497CHAPTER 44 Overview of Breathing Failure disorders of muscle (e g , Duchenne muscular dystrophy, con genital myopathies) The hallmark of these diseases is muscle pa resis or paralysis Hypercapnia i[.]
CHAPTER 44 Overview of Breathing Failure disorders of muscle (e.g., Duchenne muscular dystrophy, congenital myopathies) The hallmark of these diseases is muscle paresis or paralysis Hypercapnia is a common feature Depending on the extent of weakness, patients will be unable to increase their work of breathing in the face of lung disease Importantly, clinical signs such as tachypnea and accessory muscle usage cannot be relied on Abnormal arterial blood gases may be the only indications of worsening breathing failure, especially in patients with altered sensorium Patients with spinal cord lesions develop muscle weakness below the level of injury For instance, patients with lower cervical cord injury may have preservation of suprasternal muscle action but have loss of other accessory muscle function as well as diaphragmatic dysfunction or paresis These patients will have vigorous suprasternal retractions that expand the lung apices but have little other movement Structural Brain Lesions Moderate brain injuries are typically associated with hyperventilation, whether the injury is traumatic, infectious, or hypoxicischemic The hypermetabolic state, lung pathology, and loss of inhibitory cortical influences probably combine to augment ventilation Even when the brain-injured patient does hyperventilate, airway protective reflexes are usually impaired, seizures may ensue, and subtle progression of the brain lesion may lead to hypoventilation Resulting hypoxia exacerbates the brain injury Nonstructural Acquired Disorders Seizures impair breathing in various ways Apnea or slowing of respirations, impairment of upper airway protective reflexes, and poor inspiratory effort are common Because infants can have subtle manifestations of seizure, the clinician must have a high index of suspicion to recognize occult seizures as a cause of hypoventilation The seizure-induced respiratory depression may be difficult to distinguish from the brain pathology predisposing to the seizure (e.g., infection, hemorrhage) as well as the respiratorydepressing effects of anticonvulsant medications Respiratory depression by analgesic, sedative, anticonvulsant, and anesthetic agents is common in the pediatric intensive care unit Relative effects on upper airway patency and hypoxic, hypercapnic, and loading responses may be dissociated Concern regarding respiratory depression does not warrant withholding analgesia, however, but rather warrants appropriate monitoring In fact, the patient experiencing pain may breathe shallowly, exacerbating hypoxia Sedatives and analgesics that are rapidly cleared after a single dose may have a more prolonged duration of action when given repeatedly or continuously Clearance rates for medications may also vary with systemic disease, immaturity, or genetic factors Sedation and analgesic-induced respiratory depression may unnecessarily prolong the need for mechanical ventilation Opioid-induced respiratory depression can be reversed with naloxone However, in patients with cardiovascular compromise (e.g., those who have had cardiac surgery), naloxone should be avoided in the immediate postoperative period because abruptly eliminating analgesia can provoke dangerous increases in catecholamines In the patient with multiple drug ingestions, naloxone may induce vomiting without improving airway protective reflexes, predisposing the patient to aspiration The benzodiazepine antagonist flumazenil reduces the respiratory depression that results from taking benzodiazepines, but little pediatric experience with this agent has been reported Flumazenil lowers the threshold 497 for seizures and may cause a more hazardous condition than the initial respiratory depression Importantly, the duration of action of antagonists is often shorter than the agent that is depressing breathing Close monitoring of the patient is essential, and repeated doses of antagonists may be necessary In other cases of drug-induced respiratory depression, mechanical ventilation provides greater safety than pharmacologic antagonists, particularly with multifactorial central depression or in severely ill patients Other medications may depress breathing without alteration of consciousness For example, prostaglandin E1, which is given to maintain patency of the ductus arteriosus in infants with congenital heart disease, is frequently associated with respiratory depression Metabolic alkalosis, which may result from diuretics and other medications, can promote respiratory depression and may contribute to prolonged dependence on mechanical ventilation When metabolic alkalosis accompanies prolonged recovery from respiratory failure, correction of the alkalosis may promote ventilator weaning Chronic Respiratory Failure Congenital or long-standing acquired disorders of the central nervous system may impair respiratory centers, leading to respiratory failure Acute respiratory insufficiency may accompany progression of a central lesion Alternatively, static regulatory impairment may be revealed by failure to compensate for acute systemic illness Structural Brain Lesions Recognition of structural brain lesions as the cause of impaired respiratory regulation is important because some of the lesions are correctable Congenital structural neurologic malformations may manifest as apnea or profound hypoventilation at birth or may be recognized later In particular, patients with Chiari malformations often have central and obstructive sleep apnea.10 Surgical decompression may be associated with improvement even when performed in adults.11 Acquired lesions, such as posterior fossa tumors, may interfere with respiratory regulation before or after surgical resection Nonstructural Congenital Disorders Some genetic conditions are associated with derangements in regulation of breathing Children with congenital central hypoventilation syndrome12 have characteristic mutations in the PHOX2B gene, which is critical for function of the retrotrapezoid nucleus.2 These patients may first come to medical attention because of growth failure, neurodevelopmental disabilities, or cor pulmonale Other abnormalities include autonomic dysfunction and cardiovascular instability, occasional association with Hirschsprung disease, tumors of neural crest origin, and impaired controls of breathing The syndrome may be recognized in the newborn period, later in childhood, and rarely in adults Sleep hypoventilation predominates in persons with congenital central hypoventilation syndrome, although some patients also experience respiratory insufficiency while awake The disorder is often fatal without mechanical ventilation Early mechanical ventilation may reduce the sequelae and improve long-term neurodevelopmental outcome Prader-Willi syndrome is a multigenic disorder initially presenting with hypotonia and then with progressive obesity, growth failure, neurodevelopmental disabilities, reduced ventilatory response to hypoxia and hypercapnia, sleep hypoventilation, and apnea.13 498 S E C T I O N V Pediatric Critical Care: Pulmonary Rett syndrome is an X-linked disorder affecting development, behavior, autonomic and respiratory regulation, and seizures Occurring only in girls, most cases involve spontaneous mutations, although occasionally other family members are affected Abnormalities are often present in the MECP2 gene, although some patients with characteristic clinical features have other genetic findings Multiple phenotypes exist in regard to the respiratory control disorder, with hyperventilation, hypoventilation while awake or asleep, and apneustic breathing.14 Many other genetic syndromes with severe neurologic manifestations have impaired upper airway motor function, respiratory cycle timing, and respiratory effort nonspecifically associated with their brain disorder Nongenetic congenital disorders may impair respiratory controls For example, children with cerebral palsy occasionally have neurologic deficits of pharyngeal tone, although the central drive to breathe is usually intact For this reason, these patients may paradoxically have improved ventilation with sedation during times of significant respiratory distress caused by noxious stimuli Nonstructural Acquired Chronic Disorders Some patients with severe chronic respiratory disease have blunted ventilatory responses to hypoxia, hypercapnia, or respiratory mechanical loads A concern regarding supplemental oxygen is sometimes raised in the care of patients with acute exacerbations of chronic respiratory disease It is sometimes argued that administration of supplemental oxygen causes respiratory failure in patients with chronic CO2 insensitivity who might depend on hypoxic drive to breathe However, the adverse effects of hypoxia are a greater concern Because the hypoxic drive to breathe increases substantially only at oxygen tension below 50 mm Hg (see Fig 44.2A), it is virtually impossible to maintain stable respiratory stimulation with mild, “safe” hypoxia without risking episodic life-threatening episodes If a patient is so poorly compensated that removal of hypoxic drive results in hypoventilation, then mechanical ventilation may be the safest management strategy unless end-of-life plans specifically exclude mechanical ventilation Several clinical conditions are commonly associated with hypoventilation Obesity causes hypoventilation by a complex interaction of factors, including mechanical loads on the respiratory system, reduction of lung volume, upper airway obstruction, and impaired respiratory regulation In obese patients, weight loss often improves hypoventilation.15 Apparently healthy preterm infants may have postanesthetic apnea until the age of 60 weeks’ postconceptional age.16,17 Apneic events occurred within hours of surgery in 72% of patients, but in the remainder, respiratory irregularity began as late as 12 hours postoperatively Both obstructive and central mechanisms of apnea were observed Continuous monitoring for at least 12 hours after anesthesia is warranted when surgery is required for infants born prematurely who are still younger than 60 weeks’ postconceptional age Sleep hypoventilation tends to occur in adults with hypothyroidism and diabetes mellitus.18 Little information is available regarding the pediatric patient or the specific role of the endocrine disorder versus the effect of obesity Respiratory Pump Failure Causes of respiratory pump (breathing muscle) failure are listed in Table 44.3, along with examples They may be crudely divided into direct causes of muscle failure (plegia, paralysis, and tetany) or muscle failure from exhaustion Exhaustion may result from impaired delivery of substrate to the muscles or from increased TABLE 44.3 Causes of Breathing Muscle Failure Mechanism Example Overwork Lung dysfunction, airway obstruction Inadequate substrate Shock, hypoxemia Muscle plegia Hypokalemia Muscle tetany Tetanus, hypocalcemia load due to increased ventilatory demands, or mechanical failure of the respiratory system, including restrictive or obstructive lung disease In most cases, patients progress through a typical phase of compensation based on the mechanism of the insult, allowing clinicians to identify and attempt to intervene before frank breathing failure Respiratory Plegia, Paralysis, and Tetany There are a few conditions in which muscle contraction may be impaired by dysfunction of the myocyte Tetrodotoxin (puffer fish) blocks the fast voltage-gated sodium channel of the muscle cell, thereby causing paralysis Hypokalemia may impair muscle contraction; hypokalemic periodic paralysis has similar effects Profound hypophosphatemia can impair the availability of highenergy substrate and is a cause of respiratory failure Tetanus and hypocalcemia are capable of causing tetanic contraction that can impair breathing All these disorders can result in hypoventilation and require intervention Muscle Exhaustion When challenged, muscle tension can be augmented by increasing either the frequency of firing or number of motor units being fired At low muscle tension, the number of motor units participating in contraction may be increased before frequency is raised Recruitment is used to increase work To achieve an even greater increase in force, the frequency of firing of individual motor units may be raised, such that while the number of motor units is held constant the work of each motor unit is increased In pediatric critical care, exhaustion is the most common cause of failure of the muscles of breathing The respiratory muscles may become exhausted when responding to excessive workload Airway obstruction (fixed or functional), lung stiffness, thoracic stiffness (e.g., anasarca), abdominal distension, air trapping, and inefficient ventilation-perfusion matching (e.g., high ventilationperfusion mismatch, which functionally wastes ventilation) are all examples Respiratory muscles may also become exhausted if their effort is not supported by adequate nutrition, blood supply, and oxygen delivery Respiratory distress can be likened to running a marathon There can be a “wall” beyond which respiratory muscle metabolism is not able to sustain further respiratory effort As discussed later, the clinical hallmarks of such failure are brief respiratory pauses followed by longer periods of apnea Without assistance, after these muscles reach their wall, respiratory arrest occurs Working skeletal muscles rely on a continuous supply of oxygenated blood Diaphragmatic function can be impaired if blood flow or oxygenation is reduced Diaphragmatic muscle cannot operate at optimal length (force-length relationship) to generate the appropriate contraction (force-velocity relationship) if energy demand outstrips energy supply The combination of suboptimal CHAPTER 44 Overview of Breathing Failure force-length and force-velocity relationships causes rapid, shallow breathing, largely from dysfunction of type IIB fast-twitch glycolic fibers Respiratory muscle fatigue also develops during exhaustive exercise Prolonged malnutrition has also been shown to affect the diaphragm’s muscle structure and impair its ability to generate force On the other hand, it has been shown that respiratory muscle training can lessen the development of respiratory muscle fatigue.19 Training of the diaphragm can increase capillary density, myoglobin content, mitochondrial enzyme concentration, and the concentration of glycogen, but persistent mechanical ventilation (particularly during deep sedation or paralysis) decreases muscle strength by allowing disuse muscle atrophy In acute illness, breathing fails if respiratory muscle demand for blood flow, metabolic substrate, and oxygen delivery outstrips supply, just as it does in exhaustive exercise.20 The point at which this occurs is influenced by many factors, including the energy cost of breathing, duration of contraction per breath, velocity of contraction, operational length of muscle fibers, energy supply, efficiency of muscles, and state of muscle training.21 In both shock and hypoxemia, oxygen delivery to respiratory muscles may prove inadequate to meet demand on a minuteby-minute basis.22 Skeletal muscle may sustain a transient oxygen debt by unloading oxygen from myoglobin, but such reserve is limited Skeletal muscle can also perform anaerobic metabolism to generate adenosine triphosphate, but only until levels of reducing substances (diphosphopyridine nucleotide) build up to such a degree that they inhibit the activity of Krebs cycle enzymes Over a range of oxygen delivery, metabolic use of oxygen is insensitive to rates of supply (delivery), but below some threshold, muscle aerobic metabolism must inevitably be reduced.23 Mitochondrial dysfunction may aggravate deficient muscle metabolism in sepsis Impaired oxygen utilization may contribute to failure of breathing in other mitochondrial crises as well One of the indications for intubation and mechanical ventilation in patients with septic shock is to prevent respiratory arrest from failure of oxidative metabolism and exhaustion of the muscles of breathing Failure of Mechanics of Breathing Mechanical factors may pose acute or chronic impediments to breathing These factors may be intrinsic or extrinsic to the lungs and airways All place a burden on respiratory muscles, increasing the propensity to become exhausted; some, when severe, cannot be overcome by any effort (e.g., laryngotracheal foreign body, tension pneumothorax) Exhaustion is the primary mechanism of breathing failure Both excessive demand and impaired supply may come into play; in many children, poor thoracic mechanics and neurologic impairment contribute to breathing failure from acute lung disease Chronic or congenital disorders—such as chest and spinal deformities, diaphragmatic eventration, prune belly syndrome (in which abdominal musculature is virtually absent), and deformities that flatten the diaphragm—may either cause chronic respiratory insufficiency or contribute to intolerance of intercurrent processes such as pneumonia Typically, lung disease increases the work of breathing Hypoxia and hypercarbia drive the respiratory muscles toward exhaustion Efficiency of the respiratory system is impaired by lung regions of high ventilation-perfusion ratio (dead space ventilation) Because these regions see scant blood flow, they actually waste ventilation and breathing effort On the other hand, low ventilation-perfusion ratio segments cause hypoxemia, which 499 impairs oxygen delivery to tissue and makes circulation inefficient When oxygen delivery is too low, muscle oxygen utilization becomes delivery dependent, and muscle work capacity declines Other factors—such as the compliance of the infant chest (which wastes breathing effort), the deformity of kyphoscoliosis (which makes breathing less efficient), the inefficiency of the infant diaphragm that operates at a wide angle to the chest wall, abdominal distension (which further widens that angle and opposes descent of the diaphragm), and the nutritional issues of chronic illness— accelerate the progression toward respiratory arrest Superimposed immaturity, neuromuscular dysfunction, or other comorbid conditions may also exacerbate breathing failure As breathing failure worsens, fatigue causes the patient’s respiratory effort to deteriorate Patients use accessory muscles less, develop brief respiratory pauses, and progress to apnea followed by respiratory arrest Respiratory pauses are a subtle but helpful warning sign that should be interpreted as impending respiratory arrest and suggest that respiratory support is indicated Another helpful sign is grunting, a low-pitched sound produced by partial or total closure of the glottis in expiration Grunting is thought to augment expiratory lung volume (functional residual capacity [FRC]) and increase arterial oxygen tension much like positive end-expiratory pressure.24 Grunting is also a warning of possible impending arrest in children and adults with respiratory failure,25 though it also often occurs immediately after birth and may quickly resolve as the neonate successfully navigates transition Patients may appear anxious and describe a feeling of air hunger Mental status changes ranging from panic to obtundation may occur Abrupt respiratory slowing and gasping are harbingers of respiratory arrest Terminal Failure of Respiratory Control and Mechanics In the advanced stage of respiratory failure, the vigorous respiratory effort of the dyspneic patient may become counterproductive Agitation increases oxygen consumption and forced respiratory efforts may cause dynamic obstruction of airways Dynamic airway obstruction in the dyspneic child may account for rapid progression of respiratory failure in some cases As the severely dyspneic patient decompensates, exhausted efforts may rapidly give way to periodic breathing and apnea While this phenomenon is commonly observed in infants with lower respiratory tract infections26 and pertussis,27 observations in adults with near-fatal asthma reveal a similar tendency for respiratory arrest to precede cardiovascular collapse.28 The mechanism of this terminal respiratory depression with severely loaded breathing is not well understood, but it appears to occur in some patients before the development of life-threatening hypoxia and hypercapnia Restrictive Versus Obstructive Respiratory Disease Mechanical causes of breathing failure can be divided into two pathophysiologic categories: restrictive and obstructive pulmonary processes Though both ultimately can lead to breathing failure, their etiologies, mechanics, and clinical manifestations often differ Restrictive diseases are those that limit lung expansion These include processes that (1) fill alveoli with blood, infectious material, edema, or other debris; (2) involve expansion or swelling of the alveolar interstitium; (3) compress the lung, as with pneumothorax or effusion; or (4) impair excursion of the chest wall or thoracoabdominal region because of neuromuscular dysfunction, 500 S E C T I O N V Pediatric Critical Care: Pulmonary skeletal deformity, abdominal distension, ascites, or anasarca These processes are characterized by a reduction in vital capacity and small resting lung volumes, but normal or near-normal airway resistance The physical examination reflects these processes Patients are tachypneic, taking rapid, shallow breaths Auscultation of the chest may reveal fine inspiratory crepitations (crackles) or rales as evidence of parenchymal lung disease Poor excursion of the chest wall is usually readily appreciated Retractions (subcostal and intercostal) are common and indicate significant respiratory effort Obstructive diseases of the lung are common in childhood and are characterized by obstruction to flow in airways In contrast to restrictive diseases, patients are often less tachypneic relative to their work of breathing In fact, with mild obstruction, the respiratory rate is often slower than normal but rises as ventilation perfusion inequality develops and increases respiratory drive Often, one phase of respiration is more prolonged than the other and may have associated adventitial sounds, which can help identify the level of obstruction, as detailed later Obstructive diseases may be classified as extrathoracic (above the thoracic inlet) or intrathoracic (below the thoracic inlet) Obstruction may be the result of occluding material or tissue in airways, reduction in lumen caliber from elevated tone of the smooth muscle of airway walls or swelling of tissue, weakness of the airway wall causing collapse and impeding gas flow, or extrinsic compression of airways In some diseases, several of these processes occur simultaneously Indeed, secondary obstruction is a common phenomenon Obstruction in a proximal airway may cause turbulent gas flow downstream in distal airways Turbulent gas flow causes the wall of the still-developing airway to flutter, further weakening the wall’s structure and exacerbating overall obstruction The hallmark of extrathoracic obstruction is inspiratory noise (stridor or stertor) and prolonged inspiration The affected airway segment lies between the nose and proximal trachea Inspiratory stridor is a vibratory sound heard because the reduction of intrathoracic pressure during inspiration narrows the extrathoracic (subglottic) airway, generating an inspiratory noise Stertor is a snoring noise generated in the nasopharynx Airways with severe obstruction flutter in both inspiration and exhalation, causing biphasic stridor or noise heard in both phases of the respiratory cycle Other characteristic sounds may help to identify the obstructed airway segment (Table 44.4) In extreme obstruction, patients may position themselves to maximize airway caliber (remaining upright, leaning slightly forward, and holding the head in the sniffing position to enhance alignment of the pharynx and larynx) With severe obstruction, there is little airflow; therefore, little noise is generated Loss of noise, despite increased effort, is an ominous sign indicative of complete obstruction and impending respiratory arrest The hallmark of intrathoracic obstruction is a prolonged expiratory phase and adventitial expiratory sounds In intrathoracic obstruction, forced expiration compresses soft airways, causing a musical wheeze This obstruction may be largely relieved by inspiration, which tends to dilate intrathoracic airways In severe intrathoracic obstruction, sounds may be heard during both phases of the respiratory cycle The classic pediatric disease of intrathoracic obstruction is asthma High-pitched wheezes on exhalation are heard early in the episode As airway obstruction worsens, wheezes are heard in inspiration as well With severe obstruction, wheezes diminish because there is little gas flow Tracheobronchomalacia may cause expiratory wheezing and/or expiratory stridor With severe collapse of the airway, complete absence of exhalation TABLE Abnormal Sounds Indicative of Extrathoracic 44.4 Airway Obstruction Sound Condition Hoarseness Unilateral vocal cord paralysis Muffled voice Supraglottic or infraglottic processes, including epiglottitis “Hot potato” voice Oral, retropharyngeal abscess Cellulitis or connective tissue infection of the floor of the mouth, also known as Ludwig angina “Barking” cough Laryngotracheobronchitis (croup) Monotone, hurried sentences Bilateral vocal cord paresis may occur With expiratory obstruction, hyperinflation of the lung can occur, promoting superimposed restrictive physiology, which promotes tachypnea that shortens the expiratory time, promoting a vicious cycle of worsening hyperinflation Compensatory Mechanisms in Breathing Failure A patient may try to compensate for the functional effects of lung disease These compensatory mechanisms generally come into play before there is evidence of breathing failure Many of the clinical signs of respiratory distress, discussed previously, are evidence of compensatory mechanisms Understanding these mechanisms improves one’s recognition of impending failure Compensatory Mechanisms in Restrictive Lung Disease Tachypnea is the patient’s primary compensation for the small lung volume of restrictive lung disease and is the earliest detectable clinical sign Additional compensation is achieved by recruitment of accessory muscles Patients with restrictive disease may take periodic sigh breaths, which are larger than tidal breaths, to recruit collapsing units Compensatory mechanisms also operate to maximize gas exchange in diseased lungs Hypoxic pulmonary vasoconstriction is an important mechanism to improve gas exchange in normal lungs Hypoxic pulmonary vasoconstriction is a direct response of the vascular smooth muscle to low Pao2 alveolar units The precapillary arteriole of such units constricts in response to low oxygen tension in the adjacent postcapillary venule, thereby directing blood away from poorly functioning alveoli In a lung with patchy disease, the overall effect of the hypoxic pulmonary vasoconstriction response is to shunt blood away from diseased segments and allow flow to healthier areas This may, however, increase pulmonary vascular resistance and oppose right ventricular ejection Inhaled nitric oxide (iNO) provides a potential exogenous means to improve ventilation perfusion matching (by preferentially dilating vessels to ventilated lung segments) without afterloading the right ventricle, although enduring benefits with iNO have not been demonstrated.29 Compensatory Mechanisms With Obstructive Lung Disease The major compensations in obstructive disease focus on maximizing airflow As previously stated, patients naturally position themselves to maximize opening of their airway If this is the case, repositioning patients, especially to the supine position, may CHAPTER 44 Overview of Breathing Failure worsen airflow For the infant, carefully monitored prone positioning may aid gas exchange and assist spontaneous breathing.30,31 Control of respiratory rate provides another means of compensation In mild obstructive disease, the respiratory rate is lower than normal As resistance to airflow rises, work of breathing also rises greatly To maximize efficiency, the respiratory rate falls Longer respiratory cycle times allow longer times for gas flow Having said this, the clinician will recognize that many patients with obstructive lung disease present with tachypnea, not decreased respiratory rates The causes of tachypnea are (1) ventilation/ perfusion mismatching with hypoxemia, and sometimes hypercarbia, driving the respiratory rate and (2) development of hyperinflation or atelectasis in unventilated lung segments, resulting in the superimposition of a restrictive process on an obstructive one Tachypnea in such patients is counterproductive, greatly increasing the work of breathing and further diminishing gas flow respiratory insufficiency In the infant with severe restrictive disease, recruitment of additional diaphragmatic and accessory muscles, use of compensatory braking and grunting maneuvers, and increases in respiratory rate may be insufficient to maintain a normal FRC Fatigue comes quickly Moreover, such work is extremely energy expensive Infants with chronic respiratory insufficiency can use as much as 50% of their caloric intake for breathing, leaving few calories for growth and other functions, resulting in failure to thrive Infants are also disadvantaged with respect to obstructive lung disease In infants and young children, a greater percentage of total airway resistance is apportioned to large airways than in adults Infants are particularly susceptible to nasal obstruction, as occurs during upper respiratory infection, because the nose may comprise as much as 50% of total airway resistance Moreover, resistance to airflow is proportional to the inverse of the airway radius to the fourth power: Special Conditions Several conditions deserve special note In these, physical findings may reflect specific aberrations that generate specific compensatory mechanisms Infancy As has been mentioned elsewhere, young infants are at a mechanical disadvantage for efficient breathing The configuration of the infant’s chest wall differs from that of adults Orientation of the ribs is more horizontal in infants than in adults, and they move less during breathing The chest wall is more compliant and is composed of more cartilaginous tissue Strength of intercostal muscles is less In the absence of muscular action, FRC is determined by the elastic forces of the lung and the chest wall, which oppose each other Accordingly, the infant’s more compliant chest wall and weaker musculature results in lower FRC Infants with lung disease use expiratory braking (grunting), which involves constriction of pharyngeal muscles and glottis, to increase endexpiratory lung volume Although this promotes higher FRC, it imposes a disadvantageous increase in muscular work Infant respiratory muscle fibers differ from those of adults The infant diaphragm contains a greater proportion of type II fibers, which are unable to sustain repeated strenuous activity.32 Hence, the infant’s diaphragm fatigues more quickly than that of the adult.33 The infant’s diaphragmatic anatomy is also disadvantageous The reduced appositional area and greater diaphragmatic angle of the infant causes less lung volume expansion with diaphragmatic contraction Because pulling the rib cage cephalad produces less outward chest displacement in the infant than in the adult, the infant’s tidal volume is, in the aggregate, more dependent on diaphragmatic contraction than that of the adult Superimposition of breathing failure on infant respiratory function exacerbates these mechanical disadvantages In normal respiration, the chest wall and abdomen move inward and outward in synchrony With restrictive lung disease, the respiratory pattern may be out of phase, called paradoxical breathing Contraction of the diaphragm pulls the compliant infant chest wall inward during inspiration and pushes abdominal contents outward This respiratory pattern is greatly exaggerated by severe restrictive lung disease or inspiratory obstruction With decreased lung compliance, the pleural pressure swing is exaggerated, pulling the chest wall farther inward Prolonged respiration in this manner can cause inward deformation of the sternum, or acquired pectus excavatum, which is a clinical sign of prolonged 501 R 8 L/ r where h is gas viscosity, L is airway length, and r is the radius of the airways Resistance is greater in infants and young children because of their intrinsically small airways Further reduction in airway caliber with obstructive disease (e.g., bronchiolitis) magnifies this problem The infant’s airways are also less endowed with cartilage than are those of the adult and may be subject to flow limitation during active expiration The trachea and bronchi may be pathologically compressed during expiration, causing severe obstruction that is worsened by expiratory effort Such regions of severe and pathologic flow limitation (tracheomalacia and bronchomalacia) may be localized Similarly, the larynx may be compressed (by atmosphere) during forced inspiration (laryngomalacia) if there is proximal (supraglottic) obstruction Sleep Sleep modifies breathing, and compensation for respiratory illness is most likely to fail during sleep.34 In some persons, ventilatory responses to hypercapnia and hypoxia diminish during sleep Sleep-induced reduction in upper airway tone and cough reflexes worsens the risk of obstruction and aspiration In infants, the thorax is compliant Awake lung volume is maintained by thoracic muscle tone and by breathing at sufficiently high frequencies that expiration seldom reaches FRC During sleep, inspiratory muscle tone diminishes and respiratory rate decreases, with resulting reduction in the infant’s expiratory lung volume The infant’s compensation for mechanical loads is compromised during the rapid eye movement stage of sleep more than during quiet sleep Thoracic Dysfunction Neuromuscular disorders generally result in restrictive lung defects Coexisting obstructive lung disease can be seen with some thoracic defects or with scoliosis Persistent atelectasis and longstanding lung hypoplasia may lead to atrophy and may destroy supporting airway architecture, resulting in air trapping Deformities of the rib cage and spinal column result in restriction to lung expansion, as occurs with isolated scoliosis.35,36 The most severe of these are classified as “asphyxiating thoracic dystrophies” in which the chest fails to expand at all during breathing As with the other forms of neuromuscular diseases, hypercapnia predominates Muscle strength may be normal, but abnormal configuration of intrathoracic muscles and diaphragm may make muscle work inefficient ... metabolism and exhaustion of the muscles of breathing Failure of Mechanics of Breathing Mechanical factors may pose acute or chronic impediments to breathing These factors may be intrinsic or extrinsic... illness, breathing fails if respiratory muscle demand for blood flow, metabolic substrate, and oxygen delivery outstrips supply, just as it does in exhaustive exercise.20 The point at which this occurs... combination of suboptimal CHAPTER 44 Overview of Breathing Failure force-length and force-velocity relationships causes rapid, shallow breathing, largely from dysfunction of type IIB fast-twitch