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A simple, practical guide to calculating fresh gas flow rates in small animal anaesthesia ✨
Fresh gas flow rates are essential for delivering adequate anaesthesia, yet they're often a source of confusion for student + newly qualified veterinary nurses.
Understanding how to confidently calculate them allows veterinary nurses to:
- deliver appropriate oxygen + anaesthetic concentrations
- prevent rebreathing
- confidently adjust flows during procedures
✨ The Basics
Fresh gas flow (FGF) is calculated using:
- tidal volume (TV)
- bodyweight (kg)
- minute volume (MV)
- respiratory rate (bpm)
- circuit factors (cf)
✨ Overview
The tidal volume is the volume of air exhaled with each breath:
10-15ml/kg x bodyweight (kg)
As a general rule:
- if a patient is less than 10kg = 15ml/kg
- if a patient weighs more than 10kg = 10ml/kg
The minute volume is the total volume of air expired per minute:
tidal volume x respiratory rate (bpm)
The respiratory rate is the number of breaths taken per minute.
To calculate the fresh gas flow rate:
minute volume x circuit factor
The circuit factor depends on what circuit is being used, for example:
t-piece or bain = 2.5 - 3
lack or mini-lack = 1-1.5
✨ Step-by-Step Method
✨ Step 1:
Calculate the tidal volume (TV):
10-15ml/kg x bodyweight (kg)
✨ Step 2:
Calculate the minute volume (MV):
tidal volume x respiratory rate (bpm)
✨ Step 3:
Calculate the fresh gas flow rate (fgfr):
minute volume x circuit factor
✨ Example:
Patient: 12kg dog for a general anaesthetic + castrate procedure
Respiratory rate: 18bpm whilst resting in the kennel
Breathing system: bain
Note: he weighs >10kg, so we must use 10ml/kg in our calculation.
First, we calculate the tidal volume:
12kg x 10ml/kg = 120ml
Then, we calculate the minute volume:
120ml x 18bpm = 2,160ml/min
Then, we round this number up to 2,200ml/min + divide by 1000 to convert ml/min into l/min:
2,160ml/min / 1000 = 2.2l/min
Then, we calculate the fresh gas flow rate, by using BOTH circuit factors:
2.2l/min x 2.5 = 5.5l/min and 2.2l/min x 3 = 6.6l/min
So, our fresh gas flow rate for this procedure should be between 5.5l/min and 6.6l/min.
It is really important to assess the respiratory rate once the patient is anaesthetised as this can change the minute volume!
✨ Nursing Actions
- confirm patient weight + breathing system before calculating
- reassess respiratory rate regularly during anaesthesia
- adjust flow rates based on patient depth + monitoring parameters
- be aware of circuit-specific requirements (factors differ between circuits)
- avoid unnecessary high flow rates once stable
✨ Common Mistakes
- using the wrong tidal volume for patient size
- forgetting to convert ml to litres
- applying the incorrect circuit factor
- not reassessing during anaesthesia