Acute respiratory distress syndrome

Summary

• Lung damage that occurs within one week of an insult, with bilateral CXR opacities, not explain by other causes
• Severity based on P:F ratio: severe <100, moderate 100-200, and mild 200-300
• Lung protective ventilation reduces mortality
  • VT 6mL/kg predicted body weight, plateau pressure ≤30 cmH2O, SpO2 88-95%, and pH 7.3-7.45

Definition

• Syndrome of diffuse alveolar and interstitial edema
• "Baby lung": small effective lung volume, because the dependent portions are filled with fluid

Berlin Definition (2012)

• Syndrome, not disease, per European task force in 2012
• Timing: within 1 week of a known insult
• Origin of edema: not cardiac or hydrostatic edema
• Chest imaging bilateral opacities not otherwise explained
• Severity based on oxygenation
  • Mild: ratio of PaO2/FiO2 between 200-300 with PEEP over 5 cmH2O (27% mortality)
  • Moderate: ratio between 100 and 200 with PEEP over 5 cmH2O (32% mortality)
  • Severe: ratio below 100 with PEEP over 5 cmH2O (45% mortality)

Causes

• Direct
  • Pneumonia
  • Aspiration
  • Inhalational injury
  • Pulmonary contusion
  • Near-drowning
• Indirection
  • Sepsis
  • Major trauma
  • Multiple blood product transfusions
  • Pancreatitis
  • Shock
  • Drug overdose
  • Amniotic fluid embolism

Pathophysiology

• Acute edema from neutrophil activation, with formation of hyaline membranes, and eventual fibrosis
• Type I pneumocytes replaced with type II
• Lungs become less compliant, due to a decrease in the available lung volume

Differential Diagnosis

• ARDS (50%)
• Pneumonia (25%)
• Pulmonary edema (11%)
• Invasive aspergillosis (6%)
• Pulmonary embolism (3%)
• Other (5%)

Clinical Manifestations

• Dyspnea and escalating ventilatory requirements following a known cause of ARDS

Investigations

• Chest x-ray showing bilateral infiltrates
• Bronchoalveolar lavage (BAL)
  • Excludes the diagnosis if less than 5% neutrophils (can be over 80% in ARDS)
  • Suggestive if lavage to plasma protein ratio > 0.7 (less than 0.5 in hydrostatic edema)

Management

• Lung-protective ventilation (see ARDS ventilation protocol)
  • Targets tidal volume of 6mL/kg predicted body weight, plateau pressure ≤30 cmH2O, SpO2 88-95%, and pH 7.3-7.45
  • Decrease tidal volume from 8mL/kg to 6mL/kg predicted body weight over 4 hours, then continue to decrease in 1mL/kg increments until plateau pressures <30 or tidal volume 4mL/kg
  • PEEP over 15 cmH2O may be protective in severe ARDS
  • Permissive hypercapnia to CO2 60-70 and pH 7.2-7.25, if needed
• Prone positioning
• Maintain euvolemia