Acute Respiratory Distress Syndrome (ARDS)
ARDS is a clinical syndrome of severe respiratory failure characterized by rapid onset of non-cardiogenic pulmonary edema, widespread inflammation, and hypoxemia.
1. Pathogenesis: The Diffuse Alveolar Damage (DAD)
ARDS is essentially the manifestation of Diffuse Alveolar Damage (DAD). The primary insult activates neutrophils and macrophages, leading to damage of the endothelium and alveolar epithelium.
- Exudative Phase (1–7 days): Damage to the alveolar-capillary interface causes protein-rich fluid to leak into the alveoli, forming hyaline membranes.
- Proliferative/Fibrotic Phase (1–3 weeks): Repair efforts by Type II pneumocytes and fibroblasts lead to interstitial fibrosis.
2. Exam Must-Knows
| Key Feature | Clinical/Pathological Detail |
|---|---|
| Hyaline Membranes | The hallmark histological feature is composed of fibrin and necrotic epithelial cell debris. |
| Pulmonary Edema | Non-cardiogenic (PCWP < 18 mmHg). It is due to increased alveolar-capillary permeability, not hydrostatic pressure. |
| Common Causes | Sepsis (most common), aspiration, trauma, pancreatitis, shock. |
3. Clinical Pearl
Diagnostic Note: On chest X-ray, look for diffuse bilateral opacities (white-out) that do not fit the criteria for cardiogenic pulmonary edema. The PCWP (Pulmonary Capillary Wedge Pressure) will be normal, which is the key differentiator from CHF.