What is the role of the pop-off valve in a rebreathing system?

• A. Shunts the waste gases to the scavenger system
• B. Prevents the animal's lungs from overinflating
• C. Prevents the concentration of anesthetic gas from increasing to dangerous levels
• D. Adjusts the system for use with animals of different sizes

Answer: (B)

The pop-off valve is a pressure-relief device in a rebreathing circuit. It opens to vent excess gas to the scavenger when circuit pressure rises, keeping airway pressure at a safe, near-atmospheric level and preventing the animal’s lungs from overinflating during breathing or manual ventilation. While it helps remove gas from the circuit, the primary safety purpose is preventing overinflation, not regulating anesthetic concentration or sizing the system. Anesthetic concentration is mainly controlled by the vaporizer setting and fresh gas flow, and adjustments for different animal sizes are achieved through circuit design and flow rates rather than the pop-off valve.