Fresh gas flow calculations

Question 1

What are Fresh gas flow calculations?

Answer 1

• During anaesthesia an animal loses the ability to regulate how much air it can breathe
• Up to the anaesthetist to provide enough gas to fulfil his/hers needs
• Our first step in making these calculations is to measure the patient’s respiration rate

Question 2

Tidal volume

Answer 2

• The volume of gas breathed in during normal respiration
• Cats and small dogs <10kg =15ml/kg
• Large dogs >10kg = 10ml/kg
• Formula = 15 or 10 x weight (kg)

Question 3

Minute volume

Answer 3

• Amount of air that moves in and out of airways and alveoli in 1 min
• Formula = TV x respiratory rate

Question 4

Circuit factor

Answer 4

• Each breathing system has a circuit factor which is a multiple of the minute volume
• Represents how much gas needs to be used with the system in order for the patient to have enough fresh gas (not to rebreathe exhaled air)
• Ayer’s t-piece, Bain = 2.5-3
• Lack, Mini lack, Magill = 1-1.5
• Circle = no circuit factor

Question 5

Fresh gas flow

Answer 5

• Formula = MV x circuit factor