**Question:** ARDS is characterised by all except

A. Pulmonary edema
B. Increased capillary pressure
C. Normal PaO2
D. Increased lung compliance

**Core Concept:** Acute Respiratory Distress Syndrome (ARDS) is a severe form of acute lung injury characterized by bilateral pulmonary infiltrates, hypoxemia, and increased lung compliance. It is a critical condition often associated with sepsis, pneumonia, or aspiration. The pathophysiology involves alveolar flooding (pulmonary edema) and increased capillary pressure, which leads to decreased oxygenation and impaired gas exchange. ARDS is often distinguished from other causes of hypoxemia by its unique clinical presentation and radiographic findings.

**Why the Correct Answer is Right:** Option C (normal PaO2) is incorrect because the main feature of ARDS is severe hypoxemia (low oxygen saturation in arterial blood). In healthy lungs, PaO2 is usually well above 80 mmHg, which is not the case in ARDS.

**Why Each Wrong Option is Incorrect:**

Option A (pulmonary edema) is partially correct, as ARDS is characterized by pulmonary edema, which leads to alveolar flooding. However, this option is incorrect because ARDS is not limited to pulmonary edema, as the syndrome encompasses other features as well.

Option B (increased capillary pressure) is incorrect because ARDS is caused by factors other than increased capillary pressure. The primary mechanism involves alveolar flooding (pulmonary edema) and impaired gas exchange due to inflammation, protein-rich fluid leaking from the capillaries into the alveoli, leading to decreased oxygenation.

Option D (increased lung compliance) is incorrect because ARDS is characterized by decreased lung compliance. Increased lung compliance is a feature of healthy lungs, while ARDS presents with decreased lung compliance due to lung inflammation and edema.

**Clinical Pearl:** A key aspect of managing ARDS is to maintain a low tidal volume (less than 6 ml/kg of ideal body weight) to avoid ventilator-induced lung injury and further damage. This approach, known as protective ventilation, is essential for minimizing lung injury and improving patient outcomes in ARDS cases.