EPIDEMIOLOGY• ARDS occurs in 1-4% of PICU admissions• 10% of PICU patients who receive mechanical ventilation meet diagnostic criteria for ARDS• Mortality varies between 20 – 75%
• Multicentered, prospective cohort study - Flori et al. -overall hospital mortality was 22% among children with a PaO2:FIO2 ratio <300 -ARDS (PaO2: FIO2 <200) had a mortality of 26% -MC diagnosis associated with ALI and ARDS among the entire study cohort pneumonia (35%)
American-European Consensus Criteria ALI and ARDS• Acute onset• Bilateral pulmonary infiltrates on chest radiography• Pulmonary artery occlusion pressure >18 mm Hg or no clinical evidence of left atrial hypertension• PaO2:FIO2 ratio <300 = ALI• PaO2:FIO2 ratio <200 = ARDS• Bernard GR, Artigas A, Brigham KL, et al. The American-European Consensus Conference on ARDS. Definitions, mechanisms, relevant outcomes, and clinical trial coordination. Am J Respir Crit Care Med 1994;149:818â€“24.
The Berlin Definition of ARDS• Respiratory symptoms must have begun within one week of a known clinical insult, or the patient must have new or worsening symptoms during the past week• Bilateral opacities consistent with pulmonary edema must be present on a chest radiograph or computed tomographic (CT) scan• The patient’s respiratory failure must not be fully explained by cardiac failure or fluid overload
• A moderate to severe impairment of oxygenation must be present(PaO2/FiO2) -Mild ARDS – The PaO2/FiO2 is >200 mmHg, but ≤300 mmHg, -Moderate ARDS – The PaO2/FiO2 is >100 mmHg, but ≤200 mmHg -Severe ARDS – The PaO2/FiO2 is ≤100 all accompanied by ventilator setting that include PEEP ≥5 cm H2O.
ETIOLOGYDIRECT INJURY INDIRECT INJURYCommon Common-Pneumonia , -Sepsis -Aspiration of gastric content -Severe traumaLess common Less common-Pulmonary concussion -Cardiopulmonary bypass-Fat embolism -Drug overdose-Near drowning -Acute pancreatitis-Inhalational injury, -Blood transfusion
Injury• Injury causes release of pro-inflammatory cytokines• Damage to the capillary endothelium and alveolar epithelium• Functional surfactant is lost• Ability to upregulate alveolar fluid clearance may also be lost
Consequences• Impaired gas exchange - ventilation-perfusion mismatching -while increased physiologic dead space impairs carbon dioxide elimination• Decreased lung compliance -stiffness of poorly or non-aerated lung• Pulmonary hypertension -hypoxic vasoconstriction, -vascular compression by positive airway pressure, -parenchymal destruction, airway collapse,
Phases of ARDS Exudative phase↓pulmonary compliance, arterial hypoxemia, tachypnea, hypocarbia. ,x ray (pulmonary edema) Fibroproliferative phase↑ alveolar dead space / refractory pulmonary hypertension due to chronic inflammation and scarring of the alveolar-capillary unit. Recovery phase restoration of alveolar epithelial barrier/ gradual improvement in pulmonary compliance resolution of arterial hypoxemia/ return to premorbid pulmonary function in many patients .
Clinical features• Fluid accumulation• Lung compliance declines and tachypnea ensues• Regional atelectasis and small-airways closure• Hypoxia / breathing labored• Hypocarbia followed by hypercarbia• Rales over areas of atelectasis or alveolar congestion and decreased air entry over areas that are largely consolidated. Occasionally rhonchi
InvestigationsChest Xray• Small volume lungs• Diffuse infiltrates• Airbronchograms , atelectasis• Fibrosis with reticular opacitiesCT scan• Heterogenous opacification in dependent regions
• PEEP -augment anatomical dead space by distending large airways -cardiovascular compromise in high PEEP• Optimal PEEP -Improves oxygenation -Displacement of fluid from alveoli -Recruitment and opening up of collapsed alveoli -Improved FRC• Permissive hypercapnia -Accept high CO2 till pH 7.2
• Most children have concomitant shock• Aggressive fluid resuscitation till stable• Excess lung water will decrease saturation• Adequate sedation and analgesia• Antibiotic therapy for primary cause• Early enteral nutrition
Prone positioning• Prone position improves V/Q mismatch• Recruitment of dependent portions• Decreases chest wall compliance (transmitting airway pressure to the alveoli more efficiently and stabilizing alveolar volume over a larger portion of previously nonaerated lung units)• If no deterioration with prone position, continue for 18-20 hours
Predictors of outcome• Disease-related -Oxygenation-PaO2/FiO2( mild, moderate, and severe ARDS had mortality rates of 27, 32, and 45 percent, respectively) -Pulmonary vascular dysfunction(elevated transpulmonary gradient (ie, ≥12 mmHg) -Underlying cause of the ARDS• Patient related
• Treatment related -Fluid balance-positive fluid balance may be associated with higher mortality (1) -Treatment with glucocorticoids -Packed red blood cell transfusion-increased likelihood of death (odds ratio 1.10 per unit transfused, 95% CI 1.04-1.17) (2)1.Rosenberg AL, Dechert RE, Park PK, et al. Review of a large clinical series: association of cumulative fluid balance on outcome in acute lung injury: a retrospective review of the ARDSnet tidal volume study cohort. J Intensive Care Med 2009; 24:35.2.Gong MN, Thompson BT, Williams P, et al. Clinical predictors of and mortality in acute respiratory distress syndrome: potential role of red cell transfusion. Crit Care Med 2005; 33:1191.