Circuits Flashcards
What is the function of a breathing system?
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
What are the two types of circuit?
» Non rebreathing circuits (high flow rates)
» Rebreathing circuits (soda lime)
What are the non-rebreathing circuits?
Magill (Mapleson A)
Lack
Mini Lack
Bain (& mini Bain)
What are the rebreathing circuits?
To and Fr
Circle
What does a Magill (Mapleson A) look like? What is its circuit factor?
– Valve near patient
– Bag at machine end
1
What does a lack look like? What is its circuit factor?
– Modified Mapleson A, Better design than Magill as the bag is at the other end away from the patient
– Parallel tubes
1
What are the pros and cons of Lacks?
– Efficient for spontaneous breathing
– Inefficient for IPPV (but ok for a short time)
When should you use a Lack?
– For animals >10kg (but also a mini lack)
What are the modifications in a mini-lack?
– Smooth bore tubing and Lightweight valve
– Smooth interior tubing provides minimal air resistance
What patients should you use a mini-lack in and what are its pros/cons?
– 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
What does a Bain look like and what is its circuit factor?
– Internal tube supplies gas
– 2.5 x MV 7-20kg
What are the pros/cons of a Bain?
– High flow rates
– Excellent for IPPV
– Lower flow rates can be used for IPPV
What is a quick flush?
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
What does a T piece look like? What is the circuit factor?
– We use the pediatric human version – has the teddy bear on it
– There is a valve by the bag to allow for scavenging
What are the Pro’s of a T piece?
– Low resistance <10kg
– High flow rates
– Good for IPPV (but not easy)
How do you calculate a flow rate for non re-breathing systems?
Assume minute volume is approx 250ml/kg (0.25L) (on the high side)
Multiply circuit factor x weight x 0.25
How can you tell if rebreathing is occurring?
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
What are the pros/cons of using a circle?
– Efficient – Warm and humidify gas – Less pollutant – Easy to use – Easy to perform IPPV – Soda lime must be changed often
What is the absorbent in a circle and how can you tell its working?
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
What do the valves fo in a circle?
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
How big should a breathing bag be?
- 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
How big should the soda lime canister be in a circle?
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
What does a closed rebreathing system mean?
You just deliver enough oxygen for the animal to survive- very small amounts of O2…
How much O2 do you deliver in a semi-closed rebreathing system?
- 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.
