CXR lags behind clinical deterioration by 12–24 hours — a normal CXR does NOT exclude early ARDS. HRCT is more sensitive and can show GGO before CXR becomes abnormal.
Cardiogenic Edema vs ARDS
| Feature | Cardiogenic Edema | ARDS (Non-Cardiogenic) |
|---|---|---|
| Distribution | Central/perihilar ("bat wing"); gravity-dependent; bilateral symmetric | Diffuse bilateral; peripheral and dependent; heterogeneous |
| Pleural effusions | Bilateral, common; often moderate-to-large | Usually absent or minimal |
| Cardiomegaly | Present (CTR >0.5 on PA CXR) | Normal cardiac size |
| Air bronchograms | Uncommon | Prominent; extensive |
| Septal lines (Kerley B) | Present; horizontal peripheral lines at lung bases | Absent |
| Vascular pedicle width | Widened (>70 mm on AP supine) | Normal |
| Response to diuresis | Rapid improvement | Persists despite diuresis |
ARDS — Berlin Definition Severity
ARDS is defined by bilateral opacities not fully explained by effusions, collapse, or nodules; onset within 1 week of clinical insult; PaO₂/FiO₂ ratio <300 with PEEP ≥5 cmH₂O:
| Severity | PaO₂/FiO₂ Ratio | Mortality |
|---|---|---|
| Mild | 200–300 mmHg | ~27% |
| Moderate | 100–200 mmHg | ~32% |
| Severe | <100 mmHg | ~45% |
ARDS — CT Phases
| Phase | Timing | CT Findings |
|---|---|---|
| Exudative (early) | Days 1–7 | Bilateral dependent GGO and consolidation; relative sparing of non-dependent lung; pleural effusions |
| Proliferative | Days 7–21 | Reticulation; traction bronchiectasis developing; GGO persists; fibrotic changes begin |
| Fibrotic (late) | >3 weeks | Architectural distortion; traction bronchiectasis; bullae; barotrauma complications (pneumatocele, PTX) |
Barotrauma complications — actively look for on serial CT: pneumothorax · pneumomediastinum · pneumopericardium · subcutaneous emphysema · pneumatoceles. Anterior pneumothorax may be missed on supine CXR.
Differential for Bilateral Opacities
When encountering bilateral opacities on CXR or CT:
- Cardiogenic edema — bat-wing distribution, cardiomegaly, effusions, Kerley B lines
- ARDS — heterogeneous, dependent, no cardiomegaly, air bronchograms
- Atypical/viral pneumonia — GGO-predominant, fever, infectious context
- DAH (diffuse alveolar hemorrhage) — hemoptysis, rapid onset, clearing after 3–5 days
- COP — migrating opacities, peribronchovascular, reverse halo sign
- PAP — crazy paving, perihilar geographic distribution, insidious onset