482 SECTION V Pediatric Critical Care Pulmonary Bhalla AK, Newth CJL, Khemani RG Respiratory support in children Paediatr Child Heal (United Kingdom) 2019;29(5) 210 217 Burke PGR, Kanbar R, Basting TM[.]
482 S E C T I O N V Pediatric Critical Care: Pulmonary Bhalla AK, Newth CJL, Khemani RG Respiratory support in children Paediatr Child Heal (United Kingdom) 2019;29(5):210-217 Burke PGR, Kanbar R, Basting TM, et al State-dependent control of breathing by the retrotrapezoid nucleus J Physiol 2015;593(13):2909-2926 Isono S, Tanaka A, Ishikawa T, Nishino T Developmental changes in collapsibility of the passive pharynx during infancy Am J Respir Crit Care Med 2000;162(3 I):832-836 Otis AB, Fenn WO, Rahn H Mechanics of breathing in man J Appl Physiol 1950;2(11):592-607 West JB Ventilation/Blood Flow and Gas Exchange 3rd ed London: Blackwell Scientific; 1979 The full reference list for this chapter is available at ExpertConsult.com e1 References Bhalla AK, Newth CJL, Khemani RG Respiratory support in children Paediatr Child Heal (United Kingdom) 2019;29(5):210-217 Williams K, Hinojosa-Kurtzberg M, Parthasarathy S Control of breathing during mechanical ventilation: Who is the boss? Respir Care 2011;56(2):127-139 Amiel J, Laudier B, Attié-Bitach T, et al Polyalanine expansion and frameshift mutations of the paired-like homeobox gene PHOX2B in congenital central hypoventilation syndrome Nat Genet 2003; 33(4):459-461 Weese-Mayer DE, Berry-Kravis EM, Ceccherini I, Keens TG, Loghmanee DA, Trang H An official ATS clinical policy statement: Congenital central hypoventilation syndrome - Genetic basis, diagnosis, and management Am J Respir Crit Care Med 2010;181(6): 626-644 Dempsey JA, Smith CA Update on chemoreception: Influence on cardiorespiratory regulation and pathophysiology Clin Chest Med 2019;40(2):269-283 Parshall MB, Schwartzstein RM, Adams L, et al An official American Thoracic Society statement: Update on the mechanisms, assessment, and management of dyspnea Am J Respir Crit Care Med 2012;185(4):435-452 Burke PGR, Kanbar R, Basting TM, et al State-dependent control of breathing by the retrotrapezoid nucleus J Physiol 2015;593(13): 2909-2926 Pattinson KTS Opioids and the control of respiration Br J Anaesth 2008;100(6):747-758 Dahan A General anesthesia and control of respiration Semin Anesth 1996;15(4):328-334 10 Guyenet PG, Bayliss D Neural control of breathing and CO2 homeostasis Neuron 2015;87:946-961 11 Fujioka M, Young LW, Girdany BR Radiographic evaluation of adenoidal size in children: Adenoidal-nasopharyngeal ratio Am J Roentgenol 1979;133(3):401-404 12 Arens R, McDonough JM, Corbin AM, et al Linear dimensions of the upper airway structure during development: Assessment by magnetic resonance imaging Am J Respir Crit Care Med 2002;165(1): 117-122 13 Wheeler M, Roth AG, Dunham ME, Rae B, Cote CJ A bronchoscopic, computer-assisted examination of the changes in dimension of the infant tracheal lumen with changes in head position: Implications for emergency airway management Anesthesiology 1988;88:1183-1187 14 Isono S, Tanaka A, Ishikawa T, Nishino T Developmental changes in collapsibility of the passive pharynx during infancy Am J Respir Crit Care Med 2000;162(3 I):832-836 15 Kwon JH, Shin YH, Gil NS, Yeo H, Jeong JS Analysis of the functionally-narrowest portion of the pediatric upper airway in sedated children Med (United States) 2018;97(27):1-6 16 Hogg JC, Williams J, Richardson JB, Macklem PT, Thurlbeck W, Path MC Age as a factor in the distribution of lower-airway conductance and in the pathologic anatomy of obstructive lung disease N Engl J Med 1970;282(23):1283-1287 Murray JF The Normal Lung Philadelphia: WB Saunders; 1986 18 Polar G, String ST The viscous resistance of the lung tissues in newborn infants J Pediatr 1966;69:787-792 19 Hoggs JC, Williams J, Richardson JB Age as a factor in the distribution in the distribution of lower airway conductance and in the pathologic anatomy of obstructive lung disease N Engl J Med 1970;282:1283-1287 20 Karlinsky JB, Snyder GL, Franzblaw C In vitro effects of elastase and collagenase on mechanical properties of hamster lungs Am Rev Respir Dis 1976;113:769-777 21 Ludwig MS, Dreshaj I, Solway J, Munoz A, Ingram RH Partitioning of pulmonary resistance during constriction in the dog: effects of volume history J Appl Physiol 2017;62(2):807-815 22 Wright JR, Hawgood S Pulmonary surfactant metabolism Clin Chest Med 1989;10:83 23 Bangham AD Lung surfactant: how it does and does not work Lung 1987;165:17-25 24 Hills BA What forces keep the air spaces of the lung dry? Thorax 1982;37(10):713-717 25 Akoumianaki E, Maggiore SM, Valenza F, et al The application of esophageal pressure measurement in patients with respiratory failure Am J Respir Crit Care Med 2014;189(5):520-531 26 Hedenstierna G Esophageal pressure: Benefit and limitations Minerva Anestesiol 2012;78(8):959-966 27 Fagan DG Post-mortem studies of the semistatic volume-pressure characteristics of infant’s lungs Thorax 1976;31(5):534-543 28 Thorsteinsson A, Larsson A, Jonmarker C The pressure-volume diagram of the thorax and lung Am J Physiol 1942;146:161-178 29 Otis AB, Fenn WO, Rahn H Mechanics of breathing in man J Appl Physiol 1950;2(11):592-607 30 West JB Ventilation/Blood Flow and Gas Exchange 3rd ed London: Blackwell Scientific; 1979 31 West JB, Dollery CT Distribution of blood flow and the pressureflow relations of the whole lung J Appl Physiol 1964;19:175-183 32 West JMB, Dollery CT, Naimarlk A Distribution of blood flow in isolated lung: relation to vascular and alveolar pressure J Appl Physiol 1964;19:713-724 33 Hughes JMB, Glazier JB, Maloney JE Effect of lung volume on the distribution pulmonary blood flow in man Respir Physiol 1968;4: 58-72 34 Edward RP Ventilation standards for use in artificial respiration J Appl Physiol 1955;7(4):451-460 35 Bhalla AK, Rubin S, Newth CJL, et al Monitoring dead space in mechanically ventilated children: Volumetric capnography versus time-based capnography Respir Care 2015;60(11):1548-1555 36 Bhalla AK, Belani S, Leung D, Newth CJL, Khemani RG Higher dead space is associated with increased mortality in critically ill children Crit Care Med 2015;43(11):2439-2445 37 Yehya N, Bhalla AK, Thomas NJ, Khemani RG Alveolar dead space fraction discriminates mortality in pediatric acute respiratory distress syndrome Pediatr Crit Care Med 2016;17(2):101-109 38 Roughton FJ Average time spent by blood in human lung capillary and its relation to the rates of CD uptake and elimination in man Am J Physiol 1945;143:621 e2 Abstract: The respiratory system’s vital function is to enable gas exchange The components responsible for effective gas exchange are complex and multifactorial They comprise central nervous system control of respiratory drive and airway tone, the resistive properties of the upper and lower airways, elastic properties of the lung and chest wall, diffusion across the alveolar-capillary membrane, transport of gases in the blood, diffusion of gases into the cells of the body, and the use of oxygen and production of carbon dioxide within cells as a by-product of metabolism The goal of this chapter is to introduce key concepts to better understand how they become deranged during critical illness and are modified by therapies applied in intensive care Key words: pediatrics, intensive care, respiratory physiology, mechanical ventilation, respiratory care 43 Noninvasive Respiratory Monitoring and Assessment of Gas Exchange DAVID F BUTLER AND KENNETH A SCHENKMAN PEARLS • • • Pulse oximetry is based on the principles that the pulsatile component of the optical absorbance detected from tissue is primarily from arterial blood and that oxyhemoglobin and reduced (deoxy-)hemoglobin have different optical absorption spectra Limitations of pulse oximetry include motion artifact, effects of ambient light, presence of pigmentation or dyes, low perfusion states, and dyshemoglobinemia Most pulse oximeters measure only oxyhemoglobin and deoxyhemoglobin However, blood CO-oximeters and noninvasive Noninvasive monitoring in the form of vital signs (i.e., heart rate, respiratory rate, noninvasive blood pressure, fluid intake and output, and temperature) has been used routinely for all patients receiving care in the intensive care unit (ICU) since the birth of the specialty Guidelines for equipment and monitoring and for levels of care for pediatric ICUs (PICUs) were specified by the American College of Critical Care Medicine in 2004.1 The majority of children admitted to the PICU present with cardiorespiratory disease or with an acute illness that may progress to involve the respiratory system, emphasizing the specific need for careful monitoring of respiratory parameters Close respiratory examination and monitoring allow titration of therapies to minimize oxygen toxicity, minimize ventilator-induced injury, optimize patient-ventilator interaction, and aid in weaning from the ventilator.2 Pulse oximetry and capnometry have significantly affected the practice of critical care medicine and are now standards of care New technologies currently under development to noninvasively monitor physiologic function may significantly decrease the need for more invasive monitoring and lessen the associated risks of such modalities Pulse Oximetry Pulse oximetry is a significant technologic advance that has improved patient safety.2–6 Its ease of application and accuracy have resulted in widespread use, and it is now a standard monitoring modality for many aspects of medical care Pulse oximetry allows • • CO-oximeters can account for other absorbing species such as methemoglobin and carboxyhemoglobin Capnography can be a good global indicator of the patient’s condition and can detect alveolar hypoventilation before changes detected by pulse oximetry The accuracy of the capnogram depends on the sampling site If the tidal volume is small and the sample flow rate is large, the gas sample may be diluted by entrained fresh gas for earlier detection of hypoxemia than clinical examination and aids the clinician in the identification and treatment of hypoxemia, with possible prevention of serious complications.7 In the ICU, pulse oximetry is also used to titrate oxygen therapy in patients, including those undergoing mechanical ventilation Within pediatrics, pulse oximetry is now used for neonatal congenital heart disease screening.8–10 Standard pulse oximeters are not accurately calibrated for the low saturations seen with some congenital cardiac lesions However, there is a “blue” sensor that is more accurate in the range of saturations encountered in patients with cyanotic congenital heart disease.11 Accurate pulse oximetry is especially vital in the neonatal population, who benefit from tight control of oxygenation in order to minimize oxidative stress and to decrease the risk of retinopathy of prematurity.12 Additionally, tight control of oxygenation (94%–99%) is now included in the Pediatric Advanced Life Support (PALS) guidelines for pediatric resuscitation.13 Principles of Pulse Oximetry Pulse oximetry is based on the principles that (1) the pulsatile optical absorbance detected in biological tissue is primarily due to arterial blood and that (2) oxyhemoglobin and reduced (deoxygenated) hemoglobin have different optical absorption spectra.6 The attenuation of light passing through blood-perfused tissue changes with pulsation of blood and the alternating component of the light attenuation results from the composition of arterial 483 484 S E C T I O N V Pediatric Critical Care: Pulmonary AC 2.0 Absorbance (OD) OD Arterial Capillary Venous Tissue DC 1.5 1.0 0.5 Time • Fig 43.1 Light passing through a pulsating tissue will be absorbed by multiple components of tissue and blood The alternating component (AC) is composed of only arterial blood DC, Direct current; OD, optical density 500 600 700 800 Wavelength (nm) blood.14 Fig 43.1 is a schematic diagram showing that the component of light attenuation as a result of pulsatility comes from arterial blood This information can be analyzed to determine the hemoglobin saturation in the arterial blood Absorption of light as a result of other tissue components and capillary and venous blood in the static (nonpulsatile) portion of the signal is ignored in the analysis Pulse oximeters typically use red (660-nm) and infrared (940-nm) wavelengths of light to determine the ratio of oxygenated to deoxygenated blood.15 Deoxygenated blood absorbs more red light, whereas oxygenated blood absorbs more infrared light The two wavelengths are passed through an arterial bed, and the ratio of infrared and red light transmitted to the photodetector is determined The absorption around 940 nm is relatively low and fairly constant over the range of saturations Thus, a change in absorbance at 660 nm can be referenced to the absorption at the 940-nm wavelength and is used to determine the saturation The ratio is calibrated against measurements of arterial oxygen saturations from human volunteers and their absorbance ratios Each pulse oximeter uses a complex algorithm to convert the change in absorbance at the two wavelengths to an absolute saturation value More wavelengths can be used to improve the accuracy of the measurement Hemoglobin has characteristic light-absorbing properties that change with oxygen binding Fig 43.2 shows absorption spectra of oxyhemoglobin and deoxyhemoglobin in the visible and nearinfrared spectral region At any given wavelength, there is a difference in absorption between oxyhemoglobin and deoxyhemoglobin except where the spectra cross at wavelengths called isosbestic wavelengths, where the absorption is the same for each state At nonisosbestic wavelengths, the difference in absorption can be used to determine the fraction of oxyhemoglobin Saturation of hemoglobin is defined as follows: Hbsat [OxyHb]/([OxyHb] [DeoxyHb]) where Hbsat is fractional saturation of hemoglobin, [OxyHb] is concentration of oxyhemoglobin, and [DeoxyHb] is concentration of deoxyhemoglobin Hemoglobin percent saturation, as commonly reported, is determined by multiplying Hbsat by 100 In the presence of other forms of hemoglobin, primarily carboxyhemoglobin or methemoglobin, the saturation of hemoglobin is correctly determined by the more complex relationship: Hbsat [OxyHb]/[OxyHb] [DeoxyHb] [MetHb] [CarboxyHb] • Fig 43.2 Absorption spectra of hemoglobin in the visible and nearinfrared spectral region The deoxy form of hemoglobin (blue) has a single peak in the visible and near-infrared region Oxyhemoglobin (red) has two peaks in the visible region but no significant peak in the near-infrared region OD, Optical density where [MetHb] is concentration of methemoglobin and [CarboxyHb] is concentration of carboxyhemoglobin Most pulse oximeters cannot accurately account for the presence of these other forms of hemoglobin Blood CO-oximeters, however, account for these species, as the latest generation of multiwavelength pulse oximeters Validation Numerous studies have been performed to validate existing pulse oximeters.16,17 Pulse oximeters also must be subjected to extensive testing before obtaining US Food and Drug Administration (FDA) approval for marketing in the United States Despite all of the current testing, difficulties in both calibration and validation remain One of the most significant issues surrounding calibration is the development of an appropriate universal test that will accurately test the pulse oximeter for a wide range of potential clinical applications Pulse oximeters must be accurate for a wide range of skin thickness and color, and over a wide range of saturations In general, pulse oximeters are most accurate at higher saturations, usually above 75%.18–20 Sources of Error Although pulse oximetry is widely accepted as a valid clinical monitor and provides valuable instantaneous clinical data, pulse oximeters are subject to multiple potential sources of error The accuracy (difference between peripheral capillary oxygen saturation [Spo2] and arterial oxygen saturation [Sao2]) reported by manufacturers is approximately 2%.21 Studies evaluating the performance of pulse oximetry in clinical applications have demonstrated an accuracy of 0.02% to 4% for a single Spo2 measurement.7,16,21,22 It is important to note that these values are based on measurements when patient Sao2 was above 90% and that precision of Spo2 worsens as Sao2 decreases.7 Additionally, Sao2 and partial pressure of arterial oxygen (Pao2) are not linearly related; the oxyhemoglobin dissociation curve is sigmoid in shape (Fig 43.3) Large changes in Pao2 at high levels of oxygen—the upper flat ... of the body, and the use of oxygen and production of carbon dioxide within cells as a by-product of metabolism The goal of this chapter is to introduce key concepts to better understand how they... Med 2012;185(4):435-452 Burke PGR, Kanbar R, Basting TM, et al State-dependent control of breathing by the retrotrapezoid nucleus J Physiol 2015;593(13): 2909-2926 Pattinson KTS Opioids and... control of respiration Semin Anesth 1996;15(4):328-334 10 Guyenet PG, Bayliss D Neural control of breathing and CO2 homeostasis Neuron 2015;87:946-961 11 Fujioka M, Young LW, Girdany BR Radiographic