of periodic breathing is 78% in full-term infants and 93% in premature infants. Periodic breathing decreases with increasing age, with an incidence of 29% by 1 year of age. Breathing 2 to 4% carbon dioxide depresses periodic breathing by stimulating respiration.

Central apnea or apnea of infancy is defined as cessation of breathing for 15 seconds or longer, or shorter periods when associated with bradycardia, cyanosis, or pallor.This is more common in premature infants and is probably related to immaturity of the central nervous system. After simple surgical procedures such as inguinal herniorrhaphy, life-threatening apnea may occur, especially in premature infants who are less than 50 weeks postconceptual age.Theophylline and caffeine are effective central respiratory stimulants that are frequently used as therapeutic agents for apnea of infancy.

Intrapleural pressure is 5 cm below atmospheric pressure in older children and adolescents, compared with 0 or 1 cm below atmospheric pressure in newborns.The outward recoil of the infant chest wall is very low, resulting in a functional residual capacity near closing volume. Because end-expiratory lung volumes in infants are so near closing volume, conditions that promote airway closure and loss of lung volume will result in atelectasis and ventilation-perfusion mismatch. Apnea, general anesthesia, and muscle relaxation are examples of such conditions frequently encountered in the operating room and intensive care unit. Abdominal surgery, abdominal distension, and thoracic surgery have a similar effect. Newborns with RDS have very stiff, noncompliant lungs that are difficult to expand, reducing lung volumes further.

Resistance in the pulmonary system is composed of resistance of the lung to distension and resistance to airflow through the airways. During laminar flow, flow resistance can be described by Poiseuillle’s law, where resistance (R) is proportional to the length of the tube (l ) and viscosity of the gas ( ), and inversely proportional

to the radius (r).The units are expressed as pressure per flow or cm H2O/L per second:

R 8l / r4

Clearly, radius of the airway has the largest effect on resistance to airflow. One millimeter of circumferential subglottic edema in an adult has a minor effect on airway resistance, whereas the same amount of edema in the small airway of an infant will cause a 16-fold increase in airway narrowing and resistance. Turbulence is much more common in the narrow airway of small infants. During turbulent flow, resistance is related to 1/r5. Breathing mixtures of helium and oxygen (at least 60% helium) decrease resistance in areas of turbulent airflow due to the lower density of helium compared with nitrogen. The nasal airway is the normal route of airflow into the respiratory system and comprises 55% of the total airway resistance.This is twice the resistance of one who mouth breathes. Placing a nasogastric tube will narrow the nasal airway even further and increase total airway resistance in an infant by 50%.

The pharyngeal airway is composed of soft tissues easily collapsed by negative intraluminal pressure, by posterior displacement of the jaw, muscular hypotonia, or paralysis. Mechanical support to maintain patency of the pharyngeal airway occurs by increasing muscle tone or contraction of the genioglossus, geniohyoid, sternohyoid, sternothyroid, and thyrohyoid muscles that make up the pharynx. These muscles synchrononously increase their tone along with diaphragmatic contraction during inspiration. Chemoreceptors responding to hypercapnia and hypoxemia also increase pharyngeal muscle tone.

The function of the larynx, the organ of phonation, is to maintain the airway open and to occlude and protect the lower airway from substances during swallowing. The cricoid cartilage forms a complete ring, preventing the proximal tracheal airway from complete collapse.The infant’s larynx is high in the neck at C3–C4 and is funnel shaped, with the smallest part inferiorly at the cricoid cartilage. The epiglottis is shaped and juts out at a 45-degree angle.With growth, the larynx descends to C5–C6, becoming cylindrical in shape, with the cords forming the narrowest portion. In adults, the epiglottis is thin, arises close to the base of the tongue, and generally parallels the course of the trachea. The cricoid cartilage surrounds the subglottic space that is easily traumatized from oversized or tight endotracheal tubes. Tight-fitting endotracheal tubes may cause mucosal ulceration and pressure necrosis, and inflammation of the cricoid cartilage. The ischemic injury may result in mucosal edema that is responsible for postintubation

croup. Subsequent fibrosis of the cricoid cartilage may lead to subglottic stenosis.

Normally, the glottis widens during inspiration, minimizing airflow resistance, but narrows during expiration that maintains positive airway pressure at end expiration and residual lung volume, the functional residual capacity (FRC).This function is lost with endotracheal intubation or tracheotomy due to loss of glottis closure. Application of positive end-expiratory pressure (PEEP) will help maintain FRC.

There are many protective reflexes. Coughing, sneezing, swallowing, and laryngeal and pharyngeal closure are all attempts to maintain the airway open and clear. Laryngospasm is marked closure of the glottis maintained by tonic contraction of adductor muscles after removal of the noxious stimuli. Hyperthermia, light anesthesia, hyperventilation, hypocapnia, and small lung volume all increase this reflex, whereas hypoxia, hypoventilation, hypercapnia, positive intrathoracic pressure, and deep anesthesia depress it. Laryngospasm can be broken with deep anesthesia or awakening. Lung inflation by application of positive airway pressure and PEEP can reduce its incidence and severity. Infants are at particular risk due to their laryngeal hyperexcitability. Premature infants with fluid in their larynx respond with apnea and breath holding (apnea at end inspiration). Fluid pooled in the hypopharynx is usually swallowed, but these small infants may develop central apnea even without fluid in the larynx. Apnea is more prevalent with plain water than with saline. Pooled secretions need to be suctioned to prevent this induced central apnea. The genioglossus displaces the hyoid and tongue anteriorly, maintaining the pharyngeal airway. This response is depressed by alcohol, sleep, and general anesthesia. Normal phasic activity of the genioglossus is easily abolished by general anesthesia, requiring high positive airway pressure or placement of an oral airway to maintain pharyngeal patency.

The diaphragm is the most important muscle for normal ventilation. Phrenic nerve injury can occur from birth injury or inadvertent surgical trauma. Unilateral phrenic nerve palsy results in paradoxical diaphragmatic movement of the paralyzed side and profound respiratory failure in young infants. Older infants have much more reserve and can maintain normal ventilation. Bilateral phrenic nerve palsy in infants or children universally requires assisted ventilation.

Ventilation may be quantified by measuring the exhaled tidal volume (VT) over the course of a minute (VE). Mathematically, this is

VE VT f,

where f is the number of breaths per minute. Both f and VT are adjusted to minimize the work of breathing.With respiratory disease that causes increased airway resistance or decreased pulmonary compliance, infants will increase their respiratory rates to more than twice normal because it requires less work to increase rate of breathing than to maintain or increase the tidal volume. The anatomical dead space may be estimated by solving the equation

VD/VT (PaCO2 PETCO2)/PaCO2,

where PETCO2 is the end-tidal carbon dioxide (CO2) measured from the end-exhaled gas.

PULMONARY CIRCULATION

The pulmonary circulation and diffusion of gases across the alveolar-capillary membrane are important determinants of pulmonary gas exchange.The alveolar capillary distance is 0.3 mm. Carbon dioxide is 20 times more diffusible than oxygen. Therefore, problems with carbon dioxide diffusion do not become apparent until there is severe disease. Decreased diffusion capacity is also seen in conditions with diminished pulmonary blood flow. In utero, the normally low PaO2 causes pulmonary vasoconstriction that results in markedly increased pulmonary vascular resistance. In the normal fetal circulation, both the right and left ventricles pump blood to the systemic circulation. Approximately 5% of the fetal cardiac output goes to the lungs, as opposed to 50% postnatally. Most of the blood pumped out of the right ventricle passes through the main pulmonary artery and through the ductus arteriosus to the descending aorta. With the newborn’s first breaths of room air, the PaO2 rises, dilating the pulmonary vasculature and increasing pulmonary blood flow. In addition, the ductus arteriosus constricts, for the first time separating the pulmonary and systemic circulations. Postnatally, PaCO2 (partial pressure exerted by carbon dioxide dissolved in arterial plasma and red blood cell water) and PaO2 still have an effect on pulmonary circulation. Elevated PaO2 and low PaCO2 dilate the pulmonary vasculature and simultaneously constrict the systemic circulation. Low PaO2 and elevated PaCO2 have the opposite effects, constricting the pulmonary vasculature, inducing pulmonary hypertension, and dilating the systemic circulation.

Arterial oxygen content is determined by hemoglobin concentration and arterial hemoglobin oxygen saturation. Hemoglobin oxygen saturation has many determinants such as arterial pH, body temperature, presence of 2,3-diphosphoglycerate (DPG), type of hemoglobin, presence of carbon monoxide, and oxidative state of the iron molecule in the hemoglobin.The ability

of a particular hemoglobin to bind oxygen is called its p50, or the PaO2 where hemoglobin is 50% saturated with oxygen.The p50 of adult hemoglobin is 27, for an infant 30, and a newborn 19. Newborns have high concentrations of fetal hemoglobin that bind oxygen well but decrease oxygen release at the tissue level.The normal newborn has a relatively high hemoglobin level, mean 17 g/dL. This is likely due to high fetal levels of erythropoietin stimulated by the low PaO2 in utero and the high oxygen-binding affinity of fetal hemoglobin. By 2 to 3 months of age, infants develop a physiological anemia with a mean hemoglobin concentration of 12 g/dL. By 6 months of age, the hemoglobin concentration reaches 12.7 g/dL and remains there until the child goes through puberty, at which point adult levels are realized.

The decision to transfuse an infant prior to surgery depends on the degree and duration of anemia, intravascular volume, extent of the proposed surgery, and probability of massive blood loss. Impairment of cardiorespiratory function would also lead to transfusion at least to a normal hemoglobin concentration for age. A hematocrit greater than 25% is probably acceptable for infants older than 3 months of age without any cardiorespiratory compromise. Halothane has a minimal effect on the p50 of hemoglobin, whereas nitrous oxide causes a reversible increase in hemoglobin affinity for oxygen, decreasing oxygen unloading at the tissue level. Alkalosis has a similar effect. Because hypoxemia is quite common in infants postoperatively, pulse oximetry should be monitored and supplemental oxygen administered when oxygen saturations fall below 95%.

Ciliated cells are located in the trachea and bronchi. Serous and mucus-producing cells are located in the submucosa. The cilia beat in a serous layer covering the epithelial line. There is a discontinuous layer of mucus above the serous layer. Ciliary function is to remove mucoid secretions and foreign particles and debris. Cilia beat at an incredible 600 to 1300 beats per minute and can move mucus 1.5 to 2.0 cm per minute.Viral infections reduce ciliary motion by as much as 50%. Repeated infections can destroy cilia entirely. Breathing warm, humidified air maintains cilia. Breathing dry air or oxygen for as little as 3 hours results in complete cessation of ciliary movement. Inhaled anesthetics, 100% oxygen, and positive-pressure breaths all diminish ciliary function.

INFECTIOUS DISEASES

Infectious disease can compromise respiration at all levels of the respiratory system.Though less common in this age of antibiotics, abscess formation in the retropharyngeal

or peritonsillar space can cause significant symptoms. Fever, dysphagia, drooling, neck rigidity, and noisy breathing are symptoms of a retropharyngeal abscess that usually occurs in children less than 5 years of age. Peritonsillar abscess occurs in older children. Symptoms include trismus, throat pain, and a muffled voice. Group A Streptococcus is the most common organism in both disorders. Some Bacteroides species also may be involved. Surgical drainage, appropriate antibiotics, and special attention to the airway are the necessary therapeutic interventions.

The next group of infectious disorders falls under the category of croup, characterized by inspiratory stridor. Supraglottitis or epiglottitis has become extremely rare due to immunization against the major causative organism; that is, Haemophilus influenzae type B. Streptococcus and Staphylococcus may also be causative organisms. Children with these disorders have high fever and are extremely anxious, usually sitting up with their neck extended. Diagnosis is made by visualization of a swollen and inflamed epiglottis. Antibiotics and placement of an emergent airway, preferably a nasotracheal tube, are therapeutic and life saving.The airway may be removed when there is an audible air leak around the endotracheal tube, indicating that the edema has resolved.

Laryngitis is due to viral infection and results in edema of the cords, with the resulting coarse, raspy voice. Therapy is symptomatic. However, inflammation of the subglottic space is common and potentially life threatening. The entire laryngotracheobronchial tree is

usually affected. As discussed previously, small amounts of edema can cause major narrowing of the airway with a marked increase in resistance to airflow. Children present with the respiratory symptoms of a high-pitched, barky cough and inspiratory stridor. Radiographs of the neck may show a “steeple” sign, signifying edema of the trachea and subglottic space (Fig. 16-1A, B). Parainfluenza type 3, influenza A and B, adenovirus, respiratory syncytial virus, and Echoviruses are the usual pathogens in infants. Mycoplasma pneumoniae has been isolated from older children. Therapy is symptomatic; however, a short course of corticosteroids along with inhaled racemic epinephrine usually helps resolve the symptoms. Heliox helium and oxygen mixtures may be used to decrease the work of breathing, but some infants will still need endotracheal intubation if they become exhausted from the increased work of breathing through a markedly narrowed airway. Spasmatic croup occurs in children from 6 months to 1 year of age, with recurrent acute attacks of stridor in the evening or night. Symptoms resolve with patience and mist.

Bacterial tracheitis presents with symptoms similar to viral laryngotracheobronchitis, which progress to severe respiratory distress with high fever. Staphylococcus aureus is the most frequent organism isolated, but H. influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae have been implicated. Routine childhood vaccinations have dramatically decreased the incidence of H. influenzae.Thick, purulent secretions are the cause of the respiratory

distress. Bacterial superinfection of viral laryngotracheobronchitis is the likely etiology.Antibiotics are chosen to cover the usual organisms, and endotracheal intubation is often required until secretions thin and there is audible air leak with resolution of the acute infection.

Pneumonia will also present with respiratory distress due to decreased lung compliance and an increase in bronchial secretions. Children with pneumonia are frequently cyanotic as well. Most pneumonias that affect children are of viral origin. Respiratory syncytial virus (RSV), parainfluenza, adenovirus, rhinovirus, and rubeola are the common viral etiologies. Ribavirin has activity against RSV but has not improved outcome of infants requiring assisted ventilation. Acyclovir is indicated for varicella pneumonia that will likely become as rare as acute epiglottitis with the institution of universal chicken pox vaccination.The usual respiratory bacterial pathogens that cause bacterial tracheitis may also cause pneumonia, and appropriate antibiotics should be administered. Chlamydia trachomatis produces pneumonia in young infants born to mothers whose genitourinary tract is infected with the same organism. Half of these patients also present with significant conjunctivitis. M. pneumoniae causes lower respiratory infection in children older than 5 years of age. Pneumocystis carinii produces pneumonia characterized by severe hypoxemia in infants and children who are immunosuppressed from chemotherapy, malnutrition, or human immunodeficiency virus (HIV) disease. Appropriate antibiotic therapy has been very successful in treating this devastating disorder.

TRACHEOTOMY

A tracheotomy can be a life-saving procedure in pediatric patients with an upper airway obstruction or can be placed in patients who require prolonged assisted ventilation. With tracheotomy comes loss of olfaction and normal phonation. Patients can learn to speak by placing a one-way valve on the tracheotomy tube, allowing the exhaled breath to flow through the larynx, producing sound. With air no longer passing through the nasal airway, there is loss of air conditioning (i.e., warming, humidification, and filtering of the inspired air).There is a beneficial decrease in airway resistance unless the cannula is small, in which case there will be increased airway resistance.There is also a reduction in anatomical dead space, up to 100 mL in an adult.

Placement of the tracheostomy tube will result in immediate relief of airway obstruction; however, this may be accompanied by the development of severe respiratory failure from postobstructive pulmonary edema (POPE). Patients who develop POPE may need

positive-pressure ventilation, PEEP, and diuresis until the respiratory distress resolves.The etiology of POPE is not entirely clear, but it appears to be related to marked negative pleural pressure that is generated during airway obstruction, pulling fluid into the thoracic space along with increased capillary-alveolar transmural pressure and increased capillary permeability. Also, breathing 100% oxygen during airway obstruction may lead to an increase in capillary permeability.

Air warming and humidification are functions performed poorly by the trachea and bronchi alone. Cool, dry air causes decreased mucus flow, making children with tracheotomy liable to have thickened secretions and mucosal metaplasia. Inspired air needs to be humidified to prevent atelectasis. These children are also prone to infection due to loss of filtering from the nasal airway. Humidification clearly is associated with less morbidity. After tracheotomy, the cough mechanism is less effective. Glottis closure during a cough results in compression of the tracheobronchial tree that results in peak air flows of 10 L per second upon release of glottis closure.With tracheotomy, glottis closure is not possible, making the cough ineffective.

A tracheostomy tube may be removed once the patient has an adequate upper airway, the larynx functions to protect the lower airway from aspiration, and the patient no longer requires prolonged ventilation. To ensure that the patient is ready for decannulation,the tracheobronchial tree must be free of infection and thickened secretions. There also should be no auscultatory signs of lower respiratory infection—rales, rhonchi, and wheezes. The patient must be able to breathe comfortably with the tracheotomy tube occluded, making it necessary to place a small enough tracheotomy tube that does not occlude the trachea. Pulmonary function should be evaluated for a normal tidal and respiratory rate for age, along with a negative inspiratory force of at least 20 cm H2O to ensure adequate respiratory muscle function. The airway must be visualized endoscopically for evidence of granulation tissue at the site of the tracheostoma, other lesions in the trachea, subglottic stenosis, and vocal cord function. Suprastomal granulomas are a common sequela following tracheotomy in children. If present, the granuloma must be dissected out prior to decannulation.

DECANNULATION

Once physical dependence has resolved, the patient may be decannulated. Psychological dependence is quite common. The child and family need to be continually prepared for decannulation from the time the tracheotomy

is performed. Decannulation is performed in the hospital within a few days of the endoscopy. Staff must be prepared to replace the tracheotomy tube or place an endotracheal tube if the child develops respiratory distress.

Decannulation failure is likely due to loss of laryngeal reflexes, assuming anatomical causes were ruled out by endoscopy. Airflow through the glottis is responsible for maintaining laryngeal reflexes necessary to protect the airway. Tracheotomy results in the loss of the laryngeal abductor reflex, which becomes an issue after decannulation. Without this reflex the larynx is not maintained open, and children frequently will have stridor postdecannulation until this reflex is reestablished. This problem may be ameliorated if the tracheotomy tube size is gradually made smaller over a few days, allowing for airflow through the glottis prior to decannulation, with the expectation that the laryngeal abductor reflex will reestablish prior to final decannulation. The laryngeal adductor reflex is also lost without laryngeal airflow.

SELF-TEST QUESTIONS

For each question select the correct answer from the lettered alternatives that follow.To check your answers, see Answers to Self-Tests on page 716.

1.Normal fetal partial pressure exerted by oxygen dissolved in arterial plasma and red blood cell water (PaO2) causes which of the following?

A.Rhythmic fetal breathing

B.Pulmonary vasoconstriction

C.Constriction of the ductus arteriosus

D.Systemic arteriolar constriction

2.In premature infants, hypoxia has which of the following effects?

A.Tachypnea

B.Apnea

C.Hyperventilation

D.Decreased frequency of periodic breathing

3.Helium and oxygen mixtures (heliox) will decrease the work of breath because of which of the following reasons?

A.Heliox decreases airflow resistance in areas of laminar flow.

B.Helium has higher density than nitrogen.

This results in a loss of glottis closure during swallowing and may increase the possibility of pulmonary aspiration despite the presence of the tracheotomy tube. Both of these reflexes may not return immediately after decannulation, leaving the child at risk for aspiration and stridor.

SUGGESTED READINGS

DeVries PA, DeVries CR. Embryology and development. In: Gans SL, ed. Othersen: The Pediatric Airway. Philadelphia: WB Saunders; 1991:3–16

Helfaer MA, Nichols DG, Rogers MC. Developmental physiology of the respiratory system. In: Rogers MC, ed.Textbook of Pediatric Intensive Care. Baltimore:Williams &Wilkins; 1996:97–126

Johnson JT, Reilly JS, Mallory GB Jr. Decannulation. In: Myers EM, Stool SE, Johnson JT, eds.Tracheostomy. NewYork: Churchill Livingstone; 1985:201–210

Motoyama EK. Respiratory physiology in infants and children. In: Motoyama EK, Davis PJ, eds. Smith’s Anesthesia for Infants and Children. St. Louis: Mosby; 1996:11–67

C.Helium increases hemoglobin oxygen saturation.

D.Heliox decreases airflow resistance in areas of turbulent airflow.

4.Which of the following is an infectious etiology of laryngotracheobronchitis, or “croup”?

A.Haemophilus influenzae

B.Staphylococcus aureus

C.Branhamella catarrhalis

D.Parainfluenza type 3

E.Varicella

5.The narrowest portion of an infant’s upper airway is

A.The carina

B.The vocal cords

C.The subglottic space

D.The nasal airway

E.The pharynx

DEVELOPMENTAL ANATOMY

CLASSIFICATION

CLINICAL PRESENTATION

DEFINITIVE TREATMENT

OTHER CONGENITAL MASSES

Congenital defects may arise as a result of disturbances of the orderly embryological development of structures originating from the primitive branchial apparatus. The branchial apparatus develops during the third and fourth fetal weeks and persists until the end of the sixth week of fetal development.The branchial arches, which form in the cephalic region, consist of five parallel bars of mesoderm from which cartilage, bone, and muscle form. Each of these bars has its own nerve and blood supply.The branchial arches are separated externally by branchial clefts or grooves consisting of ectoderm.They are separated internally by branchial or pharyngeal pouches lined by entoderm. Each of the clefts and pouches differentiates into various anatomical structures.

Branchial cleft anomalies represent disturbances in the mechanics of precise embryological development in the head and neck region.A clear understanding of the developmental anatomy is necessary to understand the clinical implications of disorders of development in the neck.

DEVELOPMENTAL ANATOMY

The branchial apparatus appears to undergo its major development and differentiation between the third and

VASCULAR MALFORMATIONS

SUMMARY

SUGGESTED READINGS

SELF-TEST QUESTIONS

seventh embryological weeks.This apparatus is composed of a group of five mesodermic arches that are paired and separated by four pairs of invaginations of ectoderm and entoderm referred to respectively as branchial cleft pouches. Each of these arches is supplied by its own artery and nerve and develops into well-defined structures of skeletal muscle and connective tissue in the fully formed fetus. Each of the clefts and pouches differentiates into various anatomical structures,as noted in Table 17-1. Over 90% of the branchial cleft defects in the human arise from the second pouch and groove and consequently are said to pass between the internal and external carotid arteries. Several theories exist as to how branchial cysts and fistulas are formed.The most widely accepted theory is that these fistulas are derived from remnants of branchial grooves and pouches that have a failure of either fusion or burying of cell rests of the branchial grooves.

Knowledge of the vascular supply of each arch is critical in understanding the implications of distortions of vascular development in the newborn. Each arch is supplied by a central artery, which then connects the paired primitive dorsal and ventral aortas.The first arch artery probably results in the development of the facial branch of the external carotid artery. The artery of the second

arch becomes vestigial but may persist as the stapedial artery on rare occasions. The arteries of the third arch fuse at the primitive cephalad extension of the dorsal aortas to form the internal carotid arteries, and the caudal ends of the dorsal aortas fuse to become the descending aorta. The ventral aortas give rise to the common and external carotid arteries.The right fourth artery becomes the subclavian artery and the left fourth becomes the aortic arch.The left fifth becomes the pulmonary artery. All other branches most likely disappear.

The nerves supplied to each of the five arches are as follows: first arch, V; second arch, VII and VIII; third arch, IX; fourth arch, X; and fifth arch, XI.

CLASSIFICATION

Numerous terms are used to describe the clinical findings in disorders of branchial (cervical) development. A branchial cyst refers to a mucosal or epithelial lined structure with no external openings. These cysts may contain respiratory epithelium as well.There are considerable amounts of subepithelial lymphoid tissue, and sebaceous glands and salivary tissue may also be present. These cysts are usually filled with a viscid fluid.

A branchial sinus refers to a tract, with or without a cyst, that communicates with the skin.These tracts may be lined by pseudostratified columnar epithelium, and lymphoid tissue may be present.

A branchial fistula refers to a tract that communicates with the pharyngeal or hypopharyngeal structures. The opening in the pharynx may extend to either a cystic structure in the neck or directly to an external opening in the skin.

CLINICAL PRESENTATION

Branchial cleft cysts represent the most common noninflammatory lateral neck masses in children.These anomalies most commonly originate from the first, second, and third branchial apparatus. Fourth branchial anomalies are extremely rare.

Anomalies of the first branchial apparatus are uncommon. They make up less than 1% of all branchiogenic abnormalities.These anomalies are usually related to the auricle and surrounding face and should be considered to be distinct from preauricular cysts and sinuses that result from failure of fusion of the aural folds of the first branchial arch.They are subdivided into two types.Type I