|The distinguishing feature of non-rebreathing circuits is that elimination of CO2 is accomplished by removing all expired gases from the system and venting them to the atmosphere. This is normally achieved by using the fresh gas flow from the anaesthetic machine to direct the expired gases out of the circuit via a valve or other arrangement.
In general, non-rebreathing systems provide good control of the inspired gas concentrations, since fresh gas delivered from the anaesthetic machine is inspired in each breath. They are, however, less economical in use than rebreathing systems because the minute volume of ventilation (or more) must be supplied to the patient to prevent rebreathing, and they contribute more to the problem of atmospheric pollution with anaesthetic agents.
Mapleson's classification of non-rebreathing circuitsMapleson's classification divides non-rebreathing circuits into functionally similar groups, on the basis of the fresh gas flow required to prevent rebreathing and the ease with which intermittent positive pressure ventilation may be performed.
Figure 1: Mapleson's classification of non-rebreathing circuits
Mapleson A - the Magill and Lack circuits
Mapleson B and C - Rebreathing of exhaled gases occurs even when very high fresh gas flow rates are used, since inspiration is taken from the same space into which the previous breath was expired. These are unsatisfactory for anaesthesia, but may be used in emergency for resuscitation.
Mapleson D - the modified Bain circuit.
Mapleson E - Ayre's T piece
"Mapleson F" - not originally classified by Mapleson, but is used to refer to Jackson-Rees' modification of Ayre's T-piece.
The Humphrey ADE circuit provides the ability to switch between the Mapleson A, D and E arrangements.