What is the classification of Respiratory Distress Syndrome (RDS), specifically Acute Respiratory Distress Syndrome (ARDS), based on?

Classification of Respiratory Distress Syndrome (ARDS)

Acute Respiratory Distress Syndrome (ARDS) is classified based on the severity of hypoxemia according to the Berlin Definition, which categorizes ARDS as mild (200 < PaO₂/FiO₂ ≤ 300 mmHg), moderate (100 < PaO₂/FiO₂ ≤ 200 mmHg), or severe (PaO₂/FiO₂ ≤ 100 mmHg), with all categories requiring a minimum PEEP of 5 cmH₂O. 1, 2

Berlin Definition Criteria for ARDS

The Berlin Definition, published in 2012, established the current classification system for ARDS with the following required elements:

• Timing: Onset within 1 week of a known clinical insult or new/worsening respiratory symptoms 1, 2
• Imaging: Bilateral opacities on chest radiography not fully explained by effusions, lung collapse, or nodules 1, 2
• Origin of edema: Respiratory failure not fully explained by cardiac failure or fluid overload 1, 2
• Oxygenation: Hypoxemia categorized by PaO₂/FiO₂ ratio with minimum PEEP of 5 cmH₂O 1, 2

Severity Classification

ARDS severity is stratified into three categories based on the degree of hypoxemia:

• Mild ARDS: 200 < PaO₂/FiO₂ ≤ 300 mmHg with PEEP ≥ 5 cmH₂O 1, 2
• Moderate ARDS: 100 < PaO₂/FiO₂ ≤ 200 mmHg with PEEP ≥ 5 cmH₂O 1, 2
• Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg with PEEP ≥ 5 cmH₂O 1, 2

Clinical Significance of Classification

The severity classification has important prognostic and therapeutic implications:

• Mortality rates increase with severity: approximately 27% for mild, 32% for moderate, and 45% for severe ARDS 2
• Duration of mechanical ventilation in survivors also increases with severity: median of 5 days for mild, 7 days for moderate, and 9 days for severe ARDS 2
• Treatment strategies are guided by severity classification:
  • Prone positioning is recommended for severe ARDS (PaO₂/FiO₂ < 100 mmHg) 3, 4
  • Higher PEEP strategies are recommended for moderate to severe ARDS 3, 4
  • Neuromuscular blocking agents may be considered in early severe ARDS 3, 5
  • VV-ECMO consideration increases with severity, particularly in severe cases unresponsive to conventional therapy 3, 6

Evolution from Previous Definitions

• The Berlin Definition replaced the American-European Consensus Conference (AECC) definition from 1994 2, 4
• Key improvements in the Berlin Definition include:
  • Elimination of the term "acute lung injury" 2
  • Specification of timing criteria (within 1 week of insult) 1, 2
  • Requirement for minimum PEEP level (≥ 5 cmH₂O) 1, 2
  • Better predictive validity for mortality compared to the AECC definition 2

Clinical Application

When applying the ARDS classification in practice:

• PaO₂/FiO₂ ratio should be calculated with patients on at least 5 cmH₂O of PEEP 1, 2
• Bilateral opacities must be present on chest imaging and not fully explained by other causes 1, 2
• Cardiac failure or fluid overload must be ruled out as the primary cause of respiratory failure 1, 2
• The classification may change during the course of illness as oxygenation improves or worsens 4

Common Pitfalls in Classification

• Misattribution of pulmonary edema: Failing to adequately rule out cardiogenic causes can lead to misclassification 1, 7
• Inconsistent PEEP application: The PaO₂/FiO₂ ratio is affected by PEEP level, so standardized measurement is important 1, 2
• Delayed recognition: ARDS may be underrecognized by clinicians due to variable interpretation of radiographic and clinical criteria 4
• Overreliance on PaO₂/FiO₂ ratio: While this is the primary classification metric, it should be interpreted in the context of the overall clinical picture 4

The Berlin Definition has demonstrated superior predictive validity for mortality compared to previous definitions, making it the current standard for ARDS classification in both clinical practice and research settings 2, 4.