e2 48 Curley MA, Hibbard PL, Fineman LD, et al Effect of prone posi tioning on clinical outcomes in children with acute lung injury a randomized controlled trial JAMA 2005;294 229 2237 49 Beck KC, Vet[.]
e2 48 Curley MA, Hibbard PL, Fineman LD, et al Effect of prone positioning on clinical outcomes in children with acute lung injury: a randomized controlled trial JAMA 2005;294:229-2237 49 Beck KC, Vetterman J, Rehder K Gas exchange in dogs in the prone and supine positions J Appl Physiol (1985) 1992;72:2292-2297 50 Mure M, Domino KB, Lindahl SGE, et al Regional ventilationperfusion distribution is more uniform in the prone position J Appl Physiol (1985) 2000;88:1076-1083 51 Musch G, Layfield JD, Harris RS, et al Topographical distribution of pulmonary perfusion and ventilation, assessed by PET in supine and prone humans J Appl Physiol (1985) 2002;93: 1841-1851 52 Cooper CJ, Landzberg MJ, Anderson TJ, et al Role of nitric oxide in local regulation of pulmonary vascular resistance in humans Circulation 1996;93:266-271 53 Bronicki RA, Fortenberry J, Schreiber M, et al Multicenter randomized trial of inhaled nitric oxide for pediatric acute respiratory distress syndrome J Pediatr 2015;166:365-369 54 Metter D, Tulchinsky M, Freeman LM Current state of ventilationperfusion scintigraphy for suspected pulmonary embolism AJR 2017;208:489-494 55 Sinzinger H, Rodrigues M, Kummer F Ventilation/perfusion lung scintigraphy Multiple applications besides pulmonary embolism Hell J Nucl Med 2013; 16:50-55 56 Frerichs I, Becher T, Weiler N Electrical impedance tomography imaging of the cardiopulmonary system Curr Opin Crit Care 2014; 20:323-332 57 Chatziioannidis I, Samaras T, Nikolaidis N Electrical Impedance Tomography: a new study method for neonatal Respiratory Distress Syndrome? Hippokratia 2011;15:211-215 58 Wolf GK, Gomex-Laberge C, Kheir JN, et al Reversal of dependent lung collapse predicts response to lung recruitment in children with early acute lung disease Pediatr Crit Care Med 2012;13:509-515 59 van Veenendaal MB, Miedema M, de Jongh FHC, et al Effect of closed endotracheal suction in high-frequency ventilated premature infants measured with electrical impedance tomography Intensive Care Med 2009; 35:2130-2134 60 Rossi FS, Yagui AC, Haddad LC, et al Electrical impedance tomography to evaluate air distribution prior to extubation in very-low-birth-weight infants: a feasibility study Clinics 2013; 68:345-350 e3 Abstract: The primary function of the lung is to exchange gas, which requires close matching of ventilation (VA) and perfusion (Q) Both ventilation and perfusion are influenced by factors extrinsic to the lung, including gravity and intrinsic lung factors such as the fractal structure of the bronchial and pulmonary vascular trees Both VA and Q increase from the apex to the base of the lung but perfusion increases more Disease can disturb VA/Q matching but hypoxic pulmonary vasoconstriction acts to counteract abnormalities by reducing flow to hypoxic regions Therapeutic maneuvers, such as prone positioning or inhaled nitric oxide, can improve VA/Q matching Key words: Acute respiratory distress syndrome (ARDS), asthma, fractal, hypoxic pulmonary vasoconstriction (HPV), perfusion, prone, shunt, ventilation, West zones 46 Mechanical Dysfunction of the Respiratory System JEFF CLARK, SAURABH CHIWANE, AND ASHOK P SARNAIK PEARLS • • • Workload and efficiency define the demands that disease imposes on the respiratory muscles and therefore are the relevant variables in the analysis of the mechanical function of the respiratory system The work done by the respiratory muscles and, by extension, the energy that must be supplied to these muscles are defined by the combined volume-pressure relationships of the lungs and chest wall Volume changes within the respiratory system are dictated primarily by the body’s need to take up oxygen and eliminate carbon dioxide and therefore are determined by factors such as physical activity or metabolic rate, which are relatively independent of the condition of the lungs and chest wall Pressure Much like the cardiovascular system, the lungs and chest wall can be likened to a mechanical pump that draws in air from the atmosphere into the alveoli and pushes it back into the atmosphere To generate the required pressure gradients, it is necessary to overcome the resistance and elastance of the thorax, airways, and lungs During spontaneous tidal respiration, the lungs and chest wall can be considered as two sets of springs in a stretched position that are exerting their recoil, under normal circumstances, in opposite directions While the chest wall is powered by muscles for inflation and deflation, the lungs operate predominantly with their inherent elastic recoil for generation of pressure Just as failure of the cardiac pump can be treated with mechanical support, the respiratory pump is also amenable to extracorporeal support to address the specific alterations responsible for its mechanical dysfunction Unlike the cardiac pump, the respiratory machine does not have an intrinsic pacemaker It is driven by a central controller (autonomic and voluntary) located in the brain, which receives feedback from a variety of sensors located centrally and peripherally in the form of chemoreceptors and mechanoreceptors The controllers, with the input from the sensors, drive the respiratory machine, employing its effectors—consisting of the diaphragm and intercostal and abdominal muscles The effectors create the • • changes, on the other hand, depend on physical processes that take place in the respiratory system’s components The efficiency of the respiratory pump is influenced by factors that the astute clinician must consider when assessing a child’s ability to sustain an increased respiratory workload These include the configuration of the diaphragm, presence of rib cage distortion, and nutritional state and conditioning of the respiratory muscles It is a common clinical observation that different types of mechanical derangement result in distinctive patterns of breathing These patterns generally agree with the principle of minimal power expenditure and can be useful to categorize the type of derangement the patient has during the initial evaluation necessary pressure gradients, which act on the targets (conducting airways and gas exchange elements) to generate airflow and move volumes of gas by overcoming airway/tissue resistance and pulmonary/ thoracic elastance Mechanical dysfunction can impede airflow (resistive dysfunction) or prevent tissue expansion (restrictive dysfunction) The efficiency of the mechanical pump in moving air across the airways and alveoli may be compromised by abnormalities of any one of the effectors or target structures, either individually or in combination A keen understanding of the functioning of individual components of the machine is necessary to address its failure Pump Dysfunction and Failure Pressures Pressure generation is a requirement for air movement to occur in the respiratory system However, some disagreement and confusion exist within scientific literature regarding the precise definitions of pressures within the respiratory system.1 Proper understanding of these pressures is essential to understanding respiratory function 509 510 S E C T I O N V Pediatric Critical Care: Pulmonary sec PAW Esophagus Intrapleural space Alveoli PALV PALV PATM PB Volume Chest wall PATM PB FEV1 IC TLC FRC VT VC ERV CC PES Time Diaphragm • Fig 46.1 Pressures in respiratory mechanics PALV, Alveolar pressure; PAW, pressure at the airway opening; PATM, atmospheric pressure; PB, body surface pressure; PES, esophageal pressure Fig 46.1 represents the important pressures generated in the respiratory system Pressure at the airway opening (PAW) during spontaneous unassisted respiration is atmospheric pressure (PATM) and usually taken as zero (cm H2O) relative to other pressures generated In mechanically ventilated patients, PAW refers to the pressure at the patient-ventilator interface, such the tracheal tube or the face mask The transrespiratory pressure (PTR) is PAW PALV (alveolar pressure) and is responsible for generating airflow With no air movement, such as at end-inspiration or end-expiration, the PTR is zero, meaning that PAW is equal to PALV As spontaneous inspiration begins, pleural pressure (PPL) decreases due to contraction of the respiratory muscles and expansion of the chest wall This negative PPL is transmitted to all structures inside the chest, including the airways and alveoli, so that a pressure gradient for air movement develops As PALV increases due to air entry and matches PAW, airflow stops During exhalation, PALV becomes positive and the process reverses as air flows from high pressure in the alveoli out the airway opening until PALV equals PAW again PPL is rarely measured directly but can be inferred using a distal esophageal pressure catheter (PES) and is negative during unassisted breathing Likewise, PALV cannot be measured directly but can be estimated using airway occlusion techniques After airway occlusion, airflow ceases and the PALV equilibrates with PAW, which can be measured Transpulmonary pressure (PTP) is the pressure gradient between PALV and PPL and is indicative of the stress experienced by the alveolar walls Transthoracic pressure (PTT) is defined as the pressure between the PPL and PATM PTT is used to calculate thoracic cage compliance Lung Volumes and Capacities Along with pressure, understanding lung volumes and capacities is crucial to understanding normal lung function and many pathologic conditions (Fig 46.2) Tidal volume (Vt) is the volume of gas moved with each breath In health, spontaneous Vt is usually between and mL/kg This volume refreshes alveolar gas during inspiration and leads to removal of carbon dioxide (CO2) during exhalation The volume of gas remaining in the lung after tidal respiration is termed functional residual capacity (FRC) FRC • Fig 46.2 Spirometry showing lung volumes and capacities CC, Closing capacity; ERV, expiratory reserve volume; FEV1, forced expiratory volume in second (measured only by gas dilution techniques); FRC, functional residual capacity; IC, inspiratory capacity; TLC, total lung capacity; VC, vital capacity; Vt, tidal volume acts as a reservoir for gas exchange between alveoli and pulmonary capillary blood throughout respiration Diseases that decrease lung compliance and lower FRC can have profound effects on oxygenation Positive end-expiratory pressure (PEEP) helps maintain end-expiratory volume and improve oxygenation Residual volume (RV) is the volume of gas remaining in the lung after a maximal forced exhalation The difference between FRC and RV is the expiratory reserve volume Inspiratory capacity (IC) is the volume that can be inspired from FRC after a maximal inspiration and total lung capacity is the total volume of gas in the lung at maximum inspiration Closing capacity (CC) is the volume of gas in the lung during exhalation when the dependent airways start to close CC can be measured only by specific gas dilution techniques and not by spirometry In healthy children and adults, CC is well below the FRC, meaning that all airways remain open during tidal respiration In intrapulmonary obstructive diseases and even in healthy neonates, dependent airways start to close during tidal exhalation before reaching FRC.2,3 Thus, alveolar ventilation below CC moves toward the nondependent, less perfused areas, away from dependent and better perfused areas, resulting in ventilation/perfusion (V/Q) mismatch and a lower partial pressure of arterial oxygen (Pao2) This also results in some amount of air trapping Flow/Volume Relationships Flow/volume relationship curves are clinically useful tools for demonstrating the effect that changes in pulmonary mechanics have on volumes and gas flow (Figs 46.3 and 46.4) These curves are generated with spirometry machines and can be used in both the outpatient setting and at the bedside Typically, a maximal inspiration is followed by maximal forced exhalation, generating a flow/volume loop Forced vital capacity (FVC) is the total volume exhaled during this maneuver Forced expiratory volume during the first second of exhalation is termed FEV1 Decreases in FVC and FEV1 result from decreased lung compliance, increased airway resistance, and decreased respiratory muscle strength The maximum flow occurs in the first phase of forced exhalation and is referred to as FEFmax (also referred to as peak flow) It is effort CHAPTER 46 Mechanical Dysfunction of the Respiratory System FEFmax FEF25% Normal flow volume loop with forced expiration and inspiration FEF25%–75% FEF75% Flow Expiration 25% 50% 75% Volume FEV1 Inspiration • Fig 46.3 Normal flow-volume loop performed by maximal inspiration followed by maximal forced exhalation to completion FEFMAX is synonymous with peak expiratory flow rate and represents the maximum expiratory flow rate during exhalation This occurs soon after the start of forced exhalation FEF25%–75% represents the mean flow between 25% and 75% total expiratory volume, also referred to as mid-maximum expiratory flow rate FEV1 is the volume of expired gas in the first second of expiration (From Kliegman RM, Stanton BF, St Geme JW, et al, editors Nelson Textbook of Pediatrics, ed 20 Philadelphia: Elsevier; 2016.) dependent but also a marker of airway obstruction The volume exhaled between 25% and 75% of the expiratory volume (FEF25%– 75%) is relatively effort independent; decreases in this are suggestive of intrathoracic airway obstruction, such as in asthma (see Fig 46.4).4 The shape of the expiratory curve gives clues to disease pathophysiology, as in obstructive disease, for which the midexpiratory curve may be increasingly concave Equation of Motion As with any pump, the respiratory system function is determined by the physical properties of its components The pressure necessary to move air into and out of the lungs is determined by two factors: (1) the elastance of the lung and chest wall and (2) airway resistance The equation of motion describes the pressure gradient required to overcome these two factors (Fig 46.5) The pressure gradient required to overcome the elastic properties of the lung and chest wall and to effect volume change represents the force necessary to overcome the static properties of the respiratory system and is measured once flow has ceased Additionally, the pressure required to create flow and move a certain amount of volume in a given time is a dynamic process that is determined by the resistive properties of the system and measured while flow is occurring Together, these are the primary forces that govern respiratory mechanics 511 Elastance of a tissue is the tendency of that tissue to resist deformation It is defined as the change in pressure applied to the system divided by the change in volume that results from this change in pressure (DP/DV) The pressure needed to overcome tissue elastance is a static property of the system and is measured when airflow has ceased Elastic recoil pressure is the pressure generated as the tissue returns to its resting state once the deforming stress (pressure) ceases Elastic recoil of the respiratory system increases as lung volume increases above FRC At end-exhalation, the lungs tend to recoil inward toward a lower volume and the chest wall tends to recoil outward toward a higher volume The balance of these forces determines FRC The primary determinants of lung recoil pressure are elastin content within the tissue and surface tension within the airspaces The chest wall recoil is determined by the integrity and maturity of the ribs, along with the mass and inherent tone of the muscles of respiration Although the lung and the chest wall have separate and opposite recoil pressures, they are linked via the pleural space and need to be considered together under most clinical situations The recoil pressure of the lung is relatively constant per unit volume over the developmental spectrum However, the chest wall compliance (expansibility) changes significantly throughout development (Fig 46.6) At birth, chest wall compliance is increased owing to a decrease in elastic elements.5 This is primarily due to lack of ossification of the ribs Based on lung and chest wall recoils alone, FRC should be much lower in the newborn However, the actual FRC measured is remarkably similar to older children when corrected for size This is because neonates, unlike older children, maintain inherent tone in the muscles of respiration at end-exhalation.6 This prevents exhalation to a lower FRC that would otherwise occur if driven by chest wall elastic recoil alone Additionally, the higher respiratory rates and shorter expiratory times of neonates prevent cessation of flow during exhalation and result in incomplete emptying of alveoli Also, neonates have a greater CC, which results in some amount of air trapping By year of age, chest wall elastance has increased sufficiently to maintain adequate FRC without the need of increased muscular tone at end-exhalation The implications of these developmental differences are apparent when considering states that decrease respiratory muscle tone, such as rapid-eye-moment sleep or pharmacologic muscle relaxation In young infants, these states may have a significant effect on FRC and overall oxygenation due to lack of end-expiratory muscle tone.6 Similarly, disease states that alter level of consciousness or affect neurologic tone or muscular strength (e.g., myopathies or neuropathies) will have similar effects It has been shown that during anesthesia and muscle relaxation, the FRC of a neonate may decrease by more than 50% in contrast to an adult in whom it declines by only 10% to 25% Adverse effects of anesthesia on lung function are well described These effects are even more pronounced in young children compared with adults.7 Compliance is the opposite of elastance and can be considered as the distensibility or stretchability of a tissue It is defined as the change in volume of a tissue divided by the change in pressure (DV/DP) In health, the respiratory system is relatively compliant, meaning that it does not require high pressures to expand As compliance worsens (or elastance increases) in disease, more pressure is necessary to expand the lungs The forces of the lungs that determine elastic recoil primarily involve surface forces at the gas-liquid interface and, to a lesser extent, elastic tissue forces Surface tension is a primary determinant of recoil pressure of the lungs and is defined as the pressure ... respiratory system However, some disagreement and confusion exist within scientific literature regarding the precise definitions of pressures within the respiratory system.1 Proper understanding of these... determines FRC The primary determinants of lung recoil pressure are elastin content within the tissue and surface tension within the airspaces The chest wall recoil is determined by the integrity and maturity... older children when corrected for size This is because neonates, unlike older children, maintain inherent tone in the muscles of respiration at end-exhalation.6 This prevents exhalation to a lower