**Core Concept**
Air embolism is a medical emergency that occurs when air enters the vascular system, potentially leading to cardiac arrest and death. Continuous monitoring is crucial to detect air embolism early, allowing for prompt intervention. The monitoring technique should ideally detect even small amounts of air, making it a reliable indicator.

**Why the Correct Answer is Right**
The correct answer is the pulse oximeter, which continuously monitors oxygen saturation in the blood. A sudden drop in oxygen saturation can indicate an air embolism. This is because air bubbles in the bloodstream can reduce the surface area available for gas exchange, leading to decreased oxygen saturation. The pulse oximeter is sensitive to changes in oxygen saturation, making it an early and reliable indicator of air embolism.

**Why Each Wrong Option is Incorrect**
**Option A:** Electrocardiogram (ECG) may show changes in heart rhythm or rate in response to an air embolism, but it is not a direct indicator of air in the bloodstream.
**Option B:** Blood pressure monitoring may show changes in blood pressure, but it is not sensitive enough to detect small amounts of air in the bloodstream.
**Option C:** Central venous pressure (CVP) monitoring may show changes in CVP, but it is not a direct indicator of air in the bloodstream.

**Clinical Pearl / High-Yield Fact**
A sudden drop in oxygen saturation on pulse oximetry should prompt immediate investigation for air embolism, including cessation of anesthesia, checking for air in the breathing circuit, and using transesophageal echocardiography (TEE) to visualize air bubbles in the heart.

**Correct Answer: D. Pulse oximeter. Pulse oximeter monitoring is an early and reliable indicator of air embolism during anesthesia.**