Anaesthesia breathing systems Flashcards
Aim of anaesthesia breathing systems
- To provide enough gas flow for ventilation to occur (to satisfy peak inspiratory flow rate) and for gas exchange to happen whilst preventing rebreathing of expired gases
- To prevent contamination of the air in the theatre
** Fresh gas flow rate is NB in non-absorber systems because carbon dioxide isn’t absorbed - We also want to assist patients by not adding more work to their breathing efforts
Function of anaesthesia breathing systems
- Delivers oxygen-containing fresh gas to the patient
- Allows us to control a patient’s ventilation
- Allows patients to control their own ventilation whilst under an anaesthetic
Non-CO2 absorber systems:
T piece Breathing System
Mapleson classification
Mapleson classification:
- A Magill’s circuit
- B & C not really used in anaesthetics
- D, E, & F all variants of T-piece circuit
Co-axial Bain System = Mapleson D
Ayres T piece = Mapleson E
Jackson Rees Modification = Mapleson F
NB! Go look at pictures of these circuits because they WILL ask them
How does Mapleson E circuit work
As a patient inspires, you are going to have negative pressure at / below point A, and the patient will inhale the FGF
When the patient breathes out, carbon dioxide will flow in reverse
It will be very concentrated in part of the rebreathing tube that is closest to the patient’s airway, but as it mixes with the FGF, it becomes less concentrated towards the exit point of the rebreathing tube.
**
If the FGF is high, most carbon dioxide will be expelled at the end of the rebreathing tube, however if the FGF is low, the patient will rebreathe some carbon dioxide. In other words, during expiration, fresh gas washes expired gas out of reservoir tube, filling it with fresh gas ready for the next inspiration
Mapleson F?
a bag was added to the end of the rebreathing tube, allowing us to manually bag / ventilate the patient
“Jackson-Rees’ modification”
Mapleson D?
A one-way valve was added above the bag allowing the expiratory gasses & extra pressure to exit the system
Bain System breathing circuit
The FGF now goes through the same tubing
less chances for disconnection
the FGF tube now runs inside of the rebreather tube. This is called “co-axial modification” of Mapleson D.
Spontaneous respiration:
- Fresh gas flow @ 2.5 – 3 x the patient’s minute volume
Controlled ventilation (IPPV):
- Fresh gas flow @ 1 x the patient’s minute volume
- Even more efficient with long expiration
What is dead space?
The portion of the tidal volume that does not take part in alveolar gas exchange
Consists of apparatus dead space and physiological dead space (airways and alveolar dead space)
Alveolar dead space has high CO2 conc
Advantages of T-piece breathing system:
- Compact and inexpensive
- Minimal dead space fresh gas flow comes out close to patient
- Low resistance (work of breathing)
- More economical for controlled ventilation NB!
Disadvantage of the T-piece breathing system:
- High fresh gas flow required for spontaneous ventilation (less economical)
- The bag may get twisted and impede breathing
Magill circuit?
Magill = me (spontaneous breathing)
Mapleson A
During inspiration, you have fresh gas throughout the reservoir bag and tube that the patient can breathe in. As the patient exhales, most of the expired gases will exit through the spring-loaded valve due to its close proximity to the patient. However, some expired gases will flow back through the tube and if you have a very low flow of fresh gas, the expired gases may even reach the reservoir bag leading to rebreathing.
Advantages of Magill’s circuit:
- Simple
- More economical for spontaneous ventilation NB!
- Fresh gas flow @ 0.7 x the patient’s minute volume
Disadvantages of Magill’s circuit:
- High fresh gas flow required for controlled ventilation (less economical)
- Large apparatus dead space
- Not practical in children = NB!
- Unable to scavenge
- No humidification
Humphrey’s Ade circuit
- Two modes which you can switch between
- A-mode patient can breathe spontaneously with minimal
wastage - D-mode can ventilate patient with minimal wastage
- Fresh gas flow @ 1 x the patient’s minute volume
Circle Absorber System
- Modification of Mapleson D
- In this circuit, you have two unidirectional valves that ensures that flow is going in the right direction
o Valve 1 ensures that flow only goes to the patient
o Valve 2 ensures that flow only goes to reservoir
Advantages of Circle Absorber circuit
we can put an absorber into the system to remove the carbon dioxide on the expiratory limb of the circuit and this allows the patient to breathe the air without becoming hypercapnic (there is no mixing of gas as is the case
in the other systems) NB!
Equations for the Soda Lime
CO2 + H2O = H2CO3
H2CO3 + 2NaOH = Na2CO3 + 2H2) + E
Na2CO3 + Ca(OH)2 = CaCO3 + 2NaOH
Soda lime granules contain
o Water
o Sodium hydroxide (NaOH) to speed up the reaction
o Calcium hydroxide (Ca(OH)2) this is the actual thing that absorbs the carbon dioxide
- The colour changes at a pH of 10.5
- Contains granules of calcium hydroxide (not calcium oxide),
- Silica is added to the granules to increase the hardness
Reaction of Soda Lime
- The reaction is exothermic (produces heat)
- Note how all three reactions are interactive the product of the one reaction then fuels another reaction etc
- As long as there is carbon dioxide present, the process will continue
- Indicator dye is added to the soda lime to indicate when it is depleted it is pH sensitive and thus the dye will become purple once the hydroxycarbonate accumulates as none of it is absorbed anymore (soda lime is initially white)
- Products of the soda lime reaction Na2CO3 + 2 H2O
Advantages of the circle system:
- Fresh gas flow rate related
- Small dead space
- Warming & humidification facilitated (NB!) because water is made during the soda lime reactions
- Pressure monitoring easy
- Safety margin with loss of oxygen supply
- Scavenging & reduced atmospheric pollution
- Spontaneous ventilation fresh gas flow relatively unimportant
- Controlled ventilation fresh gas flow relatively unimportant
