Fresh Gas Flow Rate Calculation Veterinary

Determine appropriate oxygen flow rates for anesthesia circuits.

Kilograms (kg) Pounds (lbs)

Rebreathing System (Circle) – Patients > 7-10kg Non-Rebreathing System (Bain/Magill) – Patients < 7-10kg

Induction / Recovery (Filling/Emptying) Maintenance (Stable Plane)

Maintenance uses lower flow rates (Sem-Closed). Induction uses high flow.

Understanding Fresh Gas Flow (FGF) in Veterinary Anesthesia

Correctly calculating the Fresh Gas Flow (FGF) rate is critical in veterinary anesthesia to ensure the patient receives adequate oxygen, to vaporize the anesthetic agent effectively, and to remove exhaled carbon dioxide (CO2) from the breathing circuit. The calculation depends heavily on the type of circuit used and the patient's weight.

1. Non-Rebreathing Systems (e.g., Bain, Jackson-Rees)

These circuits are typically used for smaller patients (under 7kg to 10kg). Because they rely on high fresh gas flow to flush out exhaled CO2 (rather than chemical absorbents like soda lime), they require significantly higher flow rates per unit of body weight.

Standard Formula: The flow rate is generally calculated as 130 to 300 mL/kg/min. A safe clinical standard often used is 200 mL/kg/min. This high flow prevents the patient from rebreathing their own exhaled CO2.

2. Rebreathing Systems (Circle Systems)

Used for larger patients (over 7kg to 10kg), these circuits use chemical absorbents to scrub CO2, allowing gases to be recirculated. This allows for lower flow rates, conserving heat and moisture.

• Induction & Recovery: During these phases, the goal is to quickly change the concentration of gas in the circuit. A higher flow rate is used, typically 50 to 100 mL/kg/min (or 3 L/min).
• Maintenance (Semi-Closed): Once a stable anesthetic plane is reached, the flow can be reduced. A standard rate is 20 to 40 mL/kg/min.

Important Safety Minimums

Regardless of the calculated weight-based value, equipment limitations must be respected. Most precision vaporizers require a minimum flow to function accurately (often 500 mL/min or 1 L/min). Even if a 10kg dog technically only needs 300 mL/min for maintenance, the flow meter should typically not be set below 500 mL/min to ensure the vaporizer delivers the correct percentage of isoflurane or sevoflurane.

Calculating Tidal Volume and Minute Volume

The underlying physics relies on Minute Volume (MV).
Tidal Volume (TV) ≈ 10–15 mL/kg
Minute Volume (MV) = TV × Respiratory Rate
Non-rebreathing circuits usually require a flow rate of 1.5 to 3 times the Minute Volume to prevent rebreathing.