A 64 year old hypertensive obese female was undergoing surgery for fracture femur under general anaesthesia. Intra-operatively her end tidal carbon dioxide decreased to 20 from 40 mm Hg, followed by hypotension and oxygen saturation of 85%. What would be the most probable cause:

**Core Concept**
The patient's sudden decrease in end tidal carbon dioxide (ETCO2) indicates a reduction in CO2 production or delivery to the lungs, which can be due to various factors such as cardiac arrest, pulmonary embolism, or a decrease in cardiac output.

**Why the Correct Answer is Right**
The patient's presentation of decreased ETCO2, hypotension, and decreased oxygen saturation suggests a massive pulmonary embolism (PE). The sudden decrease in ETCO2 is likely due to the blockage of pulmonary arteries, leading to a decrease in blood flow to the lungs, resulting in a reduction in CO2 delivery. This, in turn, causes hypoxemia and hypotension due to the decreased cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** Cardiac arrest would typically present with a more immediate and dramatic decrease in ETCO2, as well as a loss of electrical activity on the electrocardiogram.
**Option B:** A decrease in cardiac output could be due to various factors such as hypovolemia or anaphylaxis, but the sudden decrease in ETCO2 and the specific combination of symptoms make PE a more likely cause.
**Option C:** A pneumothorax would typically present with a sudden decrease in lung sounds and a shift of the mediastinum, which was not mentioned in the scenario.

**Clinical Pearl / High-Yield Fact**
Intraoperative hypoxemia and decreased ETCO2 can be a sign of a massive pulmonary embolism, which requires immediate recognition and treatment.

**Correct Answer:** C.

✓ Correct Answer: B. Fat embolism