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Bonnet S Apabetalone (RVX-208) alone and in combination with standard of care improves experimental PAH in sugen-hypoxia rat model Am J Respir Crit Care Med 2018;197:A4628 225 Furman L, Baley J, Borawski-Clark E, Aucott S, Hack M Hospitalization as a measure of morbidity among very low birth weight infants with chronic lung disease J Pediatr 1996;128(4):447-452 226 Smith VC, Zupancic JA, McCormick MC, Croen LA, Greene J, Escobar GJ, Richardson DK Rehospitalization in the first year of life among infants with bronchopulmonary dysplasia J Pediatr 2004;144(6):799-803 227 Baraldi E, Filippone M Chronic lung disease after premature birth N Engl J Med 2007;357(19):1946-1955 228 Lewandowski AJ, Bradlow WM, Augustine D, et al Right ventricular systolic dysfunction in young adults born preterm Circulation 2013;128(7):713-720 229 Naumburg E, Axelsson I, Huber D, Soderstrom L Some neonatal risk factors for adult pulmonary arterial hypertension remain unknown Acta Paediatr 2015;104(11):1104-1108 230 Stocks J, Hislop A, Sonnappa S Early lung development: lifelong effect on respiratory health and disease Lancet Respir Med 2013;1(9):728-742 231 Spiekerkoetter E, Kawut SM, de Jesus Perez VA New and Emerging Therapies for Pulmonary Arterial Hypertension Annu Rev Med 2019;70:45-59 232 Leary PJ, Maron BA, Tedford RJ, Lahm T Pulmonary Hypertension: Good Intentions, But a Questionable Approach Ann Am Thorac Soc 2018;15(6):664-666 ee7 Abstract: This chapter addresses the pediatric aspects of pulmonary hypertension (PH) Definition, classification, diagnosis, current treatment, and advances of molecular mechanisms for new therapeutic approaches of PH are briefly discussed This background helps in understanding the pathophysiology and treatment of pulmonary vascular disorders in children Key Words: pulmonary hypertension, pulmonary vascular disease, pediatrics, right ventricular hypertrophy, hypoxia 54 Mechanical Ventilation and Respiratory Care KYLE J REHDER AND IRA M CHEIFETZ PEARLS • Modern respiratory care requires a major institutional commitment in resource allocation for state-of the-art management of the pediatric patient requiring mechanical ventilation • A comprehensive and unified approach in which the roles and responsibilities of each of the team members (physicians, respiratory therapists, bedside nurses, and the family) are clearly defined and respected is required • Every team member must be aware of the current technologic advances in the design and implementation of mechanical ventilation as well as the unique physiology of pediatric patients This chapter reviews the applied physiology relevant to mechanical ventilation and design and function of ventilators, including a modern classification of the modes; guidelines for the use of mechanical ventilation based on pathophysiology, philosophy, and practice of weaning; respiratory care adjuncts to mechanical ventilation; high-frequency ventilation; and the adverse effects of mechanical ventilation The topic of noninvasive mechanical ventilation, including negative-pressure ventilation, is covered in Chapter 55 lung at the end of a normal expiration FRC results from the balance between forces that favor alveolar collapse and those that maintain alveolar inflation, with normal FRC being approximately 30 mL/kg Closing volume refers to the volume of gas present in the lung at which small conducting airways begin to collapse When FRC exceeds closing volume, the small airways and alveoli remain open On the other hand, when closing volume exceeds FRC, the small airways tend to collapse, followed by the alveoli, owing to the continued absorption of gases into the bloodstream Because of the highly compliant chest wall of infants and children younger than years, the closing volume can exceed FRC This is one reason for the propensity for atelectasis in infants and young children With development, FRC exceeds closing volume in children older than years Applied Respiratory Physiology This section reviews principles of respiratory physiology necessary to understand and optimize mechanical ventilation A more in-depth description of respiratory physiology is available in Chapters 42, 45, and 46 Lung Volumes and Capacities Breathing fulfills the physiologic necessity of gas exchange Oxygen is delivered to the blood with each inspiration, and carbon dioxide is removed with each exhalation Lung volumes increase during inspiration and fall during expiration (Fig 54.1) Tidal volume (Vt) is the volume of gas that moves in and out of the lungs with each breath and is normally to mL/kg for a spontaneous breath, regardless of age.1 Total lung capacity (TLC) is the volume of gas present in the lung with maximal inflation (60–80 mL/kg) Vital capacity is the volume of gas that can be maximally expired from TLC (40–50 mL/kg) Residual volume is the volume of gas present in the lung at the end of a maximal expiratory effort and represents anatomic dead space Functional residual capacity (FRC) is the volume of gas that is present in the Lung Inflation and Deflation Thoracic structures impede lung inflation; a certain amount of force is required to overcome this impedance Elasticity of the lung and chest wall is typically the primary factor that resists lung inflation, and respiratory system compliance dictates how much pressure is required to deliver a certain Vt to the lungs Total respiratory system compliance is a sum of the compliance of both the lungs and chest wall Disease processes that result in an abnormal lung or chest wall compliance fall into the category of restrictive lung disease as listed in Table 54.1 Total respiratory system resistance is the second factor that impedes inflation Resistance is defined as the pressure required to produce flow into the lungs It can be partitioned into airway resistance and frictional resistance to deformation of the lungs, chest wall, and abdominal contents (also known as nonelastic frictional resistance).2 In the infant, airway resistance is equally 625 626 S E C T I O N V Pediatric Critical Care: Pulmonary lnspiratory reserve volume (IRV) Inspiratory capacity (IC) Volume Tidal volume (VT) Vital capacity Total lung (VC) capacity (TLC) Expiratory reserve volume (ERV) Functional reserve capacity (FRC) Residual volume (RV) • Fig 54.1 Lung volumes TABLE Factors Associated With Decreased Respiratory Compliance or Increased Respiratory Resistance 54.1 RESTRICTIVE LUNG DISEASE Decreased Lung Compliance Reduced Chest Wall Compliance Obstructive Lung Disease (Increased Resistance) Surfactant deficiency or inactivation Interstitial inflammation Diffuse pneumonitis Interstitial or alveolar edema Acute respiratory distress syndrome Interstitial fibrosis Atelectasis Hyperinflation Pleural fibrosis Pleural effusion Pneumothorax Increased intercostal muscle tone Upper motor neuron disease Drug effects (e.g., fentanyl) Deformations—kyphosis, scoliosis, or both Restrictive bandages Abdominal distension or binding Upper airway obstruction Bronchospasm (including asthma) Airway malacia Anaphylaxis Airway compression (secondary to masses or vasculature) Bronchiolitis Foreign body distributed between the upper and lower airways However, with increasing age, most of the airway resistance resides in the upper airways Airway resistance is primarily a function of the caliber of the airways but is also dependent on the viscosity and density of the gas and whether the gas flow is in a laminar or turbulent state In obstructive lung disease, airway resistance can be dramatically increased in the setting of bronchospasm, upper airway edema or fixed obstruction, and airway malacia (see Table 54.1) In certain pathologic conditions—such as pulmonary edema, interstitial lung disease, and pulmonary fibrosis—frictional resistance may also be increased Normal expiration is passive and dependent on the elastic recoil of the lung, which is attributable to alveolar surface tension and tissue elasticity Surface tension is greatest at high lung volumes and lowest at FRC.3 Elastic recoil of the lung provides most of the force required to expel the gas from the lungs, but expiration may be aided by respiratory muscles during forced exhalation Airway resistance will impede expiration in that higher resistance will slow the flow of gas out of the lungs until closing volume is reached and airflow will cease Time Constant Understanding the concept of time constants is necessary to optimize ventilator settings, particularly ventilation rate With a constant inflation pressure, it takes a finite amount of time to inhale or exhale a given volume of gas The rate of inflation and deflation of the lung is approximately mono-exponential and is directly proportional to compliance and resistance.3 Inspiratory and expiratory time constants will be different as the inspiratory and expiratory resistances differ Time constant is calculated as the product of compliance and resistance Each time constant represents the amount of time required for 63% of change in lung volume As such, it takes one, three, and five expiratory time constants to reach a 63%, 95%, and 99% exhalation, respectively (Fig 54.2).2 Work of Breathing During normal breathing, work of breathing is performed entirely by inspiratory muscles Nearly half of the work of breathing during normal inspiration is dedicated to overcoming frictional ... respectively (Fig 54.2).2 Work of Breathing During normal breathing, work of breathing is performed entirely by inspiratory muscles Nearly half of the work of breathing during normal inspiration is... FRC This is one reason for the propensity for atelectasis in infants and young children With development, FRC exceeds closing volume in children older than years Applied Respiratory Physiology This... implementation of mechanical ventilation as well as the unique physiology of pediatric patients This chapter reviews the applied physiology relevant to mechanical ventilation and design and function