What is the induction flow rate for a circle circuit?

• A. 100 mL/kg/min
• B. 10 mL/kg/min
• C. 4 L/min
• D. 1 L/min

Answer: (A)

Induction flow rate in a circle breathing system is about delivering a relatively high amount of fresh gas during the induction phase to quickly reach the target anesthetic concentration in the lungs. Using a higher flow helps purge the circuit of any residual gases, minimizes rebreathing of exhaled CO2, and allows the animal to reach the desired depth of anesthesia more rapidly.

The commonly taught value is 100 mL/kg/min because it scales with the patient’s size, so smaller animals get a brisk but manageable flow and larger animals receive enough gas to wash through the circuit effectively. For example, a 10 kg animal would be about 1 L/min, a 20 kg animal about 2 L/min, and so on. After induction, the flow is typically reduced to a maintenance level, which is lower and more economical while keeping the animal anesthetized.

Smaller, fixed numbers like a flat 4 L/min or 1 L/min don’t adjust for size and can be insufficient or wasteful depending on body weight, while a very low rate such as 10 mL/kg/min would be too slow to achieve rapid induction.