What is Acute Respiratory Distress Syndrome (ARDS)?

Acute Respiratory Distress Syndrome (ARDS)

ARDS is a severe form of acute lung injury characterized by bilateral pulmonary infiltrates, profound hypoxemia, and non-cardiogenic pulmonary edema that requires immediate lung-protective ventilation strategies to reduce mortality. 1, 2

Definition and Diagnostic Criteria

• ARDS is defined by the Berlin Definition as an acute inflammatory syndrome with increased pulmonary capillary leakage leading to interstitial and alveolar pulmonary edema 3
• Diagnostic criteria include:
  • Acute onset within one week of a known insult or new/worsening respiratory symptoms 1
  • Bilateral pulmonary opacities on chest radiography 1, 4
  • Respiratory failure not explained by cardiac failure or fluid overload 1, 4
  • Profound hypoxemia measured by PaO₂/FiO₂ ratio 1

Classification of Severity

• ARDS is classified based on the degree of hypoxemia using the PaO₂/FiO₂ ratio with a minimum PEEP of 5 cmH₂O 1:
  • Mild: 200 < PaO₂/FiO₂ ≤ 300 mmHg
  • Moderate: 100 < PaO₂/FiO₂ ≤ 200 mmHg
  • Severe: PaO₂/FiO₂ ≤ 100 mmHg

Epidemiology and Outcomes

• ARDS accounts for approximately 10% of all ICU admissions and 25% of mechanical ventilations 4
• The mortality rate remains high, with in-hospital mortality for severe ARDS ranging from 46% to 60% 4
• The development of ARDS in sepsis patients occurs in 25-42% of cases, with increased risk in those with persistent arterial hypotension 5

Pathophysiology

• ARDS develops when pulmonary or extrapulmonary insults trigger an inflammatory cascade 4
• Key pathophysiological mechanisms include:
  • Alveolar epithelial inflammation 5
  • Airspace flooding with plasma proteins and cellular debris 5
  • Surfactant depletion and inactivation 5
  • Loss of normal endothelial reactivity 5
• The inflammatory response involves neutrophils, cytokines, and oxidant stress 5
• The principal cause of hypoxemia is extensive right-to-left intrapulmonary shunting of blood flow 5

Common Causes

• Sepsis (most common cause, accounting for approximately 40% of cases) 5
• Pneumonia 4
• Trauma 4
• Multiple blood transfusions 4
• Pancreatitis 4
• COVID-19 pneumonia 6

Management Strategies

• Lung-protective ventilation is the cornerstone of ARDS management:

  • Low tidal volumes (4-8 ml/kg predicted body weight) 2
  • Limiting plateau pressures (<30 cmH₂O) 2
  • Higher PEEP for moderate to severe ARDS 2

• Prone positioning is recommended for:

  • All severe ARDS cases (PaO₂/FiO₂ <100 mmHg) 1
  • Should be implemented for >12 hours per day 2

• Adjunctive therapies based on severity:

  • Neuromuscular blocking agents for early severe ARDS 2
  • Corticosteroids (conditional recommendation) 2
  • VV-ECMO for selected patients with severe ARDS who fail conventional therapy 2

Monitoring and Prognostic Indicators

• The Lung Injury Score (LIS) can help grade severity and provide prognostic information 5:

  • LIS >3.5: 18% survival rate
  • LIS 2.5-3.5: 30% survival rate
  • LIS 1.1-2.4: 59% survival rate
  • LIS <1.1: 66% survival rate

• Changes in oxygenation over the first 48 hours appear to be more prognostically valuable than initial hypoxemia 5

Common Pitfalls and Caveats

• Misattribution of pulmonary edema can lead to misclassification - cardiogenic causes must be ruled out 1
• Underutilization of evidence-based strategies is associated with increased mortality 2
• Excessive tidal volumes and inspiratory pressures can cause ventilator-induced lung injury 2
• Delaying prone positioning in severe ARDS may miss the window of opportunity for this effective intervention 2
• Initiating corticosteroids too late (>2 weeks after ARDS onset) may be harmful 2

Long-term Outcomes

• Survivors of ARDS are at risk for:
  • Diminished functional capacity 4
  • Mental health issues 4
  • Decreased quality of life 4
  • Ongoing primary care is beneficial for these patients 4