In general anaesthesia, recovery and
emergency resuscitation procedures involve the use of a breathing
system. The principles of anaesthetic systems is to efficiently
eliminate exhaled carbon dioxide, without greatly increasing dead
space or resistance. In 1954 Mapleson classified breathing systems
based on their efficiency in eliminating carbon dioxide during
spontaneous respiration. The breathing systems are classified in
order of increased requirement of fresh gas flow (FGF) to prevent
rebreathing during spontaneous respiration. System A requires 0.8 -
1 times, B and C require 1.5 - 2 times and D, E and F require 2 - 4
times of the patient's minute volume.
| a |
This system consists of a reservoir bag close to the
fresh gas outlet with the APL valve placed at the patient end
(Magill A, or expiratory limb - Parallel Lack). During inspiration,
as the patient's inspiratory flow is greater than the fresh Gas
Flow, additional gases will be drawn out from the reservoir bag.
During the expiratory phase, exhaled gases will force the fresh gas
back into the circuit and reservoir bag. This will result in
increasing the pressure in the breathing system and thus forcing
the exhaled gases to escape through the APL valve (scavenging
port). the system requires a low flow (0.8 - 1 times of Minute
Volume, approximately 6-8 litres/minute) during spontaneous
respiration to prevent rebreathing. This system is not recommended
for intermittent positive pressure ventilation (IPPV). |
| b |
In this system the reservoir bag, fresh gas supply and
APL valve are closer to the patient. This will cause mixing of
inspiratory and expiratory gases and therefore a higher flow rate
(1.5 - 2 times of the patient's minute volume, i.e. 12 - 16
litres/min) is required to prevent rebreathing during spontaneous
respiration. Due to the risk of rebreathing and reduced delivery of
oxygen rich gases to the patient this system is no longer
used. |
| b |
This system is similar to Mapleson B, however, the bag is
positioned very close to the patient. This system is used for
manual ventilation during resuscitation. A flow rate of 1.5 - 2
times of the minute volume is required to avoid
rebreathing. |
| b |
This system is mainly used with spontaneous respiration.
The Fresh Gas Flow is close to the patient and the APL valve is
placed away from the patient. The risk of rebreathing in this
circuit will be high especially in patients who have a short
expiratory pause or do not have an expiratory pause (infants). To
overcome this problem a high FGF of 2 - 4 times of the patient's
Minute Volume is required to prevent all
rebreathing. In practice some rebreathing is tolerated and in an
adult an FGF of 6 - 7 litres/minute will maintain a normal arterial
CO2 tension |
| b |
This system is primarily for use in neonates and
paediatrics, where low resistance is of great importance. There is
no APL valve (to reduce resistance) and a high FGF, 2 - 4 times of
the patient's Minute Volume (with a minimum flow of 3
litres/minute) is required to eliminate rebreathing risk during
spontaneous ventilation. This system is recommended for up to 22kg
(approximate tidal volume of 140ml). |
| b |
Jackson Rees made a great improvement to T-Pieces
(Mapleson E) by adding an open tail 0.5 litre reservoir bag to the
end of the expiratory (reservoir) limb. This allows manual
ventilation and the application of Positve End Expiratory Pressure
(PEEP) to help maintain open airways. For controlled ventilation,
normocapnia can be maintained with a FGF of 1000ml + 100ml/kg body
weight |