Circuits

Question 1

What is the function of a breathing system?

Answer 1

To supply oxygen
To supply anaesthetic agent
To remove carbon dioxide
•	Via high flow rates i.e. flushed out  
•	Via absorption by soda lime facilitating lower flow rates

Question 2

What are the two types of circuit?

Answer 2

» Non rebreathing circuits (high flow rates)

» Rebreathing circuits (soda lime)

Question 3

What are the non-rebreathing circuits?

Answer 3

Magill (Mapleson A)
Lack
Mini Lack
Bain (& mini Bain)

Question 4

What are the rebreathing circuits?

Answer 4

To and Fr

Circle

Question 5

What does a Magill (Mapleson A) look like? What is its circuit factor?

Answer 5

– Valve near patient
– Bag at machine end
1

Question 6

What does a lack look like? What is its circuit factor?

Answer 6

– Modified Mapleson A, Better design than Magill as the bag is at the other end away from the patient
– Parallel tubes
1

Question 7

What are the pros and cons of Lacks?

Answer 7

– Efficient for spontaneous breathing

– Inefficient for IPPV (but ok for a short time)

Question 8

When should you use a Lack?

Answer 8

– For animals >10kg (but also a mini lack)

Question 9

What are the modifications in a mini-lack?

Answer 9

– Smooth bore tubing and Lightweight valve

– Smooth interior tubing provides minimal air resistance

Question 10

What patients should you use a mini-lack in and what are its pros/cons?

Answer 10

– 1/3 flow rates of a T piece (££ saving gas & inhalant)
– For patients 2-10kg
– NOT suitable for longterm IPPV: Unless FGF increased to 600ml/kg/min

Question 11

What does a Bain look like and what is its circuit factor?

Answer 11

– Internal tube supplies gas

– 2.5 x MV 7-20kg

Question 12

What are the pros/cons of a Bain?

Answer 12

– High flow rates
– Excellent for IPPV
– Lower flow rates can be used for IPPV

Question 13

What is a quick flush?

Answer 13

The quick flush valve : provides a rapid source of anesthetic-free oxygen to the patient when required. Most quick-flush valves are of the spring-loaded, push-button type, but some lever-action valves may be encountered. Do not use the quick flush valve with a non re breathing circuit, especially a Bain

Question 14

What does a T piece look like? What is the circuit factor?

Answer 14

– We use the pediatric human version – has the teddy bear on it
– There is a valve by the bag to allow for scavenging

Question 15

What are the Pro’s of a T piece?

Answer 15

– Low resistance <10kg
– High flow rates
– Good for IPPV (but not easy)

Question 16

How do you calculate a flow rate for non re-breathing systems?

Answer 16

Assume minute volume is approx 250ml/kg (0.25L) (on the high side)
Multiply circuit factor x weight x 0.25

Question 17

How can you tell if rebreathing is occurring?

Answer 17

The capnograph is the only way to tell if rebreathing is occurring i.e. the flow rate is not high enough to flush out the CO2

Question 18

What are the pros/cons of using a circle?

Answer 18

–	Efficient
–	Warm and humidify gas
–	Less pollutant
–	Easy to use
–	Easy to perform IPPV
–	Soda lime must be changed often

Question 19

What is the absorbent in a circle and how can you tell its working?

Answer 19

The absorbent is usually calcium hydroxide (soda lime and barium hydroxide lime)
Incorporates an indicator to show activity of the soda lime)
Active soda lime generates heat and this will be detectable by touching the canister

Question 20

What do the valves fo in a circle?

Answer 20

Pressure generated by the patient’s breathing causes the disc to move and allow gas to pass in one direction only
The transparent dome allows seeing the valve moving up and down
Valves/discs can be made of deformable rubber or mica or plastic
Valves must be checked prior to use, If they are sticking they should be cleaned and dried or else replaced

Question 21

How big should a breathing bag be?

Answer 21

• The volume of the breathing bag must be greater than the patient’s inspiratory capacity
• This is usually estimated at 30 ml/kg body weight

Question 22

How big should the soda lime canister be in a circle?

Answer 22

Soda lime contains 50% - 70% air around the granules, the volume of the absorber canister should be at least double that of the tidal volume of the patient for optimal efficiency

Question 23

What does a closed rebreathing system mean?

Answer 23

You just deliver enough oxygen for the animal to survive- very small amounts of O2…

Question 24

How much O2 do you deliver in a semi-closed rebreathing system?

Answer 24

• In a semi-closed system, the O2 flow rate must exceed the patient’s O2 consumption
• When using an out-of-circuit vaporiser, the fresh gas flow rates employed are a compromise between achieving a reasonable rate of change of anaesthetic concentration and economy of anaesthetic consumption- Any excess is simply lost via the pressure relief valve

As a general rule, a flow rate of 2 to 3 L/min initially, and 500 ml to 1 L/min during maintenance of anesthesia, will usually prove satisfactory for dogs. For other large animals, e.g. horses start at 8-10L/min, then reduce.

Question 25

Why do we swap to a lower flow in maintenance on a circle?

Answer 25

– Less wastage
– Cheaper (less inhalant and carrier gases used)
– Less pollutant
– Gases are warmed and humidified

An out of circuit vaporiser may be used to introduce a precisely known concentration of anaesthetic into the circle circuit. However, since this delivered concentration is diluted by the gas already contained in the circle circuit, the concentration inspired by the patient is not known with certainty

Question 26

The rate of change of anaesthetic concentration in the circle circuit depends upon the fresh gas flow rate: a high fresh gas flow rate will….

Answer 26

…achieve equilibration much faster than if a low fresh gas flow rate is used.
….hence why we induce on high flow

Question 27

How do we alter flow rates and change of % inhalant during anaesthesia?

Answer 27

• Start at a higher fresh gas flow rate and vaporiser % for first 10 mins
• Then reduce FGF & % inhalant when plane of anaesthesia is stable
• If you want to change the plane of anaesthesia during the procedure, may need to change FGF (increase) and % inhalant (increase or decrease) for a rapid change
• At end, switch up FGF & % inhalant down, to speed recovery

Question 28

What are the advantages of a Humphrey ADE Circle (combination system)?

Answer 28

• Compact, very robust, easy to clean
• Versatile 2Kg-100Kg
• Has a PEEP valve, IPPV excellent
• Low flow rates

Question 29

What are the flow rates on a Humphrey?

Answer 29

– Dogs< 10kg (100ml/kg/min)
– Dogs >10kg (1l/min) + canister
– Cats (100ml/kg/min) up to 600ml/min

Question 30

In an induction chamber where do you attach the scavenge?

Answer 30

Scavenge from the bottom