All are seen in ARDS, except :
**Core Concept**
Acute Respiratory Distress Syndrome (ARDS) is a life-threatening condition characterized by rapid onset of non-cardiogenic pulmonary edema, leading to impaired gas exchange and hypoxemia. The pathophysiology involves inflammation, increased permeability of alveolar-capillary membrane, and accumulation of edematous fluid in the alveoli.
**Why the Correct Answer is Right**
Hypercapnia, or elevated carbon dioxide levels in the blood, is not a characteristic feature of ARDS. This is because, although ventilation-perfusion mismatch and decreased lung compliance can impair oxygenation, they typically lead to hypoxemia (low PaO2) rather than hypercapnia (high PaCO2). In ARDS, hypoxemia is usually more pronounced than hypercapnia due to the compensatory increase in ventilation, which helps to maintain or even decrease PaCO2 levels.
**Why Each Wrong Option is Incorrect**
**Option A:** Pulmonary edema is a hallmark of ARDS, resulting from increased permeability of the alveolar-capillary membrane and leakage of fluid into the alveoli.
**Option B:** Decreased tidal volume is often seen in ARDS due to the associated lung injury and decreased lung compliance, which can lead to difficulty in expanding the lungs during inspiration.
**Option D:** Decreased compliance is a characteristic feature of ARDS, resulting from the inflammation, edema, and atelectasis that occur in the lungs.
**Clinical Pearl / High-Yield Fact**
In ARDS, the ratio of arterial oxygen to inspired oxygen (PaO2/FiO2) is often used to assess the severity of the condition. A ratio of less than 300 is considered indicative of ARDS.
✓ Correct Answer: C. Hypercapnia.