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Non-rebreathing Circuits

The distinguishing feature of non-rebreathing circuits is that elimination of carbon dioxide is accomplished by removing all expired gases from the system and venting them to atmosphere. This is normally achieved by using the fresh gas flow from the anesthetic 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 anesthetic machine is inspired in each breath. They are, however, are 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 anesthetic agents. They are also less forgiving of operator error since an inadequate fresh gas supply will result in rebreathing

Mapleson's Classification of Non-rebreathing Circuits

Mapleson'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.

Mapleson's Classification

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 anesthesia, but may be used in emergency for resuscitation.
Mapleson D - the modified Bain circuit.
Mapleson E - Ayre's T piece and the Bain circuit.
"Mapleson F" - not originally classified by Mapleson, but is used to refer to Jackson-Rees' modification of Ayre's T-piece.

The Humphrey ADE is a new circuit that provides the ability to switch between the Mapleson A, D and E arrangements.

Non-standard circuits
There is a vast number of ways in which tubes, bags and valves may be inter-connected to fabricate an anesthetic breathing circuit. Caution should be exercised when using any "new" circuit, since it may not work in the way its designer intended.


Comments on this article should be addressed to Dr Guy Watney
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