Anaesthetic Circuits Flashcards
What is the other name for a closed and semi-closed circuit
Closed - usually re-breathing
Semi-Closed - non-rebreathing
What is the function of the APL valve
Safety feature
Will open once a high pressure is reached in the circuit
How do you choose the correct reservoir bag?
Should be roughly 3-6 x patients tidal volume (VT)
What factors should you consider when choosing the right circuit
aim to tailor the circuit size to the patient
Minimise airway resistance
Minimise deadspace
Choose correct reservoir bag size
Name 2 points about non-rebreathing circuits
- Do not require a CO2 absorbent canister
- Waste CO2 and waste inhalational gases are flushed to scavenging system
- Rely on high pressure gas flow to flush exhaled gases from the circuit so they cannot be rebreathed by the patient
What is the circuit factor for ayres T-piece and Bane
2.5-3
How does the ayres T-piece inspiration work:
Patient inhales FGF coming down the inspiratory limb from the anaesthetic machine
How does ayres T-piece expiration work:
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
What weight is suitable for Ayres T-piece?
under 8-10kg
What weight is suitable for bain circuit?
over 10kg
What kind of system is a bain seen as?
Coaxial system. Tip: Always check inner tubing is in palce
How does bain inspiration works?
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.
How does the bain expiration work?
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
What is the circuit factor for a lack and magill?
1-1.5
What weight is a lack and mini lack suitable for?
Lack - over 10kg
Mini-lack - under 10kg
How does the inspiration on a lack work?
Gases inhaled from inspiratory limb and reservoir bag
How does expiration on a lack work?
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
Name two advantages of a non-rebreathing circuit
- 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
Name 2 disadvantages for non-rebreathing circuits
- 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
What do rebreathing circuits always include that non-rebreathing circuits do not?
CO2 absorber
When is a circle circuit suitable?
for animals over 10-15kg
Does the circle circuit have high or low resistance?
High
What flow rate should be run for a circle circuit?
nitially use higher flow rates for 10-15 minutes to allow de-nitrogenation then 10ml/kg/min
What indicates soda lime is exhausted?
Colour change either pink to white, or white to purple
When should soda lime cannisters be checked and when should it be changed
before each procedure and at end of day
Change when 2/3 or more is exhausted
What checks for soda lime cannisters should you always follow
always wear gloves when changing the canister
Always check cannisters for leaks
Name 2 advantages of rebreathing systems
- 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
Name 2 disadvantages of rebreathing systems:
- 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
How can a humphrey ADE be used
either as a rebreathing or non-rebreathing circuit depending on lever position and additional of soda lime
What weight does Humphrey ADE claim to be useful for
3-100kg
What criteria is there for an anaesthetic chamber?
- 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
What safety checks are there for anaesthetic circuits
- 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
