A 52-year-old man with community-acquired pneumonia is admitted to the ICU. Over the next 48 hours, he develops progressive hypoxemia, bilateral infiltrates on chest X-ray, and a PaO₂/FiO₂ ratio of 180 mmHg despite supplemental oxygen. He meets diagnostic criteria for ARDS. Which is the most common cause of ARDS in this clinical scenario?

Explanation

## Most Common Cause of ARDS **Key Point:** Sepsis (particularly from pulmonary and intra-abdominal sources) accounts for approximately 40–50% of all ARDS cases, making it the single most frequent etiology. ### Epidemiology of ARDS Causes | Cause | Frequency | Clinical Context | |-------|-----------|------------------| | **Sepsis** | 40–50% | Pneumonia, intra-abdominal infection, UTI, catheter-related | | Aspiration | 10–15% | Altered mental status, dysphagia, anesthesia | | Acute pancreatitis | 5–10% | Gallstones, alcohol, post-ERCP | | Massive transfusion | 5–10% | Trauma, massive hemorrhage | | Trauma (non-pulmonary) | 5–10% | Polytrauma, fat embolism | | Drug toxicity | 5% | Chemotherapy, antibiotics | **High-Yield:** In the given case, community-acquired pneumonia with sepsis is the most likely trigger. Pneumonia is the most common infectious cause of sepsis-related ARDS. ### Pathophysiology of Sepsis-Induced ARDS 1. Bacterial endotoxin (LPS) activates TLRs on macrophages and endothelial cells 2. Release of TNF-α, IL-1, IL-6, and IL-8 3. Neutrophil recruitment and activation in lung parenchyma 4. Increased capillary permeability → protein-rich edema fluid 5. Surfactant dysfunction and alveolar collapse 6. Ventilation–perfusion mismatch and hypoxemia **Clinical Pearl:** Sepsis-related ARDS typically has a worse prognosis (mortality ~45%) compared to non-sepsis ARDS (~35%), largely due to multi-organ dysfunction. **Warning:** Do not confuse the most common *cause* of ARDS with the most common *risk factor*. While sepsis is the leading cause, aspiration and trauma are also frequent precipitants in ICU populations. [cite:Harrison 21e Ch 297]