Chapter 23: Acute Respiratory Distress Syndrome

PATHOPHYSIOLOGY OF ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

• Acute inflammation occurring as a result of either direct or indirect injury

• Hallmark of acute phase is diffuse alveolar damage via altered permeability of pulmonary endothelial and epithelial cells

  • Leads to accumulation of protein-rich fluid in alveoli

  • Inactivates surfactant

  • Leads to hypoxia and impaired lung compliance

DEFINITIONS

• P/F Ratio

  • PaO₂/FiO₂ is a measure of oxygenation

  • A lower P/F ratio is reflective of more impaired oxygenation

  • Does not incorporate mean airway pressure

• Oxygenation Index (OI)

  • Measure of oxygenation developed for use in neonates with persistent pulmonary hypertension

  • Recommended for use in pediatric ARDS over P/F ratio

    $OI=[meanairwaypressures×FiO2÷PaO2]×100$

DIAGNOSTIC CRITERIA

HISTORICAL DEFINITION

• Acute lung injury (ALI)

  • Acute onset of respiratory failure

  • Bilateral infiltrates on chest x-ray (CXR)

  • PaO₂/FiO₂ <300

  • Absence of left ventricular failure

• Acute respiratory distress syndrome (ARDS)

  • Acute onset of respiratory failure

  • Bilateral infiltrates on CXR

  • PaO₂/FiO₂ <200

  • Absence of left ventricular failure

BERLIN DEFINITION OF ARDS (2012)

• Respiratory failure within one week of a known clinical insult OR new or worsening respiratory symptoms

• Bilateral opacities on CXR not explained by effusions, collapse, or nodules

• Respiratory failure not fully explained by cardiac failure or fluid overload

• Impaired oxygenation with positive end-expiratory pressure (PEEP) ≥5 cm H₂O

  • Mild: P/F ratio 200 to 300

  • Moderate: P/F ratio 100 to 200

  • Severe: P/F ratio <100

PEDIATRIC ACUTE LUNG INJURY CONSENSUS CONFERENCE (2015)¹ (TABLE 23-1)

• Excluded patients with perinatal lung disease

• Recommendation to use OI preferentially over P/F ratio as metric of severity of lung disease

• Provision for noninvasively ventilated patients