**Core Concept**
Air embolism is a medical emergency caused by the entry of air into the circulatory system, leading to potential collapse of the lungs and cardiac arrest. Early detection and diagnosis are crucial for effective management.

**Why the Correct Answer is Right**
A Doppler study, specifically a transesophageal echocardiogram (TEE), is the most sensitive and specific method for detecting air embolism. This is because air bubbles produce a characteristic signal on Doppler ultrasound due to their high reflectivity and low density. The Doppler study can detect even small amounts of air in the heart and great vessels, allowing for prompt intervention.

**Why Each Wrong Option is Incorrect**
**Option A:** End-tidal CO2 (ETCO2) is a measure of carbon dioxide elimination from the lungs, but it is not sensitive for detecting air embolism. A sudden drop in ETCO2 may indicate air embolism, but this is not a diagnostic test.

**Option B:** End-tidal N2 is not a commonly used measurement in clinical practice, and it is not relevant to the diagnosis of air embolism.

**Option D:** Ultrasound can be used to detect air embolism, but it is not as sensitive or specific as Doppler study. A Doppler study can detect air bubbles in real-time, whereas ultrasound may require imaging and interpretation.

**Clinical Pearl / High-Yield Fact**
Remember the "3 Ps" of air embolism: Prevention (using filters and avoiding needle insertion), Prompt recognition (using Doppler study), and Prompt treatment (using anticoagulants and surgical intervention).

**Correct Answer Line**
✓ Correct Answer: C. Doppler study