Kerley B line is seen on chest x ray when left atrial pressures exceed:
**Core Concept**
Kerley B lines are radiographic signs of pulmonary congestion, which occur due to increased pressure in the left atrium. The left atrium is a small, muscular chamber that receives oxygenated blood from the lungs and pumps it into the left ventricle. Elevated left atrial pressures can lead to fluid buildup in the lungs, causing Kerley B lines to appear on chest X-rays.
**Why the Correct Answer is Right**
Kerley B lines are seen on chest X-rays when the pressure in the left atrium exceeds 15-20 mmHg. This increased pressure causes fluid to leak from the pulmonary capillaries into the interstitial space, leading to the appearance of Kerley B lines. The interstitial fluid accumulation causes the lines to appear as short, horizontal lines at the lung periphery, usually in the lower lobes. The mechanism behind this is related to the increased hydrostatic pressure in the pulmonary capillaries, which exceeds the oncotic pressure, allowing fluid to leak out.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not correct because Kerley B lines are not typically seen at pressures below 15 mmHg. At lower pressures, the fluid leakage from the pulmonary capillaries is minimal, and Kerley B lines may not be visible on chest X-rays.
**Option B:** This option is incorrect because Kerley B lines are not typically seen at pressures above 25 mmHg. At higher pressures, the fluid leakage from the pulmonary capillaries is more severe, and Kerley B lines may be accompanied by other signs of pulmonary edema, such as alveolar edema and Kerley A lines.
**Clinical Pearl / High-Yield Fact**
Kerley B lines are a sensitive indicator of left atrial hypertension but are not specific. They can be seen in other conditions, such as interstitial lung disease, and are often accompanied by other radiographic signs of pulmonary congestion.
**Correct Answer:** C. 20