The future of low flow breathing systems in anaesthesia

The traditional circle system was devised as a means of purifying and recycling used anaesthetic gases and vapours without the requirement of having the absorber positioned close to the patient´s head. The theory of circle system agent delivery was founded on the concept of a known fresh gas mixture being used to replace agents taken up by the patient. This focuses our techniques of anaesthetic administration on the fresh gas constitution entering the circle as this is the factor that is able to be adjusted by the anaesthetist. However, what truly matters to the conduct of the anaesthetic is the specific balance of the inhalational components that is presented to the patient through the patient connection. Until the advent of system gas and volatile analysers within the theatre, the manual adjustment of the gaseous and volatile input into the circle was the only mechanism of controlling the mixture presented to the patient albeit through the buffering mechanism of the circle system itself. The technology is now available to close the servo-feedback loop with the mixture sensed at the patient connection. Accurate and reliable agent delivery systems and computer coordinated modulation are vital links in this process. It is therefore now a logical step for anaesthetists to think in terms of target patient end-tidal volatile and gas values. The anaesthetist is accustomed to monitoring such values during routine anaesthesia. A servo system would be able to achieve a targeted value and equally importantly maintain a steady value throughout the constantly changing patient uptake