breathing systems Flashcards
advantages of a circle breathing system…
efficient - low flows and recycled gas’s , less waste hence better for environment
also means conservation of heat and humidity of gases
what happens if unidirectional valves in a circle system malfunction?
Malfunction of the unidirectional valve leads to mixing of inspired gas with expired gas containing CO2and results in hypercapnIa.
size of MESH granules in CO2 canister
4 to 8 mesh
i,e, particles that could fit through a mesh that has four equal strands per linear inch in both vertical and horizontal axes = 4 mesh.
- Inspiratory limb; 2. Expiratory limb; 3. Unidirectional valve; 4. Fresh gas flow; 5.
Scavenging; 6. Reservoir bag; 7. Soda lime canister; 8. Water drain; 9. Simulated
lung; 10. APL valve
what are the features of an ideal breathing circuit?
an ideal breathing system has 3 main functions - deliver O2, deliver anaesthetic gas, remove CO2 and waste or avoid rebreathing.
environmental
* produce minimal anaesthetic waste - low flows
* have the option of being attached to scavenging
* cheap to produce, with some parts being reusable by use of a filter.
user friendly
* compact
* intuitive
patient features
* efficient for both SV and CV
* no rebreathing , low deadspace
* low resistance to flow
* suitable for all ages
* protects from barotrauma
how can breathing systems be classified?
open and closed, with 2 classifications in between - semi open and semi closed.
open - patients airway is open to atmosphere. schimelbush mask. no rebreathing but no control.
semi open - volatiles carried by FGF and diluted by air e.g. hudson mask/ venturi mask. very inefficient, need high FGF dilution.
semi closed - volatiles carries by FGF but not diluted by room air. may be further categorised to rebreathing and non rebreathing. rebreathing includes circle system with APL valve open. non rebreathing is the mapelson classification.
closed - circle system with completely closed APL valve. hence rebreathing system and therefore must have CO2 absorber. usually never fully closed as risk of high pressures building up.
Draw mapelsons classification…
classification of semi-open breathing systems. labelled A to E, later F was added as the jackson rees modification.
based on position of key components.
mapelson A a.k.a magill
C = water circuit
E= ayers T piece
F = jackson reese modification
what is meant by a co-axial system?
the inspiratory and expiratory tubing are within one another
e.g. mapelson A has a co-axial system = lacks
e.g. mapelson D has a co-axial system = bains
A - the expiratory limb is inside
D - the inspiratory limb is inside
in lacks, the reservoir bag is within the inspiratory limb whereas for bains its within expiratory.
what FGF is required for mapelson A and D ?
mapelson A more efficient for SV, D for CV
A:
* SV = MV = 70ml/kg
* CV = 2-3x MV = 200ml/kg
D:
* SV = 2-3x MV = 200ml/kg
* CV = MV = 70ml/kg
explain how the magils A works for SV?
patient inspires,
inspiratory pause, bag starts to fill FGF
during expiration dead space gas exists first and enters and fills the bag.
by the time alveolar gas high in CO2 exits, it is preferentially removed via APL
in next inspiration bag is emptied containing FGF and dead space gases so there is minimal rebreathing of CO2 if the FGF matches the MV.
what are the problems with mapelson A
bulky
APL valve near patient so hard for scavenging. the co-axial version helps with this.
no good for CV
tell me about mapelson B
not really used anymore
inefficient - rebreathing at high FGF - both SV and CV around 2-3x MV
when is mapelson C used?
very inefficient however much less bulky so used for convienence in emergencies/ transfers
FGF 2.5xMV for SV/CV
another advantage = less dead space
tell me about the breathing circuits used in paediatrics (under 20kg) …
typically E and F
this is because no valves and low resistance to breathing. v light weight. minimal deadspace.
Jackson rees mod includes an open ended bag such that control ventilation + PEEP could be delivered.
both require high FGF - 2-3xMV
therefore wasteful and can result in theatre pollution.
both are more efficient for CV than SV
what are the pros and cons of mapelson D …
pros = efficient for CV , can be used as co-axial system (bain) more compact and handy. APL valve position is better for scavenging
cons - not good for SV , more deadspace compared to C.
what is the main risk of a co-axial system?
disconnection of inner tube supplying FGF. this would convert entire thing to deadspace and would get significant rebreathing.
kinking or damage to inner tube which cant be seen. - no FGF
what FGF is required for a 100kg person using Bain?
CV
70ml/kg/min
7L/min
around 1x MV
what methods prevent re-breathing in mapelson system?
there is no CO2 absorber so use other ways
- FGF adequate to replace exhaled air
- reservoir bag to fill with FGF during inspiratory phase - otherwise FGF would have to be very high - i.e. peak inspiratory rate
- APL valve to vent off waste gases
what is rebreathing
rebreathing is the process of inhaling previously exhaled gases. it occurs if the deadspace of a breathing apparatus is more than TV.
expired gases consist of deadspace gas and alveolar gas. if rebreathing is enough to include alveolar gas It can result in accumulation of CO2.
extent of rebreathing depends on
* dead space of the circuit,
* its design i.e. positioning of FGF vs APL v,
* patients MV ,
* FGF rate
* mode of ventilation
what is this…
Humphreys ADE circuit
uses a lever to switch between A , D and E circuits.
hence can be used in SV, CV and paediatrics.
the lever determines direction of FGF
consists of:
* inspiratory and expiratory tubing
* lever
* FGF
* reservoir bag
* APL valve
what is a parallel lacks circuit?
what is an APL valve?
adjustable one way spring loaded disc valve
when pressure increases , enough force to push the spring up and open a gap to allow gas to escape
the valve can be opened which adjusts the spring e.g. if opened to 1cmH20 only this amount of extra pressure needed from the gas to open the valve
if closed down, then more pressure is required to push spring open. e.g. 70cmH20
helps prevent barotrauma, can be used to give PEEP, can be attached to scavenging.
describe the circle breathing system…
This is a closed breathing system
used on most modern anaesthetic machines to reduce flow rates and minimise pollution
it has a number of key components:
1. patient expires through 1 wave expiratory valve
2. gas flows down expiratory limb
3. APL valve positioned to vent excess gas
4. reservoir bag, ventilator are next
5. soda lime removes CO2
6. FGF added
7. down inspiratory limb via 1 way valve
since CO2 is being removed, the other gases and humidifed air can be recycled. this means low FGF can be used, and less waste
theoretically at equilibrium could have only O2 being added as anaesthetic gases have equilibrated to brain. however since a small % is metabolised, always will need to add some.
