According to the Berlin Definition, ARDS is classified into three severity grades based on the PaO₂/FiO₂ ratio. Which of the following represents the PaO₂/FiO₂ ratio threshold for moderate ARDS?

Explanation

## Berlin Definition of ARDS Severity The Berlin Definition (2012) replaced the older American-European Consensus Conference (AECC) definition and stratifies ARDS into three mutually exclusive categories based on the PaO₂/FiO₂ ratio, measured with PEEP ≥ 5 cm H₂O (or CPAP ≥ 5 cm H₂O). ### ARDS Severity Classification | Severity | PaO₂/FiO₂ Ratio | Mortality Risk | Clinical Features | |----------|-----------------|----------------|-------------------| | **Mild** | 201–300 mmHg | ~27% | Minimal hypoxemia, often managed with supplemental O₂ | | **Moderate** | 101–200 mmHg | ~32% | Significant hypoxemia, requires higher PEEP/FiO₂ | | **Severe** | ≤ 100 mmHg | ~45% | Profound hypoxemia, high mortality, often refractory | **Key Point:** The PaO₂/FiO₂ ratio of **101–200 mmHg defines MODERATE ARDS** (Option A). The range 201–300 mmHg corresponds to **MILD** ARDS, not moderate. The original answer key incorrectly marked Option B (201–300 mmHg) as correct — this is the mild category. **High-Yield:** All three grades require: - Acute onset (≤ 1 week of a known clinical insult or new/worsening respiratory symptoms) - Bilateral opacities on imaging (CXR or CT) not fully explained by effusions, lobar/lung collapse, or nodules - Respiratory failure not fully explained by cardiac failure or fluid overload - PEEP ≥ 5 cm H₂O (or equivalent CPAP) **Clinical Pearl:** The severity grade at presentation correlates with mortality and ICU length of stay. Severe ARDS (PaO₂/FiO₂ ≤ 100 mmHg) may require rescue therapies such as prone positioning, ECMO, or inhaled nitric oxide. (Reference: ARDS Definition Task Force, JAMA 2012; Harrison's Principles of Internal Medicine, 21st ed.)