Which combination of signs is most characteristic of an air embolism during surgery in the sitting position?

• A. Sudden drop in end-tidal CO2 and hypotension with no other signs
• B. Sudden drop in end-tidal CO2, mill wheel murmur, hypotension
• C. Increase in EtCO2 and hypertension
• D. Increased SpO2 and stable hemodynamics

Answer: (B)

In a sitting-position operation, air can be entrained into the venous system and travel to the heart and lungs. The most characteristic picture of venous air embolism is a sudden fall in end-tidal CO2 because the air obstruction reduces pulmonary blood flow and cardiac output, so less CO2 is delivered to the lungs for exhalation. At the same time, air in the right heart and pulmonary circulation creates a distinctive murmur often described as a mill-wheel sound, reflecting the air-fluid interface, and the patient can become hypotensive from the drop in venous return and the resulting shock. This combination of a rapid EtCO2 drop, the mill-wheel murmur, and low blood pressure strongly points to air embolism in this setting.

Other patterns don’t fit as well: a lone EtCO2 drop with no other signs could be due to many issues; a rise in EtCO2 with hypertension isn’t typical of embolism and suggests other processes; increased SpO2 with stable hemodynamics would not signal an embolic event.