What is the commonly used induction fresh gas flow rate for a circle breathing circuit?

• A. 50 mL/kg/min
• B. 100 mL/kg/min
• C. 10 mL/kg/min
• D. 300 mL/kg/min

Answer: (B)

During induction in a circle breathing circuit, you want the alveolar concentration of the inhaled anesthetic to rise quickly to match the inspired level. Using a higher fresh gas flow during this phase helps purge exhaled gas and deliver anesthetic rapidly, speeding up the wash‑in process and reducing the time of induction.

The commonly used induction fresh gas flow rate of about 100 mL/kg/min strikes a practical balance: it’s high enough to achieve rapid onset without being unnecessarily wasteful. It avoids prolonged induction that would occur with a much lower flow, and it isn’t so excessive that it wastes gas or complicates control of the circuit.

A flow of 50 mL/kg/min would slow induction, as would 10 mL/kg/min, which could lead to more rebreathing and slower achievement of the desired alveolar concentration. An extremely high rate like 300 mL/kg/min isn’t necessary and wastes gas, offering little added benefit during induction.