The Ruben circle anaesthesia system. An investigation of reverse flow of patient expired gas during spontaneous breathing.

The Ruben circle anaesthesia system was studied in the spontaneous breathing mode, and under certain conditions there was incompetence of the replaceable mushroom control valve. This causes a reverse flow of gas which results in rebreathing of expired gas when the system is used on spontaneously breathing patients. A patient simulator was used to investigate the way in which the reverse flow of gas depends on ventilatory parameters and fresh gas flow. The inspired concentration of carbon dioxide increased for increased fresh gas flow and for decreased tidal volume. These results were confirmed by observations on anaesthetised adult and paediatric patients during spontaneous breathing. We conclude that the system in its present form, is not suitable for use on spontaneously breathing paediatric patients.