In rebreathing machines, the size of the rebreathing bag should be determined by which factor?

• A. The anesthetic gas used
• B. The concentration of anesthetic gas used
• C. The total length of tubing
• D. The size of the animal

Answer: (D)

The amount of gas the animal needs per breath scales with the animal’s size because tidal volume (how much gas is moved in and out with each breath) increases as the animal gets larger. The rebreathing bag serves as the breathing reservoir for that gas, so its volume should match the animal’s tidal volume to provide a comfortable, effective breath and to keep CO2 under control. If the bag is too small for the animal, CO2 can accumulate and breathing becomes difficult; if it’s too large, the circuit’s dead space increases and can slow the rise of the anesthetic concentration. The anesthetic gas type or concentration and the tubing length don’t determine the bag size—the size of the animal does, because it dictates how much gas will be rebreathed and how much reservoir the patient requires.