CLOSED CIRCUIT

Typical Number in Hospital:

Cost Bands: 1

References: 2

Although this could apply to an electrical circuit its common use in medical equipment is for an anaesthetic breathing circuit in which part or all of the expired gases are intentionally re- inspired. In such rebreathing circuits carbon dioxide is removed in an absorber. Closed circuits may be either 'to and fro' such as with the Waters canister, or 'circle' types. The advantages of the closed or partially closed circuits are that very little of the anaesthetic gases are wasted (they are rebreathed), pollution is reduced, and warming of the gases and humidification is not usually required. They are particularly attractive when using artificial ventilation.

Double pass 'to and fro' systems have the disadvantage that there is considerable dead space, there is a tendency to waterlogging, and 'channelling' may occur because the granules may shake down. They can, however, be readily sterilized. Circle systems are the most common since the dead space is reduced and the gases only make a single pass. The circle circuit conveys the gases around a circle off which there are ports for the rebreathing bag, the patient connection, and the fresh gas input. The circle absorber does not suffer from 'channelling' because of its vertical position. It can also be switched out of circuit. A vaporizer may also be included in the circuit but this must be a low resistance type, and wick types are unsuitable because of water condensation.