512 SECTION V Pediatric Critical Care Pulmonary FEFmax FEF25% FEF25%–75% FEF75% FEV1 Expiration Inspiration A B Intrapulmonary airway obstruction VolumeF lo w FEFmax FEF25% FEF25%–75% FEF75% FEV1 Expi[.]
512 S E C T I O N V Pediatric Critical Care: Pulmonary FEFmax FEF25% FEFmax Intrapulmonary airway obstruction FEF25%–75% Restrictive disorder FEF25% FEF25%–75% FEF75% Expiration Flow Flow Volume FEV1 Inspiration Expiration FEF75% FEV1 Volume Inspiration A B • Fig 46.4 (A) Flow-volume loop in disease Intrapulmonary airway obstruction Note the decrease in FEFMAX (maximum expiratory flow rate during exhalation) and FEF25%–75% (mean flow between 25% and 75% total expiratory volume) and the concavity of the mid-expiratory curve (B) Restrictive disorder Note the more vertical and narrow shape Forced vital capacity is decreased, but flow rates are relatively less affected FEV1, 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.) P= ΔV + ΔC Elastic properties flow × R Flow-resistive properties P = pressure gradient V = volume C = compliance R = resistance Δ pressure Δ flow Dynamic process Flow Resistance = Elastance (inverse of Compliance) Volume Compliance = Δ volume Δ pressure Static process exerted by liquid at the air-liquid interface along the lining of the alveoli Liquid molecules at this interface are attracted to themselves to a much greater degree than to the adjacent air This creates tension within the alveoli and a natural tendency for them to collapse as the liquid lining its surface is drawn toward itself This property was first identified in the lung in the 1920s, when it was shown that a lung completely filled with fluid instead of air has a much lower elastance than lungs filled with air.8 As alveoli become smaller, the liquid layer along their surface becomes thicker and the attraction force among the water molecules becomes greater As this occurs, surface tension increases The pressure developed can be predicted using the Laplace equation, which describes the pressure inside a liquid bubble This pressure is directly related to the surface tension of the liquid and inversely related to the radius: P 2T/r • Fig 46.5 Equation of motion A pressure gradient is necessary to move air into the lungs This movement is opposed by elastic and flow resistive properties of the system To move air, the pressure gradient must overcome lung and chest wall elastance (static component) and flow resistance (dynamic component) (From Kliegman RM, Stanton BF, St Geme JW, et al, editors Nelson Textbook of Pediatrics, ed 20 Philadelphia: Elsevier; 2016.) As the radius decreases, pressure increases as the liquid film lining the alveoli becomes thicker and more water molecules interact When considering individual alveoli, those that are smaller tend to collapse more than those that are larger in diameter (Fig 46.7) This represents an unstable condition in which alveoli that are smaller would keep getting smaller and empty into low-pressure larger alveoli In intact lungs, this does not occur for two reasons: the action of surfactant and the interdependence of alveoli CHAPTER 46 Mechanical Dysfunction of the Respiratory System Infant Chest wall Volume (percent of total lung capacity) 100 Adult Lung 80 60 60 40 40 FRC +20 +10 –10 –20 –30 –40 Pressure (Cm H2O) Chest wall 100 80 20 513 Lung FRC 20 +30 +20 +10 –10 –20 –30 –40 Pressure (Cm H2O) • Fig 46.6 Developmental changes in lung and chest wall elastance Interaction between chest wall and lung recoil pressures in infants compared to adults The lower recoil pressure of the chest wall in infants favors a lower functional residual capacity (FRC) in infants (From Kliegman RM, Stanton BF, St Geme JW, et al, editors Nelson Textbook of Pediatrics, ed 20 Philadelphia: Elsevier; 2016.) Laplace’s Law P= 2T r Surfactant P1 P2 Unless surfactant is present, the smaller alveolus would empty into the larger one due to higher pressure Surfactant decreases surface tension more in the smaller alveolus as its surface concentration increases • Fig 46.7 Surface tension P, Pressure; P1, pressure in larger alveolus; P2, pressure in smaller alveolus; r, radius of alveolus; T, wall tension Surfactants are phospholipids and proteins that decrease surface tension of liquids They significantly reduce surface tension in the lung by reducing the attraction of the water molecules along the air-liquid interface Surfactant is produced in the type II alveolar cells and released onto the alveolar surface in response to alveolar stretch The phospholipids align along the interface with their hydrophobic tails facing the alveolar cavity and their hydrophilic heads embedded in the liquid layer As alveolar volume increases, the surfactant concentration at the interface decreases as it is spread over a larger surface area, and its concentration increases as alveolar volume falls With increasing concentration, the surfactant effect of lowering surface tension increases This allows smaller alveoli to maintain the same cavitary pressure as larger alveoli despite their lower volume, helping to prevent a further decrease in volume Although the function of surfactant has helped explain lung compliance and alveolar patency and has improved our understanding of lung function in health and disease for decades, the properties of a single alveolus and the law of Laplace oversimplify the forces acting on the intact lung, as alveoli are not independent of each other Alveolar interdependence is thought to be another important factor in determining alveolar patency As alveoli are adjacent and connected to each other, they exert tension on each other, and their interconnections tend to stabilize groups of alveoli As the volume of a single alveolus decreases, the recoil forces of adjacent alveoli pulling it back open will increase, helping to maintain its patency In addition to the surface properties, the elastic elements of the tissues involved also help determine overall respiratory compliance The interstitial components—such as connective tissue elastin, collagen, and other fibrous tissue—usually not contribute to lung recoil at low lung volumes or FRC At higher lung volumes, these tissues begin to add to recoil pressure as they become stretched With age, these tissues become less elastic and can contribute to increased lung compliance.9 Interstitial fluid and pulmonary blood volume also contribute to lung recoil, increasing elastance As interstitial fluid increases, such as in states of capillary leak or diseases of increased capillary hydrostatic pressure (e.g., heart failure), lung compliance will decrease Resistance is a dynamic property of the respiratory system and is defined as the amount of pressure necessary to generate a given amount of flow (volume/time) It is calculated as R DP/Dflow (Ohm’s law) and is expressed as cm H2O/L per second Resistance is directly proportional to the pressure gradient driving gas flow and inversely proportional to the rate of flow When pressure gradient is zero and flow ceases, resistance is zero Because resistance exists only while flow is occurring, it is often termed flow resistance Resistive Forces Although tissue (viscous) resistance is a component of total pulmonary resistance, it is relatively minor compared with airway resistance The contribution of viscous resistance and tissue inertia is more important in neonates and in conditions with increased alveolar/interstitial fluid Resistance encountered by the respiratory system during air movement is determined primarily by airway diameter and flow dynamics Laminar flow occurs when gas molecules are moving parallel to each other Laminar flow travels through a tube as a series of concentric layers of gas, the outermost of which travels the slowest owing to friction with the walls of the tube, and the innermost travels the fastest (Fig 46.8) When flow is laminar, movement is primarily determined by the gradient in pressure, with higher gradients producing increased flow Turbulent flow is the loss of this parallel arrangement, which occurs when the airways become acutely narrowed or at branch points or with sudden change in velocity, such as at airway junctions or bends S E C T I O N V Pediatric Critical Care: Pulmonary ∆ Volume 514 Laminar flow V3 AT T CS CDYN A V2 B V1 C CSTAT (A) = V3/∆P CDYN (B) = V2/∆P CDYN (C) = V1/∆P ∆ Pressure Turbulent flow • Fig 46.8. Laminar versus turbulent flow Laminar flow can be predicted using the Hagen-Poiseuille law, which states: ˙ ) (8lh/πr4) P (V where P is the pressure gradient, V˙ is flow, l is length of the tube, h is the viscosity of the gas, and r is the radius of the tube ˙, the equation can be written as: Since resistance P/ V R 8lh/πr4 Resistance in the setting of laminar flow is proportional to viscosity of the gas and length of the tube and is inversely proportional to the radius to the fourth power This demonstrates clearly the importance of radius on resistance If the radius of a tube is decreased by 50%, the resistance increases by 16 times When flow becomes turbulent, it becomes less dependent on pressure and more dependent on gas density Therefore, increases in driving pressure result in relatively less increase in flow This highlights the importance of maintaining laminar flow for minimizing work of breathing (WOB) and maximizing air movement Whether flow will remain laminar or turbulent can be predicted by Reynolds number (Re) Re 2rvD/h where r is the radius, v is the average velocity, D is the density of gas, and h is the viscosity of gas Re is a dimensionless number that predicts flow dynamics through a tube Numbers below 2000 are predictive of laminar flow Although calculation of Reynolds number is not clinically practiced, the equation illustrates the effect that radius, density, and viscosity have on maintenance of laminar flow A clinical example of this effect is the use of helium-oxygen mixture in the setting of airway obstruction Helium is much less dense and slightly more viscous than nitrogen; when it replaces nitrogen in inspired gas, it will improve laminar flow across obstructed airways.10 The airways can be divided into the extrathoracic, intrathoracic extrapulmonary, and intrathoracic intrapulmonary airways The airways branch dichotomously into an average of 23 generations The first 14 generations are purely conductive, and the latter are acinar airways that participate in gas exchange There is fair variation in airway branching and caliber, which accounts for heterogeneity in airway resistance and gas exchange among individuals From the terminal bronchiole onward, the airway is devoid of the supporting cartilage This predisposes the smaller airways to collapse with greater transmural pressure gradient, especially during exhalation The extrathoracic airway includes the nasal and oropharyngeal airway, pharynx, larynx, and proximal trachea Nasal passages • Fig 46.9 Static and dynamic pressure-volume relationship CDYN, Dynamic compliance; CSTAT, static compliance account for one-half to two-thirds of total airway resistance when breathing through the nose Newborns are relatively obligate nose breathers; nasal obstruction can significantly impact airway resistance and respiratory distress When mouth breathing, the extrathoracic airways account for one-third of total airway resistance.11 The main site of intrathoracic airway resistance is the mediumsized bronchi Although they have smaller radii, small-sized bronchi have a much larger total cross-sectional area and also maintain predominantly laminar flow Therefore, they are not major contributors to overall resistance in healthy older children However, in newborns and young infants, small-airway resistance is a greater proportion of overall resistance and may be up to four times greater than in adults.12 This, in part, explains why small-airway disease is so much more symptomatic in infants than older children Static and Dynamic Compliance The pressure needed to overcome elastic recoil and move air is measured once pressure has equilibrated and airflow has stopped When compliance (DV/DP) is measured in this manner, it is termed static compliance (CSTAT) Additional pressure is necessary to overcome resistance when air is flowing The effect of resistance on compliance can be demonstrated using a pressure-volume plot (Fig 46.9) The static relationship between pressure and volume is represented by the line A in Fig 46.9 At any given change in pressure, a corresponding change in volume is achieved once airflow has stopped The additional pressure necessary to overcome resistance is represented by the curves B and C in Fig 46.9 when flow is occurring During airflow, the same change in pressure results in less change in volume depending on the amount of resistance The DV/DP relationship during flow is termed dynamic compliance (CDYN) The CDYN for curve C is lower than for curve B because of increased resistance Therefore, the difference between CDYN and CSTAT represents degree of resistance Clinically, the difference between static and dynamic compliance can be measured during mechanical ventilation When patients are receiving a set Vt using constant flow (volume control mode), the difference between peak pressure and the plateau pressure obtained using an inspiratory hold maneuver can give an estimate of the severity of airway obstruction (Fig 46.10) CDYN can be calculated as the Vt divided by the DP (difference between the peak inspiratory pressure and the end-expiratory pressure) CSTAT is calculated as the Vt divided by DP (the difference between the plateau pressure and end-expiratory pressure) CDYN is always lower than CSTAT, and the degree of difference is dependent on the degree of airway obstruction CHAPTER 46 Mechanical Dysfunction of the Respiratory System 515 Pressure Peak inspiratory pressure (PIP) Plateau pressure Flow CSTAT = Volume CDYN = VT PPLAT – PEEP VT PIP – PEEP Inspiratory hold maneuver Time • Fig 46.10 Static (CSTAT) and dynamic (CDYN) compliance Pressure, flow, and volume versus time curves for mechanically ventilated patient receiving volume-control breaths with constant flow The first breath is normal inflation and exhalation The second breath has an inspiratory hold maneuver added The peak inspiratory pressure (PIP) is achieved at end of tidal inflation The plateau pressure (Pplat) is achieved once pressure has equilibrated PIP is higher than plateau pressure due to the presence of resistance during airflow CSTAT is calculated using plateau pressure CDYN is calculated using peak inspiratory pressure PEEP, Positive end-expiratory pressure; Vt, tidal volume Airway Dynamics Normally, resistance to airflow is lower during inspiration than exhalation During inspiration, PPL becomes more negative This negative pressure is transmitted to all structures inside the chest, including the airways, which get larger in diameter Conversely, during exhalation, the pressure inside the lung becomes positive relative to PATM, driving air from the lungs As gas flows, the pressure inside the airways decreases more so than outside, leading to a tendency of airways to narrow In normal individuals, the changes in airway diameter are relatively small and of little significance during restful breathing The changes in airway diameter during inspiration and exhalation are dependent on two factors: (1) the magnitude of changes in transmural pressures experienced by the airways and (2) softness (compliance) of the airways, primarily determined by structural support The changes in airway diameter are exaggerated in any form of airway obstruction, where greater intrathoracic pressure must be created to overcome airway obstruction, and in infants and young children whose airways are more compliant because of less cartilaginous support With extrathoracic airway obstruction (choanal atresia, retropharyngeal abscess, vocal cord dysfunction, laryngotracheitis, and so on), highly negative intrapleural pressure during inspiration is transmitted up to the site of obstruction beyond which the pressure drops precipitously The intramural pressure in the extrathoracic airway below the site of obstruction is much more negative than the PATM resulting in its collapse (Fig 46.11) This results in increased inspiratory resistance, prolongation of inspiration, stridor, and suprasternal retractions During exhalation, the increased positive intrapleural pressure is similarly transmitted to the site of obstruction, distending the extrathoracic airway and ameliorating obstruction It should be noted that for stridor to manifest, sufficient air needs to move across the obstructed airway This may not occur in very young babies whose only manifestation may be a pattern of seesaw or paradoxic respiration as the compliant chest wall retracts inward during inspiration in response to negative intrathoracic pressure and bulges outward during exhalation because of positive pressure in the thorax The abdomen moves in the opposite direction of the thorax because of the movement of the diaphragm With intrathoracic airway obstruction, both extrapulmonary (vascular ring, mediastinal tumors, and so forth) and intrapulmonary (asthma, bronchiolitis, and so forth), the increased negative intrapleural pressure helps distend the intrathoracic airway during inspiration, thus ameliorating the obstruction somewhat (Fig 46.12) During exhalation, the increased positive intrathoracic pressure is transmitted up to the site of obstruction, after which there is a rapid decline in intraluminal pressure Therefore, the intrathoracic airway above the site of obstruction is subjected to an excessively increased transmural pressure, resulting in collapse This leads to a marked increase in expiratory resistance, prolonged expiration, and wheezing As obstruction becomes more severe, the expiratory time may become insufficient to allow for complete alveolar emptying, resulting in air trapping and auto-PEEP The site at which the intraluminal pressure is equal to extraluminal pressure during normal forced exhalation is referred to as the equal pressure point (EPP; Fig 46.13) With intrathoracic airway obstruction, the EPP is shifted distally toward the alveolus, causing intrathoracic airway collapse proximal to the EPP In infants, owing to the relatively weaker cartilaginous support, the changes in the intrathoracic tracheal lumen may be exaggerated in response to these wide swings in transluminal pressures This is often referred to as collapsible trachea, but the tracheal collapse is a result of distal airway obstruction This tracheal collapse can contribute to the increase in airway resistance and expiratory difficulty, but it is rarely the primary abnormality Time Constant Air flows into and out of the lungs as long as a pressure gradient exists Once pressure equilibrates, the flow ceases However, pressure equilibration does not occur instantaneously It takes time, 516 S E C T I O N V Pediatric Critical Care: Pulmonary Retropharyngeal abscess Vocal cord paralysis Subglottic stenosis Croup Worse during inspiration • prolongation of inspiration • Inspiratory stridor • Chest wall retractions • Paradoxical (see-saw) respiration –40 –40 –30 –40 Airway collapse +5 +40 +40 +40 +40 Airway distension +40 +40 –40 –40 +5 –35 –40 A Retropharyngeal abscess Vocal cord paralysis Subglottic stenosis Croup –5 +40 +5 +45 +40 –40 Extrathoracic airway obstruction Inspiration B +40 Extrathoracic airway obstruction Expiration • Fig 46.11 Airway dynamics in extrathoracic airway obstruction (A) During inspiration, increased nega- tive pleural pressure is transmitted to all airways, including the extrathoracic This results in collapse of the extrathoracic airways distal to the site of obstruction The end result is increased inspiratory resistance and worsening of obstruction (B) During expiration, positive pleural pressure is transmitted to all airways, including the extrathoracic This results in distention of the airway below the site of obstruction and improvement 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.) which is determined by the compliance and resistance of the lung and airways Greater resistance results in greater time for pressure equilibration and flow cessation By the same token, the greater the compliance, the more time is needed to either fill or empty the alveoli Time constant is a representation of the time needed to move air into and out of the lung for a given breath It is the product of compliance and resistance (T C R) Since compliance is measured as V/P and resistance as P/V per second, their product is simply measured in seconds A single time constant represents the time necessary to complete 63% of the Vt gas flow (or pressure equilibration) for that breath Three time constants represent 95% and four constants represent 98% completion, respectively Because resistance is higher during exhalation, the expiratory time constant is longer than inspiratory under normal conditions Diseases of decreased compliance (increased elastance) result in decreased time constant When acted upon by a driving pressure (either negative or positive), lungs with low compliance not stretch as well During inspiration, pressure equilibrates more quickly and flow stops During exhalation, the greater elastic recoil returns the lung to its original state more quickly; thus exhalation is short as well This results in a very short respiratory cycle with relatively low Vt With lower Vt, minute ventilation is maintained by increased respiratory rate and, indeed, the hallmark breathing pattern of low-compliance respiratory disease is low Vt and high respiratory rate Conversely, in the setting of airway obstruction, pressure equilibration and the completion of gas flow take longer than normal; therefore, time constants are prolonged Although disease with high resistance may have respiratory rates elevated above normal, they are typically not as elevated as in diseases of low compliance Failure of pressure equilibration during exhalation due to inadequate exhalation time can result in hyperinflation and air trapping This can occur in patients who are mechanically ventilated with high respiratory rates or in whom significant airway obstruction exists Failure of complete exhalation results in auto-PEEP In the setting of auto-PEEP, the lung will be less receptive to receiving the subsequent Vt because of hyperinflation and increased alveolar pressure As this perpetuates, compliance worsens and Vt declines This effect is more pronounced at higher rates and lower expiratory times The dependence of CDYN on respiratory rate and adequate lung emptying is termed frequency dependence of compliance If auto-PEEP becomes severe enough, ventilatory failure and lung injury may result The tendency of neonates to have higher respiratory rates and incomplete lung emptying during exhalation also results in air trapping This is termed dynamic hyperinflation, which helps to increase FRC Work of Breathing Respiratory distress is a subjective term often used to describe the perceived amount of effort that patients expend to breathe It often takes into account degrees of tachypnea, retractions, and accessory muscle use In contrast, respiratory work or work of breathing (WOB) is an important physiologic concept that can be defined and is helpful in understanding both the physiology of respiratory disease and benefits of therapeutic interventions All diseases of compliance and resistance result in increased WOB; the primary goal of interventions for these diseases is, even indirectly, to decrease WOB ... pressure result in relatively less increase in flow This highlights the importance of maintaining laminar flow for minimizing work of breathing (WOB) and maximizing air movement Whether flow... CDYN, Dynamic compliance; CSTAT, static compliance account for one-half to two-thirds of total airway resistance when breathing through the nose Newborns are relatively obligate nose breathers; nasal... significantly impact airway resistance and respiratory distress When mouth breathing, the extrathoracic airways account for one-third of total airway resistance.11 The main site of intrathoracic airway