517CHAPTER 46 Mechanical Dysfunction of the Respiratory System Airway distention –5 –3 –30 0 0 –40 –40 –40 –40 –40 –40 –40 +5 –35 Vascular ring Pulmonary sling Mediastinal tumor Large heart Intrathora[.]
CHAPTER 46 Mechanical Dysfunction of the Respiratory System Vascular ring Pulmonary sling Mediastinal tumor Large heart Vascular ring Pulmonary sling Mediastinal tumor Large heart –3 0 –40 –40 –5 +40 Airway distention –30 –40 +5 +40 Airway collapse above EPP +40 +40 –40 –40 +5 –35 –40 +3 +40 +40 +5 +45 +40 –40 +40 Worse during expiration • prolongation of expiration • Expiratory wheezing All that wheezes is not asthma Intrathoracic-extrapulmonary airway obstruction Inspiration Intrathoracic-extrapulmonary airway obstruction Expiration A B Asthma Bronchiolitis Aspiration Large heart Early pulmonary edema –3 –40 –5 –40 +40 Airway distention +40 –40 –40 +5 –35 –40 +3 Intrapulmonary airway obstruction Inspiration C +40 Airway collapse above EPP +40 +40 –40 +5 +40 +5 +45 +40 Worse during expiration • prolongation of expiration • Expiratory wheezing • Air trapping, AutoPEEP Intrapulmonary airway obstruction Expiration D • Fig 46.12 (A–B) Airway dynamics in intrathoracic extrapulmonary airway obstruction (A) During inspiration, increased negative pleural pressure is transmitted to all structures inside the chest, including the airways up to the site of obstruction, beyond which it is rapidly dissipated This results in distension of the intrathoracic airways proximal to obstruction, as it is surrounded by even greater negative intrathoracic pressure (B) During expiration, the increased airway pressure rapidly dissipates above the obstruction There is a collapse of the intrathoracic airway above the obstruction because of markedly increased positive intrathoracic pressure outside the airway, making the obstruction worse during exhalation The equal pressure point (EPP) is the point at which intra- and extraluminal pressures during exhalation are equal Pressures are presented relative to atmospheric pressure (0 cm H2O) Distal airway pressures are taken as pleural pressure plus lung recoil pressure (arbitrarily taken as 15 cm H2O for simplicity) (C–D) Airway dynamics in intrathoracic intrapulmonary airway obstruction (C) During inspiration, increased negative pleural pressure is transmitted to all structures inside the chest, including the airways The intrathoracic extraluminal airway pressure is more negative, especially above the site of obstruction, resulting in airway distension (D) During expiration, the positive intrathoracic pressure rapidly dissipates above the site of obstruction and the EPP moves distally toward the alveoli The end result is widespread airway collapse and worsening of symptoms Pressures are presented relative to atmospheric pressure (0 cm H2O) Distal airway pressures are taken as pleural pressure plus lung recoil pressure (arbitrarily taken as 15 cm H2O for simplicity) (From Kliegman RM, Stanton BF, St Geme JW, et al, editors Nelson Textbook of Pediatrics, ed 20 Philadelphia: Elsevier; 2016.) –40 Asthma Bronchiolitis Aspiration Large heart Early pulmonary edema 517 518 S E C T I O N V Pediatric Critical Care: Pulmonary 10 15 20 25 +40 30 Intrapulmonary airways Intrathoracic-extrapulmonary airways Extrathoracic airway Equal pressure point Collapse due to transmural pressure Anatomic obstruction +40 +40 35 +40 50 20 +40 Chest wall 45 Lung +40 parenchyma +15 Lung parenchyma +40 +15 Chest wall 50 Lung +40 parenchyma +15 Lung parenchyma +40 +15 40 + 15 = 55 +40 +40 +40 45 40 + 15 = 55 +40 +40 10 40 +40 • EPP moved distally • Increased transmural pressure • Auto-PEEP • Dynamic hyperinflation +40 +40 +40 +40 +40 +40 +40 Diaphragm Diaphragm Normal forced exhalation Intrathoracic airway obstruction • Fig 46.13 Equal pressure point Assume intrapleural pressure of 40 cm H2O and lung recoil pressure of 15 cm H2O during forced exhalation PEEP, Positive end-expiratory pressure Total WOB/min = WOBeach breath × (respiratory rate/min) Normal A Restrictive B D Obstructive C C D C E B Volume Volume Volume E C D F E B B A A A –5 –10 –15 –20 Δ Pleural pressure (cm H2O) –5 –10 –15 –20 +15 +10 Δ Pleural pressure (cm H2O) Static pressure-volume relationship (no flow) Elastic work: ACDA Inspiratory flow-resistive work: ABCA +5 –5 –10 –15 –20 Δ Pleural pressure (cm H2O) Expiratory flow-resistive work (active): AEFA Note that expiratory flow-resistive work in area ACEA is accomplished by passive lung recoil • Fig 46.14 Respiratory work WOB, Work of breathing Generically, work can be defined as the energy necessary to move an object between two points It is the product of force and distance In the respiratory system, this distance is the change in volume of the thorax (work DP DV) and represents the energy necessary to move air into and out of the lungs To achieve this volume change, both elastic and resistive properties must be overcome Graphically, work can be represented using a pressurevolume relationship curve (Fig 46.14) The line AC in Fig 46.14 represents the static pressure-volume (elastic) relationship of the lung At any given change in pressure, a certain change in volume results The work required to overcome elastic recoil (the elastic WOB) during inspiration is described by the area of the trapezoid ACDA in Fig 46.14 The additional work required to overcome flow resistance (the resistive WOB) is represented by the area ABCA in Fig 46.14 The greater the resistance, the greater the excursion of the curve ABC from line AC during inspiration The total work for a given breath is described by combining the two areas, ABCDA During normal respiration, the expiratory work (ACEA) falls within the area for inspiratory work, indicating that no additional work is required for passive exhalation since it is CHAPTER 46 Mechanical Dysfunction of the Respiratory System accomplished by passive lung recoil Exhalation is accomplished using energy stored by the system during inspiration Total WOB per minute is calculated as WOB per breath respiratory rate The cost of breathing in healthy children is quite low and may be less than 3% of total energy expenditure However, in the setting of acute or chronic lung disease, it may increase to as high as 40% or more Neonates have a higher elastic WOB due to the high compliance (i.e., low elastance) of their chest wall For them, the breathing pattern that requires the least work is relatively faster and with lower Vt As children get older and elastic recoil of the chest wall increases, Vt increases and respiratory rate decreases Diseases of decreased lung compliance, such as pneumonia or pulmonary edema, result in increased elastic WOB with relatively lower resistive work (see Fig 46.14B) The respiratory pattern that minimizes work in this setting is lower Vt and a higher rate Diseases resulting in airway obstruction, such as asthma or bronchiolitis on the other hand are characterized by increased resistive work relative to elastic work These patients will therefore exhibit a relatively higher Vt and lower rates compared with diseases with decreased lung compliance In addition, diseases of intrathoracic airway obstruction may result in the need for active exhalation and result in active expiratory work denoted by area AEFA (see Fig 46.14C) Clinical Manifestations of Mechanical Dysfunction Mechanical dysfunction of the respiratory system results in abnormalities in breathing patterns, generation of abnormal respiratory sounds, or demonstration of “respiratory distress.” Although distress is a subjective term, it is often used to indicate the presence of nasal flaring, chest wall retractions, tachypnea, or overall increased WOB Sounds suggestive of mechanical dysfunction may include wheezing, stridor, or grunting The degree of tachypnea and retractions along with the adventitious sounds generated are useful in helping to identify the type of mechanical dysfunction present and can be a marker of changing disease severity (Table 46.1) For example, stridor usually indicates the presence of extrathoracic airway obstruction, whereas wheezing is a manifestation of intrathoracic obstruction Although grunting may be a sign of pain or agitation, it often indicates the body’s attempt to increase end-expiratory pressure and FRC to mitigate hypoxemia in diseases of decreased compliance These clinical signs and their severity are helpful not only in quantifying disease severity but also in evaluating response to therapeutic interventions 519 Effectors of the Respiratory Pump Lungs inflate and deflate in response to changes in the shape and size of the thoracic cage The lungs are not attached to the thoracic cage but are rather suspended at their hila from the mediastinum and move in concert with it They are surrounded by visceral and parietal pleura, which, in turn, are attached to the chest wall The diaphragm is a dome-shaped, fibromuscular structure that is the principal muscle of inspiration It has a costal portion, crural portion, and a central tendon During inspiration, the diaphragm contracts and moves downward, the thoracic cage elongates, and the PPL becomes increasingly negative During exhalation, the diaphragm relaxes and the elastic recoil of the lungs and abdominal contents compress the lungs and lead to exhalation passively Normal quiet breathing, which we usually are unaware of, is accomplished by movement of the diaphragm Owing to its continual use, the diaphragm needs to be fatigue resistant There are two factors that contribute to the fatigue resistance of the diaphragm compared with other skeletal muscles First, the smaller size of the diaphragm muscle fibers and the well-developed capillary network decrease the diffusion distance of oxygen between capillaries and the muscle fibers and increase efficiency of oxygen delivery Second, as compared with skeletal muscles, which contain primarily fast-twitch, fatigable type 2x and 2b fibers, the predominant muscle fiber types in the diaphragm are the fatigue-resistant slow-twitch type and fatigable fast-twitch type 2x, which has intermediate fatigue resistance.13 The type fibers have slower shortening velocities than fast-twitch type 2x fibers but are highly fatigue resistant owing to their lower ATP consumption and their reliance almost exclusively on aerobic metabolism Type 2x fibers, on the other hand, are fast twitch, less oxidative, and are more prone to fatigue This combination of fiber types allows for good fatigue resistance and increased force generation when necessary Muscles types in the diaphragm change with age and are modifiable via exercise or disuse Neuromuscular disease, such as spinal muscular atrophy or Duchenne muscular dystrophy, have a decreased proportion of fast-twitch fibers, thus decreasing force generation and potentially increasing the risk of respiratory failure Diaphragms of newborns and infants have a lower muscle mass when indexed for body size In addition, they have lower percentages of fatigue-resistant type fibers The diaphragm of preterm infants contains only 10% type fibers This increases to 25% in term neonates and 55% in children greater than years of age.14 These developmental differences predispose neonates and infants to respiratory muscle fatigue and respiratory failure Accessory Muscles of Respiration TABLE Clinical Signs of Mechanical Dysfunction 46.1 Site of Dysfunction Rate Retractions Sounds Extrathoracic airway h hhhh Stridor Intrathoracic extrapulmonary airway h hhh Wheezing Intrapulmonary airway hh hh Wheezing Alveolar/interstitial hhh hhh Grunting Contraction of accessory muscles causes elevation or depression of the ribs, like a bucket handle, resulting in an increase or decrease in the anteroposterior diameter of the thoracic cage At end-expiration, the ribs slant downward and the sternum falls back toward the vertebrae When the ribs are elevated, they move forward, the intercostal space increases, and the sternum moves away from the vertebrae This results in an increase in the anteroposterior diameter of the thoracic cavity The dominant accessory muscles of inspiration are the external intercostals; their contraction elevates the ribs The sternocleidomastoids help lift the sternum, the serratus anterior muscles help elevate posterior ribs and scapulae, and the scalene muscles help elevate the first two ribs Muscles that depress the ribs will augment exhalation These are the rectus abdominis and internal intercostal muscles During 520 S E C T I O N V Pediatric Critical Care: Pulmonary active exhalation, they pull the ribs down and compress the contents of the abdomen toward the diaphragm During normal quiet respirations, these accessory muscles not play a major role in increasing the thoracic cage volume Their main role is to stabilize the chest wall from the collapsing effect of the negative intrapleural pressure Any condition that leads to an increase in respiratory drive will lead to accessory muscle activation Commonly, this results from increases in metabolic demands or changes in respiratory mechanics Chest Wall The chest wall consists of ribs, the spine and it is joints, intercostal muscles and nerves, the sternum, and the abdominal wall muscles In health, the work required to expand the chest wall is minimal Compared with an adult thorax, which is ellipsoidal, an infant’s thorax is more circular This is due primarily to the fact that the ribs in an infant are more horizontally oriented compared with the obliquely placed ribs of an adult Because of this, the intercostal muscles of an infant chest are shorter and less effective at elevating ribs and increasing intrathoracic volume In addition, the costal attachments of the diaphragm in an infant are more horizontal compared with an adult When the diaphragm contracts, the lower ribs tend to move inward A compliant chest wall only further promotes this inward movement, making the chest wall excursion less efficient and promoting the abdominal breathing pattern seen in newborns and young infants Disease of the Chest Wall Kyphoscoliosis is a group of spinal diseases associated with abnormal spinal curvature either in the lateral plane (scoliosis) or in the sagittal plane (kyphosis) These are either idiopathic or associated with neuromuscular disorders Severe kyphoscoliosis often impairs chest wall expansion and leads to restrictive lung disease This decrease in compliance leads to smaller change in volume for any given change in pressure (see Fig 46.4B), which results in decreased efficiency and increased work All lung volumes are reduced—including FVC, total lung capacity, and FRC—but their relative proportions are often preserved As ribs ossify with age and chest wall compliance decreases further, the mechanics of respiration often worsen even without progression of spinal deformity Associated neuromuscular weakness will exacerbate this and lead to more rapid deterioration of respiratory status compared with idiopathic kyphoscoliosis Flail chest results from a pattern of fractures involving the ribs and/or the sternum that creates a segment of the thoracic cage that is uncoupled and moves paradoxically from the remainder of the thorax This most often results from blunt thoracic trauma Because of its extreme softness, the chest wall of premature babies often behaves as a flail chest, especially when dealing with altered pulmonary mechanics Movement of this flail segment depends entirely on intrapleural pressure changes During inspiration as the intrapleural pressure becomes negative, the flail segment moves inward (Fig 46.15) During expiration, as intrapleural pressure increases, the flail segment moves outward This movement is paradoxic compared with the rest of the ribcage This paradoxic movement increases chest wall compliance and decreases respiratory efficiency Any condition that exaggerates the changes in PPL, such as pulmonary contusion, will make the flail chest worse A similar effect is encountered in neuromuscular diseases (e.g., spinal muscular atrophy, muscular dystrophy, myasthenia gravis), in which the entire thoracic cage moves paradoxically inward in response to negative intrathoracic pressure generated by the diaphragm –3 –15 –15 –5 –15 –15 –15 +5 –10 –15 –15 Flail chest Inspiration • Fig 46.15 Flail chest During inspiration when pleural pressure becomes negative, the unsupported flail segment of the chest moves inward During expiration, the flail segment moves outward as pleural pressure becomes less negative or positive This paradoxic movement during both phases of respiration reduces efficiency and increases work Therapeutic Maneuvers to Improve Mechanical Dysfunction of the Respiratory System Various invasive and noninvasive respiratory support modalities can be employed to treat respiratory dysfunction Mechanical support can consist of varying amounts of pressure, rate, and time for respiration The optimum application of these modalities is determined by the underlying pathophysiology of mechanical dysfunction Positive End-Expiratory Pressure/Continuous Positive Airway Pressure The application of positive-pressure support during spontaneous or mechanical ventilation is one of the most common types of mechanical support, which can have significant benefits if applied properly There are two modalities of applying positive airway pressures Continuous positive airway pressure (CPAP) is application of positive pressure throughout all phases of respiration in a spontaneously breathing patient PEEP is the application of positive pressure at the end of exhalation during spontaneous or mechanical ventilator breaths In diseases of decreased CSTAT, the primary cause for hypoxemia is reduced FRC leading to intrapulmonary shunt End-expiratory pressure helps recruit alveoli, improving FRC and lung compliance The effect of PEEP on compliance and FRC can be demonstrated using pressure volume curves (Fig 46.16) Alveoli are most likely to collapse at end-exhalation Collapsed alveoli need to open prior to inflating and therefore require a greater driving pressure to fill during inspiration Critical opening pressure, also referred to as lower inflection point, is the pressure at which collapsed alveoli begin to open and at which relatively less pressure results in greater volume change The goal when applying end-expiratory pressure is to minimize alveolar collapse at end exhalation This can be evaluated clinically by examining the pressure volume CHAPTER 46 Mechanical Dysfunction of the Respiratory System curve generated by the ventilator to ensure that inspiration begins above the lower inflection point If it does not, additional end-expiratory pressure may be required If a pressure-volume curve is not available, adjustment of PEEP and noting the change in exhaled volume can also be helpful In addition to improving FRC and lung compliance by preventing atelectasis at end-exhalation, PEEP redistributes lung water by displacing it from alveolar to extra alveolar spaces, improving oxygenation In the setting of airway obstruction, positive airway pressure also provides benefits In extrathoracic obstruction, positive Normal lung Upper inflection point (PFLEX) ARDS “Safe” zone of ventilation Critical opening pressure Lower inflection point (PFLEX) Effect of positive end-expiratory pressure on the pressurevolume relationship in normal lungs and in acute respiratory distress syndrome (ARDS) In ARDS, atelectatic alveoli require a considerable amount of pressure to open Critical opening pressure, also referred to as lower PFLEX, is the airway pressure above which further alveolar expansion occurs with relatively less pressure The upper PFLEX is the airway pressure above which further increase in pressure results in less alveolar expansion This is the area of alveolar overdistention Keeping tidal volumes between the upper and lower inflection points is considered less injurious for the lung (From Kliegman RM, Stanton BF, St Geme JW, et al, editors Nelson Textbook of Pediatrics, ed 20 Philadelphia: Elsevier; 2016.) inspiratory airway pressure helps prevent the airway collapse that occurs due to negative intraluminal pressure generated during spontaneous breathing With intrathoracic obstruction, positive inspiratory pressure helps unload respiratory muscles by decreasing negative PPL needed to move gas across obstructed airways In addition, application of PEEP moves the EPP away from the smaller unsupported airways and closer to the larger cartilaginous airways, preventing airway collapse, improving hyperinflation, and decreasing the effort needed for active exhalation By these effects, positive-pressure support can have significant benefits on WOB (Fig 46.17).15 Another clinical scenario that benefits from the application of positive airway pressure is muscular weakness or sedation As discussed, neonates and infants are at a high risk for atelectasis during general anesthesia and neuromuscular blockade due to high chest wall compliance The increased compliance and alveolar collapse that occurs due to a relaxed or weak chest wall is counteracted by the application of PEEP In addition, application of PEEP increases the FRC above closing capacity, allowing for ventilation of dependent areas of the lung, improving V/Q matching Rate Pressure • Fig 46.16 521 Adjustment in respiratory rate is necessary in the setting of both low-compliance and high-resistance diseases Children with intrathoracic airway obstruction have long time constants with expiratory time constants being longer than inspiratory time constants These patients are best ventilated with relatively slower rates This allows for long expiratory times necessary for complete exhalation and adequate inspiratory time to deliver higher Vt to ensure optimal alveolar ventilation Adequacy of exhalation time can be assessed by performing an expiratory hold maneuver to evaluate for air trapping manifested by autoPEEP This can also be accomplished by measuring exhaled Vt with adjustments of expiratory time Diseases of decreased respiratory compliance have fast time constants for inspiration and exhalation These patients are most efficiently ventilated with higher rates and lower Vt Spontaneous Tidal volume (ml) EPAP a tic V P- l re p hi ns tio a St IPAP + + Spontaneous +30 +20 +10 –10 –20 –30 –40 Intrapleural pressure (cm H2O) • Fig 46.17 Effect of continuous positive airway pressure on work of breathing in intrathoracic airway obstruction Work of breathing in intrathoracic airway obstruction Application of end-expiratory pressure stents airways open during exhalation, reducing airway collapse and auto-PEEP, decreasing the need for active exhalation The improvement in hyperinflation also improves inspiratory compliance Inspiratory positive pressure unloads respiratory muscles by decreasing negative pleural pressure required to move air By these mechanisms, inspiratory and expiratory work are decreased EPAP, Expiratory positive airway pressure; IPAP, inspiratory positive airway pressure; PEEP, positive end-expiratory pressure; WOB, work of breathing (From Kliegman RM, Stanton BF, St Geme JW, et al, editors Nelson Textbook of Pediatrics, ed 20 Philadelphia: Elsevier; 2016.) +40 Resistive inspiratory WOB with IPAP Resistive expiratory WOB with EPAP Spontaneous resistive inspiratory WOB Spontaneous resistive expiratory WOB ... Work of breathing Generically, work can be defined as the energy necessary to move an object between two points It is the product of force and distance In the respiratory system, this distance... thorax is more circular This is due primarily to the fact that the ribs in an infant are more horizontally oriented compared with the obliquely placed ribs of an adult Because of this, the intercostal... A compliant chest wall only further promotes this inward movement, making the chest wall excursion less efficient and promoting the abdominal breathing pattern seen in newborns and young infants