522 SECTION V Pediatric Critical Care Pulmonary Tidal Volume Patients with diseases of decreased CSTAT (e g , acute respiratory distress syndrome [ARDS], pulmonary edema, pneumonia) are best ventilate[.]
522 S E C T I O N V Pediatric Critical Care: Pulmonary Areas with low resistance are preferentially filled throughout inspiration (both early and late) resulting in uneven ventilation especially in obstructive lesions Early inspiration: areas with short time constants fill up quickly and equilibrate with proximal airway pressure Late inspiration: areas with prolonged time constants receive more volume with slower equilibrium of pressure Result: more even gas distribution compared to volume-controlled ventilation especially in obstructive lesions Early phase Pressure equilibrium Max volume reached P Late phase Pressure and volume equilibrium still occurring P A Constant flow (volume control ventilation) B P P Variable flow (pressure control or PRVC ventilation) • Fig 46.18 Effect of inspiratory flow characteristics (A) In volume control ventilation, tidal volume (Vt) is delivered to the less-obstructed areas throughout inspiration Therefore obstructed areas of the lung receive a lower proportion of Vt, resulting in uneven ventilation (B) In pressure control ventilation or pressure-regulated volume control (PRVC), less-obstructed areas equilibrate with inflation pressure and therefore receive most of their Vt early during inspiration More obstructed areas, with prolonged time constants, require longer time for pressure equilibration and therefore continue to receive a portion of their Vt later during inspiration The entire Vt is more evenly distributed than with volume-cycled ventilation (From Kliegman RM, Stanton BF, St Geme JW, et al, editors Nelson Textbook of Pediatrics, ed 20 Philadelphia: Elsevier; 2016.) Tidal Volume Patients with diseases of decreased CSTAT (e.g., acute respiratory distress syndrome [ARDS], pulmonary edema, pneumonia) are best ventilated with a smaller Vt (6–8 mL/kg) to minimize peak inspiratory pressure and barotrauma This, again, can be demonstrated by examining the inspiratory pressure-volume curve (see Fig 46.16) After alveoli are inflated to a certain point, the pressure-volume curve flattens and application of higher pressure results in less volume change and risks overdistention of alveoli This point is termed the upper inflection point Avoiding atelectasis and overdistention by maintaining mechanical ventilation between the lower and upper inflection points is desirable Studies evaluating the efficacy of low Vt and high rate compared with standard ventilation have demonstrated improved outcomes in adults with ARDS,16 and this strategy is recommended in children receiving mechanical ventilation for ARDS.17,18 Maintaining ventilation in this “safe” zone can be assessed by examining the pressure-volume curve on the ventilator while in volume-control mode Conversely, with diseases of increased airway resistance and longer time constants, such as intubated asthma, lower respiratory rates are necessary to allow for pressure equilibration and gas flow As alveolar ventilation is dependent on rate and Vt, processes that require slow rates also require increases in Vt (often 10–12 mL/kg) to maintain adequate ventilation Inspiratory and Expiratory Times Manipulation of inspiratory and expiratory times during mechanical ventilation can have beneficial effects in both low-compliance and high-resistance lung disease Inspiratory time (TI) can be set on pressure-control and pressure-regulated volume-control modes In volume-control ventilation, TI can be adjusted by varying the inspiratory flow Expiratory time (TE) is not directly manipulated but varies based on respiratory rate and TI In patients with decreased FRC, hypoxia can often be improved by increasing TI and exposing the pulmonary capillary blood to a higher oxygen environment for a longer period of time Prolongation of TI also increases mean alveolar pressure, resulting in alveolar expansion and improving FRC This strategy can be effective as long as TE is sufficient for alveolar emptying, which is more likely in diseases with shorter time constants In diseases of increased airway resistance, a longer TI allows for better equilibration of pressure and therefore delivery of Vt Again, sufficient TE must be allowed for alveolar emptying In the setting of intrathoracic airway obstruction, this will necessitate slower respiratory rates The desirable effects of these maneuvers should be monitored by measuring auto-PEEP and exhaled Vt Inspiratory Flow Patterns Manipulation of inspiratory flow patterns may be beneficial in the setting of nonhomogenous lung disease in which different areas of the lung have different time constants A decelerating inspiratory flow pattern is characterized by a rapid rise to maximum flow very early in inspiration followed by a gradual decline in flow until exhalation starts This allows peak pressure to be achieved very early and held nearly constant throughout inspiration This flow pattern is inherent to pressure-control ventilation (PCV) and pressure-regulated volume control but not to standard volumecontrol ventilation, in which flow remains constant and peak inspiratory pressure is achieved very late in inspiration Compared with a constant flow pattern, decelerating flow is associated with lower peak inspiratory pressure while maintaining similar mean airway pressure.19 In lung disease characterized by regions with variable time constants, decelerating inspiratory flow patterns have been advocated and may result in more homogenous ventilation When peak pressure is achieved early in inspiration, less obstructed regions of the lung with shorter time constants will equilibrate early As pressure is held constant throughout inspiration, obstructed regions with long time constants will continue to receive volume and equilibrate later (Fig 46.18) This may result in more uniform gas distribution, reduce dead space, and improve CDYN PCV has been shown to be a safe and effective strategy in children with respiratory failure from status asthmaticus.20 CHAPTER 46 Mechanical Dysfunction of the Respiratory System Key References Lanteri CJ, Sly PD Changes in respiratory mechanics with age J Appl Physiol (1985) 1993;74(1):369-378 Papastamelos C, Panitch HB, England SE, Allen JL Developmental changes in chest wall compliance in infancy and early childhood J Appl Physiol (1985) 1995;78(1):179-184 Randolph AG Management of acute lung injury and acute respiratory distress syndrome in children Crit Care Med 2009;37(8):2448-2454 523 Sarnaik AA, Sarnaik AP Noninvasive ventilation in pediatric status asthmaticus: sound physiologic rationale but is it really safe, effective, and cost-efficient? Pediatr Crit Care Med 2012;13(4): 484-485 West JB Respiratory Physiology: the Essentials 9th ed Baltimore, MD: Lippincott, Williams and Wilkins; 2012 The full reference list for this chapter is available at ExpertConsult.com e1 References Wolfe DF, Sorbello JG Comparison of published pressure gradient symbols and equations in mechanics of breathing Respir Care 2006;51(12):1450-1457 Mansell A, Bryan C, Levison H Airway closure in children J Appl Physiol 1972;33(6):711-714 Bourdin A, Paganin F, Préfaut C, Kieseler D, Godard P, Chanez P Nitrogen washout slope in poorly controlled asthma Allergy 2006;61(1):85-89 Arets HG, Brackel HJ, van der Ent CK Forced expiratory manoeuvres in children: they meet ATS and ERS criteria for spirometry? Eur Respir J 2001;18(4):655-660 Papastamelos C, Panitch HB, England SE, Allen JL Developmental changes in chest wall compliance in infancy and early childhood J Appl Physiol (1985) 1995;78(1):179-184 Lopes J, Muller NL, Bryan MH, Bryan AC Importance of inspiratory muscle tone in maintenance of FRC in the newborn J Appl Physiol Respir Environ Exerc Physiol 1981;51(4):830-834 Trachsel D, Svendsen J, Erb TO, von Ungern-Sternberg BS Effects of anaesthesia on paediatric lung function Br J Anaesth 2016; 117(2):151-163 West JB Respiratory Physiology: the Essentials 9th ed Baltimore, MD: Lippincott, Williams and Wilkins; 2012 Lanteri CJ, Sly PD Changes in respiratory mechanics with age J Appl Physiol (1985) 1993;74(1):369-378 10 Gupta VK, Cheifetz IM Heliox administration in the pediatric intensive care unit: an evidence-based review Pediatr Crit Care Med 2005;6(2):204-211 11 Ferris BG, Jr, Mead J, Opie LH Partitioning of Respiratory Flow Resistance in Man J Appl Physiol 1964;19:653-658 12 Stocks J, Godfrey S Specific airway conductance in relation to postconceptional age during infancy J Appl Physiol Respir Environ Exerc Physiol 1977;43(1):144-154 13 Polla B, D’Antona G, Bottinelli R, Reggiani C Respiratory muscle fibres: specialisation and plasticity Thorax 2004;59(9):808-817 14 Meznaric M, Cvetko E Size and Proportions of Slow-Twitch and Fast-Twitch Muscle Fibers in Human Costal Diaphragm Biomed Res Int 2016;2016:5946520 15 Sarnaik AA, Sarnaik AP Noninvasive ventilation in pediatric status asthmaticus: sound physiologic rationale but is it really safe, effective, and cost-efficient? Pediatr Crit Care Med 2012;13(4):484-485 16 Amato MB, Barbas CS, Medeiros DM, et al Effect of a protectiveventilation strategy on mortality in the acute respiratory distress syndrome N Engl J Med 1998;338(6):347-354 17 Dellinger RP, Levy MM, Rhodes A, et al Surviving sepsis campaign: international guidelines for management of severe sepsis and septic shock: 2012 Crit Care Med 2013;41(2):580-637 18 Randolph AG Management of acute lung injury and acute respiratory distress syndrome in children Crit Care Med 2009;37(8): 2448-2454 19 Prella M, Feihl F, Domenighetti G Effects of short-term pressurecontrolled ventilation on gas exchange, airway pressures, and gas distribution in patients with acute lung injury/ARDS: comparison with volume-controlled ventilation Chest 2002;122(4):1382-1388 20 Sarnaik AP, Daphtary KM, Meert KL, Lieh-Lai MW, Heidemann SM Pressure-controlled ventilation in children with severe status asthmaticus Pediatr Crit Care Med 2004;5(2):133-138 e2 Abstract: Much like the cardiovascular system, the lungs and the 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 Key words: Mechanical dysfunction, respiratory system, compliance, resistance, time constant, work of breathing 47 Diseases of the Upper Respiratory Tract TODD OTTESON, CLARE RICHARDSON, AND JAY SHAH PEARLS • Laryngomalacia is the most common congenital anomaly of the Diseases leading to compromise of the airway are a frequent cause of morbidity and mortality in pediatric patients The upper airway in particular plays a critical role in basic functions, such as feeding, breathing, and voicing Pathology related to the upper airway often affects all three of these processes Furthermore, airway compromise is the most frequent cause of cardiac arrest in pediatric patients Respiratory failure can be caused by obstruction, mechanical impairment of ventilation, neuromuscular failure, and failure of oxygen delivery to the tissue.1 Specific diseases of the upper airway leading to obstruction are discussed initial more cephalad position brings the epiglottis into contact with the soft palate, making the neonate an obligate nasal breather for the first months of life As the child matures to an adult, the larynx descends to the level of the seventh cervical vertebra Traditional teaching has been that the larynx and trachea differ greatly between young children and adults, with the immature airway having a conical shape and the mature airway a cylindrical shape However, observations from modern imaging techniques, including bronchoscopy, computed tomography (CT), and magnetic resonance imaging (MRI), provide evidence that contradicts previous dogma.2–4 In fact, the immature larynx is roughly cylindrical and similar to that of the adult The glottis of a newborn measures mm in the anteroposterior dimension and mm in the transverse dimension The subglottis typically measures between and mm.5 Reduction of the subglottic lumen due to structural dysfunction or edema of even mm can reduce the lumen by approximately 30% Poiseuille’s law stipulates that laminar flow of gas through a tube is inversely proportional to the fourth power of the radius of the lumen: Anatomy and Physiology Distinct differences in the pediatric airway and adult airway exist that may predispose patients to acute airway obstruction The upper airway is composed of multiple subsites, including the nasal cavity, nasopharynx, oral cavity, oropharynx, and larynx The larynx can be divided into three separate subsites The supraglottis includes the epiglottis, arytenoid cartilages, aryepiglottic folds, and a portion of the false vocal cords The glottis is composed mainly of the true vocal cords, which are the main mechanism of phonation The subglottis consists of the area immediately below the true vocal cords, bounded by the cricoid cartilage The infant larynx is situated higher in the neck than that of an adult, between the second and fourth cervical vertebrae This 524 larynx, characterized by excess supraglottic tissue, shortened aryepiglottic folds, and low laryngeal tone It is also the most common cause of stridor in infants • Propanolol has become a mainstay in the medical therapy for children with subglottic hemangioma • Laryngotracheal stenosis or subglottic stenosis can present as a congenital airway obstruction and occurs most commonly at the level of the cricoid cartilage, which is a complete circumferential ring of cartilage Characteristics and size of endotracheal tubes may contribute to scar tissue in the subglottis with subsequent stenosis There are distinct differences in the pediatric and adult airway, including the fact that the infant airway is situated higher in the neck (between the second and fourth cervical vertebrae) than in the adult • Principles related to Poiseuille’s law state that a narrow airway lumen results in more turbulent air flow and higher resistance to air movement creating a small lesion, which may not even be symptomatic in an adult, that causes significant respiratory compromise in a pediatric patient • Symptoms of respiratory distress are closely associated with issues of the digestive system and may present in conjunction with aspiration and failure to thrive • Choanal atresia presents as a mixed bony/membranous atresia in 70% of patients and as a pure bony atresia in 30%, with a 2:1 predominance for females • R5 8ln/πr4 where R is the resistance to gas flow, l is the length of the tube, n is the viscosity of the gas, and r is the radius In translation, a narrow airway lumen results in more turbulent flow and higher resistance to air movement Because of this, a small lesion, which ... reference list for this chapter is available at ExpertConsult.com e1 References Wolfe DF, Sorbello JG Comparison of published pressure gradient symbols and equations in mechanics of breathing Respir... Mechanical dysfunction, respiratory system, compliance, resistance, time constant, work of breathing 47 Diseases of the Upper Respiratory Tract TODD OTTESON, CLARE RICHARDSON, AND JAY SHAH PEARLS... patients The upper airway in particular plays a critical role in basic functions, such as feeding, breathing, and voicing Pathology related to the upper airway often affects all three of these processes