e66 SECTION XV Pediatric Critical Care Board Review Questions Krebs cycle production of adenosine triphosphate is very ineffi cient Most Krebs cycle adenosine triphosphate is generated by oxidation of[.]
e66 S E C T I O N XV Pediatric Critical Care: Board Review Questions Krebs cycle production of adenosine triphosphate is very inefficient Most Krebs cycle adenosine triphosphate is generated by oxidation of Krebs cycle reducing fragments When there is insufficient oxygen to feed oxidative phosphorylation, reducing fragments build up and inhibit Krebs cycle enzymes Anaerobic metabolism cannot prevent respiratory muscle exhaustion Muscle training improves the ability of respiratory muscles to meet excessive demand, although only to a point Rapid shallow breathing is a compensatory mechanism to deal with elevated work of breathing and occurs before respiratory muscles become exhausted Which of the following values is the least likely to stimulate the peripheral or central chemoreceptors, resulting in an increase in respiratory rate? A Low hemoglobin B Paco2 C Pao2 D pH Preferred response: A Rationale Hypoxemia is a powerful stimulus to ventilation mediated by sensory input originating in the carotid body chemoreceptor Peripheral chemoreceptor activity (reflected in minute ventilation) increases slightly with decrements in Pao2 below 500 mm Hg and rises steeply as Pao2 falls below 50 mm Hg Low oxygen tension, rather than low oxygen content, is the important ventilator stimulus Little carotid body response results from profound anemia Carotid chemoreceptor response also contributes to arousal from sleep during episodes of hypoxia Hydrogen ion concentration and carbon dioxide tension independently activate chemoreceptors in the carotid body and in the brainstem The simultaneous presence of hypoxia augments the hypercapnic ventilatory response Chapter 45: Ventilation/Perfusion Inequality The primary abnormality of gas exchange in all patients with respiratory distress syndrome (ARDS) include A Decreased alveolar-end capillary diffusion capacity B Decreased ventilation/perfusion (V/Q) matching C Increased intrapulmonary shunt D Hypoventilation Preferred response: C Rationale The primary gas exchange abnormality of ARDS is intrapulmonary shunt Some, but not all, patients will have areas of low V/Q in addition to shunt Consequently, increases in the fraction of inspired oxygen (Fio2) usually have little influence on arterial oxygenation Patients with ARDS not suffer from abnormalities in alveolar-end capillary diffusion capacity or hypoventilation unless there are other conditions complicating the ARDS Patients with ARDS experience elevated alveolar Pco2 and hypercapnia, which may be made worse if excessive levels of positive-end expiratory pressure (PEEP) are employed According to the alveolar gas equation, elevations in the alveolar partial pressure of carbon dioxide (Paco2) will result in a fall in alveolar and arterial oxygenation These decrements are small, responsive to increases in Fio2, and not the primary cause of hypoxemia in patients with ARDS 2 The abnormalities of gas exchange and the site of airflow obstruction in patients with asthma are, respectively: A Atelectasis and distal airways B Decreased alveolar-end capillary diffusion capacity and distal airways C Decreased ventilation/perfusion (V/Q) matching and proximal airways D Hypoventilation and distal airways E Intrapulmonary shunt and proximal airways Preferred response: C Rationale The primary gas exchange abnormality of asthma is V/Q mismatch which occurs in the distal airways due to edema and/or mucus production while airflow obstruction occurs due to bronchoconstriction in larger more proximal airways Although the same pathology produces the two pathophysiologic mechanisms, no correlation exists between measurements of airway obstruction and gas exchange Bronchodilators may augment flow to areas of low V/Q match, temporarily worsening hypoxemia Which of the following is most accurate regarding West zones of the lung A In zone 2, perfusion is determined by the difference between alveolar pressures and pulmonary venous pressures B In zone 3, the greatest pressure is the alveolar pressure C Zone approximates dead space areas of the lung D Zone conditions are commonly seen with increased pulmonary blood flow Preferred response: C Rationale The West zones describe the forces affecting pulmonary blood flow (PBF), which are determined by the gravitational influences on the lung In zone 1, the alveolar pressure (PA) exceeds both the pulmonary arterial (PPA) and venous pressures (PV) In this zone there is little to no blood flow so it approximates dead space, but such conditions are rare except in cases of diminished PBF In zone 2, PPA exceeds PA, which is greater than PV, and perfusion in this zone is determined by the pressure difference between alveolar and pulmonary venous pressures In zone 3, the pressures descend from PPA to PV and finally PA Shunt fraction can be calculated Which of the following data would be required? A Arterial oxygen content, alveolar-arterial oxygen difference, cardiac output B Arterial oxygen content, venous oxygen content, assumption of oxygen content of blood undergoing gas exchange C Cardiac output, arterial oxygen saturation, and oxygen consumption D Dead space fraction, minute ventilation, and cardiac output Preferred response: B Rationale The equation for calculating the shunt fraction is based on the Fick equation Q s / Q t C c O2 C a O2 C c O2 C V O2 CHAPTER 136 Board Review Questions where Qs and Qt represent shunt and total pulmonary blood flow, respectively, and CaO2, CvO2, and CcO2 are the arterial, venous, and pulmonary capillary oxygen contents, respectively The dead space fraction can be derived from measurement of mixed endtidal CO2 and arterial Pco2 but is not needed for calculation of shunt fraction Minute ventilation also is not needed for the calculation of the shunt fraction, but total cardiac output is part of the equation as seen above A similar equation employing carbon dioxide contents in place of oxygen contents could be constructed Although cardiac output is derived in the calculation of shunt fraction, response C is incomplete as the equation also requires contents Alveolar ventilation and barometric pressure are not required for the calculation Response A has only one of the necessary variables for the equation Establishment of a significant shunt fraction will inform the clinician that there will be decreased alveolar-arterial oxygen difference locally Regarding the effect of intrapleural pressures and alveolar size, which of the following statements is accurate? A The intrapleural pressure at the apex is more negative than at the base, so the alveoli are larger at the apex B The intrapleural pressure at the apex is more positive than at the base, so alveoli are smaller at the apex C The intrapleural pressures not vary between the apex of the lung and the base and therefore not affect alveolar size D The intrapleural pressure at the base is more positive than at the apex, so alveoli are larger at the base Preferred response: A Rationale The lung is a viscoelastic structure encased in the supporting chest wall with gravity imposing a globular shape on the lung Pleural pressure is more negative at the apex of the lung compared with the base, increasing approximately 0.25 cm H2O per centimeter of vertical distance toward the lung base Thus transpulmonary pressure is more marked at the apex so apical alveoli are large and at the upper end of the normal pressure-volume curve They distend less for a given pressure change, that is, they are less compliant In the spontaneously breathing upright human, maximal gas distribution occurs at the base and progressively diminishes toward the lung apex This gradient also exists when inhalation occurs in the supine or lateral decubitus position, although to a lesser degree A 7-year-old child with status asthmaticus is undergoing treatment in your pediatric intensive care unit with systemic corticosteroids, b2-agonists, ipratropium, and 0.60 fraction of inspired oxygen She has moderate air entry, bilateral wheezes, no nasal flaring, and mild intercostal retractions Her respiratory rate is 22 per minute Her pulse oximetry saturations prior to and after initiation of therapy were 91% and 86%, respectively Which of the following is the most likely explanation for this observed change in oxygen saturation? A Excessive fatigue with hypoventilation and resultant hypoxemia B Increase in airway secretion due to the institution of ipratropium C Increase in ventilation/perfusion mismatch due to b2agonist D Mucus plugging of the airways due to institution of ipratropium Preferred response: C e67 Rationale In persons with asthma, high inspired oxygen concentrations may prevent hypoxic pulmonary vasoconstriction and place low alveolar ventilation (Va)/perfusion (Q) regions at risk for absorption atelectasis, and high doses of bronchodilators may enhance the perfusion of low Va/Q areas, exacerbating Va/Q mismatch However, the beneficial effects of bronchodilators on airway resistance generally outweigh the worsening in Va/Q mismatch This child is not showing signs of excessive fatigue There is no nasal flaring, and retractions are only mild The air entry is moderate and wheezes are present, therefore response A is not correct Ipratropium is an anticholinergic that causes a decrease in airway secretion (thus response B is incorrect), and there are no clinical signs that this child has a mucus plug in her airway (thus response D is incorrect) One of the proposed mechanisms for improvement in oxygenation in patients with acute respiratory distress syndrome (ARDS) placed in the prone position is: A Perfusion is greater in the nondependent areas in the prone position B Perfusion is greater in the dependent areas in the prone position C Ventilation is greater in the dependent areas in the prone position D Ventilation is lower in the nondependent areas in the prone position Preferred response: A Rationale In ARDS and other lung injury models, nonaerated or poorly aerated portions of the lung are found mainly in the dependent areas Perfusion is largely gravity-independent, especially in West zone conditions The majority of perfusion goes through dorsal lung regions, whether in the prone or supine position Consequently, perfusion is greatest to the dependent lung in the supine position and to the nondependent lung in the prone position Positive pressure, especially positive end-expiratory pressure, redistributes perfusion toward the dependent portion of the lungs by creating the condition of West zones and This redistribution may increase the vertical perfusion gradient in the supine position but may reduce it in the prone position These various physiologic factors contribute to the increase in the uniformity of perfusion in the prone position You are caring for a 4-month-old child with bronchiolitis who has developed respiratory failure You instruct the medical student rotating on your service that bronchiolitis has features of both restrictive and obstructive lung diseases Common pulmonary abnormalities that occur in both restrictive as well as obstructive lung diseases are: A Altered closing capacity especially with respect to functional residual capacity (FRC) B Increased intrapulmonary shunt fraction C Increased time constants D Increased zone pulmonary blood flow conditions Preferred response: A Rationale In almost all lung diseases alterations in closing capacity and functions residual capacity (FRC) occur resulting in altered gas exchange Obstructive lung diseases (e.g., asthma) are characterized by increases in FRC and TLC, whereas in restrictive lung diseases e68 S E C T I O N XV Pediatric Critical Care: Board Review Questions (e.g., ARDS) both capacities are decreased However, closing capacity changes as well and so in asthma there is increased Va/Q mismatch, whereas in ARDS there is increased intrapulmonary shunt Many other lung diseases result in Va/Q mismatch or increased intrapulmonary shunt, and some of these diseases are discussed further in the chapter Increased alveolar-capillary diffusion time is seen in diseases that result in abnormalities of the alveolar-capillary interface, such as pulmonary fibrosis, and is not a defining feature of either obstructive or restrictive lung diseases The time constant of lung segments is defined by the equation R C where the time constant (t) is the product of resistance (R) and compliance (C) or R divided by elastance (E), the inverse of compliance In general the time constant is increased in obstructive lung diseases and decreased in restrictive lung disease However, lung diseases tend to be inhomogeneous and so some local areas may have time constants that differ markedly from the lung as a whole This is especially true in bronchiolitis Finally, zone in the West model of pulmonary blood flow occurs when pulmonary arterial and venous pressures exceed pulmonary alveolar pressures In obstructive lung disease, airways become hyperinflated, increasing alveolar pressure, and this will likely diminish the amount of zone areas Similarly, in restrictive diseases, there is loss of lung volume or pulmonary edema that increases the amount of zone regions A 7-month-old girl with severe pneumococcal pneumonia develops respiratory failure requiring intubation of her trachea and institution of mechanical ventilation Shortly after intubation an arterial blood gas shows the following results: pH 7.32, paco2 36 mm Hg, pao2 57 mm Hg on and Fio2 of 0.6 Her pulse oximeter reads 88% Explanation for differences in the abnormalities seen in the levels of carbon dioxide and oxygen include: A Carbon dioxide has greater diffusion in blood than oxygen does B The difference is entirely explained by the alveolar gas equation C The greater density of CO2 results in better gas exchange in gravitationally dependent areas of the lung D The hemoglobin-oxygen binding curve has limited influence on blood CO2 levels Preferred response: D Rationale Carbon dioxide does have greater density than oxygen and does diffuse more readily than oxygen However, neither of these features of carbon dioxide would explain the differences in levels of oxygen and carbon dioxide in this patient with pneumonia In patients with diffusion abnormalities (e.g., pulmonary fibrosis), especially in states of increased cardiac output, pulmonary capillary blood will equilibrate with the CO2 in alveolar gas, but equilibrium may not be reached with the oxygen in alveolar gas Still, pneumonia is not distinguished by decreased diffusion capacity In patients with gas intrapulmonary shunt or Va/Q mismatch abnormalities the carbon dioxide levels in affected pulmonary capillary units will be elevated, but often this can be corrected by increases in alveolar minute ventilation In more normal areas the CO2 in the pulmonary capillary blood is below normal and corrects the high CO2 of these areas when the blood mixes in the large pulmonary veins, and this probably is the case for this infant In contrast, since the majority of oxygen in the blood is carried by hemoglobin, areas of abnormal gas exchange are characterized by pulmonary capillary blood that is not fully saturated This cannot be corrected by less diseased areas of the lung because the small amount of dissolved oxygen in areas that are more normal will not improve the Po2 enough to bring the saturations of arterial blood to normal levels 10 The correct statement regarding alveolar size and compliance in different lung regions in a healthy child in the upright position is: A Apical alveoli are larger and therefore more compliant than alveoli at the base of the lungs B Alveolar size is the same at the apex and the base of the lungs, so compliance is equal C Alveoli at the base of the lungs are larger and therefore more compliant than apical alveoli D Alveoli at the base of the lungs are smaller and therefore more compliant than apical alveoli Preferred response: D Rationale Apical alveoli are large and at the upper end of the normal pressure-volume curve They distend less for a given pressure change, that is, they are less compliant Alveoli at the base are smaller and more compliant than apical alveoli 1 What is the definition of time constant? A The time required to inflate 63% of final lung volume and is equal to the product of resistance and compliance B The time required to inflate 95% of final lung volume and is equal to the product of resistance and compliance C The time required to inflate 63% of final lung volume and is equal to resistance/compliance D The time required to deflate 95% of final lung volume and is equal to resistance/compliance Preferred response: A Rationale The time constant (the product of resistance and compliance) is defined as the time required for inflation to 63% of final lung volume, inflation being indefinitely prolonged Therefore a given lung unit with a slow time constant will fill more slowly than one with a fast time constant and also will empty more slowly Should the time constants of different lung units vary, as frequently happens in pulmonary illness, gas distribution will be determined in part by the rate, duration, and frequency of inhalation 12 In an upright individual with normal lung perfusion, which West zone represents a region where PA PPV PPA, where PA is alveolar pressure, PPA is pulmonary artery pressure, and PPV is pulmonary venous pressure? A Zone I B Zone II C Zone III D This representation does not exist Preferred response: D Rationale The three-zone model of pulmonary blood flow has been widely used to explain the heterogeneity of perfusion within the lung CHAPTER 136 Board Review Questions Three variables comprise the components of this model: pulmonary arterial (Ppa), alveolar (Pa), and pulmonary venous (Pv) pressures The degree of blood flow within the lung depends on the relative magnitudes of these pressures within that zone Zone (Pa Ppa Pv) has negligible blood flow, because the higher alveolar pressure is believed to compress collapsible capillaries This region is one of minimal gas exchange and “wasted” ventilation Zone conditions are rare except in cases of diminished pulmonary blood flow (e.g., hypotension and cardiac failure) or increased Pa encountered during positive pressure ventilation Zone consists of the mid portions of the lungs in which Ppa Pa Pv, where flow rate is determined by the difference between pulmonary arterial and alveolar pressure Venous pressure does not influence the flow rate Blood flow progressively increases with descent through this zone, because Ppa increases whereas Pa remains relatively constant In the lowest zone of the lung described by West, zone 3, Ppa Pv Pa, therefore the arteriovenous pressure gradient (Ppa-Pv) determines flow rate This gradient remains relatively constant descending through this zone, although because pleural pressures increase less, blood flow is greater in more dependent areas of zone A zone region in the most dependent areas of lung also has been described In this region, transudated pulmonary interstitial fluid increases interstitial pressures, thereby reducing blood flow; this effect is exaggerated as lung volume diminishes from total lung capacity to residual volume 13 Which West zone represents a region where Ppa Pa Pv, where Pa is alveolar pressure, Ppa is pulmonary artery pressure, and Ppv is pulmonary venous pressure? A Zone I B Zone II C Zone III D This representation does not exist Preferred response: B Rationale See the rationale for Question 12 Chapter 46: Mechanical Dysfunction of the Respiratory System A child with advanced cirrhosis is admitted to the ICU for management of acute gastrointestinal bleeding The child has severe ascites and has become progressively more obtunded and hypoxemic Which of the following mechanisms is most likely to result in a negative cardiovascular response to the institution of positive pressure ventilation in this patient? A A disproportionate increase in pleural pressure as lung volume increases B An increased need for sedation after endotracheal intubation C A negative effect of PEEP on left ventricular contractility D A reduction in lung compliance after the addition of PEEP Preferred response: A Rationale Abdominal distention decreases chest wall compliance and, as a result, pleural pressure increases markedly as the lungs expand, decreasing venous return e69 Which of the following statements describes most accurately the relationship between the pressure displayed by the ventilator (measured at the endotracheal tube connector) and alveolar pressure during volume-controlled (constant flow) and pressure-controlled (decelerating inspiratory flow) mechanical ventilation? A The difference between ventilator pressure and alveolar pressure is determined by lung compliance in both modes B The difference between ventilator pressure and alveolar pressure is not affected by air leaks around the endotracheal tube during pressure-controlled ventilation C Ventilator pressure is higher than alveolar pressure at endinspiration during volume-controlled ventilation D Ventilator pressure is higher than alveolar pressure at endinspiration during pressure-controlled ventilation Preferred response: D Rationale During volume-controlled ventilation, inspiratory flow continues throughout inspiration Consequently, at the end of inspiration, there is a gradient of pressure between the endotracheal tube connector and the alveoli During pressure-controlled ventilation, flow decreases and ultimately ceases during inspiration Thus the two pressures tend to equilibrate at the end of inspiration Which of the following conditions is most likely to increase the volume displacement of the diaphragm in a spontaneously breathing infant? A Abdominal distention B Croup C Pulmonary edema D Spinal cord section at the level of C8 Preferred response: D Rationale Intercostal muscle paralysis causes severe rib cage distortion during inspiration, adding to the volume that the diaphragm has to displace during a breath All the other conditions tend to decrease tidal volume Regarding transmural pressures of thorax (PTH), lung (PL), and chest wall (PW), which of the following is correct (PA alveolar pressure, Pb atmospheric pressure, and Ppl pressure at pleural surface)? A PL PA Pb B PL PA Ppl C PTH Ppl Pb D PW PA Ppl Preferred response: B Rationale Different pressures are needed to inflate the thorax, the lungs, and the chest wall When the respiratory muscles are completely relaxed, the thorax, the lungs, and the chest wall are all held at their respective volumes by outward-acting pressure gradients across their walls These pressure gradients or transmural pressures are defined by the following equations (see Fig 136.18): (1) PTH PA Pb, (2) PL PA Ppl, and (3) PW Ppl Pb e70 S E C T I O N XV Pediatric Critical Care: Board Review Questions Which term is used to describe the property responsible for the development of pressure-volume loops of the respiratory system during breathing? A Compliance B Elastance C Hysteresis D Resistance Preferred response: C Lung volume Rationale Analysis of the volume-pressure relationships of the thorax and its components becomes more complicated when pressure changes generated by gas flow and by the movement of the lung and chest wall tissue as the lungs inflate and deflate are considered These pressure changes result from molecular interactions between the gas and the airway walls, within the gas stream itself, and among the components of the gas-liquid interface and the tissue These molecular interactions always result in a net loss or dissipation of energy from the respiratory system The lost energy can no longer be used to perform work, and consequently dissipative pressure losses cause the volume-pressure relationships of the respiratory system to follow a different trajectory depending on the direction of the volume change This property, known as hysteresis, is responsible for the development of loops when the volume-pressure relationships are plotted continuously during a breath In this graphic representation, the dissipative pressures can be easily identified as the horizontal distance between the volume-pressure tracing and the corresponding point on the elastic volume-pressure relationship The work done against these pressures can be quantified as the area enclosed by the loop (see Figure 136.18, below) Pres Pressure • Fig 136.18 Regarding elastic properties of the lung and chest wall, which of the following statements is correct? A Elasticity is a dissipative property of the lungs and chest wall B Outward elastic recoil of the lungs is counterbalanced by the inward elastic recoil of the chest wall C The relaxation volume of the lungs in an adult is the residual volume D The negative pressure generated between the lung tissue and chest wall contributes to venous return at normal lung volumes Preferred response: D Rationale Elasticity is typically a nondissipative process because the energy needed to produce elastic deformation during inspiration is accumulated in the tissues and then used to empty the lungs during expiration In contrast, all resistive processes are dissipative: The energy liberated by the friction of the gas against the airway walls or by the molecular interactions within the tissue is transformed into heat and transported outside of the system by the blood or the expired gas Elastic pressures result from the tendency of the components of the lungs and chest wall to recover their original shape after undergoing deformation By definition, elastic recoil drives the thorax and its individual components to adopt a volume, known as the relaxation volume, at which recoil itself is extinguished The relaxation volumes of the lungs and the chest wall are the volumes that each of these components would adopt if all the mechanical constraints imposed by their mutual attachments and interactions were removed The relaxation volume of the thorax, in contrast, is defined by the mechanical interaction of the lungs and the chest wall It coincides with the point at which the opposing elastic recoils of these two components neutralize each other In the adult, the relaxation volume of the lungs is lower than the residual volume (the volume of gas contained in the lungs at the end of a forced expiration) The relaxation volume of the chest wall, by contrast, exceeds 50% of the vital capacity (the maximal volume of gas that can be inhaled from residual volume) This discrepancy in the relaxation volumes of the lungs and chest wall has three important consequences First, it forces the relaxation volume of the thorax as a whole to occupy a position intermediately between the relaxation volumes of the lungs and the chest wall (at approximately 35% of vital capacity) Under most circumstances, this volume coincides with the functional residual capacity (FRC), which is the volume contained in the lungs at the end of a tidal expiration Second, as the thorax starts to rise above its relaxation volume during inspiration, the outward recoil of the chest wall contributes to the expansion of the lungs, thereby reducing the work that the respiratory muscles need to perform during normal circumstances Finally, at normal breathing volumes, the opposing actions of the lungs and chest wall recoils create a negative pressure at the boundaries of the lung tissue with the chest wall and the other intrathoracic structures This negative pressure is an important contributor to the return of venous blood to the chest In an infant, the chest wall generates remarkably little outward recoil within the normal range of breathing volumes Because the inward recoil of the lungs varies little with respect to lung size and age during development, the relaxation volume of the infant’s thorax is proportionally smaller than that of the adult If, as occurs in the adult, the FRC coincided with this relaxation volume (15% of vital capacity compared with 35% in the adult), then the infant would be at a definite disadvantage in terms of alveolar stability and oxygenation Newborns of most mammalian species have developed physiologic strategies to maintain their FRC above the relaxation volume of the thorax Maintenance of alveolar stability and oxygenation in neonates depend on physiologic strategies to which of the following? A Increase closing capacity B Increase residual volume C Increase vital capacity D Maintain the FRC above the relaxation volume of the thorax Preferred response: D ... of this model: pulmonary arterial (Ppa), alveolar (Pa), and pulmonary venous (Pv) pressures The degree of blood flow within the lung depends on the relative magnitudes of these pressures within... the high CO2 of these areas when the blood mixes in the large pulmonary veins, and this probably is the case for this infant In contrast, since the majority of oxygen in the blood is carried by... Zone III D This representation does not exist Preferred response: D Rationale The three-zone model of pulmonary blood flow has been widely used to explain the heterogeneity of perfusion within the