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SummaryFive anaesthetic breathing systems (Magill, Lack, Humphrey ADE, enclosed Magill and Bain) were compared using spontaneous ventilation in a simple lung model. The fresh gas flow at which rebreathing occurred was determined for each system by the application of four modiJed definitions of rebreathing. Two were based on the measurement of minimum inspired and two on end-expired carbon dioxide. The four A systems performed similarly with each individual definition. The rebreathing points found for each individual breathing system differed markedly between definitions. with those determined by the minimum inspired CO, occurring at low, and probably misleading, FGF/ VE ratio. The Bain system demonstrated rebreathing at considerably higher fresh gasjows whichever definition was used.

The Enclosed Magill anaesthetic breathing system may be used for both spontaneous ventilation and controlled ventilation with similar fresh gas flows. During spontaneous ventilation, a fresh gas flow between estimated alveolar ventilation and minute ventilation is adequate and the system performs as a Mapleson A type breathing system. For controlled ventilation, a fresh gas flow of 70-100 ml kg-1 min-1 produces normocapnia in most subjects, as demonstrated in this limited study. These values are similar to those demonstrated for type D breathing systems. It is suggested from this preliminary study that the breathing system conforms to the requirements of a universal breathing system in as much as similar fresh gas flows are used in both modes of ventilation.

A mechanical lung model was used to investigate the effect of varying carbon dioxide production and deadspace on the end-tidal carbon dioxide levels achieved during mechanical ventilation when using the Bain, Humphrey ADE, and circle systems. Both factors had significant influence on end-tidal cardon dioxide concentration and could result in values in excess of those considered acceptable in clinical practice. The implications of the results are discussed.

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