Anaesthetic Circuits

• Identify non-rebreathing circuits?

  • Ayres T-pieces
  • Bain
  • Magill
  • Lack
  • Mini-Lack
• Ayres T-piece:

  CF: 2.5-3

  Simple system, Low resistance.

  FGF passes down inspiratory limb.

  Relies on high FGF.

  Suitable for animals under 8-10kg

  Suitable for IPPV.

  Has reservoir bag on expiratory limb.
• Bain:

  CF: 2.5-3

  Reservoir bag on expiratory limb.

  Has APL valve.

  Suitable for IPPV.

  Suitable for animals over 10kg.
• Magill:

  Suitable for animals over 10kg.

  Reservoir bag on inspiratory limb.

  APL valve located near patients head.

  Unsuitable for prolonged IPPV, rebreathing will occur.

  Circuit factor: 1-1.5.
• Lack:

  Suitable for animals over 10kg.

  reservoir bag on inspiratory limb.

  Not suitable for long term IPPV.

  ALP valve close to anaesthetic machine.

  CF: 1-1.5
• Mini-Lack:

  CF: 1-1.5

  Suitable for animals under 10kg.
• Advantages of Non-rebreathing circuits:

  Cheap

  Simple construction for cleaning and maintenance.

  Soda lime not required.

  Low resistance, ideal for smaller patients.

  Inspired gas content like that on vaporiser.

  Can change the level of volatile agent in circuit quickly.
• Disadvantages of non-rebreathing circuits:

  High carrier gas flow required, Expensive.

  High volatile agent consumption rate, Expensive.

  Expired moisture and heat are lost.

  Different flow requirements for different circuits

  Bad for the environment.
• Identify rebreathing circuits?

  To and fro

  Circle

  Humphrey ADE
• To and fro:

  Less common.

  Water canister contains CO2 absorbent.

  Exhaled gas moves across soda lime and then is re-inhaled minus the CO2.

  Low flow rate, 0.5-2l/min. (depends on patient size)

  Retains heat well.
• Circle:

  Suitable for animals over 10-25kg.

  Highly resistant, one way valve to ensure gas travels in one direction.

  Suitable for IPPV.
• What is/Characteristics of soda lime?

  Colour change indicates exhaustion (pink/white or purple/white)

  Canister inspected before each procedure and end of day.

  Wear gloves when changing canister.

  Always check canister for leaks.
• Humphrey ADE:

  Can be used as both rebreathing and non-rebreathing circuit.

  Tubing allows it to run as a lack or a circle by adding soda limb.

  Suitable for IPPV.

  Used in animals between 3-10kg.
• Advantages of rebreathing circuits:

  Lower FGF, cheaper.

  Lower volatile agent consumption, cheaper.

  Expired moisture and heat are conserved.

  Less pollution, closed or low-flow option.
• Disadvantages of rebreathing circuits:

  High resistance

  Expensive to buy

  Soda limb must be replaced when exhausted.

  Changes in vaporiser levels are slow to change depth of anaesthesia.

  N2O cannot be used safely in closed low flow systems.
• What are characteristics anaesthetic chambers:

  Must be durable and airtight.

  Ideally transparent to allow obserbation of patient.

  Should allow for gas inlet and scavenging outlet.
• What anaesthetic circuit checks should be done:

  Circuit assembled correctly.

  APL valve in correct position, open.

  Kinks, cracks or contamination to the circuit tubing.

  Tested for leaks in system.

  Soda lime- Cracks or exhaustion.

  Chamber- connections secure and air tight.