The Mapleson classification (Mapleson 1954) is the most widely used classification of breathing systems and is shown in Figure EQ.4
The components of the breathing attachment are connected together by standard male to female conical fittings with a nominal diameter of 22 mm. The endotracheal tube connection and some paediatric circuits have a diameter of 15 mm. To achieve a gas tight seal and to prevent unintentional disconnections, the fittings should be joined with a push-and-twist action. The common gas outlet of the anaesthetic machine has a 22 mm external diameter and 15 mm internal diameter. Connections to the scavenging system are made by 30 mm diameter fittings which feature turn and pull removal.
The reservoir bag can be made from anti-static rubber or plastic and is made in a range of sizes including 0.5, 1.0 and 2 litres. An appropriate size bag should be used for each patient: too small a bag does not store enough gas for a large breath but too large a bag cannot be seen to move with respiration. The bag also acts as a safety device preventing the build up of excessive pressure since it is designed to rupture at about 50 cmH2O (5 kPa). The connecting pipes were once made of black anti-static rubber but are now made from disposable lightweight plastic. The internal diameter of the tubing (22 mm) equates to a volume of about 400 ml per metre. Flow resistance can be significant and is minimised by using only as much length as necessary. Unidirectional valves control the flow of gas round the circle system and low resistance to flow is assured by making the valve area large and the discs as light as possible. They are housed in paired transparent domes to permit inspection.
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