536 48 Pediatric Acute Respiratory Distress Syndrome and Ventilator Associated Lung Injury COLIN J SALLEE, ROBINDER G KHEMANI, AND LINCOLN S SMITH • Acute respiratory distress syndrome (ARDS) is a res[.]
48 Pediatric Acute Respiratory Distress Syndrome and Ventilator-Associated Lung Injury COLIN J SALLEE, ROBINDER G KHEMANI, AND LINCOLN S SMITH • Acute respiratory distress syndrome (ARDS) is a restrictive lung disease with severe hypoxemia and often hypercarbia Diffuse disruption of the alveolar epithelial-endothelial barrier resulting in noncardiogenic pulmonary edema is the main pathologic finding Lung-protective ventilation strategies that use lower tidal volume (Vt, lower inspiratory driving pressure, and higher positive end-expiratory pressure (PEEP) have been supported in the adult literature Differences in practice patterns, comorbidities, and outcomes specific to pediatric patients led to a pediatric definition of ARDS in 2015.1 In 1821, Laënnec published the “Treatise on Disease of the Chest” in which he described the gross pathology of the heart and lungs in an entity he termed “idiopathic anasarca of the lungs”— pulmonary edema without heart failure.2 This was perhaps the first scientific description of the same condition that has become a significant focus of contemporary medical research This entity was described by the name of the inciting injury (e.g., shock lung, posttraumatic lung, DaNang lung) for many years.3 Finally, in 1967, Ashbaugh and colleagues published a case series of patients 536 • • Acute respiratory distress syndrome (ARDS) is a restrictive lung disease with clinical manifestations of hypoxemia, low respiratory system compliance, and new pulmonary infiltrates Pathologically, ARDS is characterized by diffuse alveolar damage, pulmonary edema, loss of end-expiratory lung volume, and presence of alveolar neutrophils Direct insults leading to ARDS include pneumonia, gastric content aspiration, lung contusion, hydrocarbon ingestion, smoke inhalation, and mechanical ventilation Indirect insults include sepsis, severe trauma and burn, blood transfusions, pancreatitis, major surgery, and ischemia-reperfusion injury Injury to the alveolar epithelium and/or endothelium results in loss of alveolar epithelial-endothelial barrier function, with resultant accumulation of proteinaceous fluid in the alveolar space and surfactant inactivation • • • PEARLS Cellular and soluble mediators of inflammation, surfactant inactivation and loss, coagulation dysfunction, alveolar fluid clearance, and apoptosis are pathobiological mechanisms of lung injury Ventilator management for children with ARDS should embrace limiting both tidal volume and inspiratory pressure, allowing for permissive hypercarbia with sufficient levels of positive endexpiratory pressure to prevent alveolar derecruitment and limit high concentrations of inspired fraction of oxygen There is not clear evidence to support the routine use of pulmonary or nonpulmonary ancillary therapies for children with ARDS, including surfactant and inhaled nitric oxide, but data supporting prone positioning is increasing with severe hypoxemic respiratory failure, poor lung compliance, and diffuse alveolar infiltrates on chest radiograph Although there was an 11-year-old among the 12 patients in the case series, they coined the term “acute respiratory distress syndrome in adults,” which was later revised to “adult respiratory distress syndrome.”4,5 Acknowledgment that this syndrome also occurred in children did not occur until the 1994 American-European Consensus Conference (AECC), where it was renamed the acute respiratory distress syndrome (ARDS).6,7 Clinical Features: Pathophysiology ARDS is a restrictive lung disease with reduced respiratory system compliance due to pulmonary edema, atelectasis, surfactant dysfunction, and chest wall restriction (e.g., chest wall edema, ascites, peritonitis) Hypoxemia results from pulmonary edema, loss of functional residual capacity (FRC), and the heterogeneous intrapulmonary shunt [(ventilation/perfusion (V/Q) 0) and V/Q ratios ,1] that occurs when FRC falls below closing capacity (CC) CHAPTER 48 Pediatric Acute Respiratory Distress Syndrome and Ventilator-Associated Lung Injury Regions with increased physiologic dead space (ventilation with reduced to no perfusion, or V/Q ratios 1) are also common in ARDS from mechanisms related to endothelial injury and coagulation, impaired cardiac output or pulmonary perfusion, and regional overdistension Chest imaging often shows evidence of patchy, asymmetric to diffuse opacities In patients for whom the syndrome does not resolve, hypoxemia and low lung compliance persist, and alveolar dead space may worsen In these patients, chest imaging may show linear opacities, formation of bullae, and pneumothoraces Definition ARDS is an acute, inflammatory, diffuse, yet heterogeneous form of lung injury However, like other clinical syndromes, ARDS lacks a definitive gold standard for clinical diagnosis The AECC defined ARDS as acute, noncardiogenic pulmonary edema with bilateral pulmonary infiltrates on chest radiograph and an arterial oxygen partial pressure to inspired oxygen fraction (Pao2/Fio2) of 200 or less.7 For nearly decades, pediatric practitioners used the AECC definition for clinical care, research, and prognostication, despite its limitations The Berlin definition in 2012 provided several modifications to oxygenation, minimum PEEP, timing of acute onset, chest radiograph, and pulmonary capillary wedge pressure criteria.8,9 However, validation of the Berlin definition in pediatrics was limited by differences in the epidemiology and management of children with ARDS compared with adults.10,11 The Pediatric Acute Lung Injury Consensus Conference (PALICC) published a pediatric definition of ARDS, termed pediatric acute respiratory distress syndrome (PARDS; Fig 48.1) in 2015.1,12,13 The major differences between the Berlin and PALICC definitions are: The PALICC definition does not require bilateral infiltrates on chest radiograph but still requires evidence of pulmonary parenchymal disease The PALICC definition allows substitution of peripheral capillary oxygen saturation (Spo2) when Pao2 is not available to assess hypoxemia severity, as long as Spo2 is 97% or less The PALICC definition introduces the use of the oxygenation index (OI) or oxygenation-saturation index (OSI) to stratify severity groups instead of Pao2/Fio2 in those requiring invasive mechanical ventilation PALICC criteria not define an upper age limit, although children with perinatal-related lung injuries are excluded In addition, the PALICC definition of PARDS identifies an at-risk group and criteria to identify PARDS in patients with chronic lung disease, cyanotic heart disease, or left ventricular dysfunction.1,13 Epidemiology Using the AECC definition, the incidence of ARDS in US, European, Australian, and New Zealand children was estimated at 2.0 to 12.8 per 100,000 person-years.14–18 Observational studies in the 1990s and 2000s found that ARDS occurred in 3% to 6% of pediatric intensive care unit (PICU) patients, and between 5% to 8% of mechanically ventilated PICU patients ARDS mortality in children appears to be lower than in adults (18%–27% vs 27%– 45%, respectively),19–23 although there are some populations in which adult and pediatric ARDS mortality appears similar (35%).17,18,24–26 Age Exclude patients with perinatal-related lung disease Timing Within days of known clinical insult Respiratory failure not fully explained by cardiac failure or fluid overload Origin of edema Chest imaging Chest imaging findings of new infiltrate(s) consistent with acute pulmonary parenchymal disease Noninvasive mechanical ventilation Invasive mechanical ventilation PARDS (no severity stratification) Oxygenation Full face mask bilevel ventilation or CPAP ≥5 cm H2O P/F ratio ≤300 S/F ratio ≤2641 Mild Moderate Severe 4≤ OI