25. Breathing Systems Flashcards
There are three main objectives when using a breathing system:
1
> To supply O2 to the patient
2
> To allow removal of CO2 from the system and avoid rebreathing
3
> To supply anaesthetic gases to the patient
Describe the movement of gas within each system.
The respiratory cycle comprises three phases:
inspiration, expiration and the expiratory pause.
1
> During inspiration, gas is
drawn in from the equipment.
2
> In quiet breathing, the average 70 kg
patient’s tidal volume is
approximately 500 mL.
At 20 breaths per minute,
their minute volume (MV) would be 10 L /min.
In order to avoid rebreathing, the fresh gas flow
rate would have to
exceed the patient’s MV.
This would result in very high volumes
of gas needing to be delivered.
This is wasteful and requires
high flow rates that would be
uncomfortable for the patient.
> With maximum effort, the average
70 kg patient can draw in
approximately 5 L of gas
over about 2 seconds.
Again, unless flow rates were
extremely high the patient would entrain air.
> To overcome these problems,
reservoir bags have been added to the
breathing systems.
> During deep inspiration the
patient can draw oxygen
and gases from these
as well as from the fresh gas flow.
> At the beginning of expiration, gas expired is from the anatomical dead space, so it does not contain CO2 and is not depleted of O2.
This gas is fit to be inhaled again.
> As expiration continues,
alveolar gas is exhaled next,
this contains CO2
and is O2 deplete.
It is desirable to rid the system
of this gas before the next inspiration.
> Adjustable pressure-relieving (APL) valves have been added to some circuits to vent waste gases and overcome the problem of rebreathing.
Mapleson A (non-co-axial ‘Magill’ system)
Spontaneous ventilation
> The patient inhales fresh gas,
from the supply and
from the reservoir bag,
which deflates proportionally.
> The patient exhales and the
dead space volume is
expelled into the breathing system,
passing down the tubing
and fills the reservoir bag again.
In addition,
the fresh gas flow will also
contribute to filling the bag.
> After the dead space gas,
the alveolar gas is exhaled.
At this stage the reservoir bag
is already filled and
so the pressure in the system begins to rise.
Because of this, the alveolar gas is vented through the APL valve and lost from the system, so avoiding rebreathing.
> If the fresh gas flow is too low,
the bag will not be filled solely
by dead space gas.
Some alveolar gas will be
able to enter the bag, and the
patient will rebreathe.
> If the fresh gas flow is too high,
the fresh gas flow will fill the bag to a degree
and dead space gas will be
vented along with alveolar gas.
While this avoids rebreathing, it is wasteful and inefficient.
Statement for describing systems to start off with
When describing breathing systems always start with the statement, ‘The
patient has just exhaled, the equipment is full of fresh gas and I put the mask
over the patient’s face. At the perfect flow rate …’
The Mapleson A
Controlled ventilation
> The anaesthetist squeezes the bag,
forcing gas into the patient.
Some gas, however, will be
vented from the expiratory valve
near the patient.
At the end of inspiration,
the reservoir bag will not be full.
> During exhalation,
dead space and alveolar gas will
move down to fill the reservoir bag.
Unless the gas flows are high, 2.5 × MV,
rebreathing will occur.
The Mapleson A is:
> Efficient in spontaneous ventilation (70 mL/kg/min)
> Inefficient for controlled ventilation (2.5 × MV).
Mapleson A (co-axial version ‘Lack’ system)
Co-axial means there is an inner tube surrounded by an outer one.
The Lack is a version of the Mapleson A,
which was designed to move the
pressure release valve away
from the patient and
so make it less awkward
and bulky to use.
The fresh gas flows down
the outside tubing,
and gas is vented via the inner tubing.
The reservoir bag is in the inspiratory limb,
while the pressure release valve
is in the expiratory limb.
The gas flows required in the system
are the same as for the standard A.
The Lack is bulkier than the Bain (see next page) because the inner tube has
to have a sufficiently large
diameter to minimise expiratory resistance.
Mapleson B and C
These are essentially the same,
but the C has shorter tubing.
The B is not used.
The C is used for transfer,
or ‘bagging’ patients on ICU.
This system needs high gas flows
to prevent rebreathing
(2.5 × MV for spontaneous
and controlled ventilation).
The Mapleson C is colloquially referred to as a ‘Waters’ circuit’, though strictly this is inaccurate as a true Waters’ circuit would include a canister of soda lime to absorb CO2 and prevent rebreathing.
These are not manufactured any more.
Mapleson D (non-co-axial system)
Spontaneous ventilation
The patient has just exhaled, the equipment is full of fresh gas and I put the mask over the patient’s face. At the perfect flow rate:
> The patient inhales fresh gas,
from the supply and from the reservoir bag,
which deflates proportionally
> The patient exhales and the
dead space volume is
expelled into the
breathing system.
The fresh gas flow and the
exhaled dead space gas mix
and both pass down the
tubing to fill the reservoir bag
> After the dead space gas,
the alveolar gas is exhaled.
At this stage the reservoir bag
is already filled and so the pressure
in the system begins to rise.
Because of this, the alveolar gas is vented through the pressure release valve and lost from the system so avoiding rebreathing
> During the expiratory pause, fresh gas continues to push exhaled alveolar gas down towards the reservoir bag (as the pressure release valve is further away than in the A) and rebreathing will occur at gas flows of < 2.5 × MV.
Mapleson D (non-co-axial system)
Controlled ventilation
> The patient exhales and
a mixture of fresh gas
and dead space gas
enters the bag, as described above.
> The anaesthetist squeezes the bag and fresh gas from the distal tubing is forced into the patient and a variable amount of gas from the reservoir enters the patient.
Following this, the pressure in the system rises
(according to the patient’s lung compliance)
and further gas gets vented
from the expiratory valve.
The Mapleson D is:
> Inefficient for spontaneous ventilation (2.5 × MV)
> Efficient for controlled ventilation (70 mL/kg/min).
Mapleson D (co-axial ‘Bain’ system)
Fresh Gas Flow
Reservoir Bag
Patient
Fig. 74.3 Co-axial Mapleson D
In this circuit the fresh gas
flows down the inner tubing,
and exhaled gas enters the outer tubing.
Both the reservoir bag and
APL valve are in the expiratory limb.
The Bain is equally efficient for
controlled or spontaneous ventilation.
During controlled ventilation at a
flow rate of 70 mL/kg/min,
the patient will in fact be rebreathing.
However, because we tend to over-ventilate
our patients, their end tidal CO2
will not actually rise
despite the fact they are rebreathing.
If we managed not to over-ventilate,
we would actually see a
rising ETCO2 as evidence of this.
To truly avoid rebreathing during controlled
ventilation in the Bain circuit,
we would need to use 2.5 × MV,
the same flow rate as is necessary
to avoid it in spontaneous ventilation.
Mapleson E
This is also called the Ayre’s T-piece
after the man who invented it.
It has no valves or
reservoir bag and so is a
very low resistance system.
This makes it suitable for use in paediatrics.
Mapleson F
This is an E with the ‘Jackson–Rees modification’:
an open-ended reservoir
bag connected to the
end of the tubing.
This allows for the application of
CPAP and controlled ventilation.
In both E and F, fresh gas flows of 2.5 × MV are required to prevent rebreathing.
Volume of fresh gas flow required to prevent rebreathing during spontaneous and
controlled ventilation using the Mapleson breathing systems
Mapleson Spontaneous controlled vent
A 70 mL/kg/min 2.5 × MV
B 2.5 × MV 2.5 × MV
C 2.5 × MV 2.5 × MV
D 2.5 × MV 70 mL/kg/min
E 2.5 × MV 2.5 × MV
< 20 kg 2.5 × MV (minimum 3 L/min)
1000 mL + 100 mL/kg/min
