Severe ARDS Classification
Severe acute respiratory distress syndrome (ARDS) is classified as a PaO2/FIO2 ratio of ≤100 mmHg (Answer D), measured with a minimum PEEP of 5 cmH2O according to the Berlin Definition. 1, 2
ARDS Severity Classification System
The Berlin Definition establishes three mutually exclusive severity categories based on the PaO2/FiO2 ratio, all requiring a minimum PEEP of 5 cmH2O: 1, 2
- Mild ARDS: PaO2/FiO2 ratio 201-300 mmHg 1, 2
- Moderate ARDS: PaO2/FiO2 ratio 101-200 mmHg 1, 2
- Severe ARDS: PaO2/FiO2 ratio ≤100 mmHg 1, 2
Clinical Implications of Severe ARDS
The severe ARDS classification triggers specific therapeutic interventions that have mortality benefit:
Prone positioning is strongly recommended for patients with severe ARDS (PaO2/FiO2 <100 mmHg or <150 mmHg depending on guideline), implemented for >12 hours daily 1, 3, 4
Neuromuscular blocking agents should be considered for ≤48 hours when PaO2/FiO2 ratio is <150 mmHg 1, 3, 4
Higher PEEP strategies are recommended in moderate to severe ARDS to prevent alveolar collapse 1, 3, 4
Critical Measurement Requirements
The PaO2/FiO2 ratio must be calculated under standardized conditions to ensure accurate classification: 2
- Minimum PEEP of 5 cmH2O must be applied 1, 2
- Measurement should ideally occur within 24 hours of ARDS diagnosis for prognostic accuracy 5, 6
- Failure to standardize PEEP can lead to misclassification, as patients on higher PEEP may have more severe lung injury than the P/F ratio alone suggests 7
Common Pitfalls
Misclassification occurs when: 2
- The PaO2/FiO2 ratio is calculated without ensuring minimum PEEP of 5 cmH2O 2
- Cardiogenic pulmonary edema is not adequately excluded 3, 2
- The ratio is measured at ARDS onset rather than at 24 hours, which has better prognostic validity 5, 6
The 150 mmHg threshold within moderate ARDS has emerged in recent research as clinically relevant for identifying patients who may benefit from advanced therapies like ECMO, though this is not part of the formal Berlin Definition 8, 6, 9