Anaesthetic Circuits

Question 1

What is the other name for a closed and semi-closed circuit

Answer 1

Closed - usually re-breathing

Semi-Closed - non-rebreathing

Question 2

What is the function of the APL valve

Answer 2

Safety feature

Will open once a high pressure is reached in the circuit

Question 3

How do you choose the correct reservoir bag?

Answer 3

Should be roughly 3-6 x patients tidal volume (VT)

Question 4

What factors should you consider when choosing the right circuit

Answer 4

aim to tailor the circuit size to the patient
Minimise airway resistance
Minimise deadspace
Choose correct reservoir bag size

Question 5

Name 2 points about non-rebreathing circuits

Answer 5

1. Do not require a CO2 absorbent canister
2. Waste CO2 and waste inhalational gases are flushed to scavenging system
3. Rely on high pressure gas flow to flush exhaled gases from the circuit so they cannot be rebreathed by the patient

Question 6

What is the circuit factor for ayres T-piece and Bane

Answer 6

2.5-3

Question 7

How does the ayres T-piece inspiration work:

Answer 7

 Patient inhales FGF coming down the inspiratory limb from the anaesthetic machine

Question 8

How does ayres T-piece expiration work:

Answer 8

o Expiration
 Patient breathes out waste gases which are flushed down the expiratory limb by the high FGF eventually leaving via the APL valve (if a paediatric modification) to scavenge
 Two parts of expiration:
• 1. Dead space gas that hasn’t participated in gas exchange therefore not a lot of CO2 and does not contain volatile agent
• 2. Voiding of waste gases and will include large amounts of CO2
o Expiratory pause
 Tubing fills with fresh gas

Question 9

What weight is suitable for Ayres T-piece?

Answer 9

under 8-10kg

Question 10

What weight is suitable for bain circuit?

Answer 10

over 10kg

Question 11

What kind of system is a bain seen as?

Answer 11

Coaxial system. Tip: Always check inner tubing is in palce

Question 12

How does bain inspiration works?

Answer 12

 Fresh gas flow from the machine travels up the inner tube. When the animal breathes in it takes fresh gas from the inner tube plus gas from the outer expiratory limb.

Question 13

How does the bain expiration work?

Answer 13

o Expiration
 Exhaled gases are forced up the expiratory outer tube
• Some do pass into the inner limb where they mix with fresh gas
o Expiratory pause
 Incoming fresh gas flushes any exhaled gas through the inner tubing and out of the circuit. therefore high fresh gas flow is required as this prevents the patient rebreathing exhaled gases

Question 14

What is the circuit factor for a lack and magill?

Answer 14

1-1.5

Question 15

What weight is a lack and mini lack suitable for?

Answer 15

Lack - over 10kg

Mini-lack - under 10kg

Question 16

How does the inspiration on a lack work?

Answer 16

 Gases inhaled from inspiratory limb and reservoir bag

Question 17

How does expiration on a lack work?

Answer 17

 Dead space gases pass back down the inspiratory limb
 Second portion of inspiration is flushed out of expiratory limb by the fresh gas flow
 Long term IPPV with this circuit will disrupt gas flow and cause rebreathing at conventional FGF rates

Question 18

Name two advantages of a non-rebreathing circuit

Answer 18

• Cheap to buy
• Simple construction for cleaning and maintenance
• Soda lime is not required
• Low resistance
  o Ideal for smaller patients
• Inspired gas content similar to that on vaporiser
• Can change level of volatile agent in circuit quickly

Question 19

Name 2 disadvantages for non-rebreathing circuits

Answer 19

• High carrier gas flow required therefore more expensive
• High volatile agent consumption rate therefore expensive
• Expired moisture and heat is lost
• There are different flow requirements for different circuits
• More likely to get pollution with these circuits

Question 20

What do rebreathing circuits always include that non-rebreathing circuits do not?

Answer 20

CO2 absorber

Question 21

When is a circle circuit suitable?

Answer 21

for animals over 10-15kg

Question 22

Does the circle circuit have high or low resistance?

Answer 22

High

Question 23

What flow rate should be run for a circle circuit?

Answer 23

nitially use higher flow rates for 10-15 minutes to allow de-nitrogenation then 10ml/kg/min

Question 24

What indicates soda lime is exhausted?

Answer 24

Colour change either pink to white, or white to purple

Question 25

When should soda lime cannisters be checked and when should it be changed

Answer 25

before each procedure and at end of day

Change when 2/3 or more is exhausted

Question 26

What checks for soda lime cannisters should you always follow

Answer 26

always wear gloves when changing the canister

Always check cannisters for leaks

Question 27

Name 2 advantages of rebreathing systems

Answer 27

-	Lower fresh gas flow required 
o	Cheaper to run 
-	Lower volatile agent consumption 
o	Cheaper to run 
-	Expired moisture and heat is conserved 
o	Not a constant cold fresh gas flow 
o	Healthier for respiratory tract 
o	Stop them dropping temperature 
-	Less pollution than other systems
-	Closed for low-flow options

Question 28

Name 2 disadvantages of rebreathing systems:

Answer 28

• Higher resistance
• Expensive to buy
• Soda lime must be replaced regularly when exhausted
• Changes in vaporiser level are slow to change depth of anaesthesia
• N2O cannot be used safely in closed low flow systems
• Cannot be used on cat
  o Can’t push air around
• Expensive to buy
  o But arguably you save in the long run

Question 29

How can a humphrey ADE be used

Answer 29

either as a rebreathing or non-rebreathing circuit depending on lever position and additional of soda lime

Question 30

What weight does Humphrey ADE claim to be useful for

Answer 30

3-100kg

Question 31

What criteria is there for an anaesthetic chamber?

Answer 31

• Must be durable and airtight
• Ideally transparent to allow observation of patient
• Should allow for gas inlet and scavenging outlet
  o E.g. cooks chamber
• Only use when necessary

Question 32

What safety checks are there for anaesthetic circuits

Answer 32

• Is circuit correctly assembled
• Is the APL valve in the correct position (OPEN)
• Are there any kinks, cracks, or contamination to the circuit tubing
• Have you tested the system for leaks
• If using soda lime have you checked for cracks in the canister or soda lime exhaustion
• If using a chamber – are the connections secure and is it air tight