5. Anaesthetic Circuits Flashcards
Circle system
fresh gas flow,
together with volatile anaesthetic (if used),
enters the inspiratory limb and travels through a one-way non-return valve to the patient.
Expired air enters the expiratory limb and passes through a second non-return valve.
Excess gas (mixed expired air) is vented through the pressure relief valve on the
reservoir bag, with the remainder passing through the CO2
absorbing soda lime and
back into the inspiratory limb (Figure 5.2).
Advantages circle
The circle is very efficient in terms of conservation of gases,
heat and moisture.
It is therefore more economical and less polluting than semi-closed systems
Disadvantages
: the circle is simple in outline theory but much more complex in
detailed execution. The uptake and excretion of the different components within the
system can vary greatly, and it is essential to monitor the FiO2, CO2 and agent
concentration within the circle. The system is slow to react to changes in the inspired
volatile concentration. If nitrous oxide is used in a closed system, then hypoxia is a
potential risk as described earlier. Should soda lime be allowed to dry it may react
with the CHF2 group of desflurane, isoflurane and enflurane to produce carbon
monoxide.
Principles of use of a circle system
Closed:
circle system can be used as a genuine closed circuit in which expired
gases and volatile agents recirculate
more oxygen is added than is required for
the patient’s metabolic demand, and CO2 is removed by passage of the gases
through soda lime.
Enter the system via vaporizers that can be sited either
outside (VOC) the circuit or, much less commonly,
within the circuit (VIC) as a drawover vaporizer
truly closed system delivering only
basal flows, it is important to realize that prolonged anaesthesia with nitrous oxide
can cause hypoxia
Semi-open (also referred to as semi-closed
circle is used
with a higher than basal FGF with the excess gas being vented through the pressure
relief spill valve.
concentrations of gases and volatile
agent in the fresh gas supply are closer to those in the circle and can be changed
more quickly. This represents a compromise between economy and ease of use
Practical considerations
total volume of the system, including the breathing
hoses, the air in the absorber, reservoir bag and the patient’s FRC, is around 5 litres.
This means that high flows must be used for the first
5–10 minutes. A VOC will always show a higher concentration than is being inspired
(unless equilibrium has been reached), and rapid changes can only be initiated by a
reversion to high flow. If a fully closed system is used, then monitors which measure
inspiratory gas and volatile concentrations as close to the patient as possible must be
used, and the system should be flushed at regular intervals to minimize the risk of
dilutional hypoxia.
The Mapleson Classification
systems were classified as A to F,
and they all potentially allow rebreathing. They are ‘semi-closed and supply more gas
than the patient needs,
with the excess being vented to the atmosphere
If rebreathing
of CO2 does occur, a healthy patient who is breathing spontaneously will respond by
increasing alveolar ventilation which will rise, by up to 20 times if necessary, to keep
the PaCO2 normal
systems are defined therefore in terms of the FGF that is
needed to maintain an unchanged PaCO2 in the face of unchanged ventilation.
Mapleson A
reservoir bag into which the FGF is directed, a length of corrugated
tubing (which is resistant to kinking) and, at the patient end, an adjustable pressurelimiting
(APL) valve
Spontaneous respiration: the system is very efficient. At the end of inspiration
the valve is closed and the reservoir bag is emptying. During expiration, the FGF is
filling the reservoir bag, while expired air (dead space gas and then alveolar gas) is
passing into the tube. Hence the pressure in both the reservoir bag and the
breathing system increase to the point at which the valve opens and vents expired
air.
Mapleson C
Mapleson C comprises an APL at the patient end with the FGF entering just proximally.
short length of tubing connects this to the reservoir
bag which, in the classic ‘Waters’ circuit, includes a CO2-absorbing canister
is used in resuscitation and in areas such as
theatre recovery
allow mixing of expired air with the FGF, which must
approximate three times minute ventilation to flush the system and prevent rebreathing.
Mapleson B
Mapleson B includes a length of corrugated tubing between the FGF and the
reservoir bag.
Mapleson D, E and F:
all function as T-pieces, being inefficient for spontaneous respiration but efficient for controlled ventilation
they require up to three times the minute ventilation to prevent
rebreathing during spontaneous respiration (150–200 ml kg1) but only 70 ml kg1
to achieve normocapnia during IPPV.
The respiratory cycle is a sinusoidal
waveform, and, in order to prevent rebreathing, the FGF must equal or exceed the
peak inspiratory flow rate (PIFR).
alveolar gas has moved into the
expiratory limb where it mixes with the FGF, and, to prevent rebreathing, the FGF
should approach three times the minute ventilation.
Mapleson D: the Bain circuit is the co-axial version of the Mapleson D circuit
Mapleson F
: this differs from the E only in that it has a reservoir bag, added by
Jackson-Rees (who was a paediatric anaesthetist in Liverpool) to allow controlled
ventilation. The system has no valves and so there is minimal expiratory resistance,
hence its traditional use in paediatric anaesthesia. Effective scavenging is
difficult from both the E and F systems.
