492 44 Overview of Breathing Failure KATHERINE V BIAGAS, MICHAEL WILHELM, AND BRADLEY P FUHRMAN PEARLS Physiology of Breathing Respiration involves movement of air (breathing), diffusion of gases betw[.]
44 Overview of Breathing Failure KATHERINE V BIAGAS, MICHAEL WILHELM, AND BRADLEY P FUHRMAN Physiology of Breathing Respiration involves movement of air (breathing), diffusion of gases between alveolus and pulmonary circulation, circulation of blood between tissue and lung, and tissue energy metabolism This chapter provides an overview of spontaneous breathing and breathing gone awry, setting the stage for later chapters on respiratory disorders The term breathing failure is used in this discussion to limit consideration to failure of the respiratory pump that drives air movement Breathing failure is arguably the most common cause of arrest in infants and children Normal breathing requires neural control to maintain airway patency and drive the muscles of the respiratory pump Powerful neural regulation of breathing maintains a constant supply of oxygen to the tissues, despite wide variations of metabolic rate and respiratory system disorders, until an advanced stage of respiratory failure is reached Each of these components of breathing may be deranged in isolation or in combination to produce disordered breathing and, ultimately, breathing failure The physiology under normal and stressed circumstances leading to these clinical states is discussed in this chapter Controls of Breathing Several neural inputs regulate the drive of the respiratory pump and the muscles that maintain airway patency Derangements of respiratory controls may be the primary cause of acute respiratory failure or one of multiple causes in a critically ill or injured patient In other patients, disorders of respiratory regulation prolong dependence on mechanical ventilation Disorders of respiratory controls may be difficult to distinguish from, 492 • • • Three pathways to breathing failure are impaired neural control, failure of the muscles of breathing, and dysfunction of the mechanics of breathing Dysfunction of the mechanics of breathing can contribute significantly to respiratory muscle workload, predominantly through imposition of inefficiencies Assisted ventilation can (1) prevent breathing failure from progressing to respiratory arrest, (2) improve gas exchange, and (3) reduce metabolic expenditure for muscle work in the patient with limited reserves • • • PEARLS Patients with neuromuscular or respiratory control disorders often not exhibit typical clinical signs of respiratory distress and may have indolent respiratory failure Infants have a number of mechanical disadvantages in breathing that predispose to breathing failure and respiratory arrest Multiple causes of breathing failure may occur concomitantly and may be synergistic in causing respiratory failure Recognizing impending respiratory failure allows institution of mechanical respiratory support prior to frank respiratory arrest and may combine with, muscle failure and altered respiratory mechanics Metabolically produced carbon dioxide (CO2), and the resultant change in pH drive breathing through central chemoreceptors The retrotrapezoid nucleus (RTN) within the ventral medulla senses partial pressure of carbon dioxide (Pco2) through its effects on local pH and stimulates respiration.1,2 The RTN, in turn, innervates the pre-Bötzinger complex of the ventrolateral medulla oblongata in addition to other brain regions, which generates the rhythmic discharges whose timing corresponds to inspiratory and expiratory phases of respiration.3,4 During resting inspiration, efferent activity stimulates diaphragm flattening Increased pharyngeal muscle tone and vocal cord abduction act to keep the upper airway patent, allowing inspiratory airflow During expiration, inspiratory muscles relax Resting expiration is passive, driven by elastic recoil of the lung and chest wall The respiratory rate, inspiratory time, and motor intensity are modified by a variety of rostral neural, chemical, and mechanical stimuli (Fig 44.1) With high-intensity stimulation, accessory muscles of breathing are activated, including intercostal and neck muscles Nasal flaring also occurs Both enhance inspiratory airflow Stimulated breathing also may include end-inspiratory (premature) vocal cord closure that prolongs lung inflation with little energy expenditure, which is clinically manifested as grunting In highly stimulated breathing, abdominal muscles contract to force expiratory airflow Other normal respiratory control behaviors include periodic deep inspirations, sneezing, and coughing to maintain expansion of basal lung areas and clear secretions from the respiratory tract Breathing is normally coordinated with swallowing, vocalizing, and airway protective reflexes to prevent aspiration.5 In extreme CHAPTER 44 Overview of Breathing Failure 493 Ventilation Rostral brain influences Brainstem respiratory centers Carotid body chemoreceptors Respiratory muscles: Upper airway diaphragm accessory 50 A 100 500 PaO2 Metabolic alkalosis Airway Lung Thorax Ventilation Brainstem chemoreceptors Alveolar ventilation pHa PaCO2 PaO2 B PaCO2 Metabolism • Fig 44.1 Elements of the respiratory control system Pao2, Partial pres- circumstances, pharyngeal muscle tone may be adjusted with neck flexion and extension to maintain airway patency Peripheral chemoreception in the carotid body modulates the ventilatory response and provides powerful stimulation in response to hypoxemia Peripheral chemoreceptor activity and ventilation increase slightly as partial pressure of arterial oxygen (Pao2) falls below 500 mm Hg However, ventilation rises steeply as Pao2 falls below 50 mm Hg and acts as a ventilatory stimulus (Fig 44.2A) Furthermore, low arterial oxygen tension, rather than low oxygen content, is the ventilatory stimulus This is supported by the observation that there is little carotid body response with profound anemia alone Hydrogen ion concentration and CO2 tension independently activate chemoreceptors in the carotid body and brainstem (Fig 44.2B) The simultaneous presence of hypoxia augments the hypercapnic ventilatory response (Fig 44.2C) Mechanical loads on breathing influence respiratory efforts immediately, independent of chemical stimuli Sensors for loadcompensating reflexes are located in respiratory muscles and the chest wall Reduction in lung volume is also detected by pulmonary stretch receptors Afferent signals from loaded breathing travel via the spinal cord, vagus nerves, and perhaps the phrenic nerves Both conscious and reflex responses are involved in compensatory increases of effort, including the recruitment of accessory muscles in response to increased respiratory resistance or to a decrease in compliance Stimulation to breathe is further augmented by hypercapnia or hypoxemia when loaded breathing reduces ventilation Mechanical loads to breathing, and the resultant Hypoxia Ventilation sure of arterial oxygen; Paco2, partial pressure of arterial carbon dioxide; pHa, arterial blood pH C PaCO2 • Fig 44.2 (A) Chemoreceptor activity, as measured by minute ventilation, varies as a function of partial pressure of arterial oxygen (Pao2) (B) Partial pressure of arterial carbon dioxide (Paco2)—superimposed metabolic alkalosis inhibits breathing (C) Paco2—superimposed hypoxia further stimulates breathing afferent stimuli, can actually promote hyperventilation and hypocapnia Dyspnea and anxiety exacerbate the tendency to hyperventilate even without a chemical ventilatory stimulus Respiratory Pump The respiratory pump comprises the diaphragm, the intercostal muscles, and the accessory muscles of breathing Each contributes to breathing to a varying degree depending on the neural input and respiratory load Diaphragm The diaphragm arises from the embryologic pleuroperitoneal fold Myoblasts migrate from cervical somites to the pleuroperitoneal 494 S E C T I O N V Pediatric Critical Care: Pulmonary Xiphoid process Central tendon Costal cartilages Inferior vena cava Esophagus Abdominal aorta Right crus Psoas major Quadratus lumborum Left crus • Fig 44.3 The diaphragm is attached circumferentially to the thoracic wall Its muscular portion extends onto the dome, where it is tendinous and flattened When the muscle contracts, the dome descends like a piston, enlarging the thorax and displacing abdominal contents downward fold, where they arrange themselves into a sheet on a mesenchymal substrate that separates the peritoneum from the abdomen Once fully formed, the diaphragm originates from bilateral tendinous crura attached to the spinal column and inserts as a costal tendon attached to the chest wall between the sixth and twelfth ribs The dome of the diaphragm remains largely tendinous This configuration, a circular attachment to the thoracic wall, vertical muscle orientation adjacent to the thorax, and attachment to a flattened central tendinous dome (Fig 44.3), works like a piston during breathing to enlarge the thorax and displace abdominal contents downward Approximately 50% of the diaphragm consists of type I fasttwitch muscle fibers, which have high endurance and are resistant to fatigue The remainder of the diaphragm is made up of type IIA and type IIB fibers, which have different properties.6 Type IIA fibers are important in achieving high levels of minute ventilation quickly, have good endurance, and can contract rapidly, but they are unable to sustain long-term power output Type IIB fibers cannot sustain their force of contraction because they possess lower oxidative capacity and are more susceptible to fatigue The greater the force of contraction required, the more motor units of the diaphragm are recruited There appears to be little difference between the activity of costal muscles and crural muscles, either during normal breathing or in response to hypoxia and hypercapnia When a patient lies supine, the diaphragm rests against the inner surface of the rib cage When the diaphragm contracts and its muscle fibers shorten, the whole diaphragm moves down, lowering pleural pressure and increasing intraabdominal pressure The increase in intraabdominal pressure generated by descent of the diaphragm acts as a caval pump to enhance cardiac filling.7 Because of its alignment against the lower ribs (zone of apposition), descent of the diaphragm also expands the caudal portion of the rib cage The muscle of the diaphragm extends from the costal insertion to the dome of the diaphragm When the diaphragm is “high,” it is loaded for greater force of contraction When it is “low” or “flat,” it is unloaded and mechanically disadvantaged The diaphragm is the major inspiratory muscle of the neonate It increases in thickness with age (as its muscle mass increases) It also becomes appositional to a longer segment of chest wall with growth, enhancing its effectiveness as an inspiratory piston.8 In the neonate, the muscular diaphragm has a greater angle from the vertical than that of the adult, which reduces its effectiveness as an air pump (Fig 44.4) This angle approaches zero with growth, increasing the diaphragm’s effectiveness with advancing age The importance of this angle as an impediment to diaphragmatic effectiveness becomes exaggerated at total lung capacity, with air trapping, and when the abdomen is distended, all of which flatten and unload the diaphragmatic muscle (Fig 44.5) Intercostal Muscles The rib cage is fixed to the spine and sternum The spine maintains a relatively fixed separation between adjacent ribs Vertical mobility of the sternum allows lift and descent of the anterior rib cage Thoracic volume is modified during breathing primarily by changing the angle of the anterior ribs to the horizontal At rest, the ribs slope caudad from their spinal attachments The rib cage tilts upward during inspiration The intercostal muscles form three functional sheets The outermost (external) sheet and the parasternal sheet act to displace the ribs cephalad as they shorten This increases both the anteroposterior and lateral dimensions of the thorax There is also a deep (internal) layer of intercostal muscle at right angles to the external sheet that acts to displace the ribs caudad when it contracts Thus, the intercostal muscles play both inspiratory and expiratory roles in breathing by reshaping the thorax CHAPTER 44 Overview of Breathing Failure B A • Fig 44.4 The angle of the muscular diaphragm to the vertical is narrow in the older child (A) and adult than it is in the infant (B) This more horizontal traction on the dome may be disadvantageous when the muscle of the infant diaphragm contracts (Courtesy Women & Children’s Hospital of Buffalo, NY.) A B • Fig 44.5 The normal position of the diaphragm (A) allows it to stretch or “load” during expiration In inspiration, its muscular attachment to the thorax pulls it vertically downward Air trapping (B), abdominal distension, or other disorders that cause flattening of the diaphragm interfere with “loading” and with the direction of contraction (Courtesy Women & Children’s Hospital of Buffalo, NY.) 495 496 S E C T I O N V Pediatric Critical Care: Pulmonary Accessory Muscles of Respiration Though quiet expiration is largely passive, resulting mostly from inward elastic recoil of the lung, active expiration may be assisted by contraction of abdominal muscles (rectus abdominis, transverse abdominis, and the obliques) During quiet breathing, abdominal muscle tone elevates the diaphragm during expiration, increasing the zone of apposition and loading the diaphragm for greater inspiratory contractile efficiency Accessory muscles of inspiration include the scalenes, sternocleidomastoids, pectoralis minor, and erector spinae, all of which elevate the ribs during contraction During quiet breathing, accessory muscles play a minor role, but during respiratory exertion, they may play a major role and act to unload and unburden the diaphragm and intercostals Even profoundly neurologically impaired children, who exhibit little volitional activity, can use accessory muscles of breathing when distressed Children with dysfunction of the primary muscles of respiration, such as with inherited myopathies or denervation, may rely on their accessory muscles even at rest Breathing Failure Several discrete mechanisms may cause breathing failure (Table 44.1) Each of these final common pathways may be triggered or compounded by other discrete adversities, and independent pathways of breathing failure may converge to cause respiratory arrest (or need for mechanical support of breathing) Failure of Respiratory Controls Disorders of respiratory controls may present with impaired respiratory cycle generation (central apnea), deficient responses to respiratory stimuli (hypoventilation during stress and failure to arouse from sleep hypoxia), or inadequate motor control of the vocal cords or pharynx (causing stridor or stertor) These patterns of regulatory dysfunction may occur individually or in combination Recognizing Depressed Respiratory Drive Respiratory neural output is not routinely directly measured in the clinical setting Interpreting ventilation as a measure of the intensity of drive to breathe is confounded by dependence of ventilation on the multiple factors of muscle strength, respiratory system mechanics, and respiratory neural motor output Nevertheless, the stressed patient who appears inappropriately comfortable, with little accessory muscle activity, or with periodic breathing, should be presumed to have depressed controls of breathing warranting assisted ventilation TABLE Failure of Breathing 44.1 Mechanism Examples Failure of neural control Uncal herniation, central hypoventilation Failure of muscles of breathing Failure of mechanics of breathing Insufficient muscle blood flow, hypoxemia Flail chest, diaphragmatic paralysis Failure of Neural Control Rhythmic control of breathing by the central nervous system may fail for a variety of reasons The brain is susceptible to injury, ischemic stroke, central nervous system hemorrhage, suppression of brain function by cold, structural dysfunction, chemical suppression, signal disruption, and inherited disorders of respiratory control Efferent neural pathways may be blocked by spinal cord lesions, spinal anesthesia, phrenic nerve injury, or neuromuscular disorders Examples are cited in Table 44.2 Note that volitional and automatic control of breathing may be separately and independently affected In Ondine’s curse (resulting from stroke involving the lateral medulla), automatic control is impaired, whereas volitional control may persist in the awake state In contrast, supratentorial stroke may impair volitional control of breathing, though automatic control of breathing may be preserved Central hypoventilation is abnormal respiratory control by the brain and results in hypercapnia Patients with central hypoventilation may have otherwise normal neurologic functioning or they may have concomitant neurologic injury Unless mixed with peripheral neuromuscular disease, patients have normal muscle strength, but the essential feature is altered responses to respiratory acidosis.9 Respiratory patterns may also be abnormal Central hypoventilation is often most apparent when patients are sleeping Apnea of prematurity, which may persist beyond term, is still not well understood but is clearly aggravated by concomitant infection Peripheral (extracranial) neural disorders include disorders of the anterior horn cells (e.g., spinal cord injury, spinal muscular atrophy, infections), disorders of the peripheral nerve (e.g., GuillainBarré syndrome), disorders of the neuromuscular junction (e.g., myasthenia gravis, anticholinesterase poisoning, botulism) and TABLE Causes of Failure of Neural Control 44.2 of Breathing Mechanism Example Brain injury Closed head trauma Ischemic stroke Severe supratentorial stroke Locked-in syndrome (loss of volitional control of breathing) Lateral medullary stroke Ondine’s curse (loss of automatic control) Subarachnoid hemorrhage Intracranial hypertension with uncal herniation Brain suppression by cold Cold-water drowning Structural brain dysfunction Brainstem glioma Chemical suppression Narcotic overdose Signal disruption Seizure Intrinsic Disorders Idiopathic Ondine’s curse Immaturity Apnea of prematurity Spinal cord lesion Poliomyelitis, traumatic cord transection Spinal anesthesia Inadvertent spinal anesthesia Phrenic nerve injury Cardiac surgical phrenic nerve injury Neuromuscular disorder Curare poisoning, botulinum, and many snake venoms ... breathing when distressed Children with dysfunction of the primary muscles of respiration, such as with inherited myopathies or denervation, may rely on their accessory muscles even at rest Breathing... muscle activity, or with periodic breathing, should be presumed to have depressed controls of breathing warranting assisted ventilation TABLE Failure of Breathing 44.1 Mechanism Examples Failure... muscles of breathing Failure of mechanics of breathing Insufficient muscle blood flow, hypoxemia Flail chest, diaphragmatic paralysis Failure of Neural Control Rhythmic control of breathing by the