Breathing circuit

A closed or semi-closed circuit will include components which remove carbon dioxide from the exhaled gas and add oxygen before it is delivered for inhalation, so that the mixture remains stable and suitable for supporting life.

Originally developed for use in anaesthesiology, many variants of breathing system are in clinical use, but most comprise a source of fresh gas flow, a length of breathing tubing to direct the gas, an adjustable pressure limiting valve to control pressure within the system and direct waste away, and a reservoir bag to allow assisted ventilation.

They have traditionally been classified by the way in which the system interacts with fresh air from the surrounding atmosphere, and by whether the patient rebreathes gases that they have previously exhaled.

[1][2][3] However, there is no international standard for classifying breathing systems, and the terms "semi-open" and "semi-closed" may cause confusion in particular between US and British usage.

They vary in their efficiency, in that some need wastefully higher fresh gas flows in certain situations to ensure that carbon dioxide is removed safely, avoiding rebreathing that can lead to hypercapnia.

Examples of commercially available anaesthetic breathing systems. From top to bottom: a Mapleson C system manufactured by Intersurgical; a Mapleson E system, to which a red Venturi valve has been fitted so as to reduce the delivered oxygen concentration; and a Mapleson F system manufactured by Intersurgical.