It is shown that there is a need for a versatile inhalation anaesthetic meter. In particular it is necessary if fully closed circuit anaesthesia, which saves anaesthetic expenditure and reduces theatre pollution, is to be used. The need has not been met by any of the available meters, with the possible exception of the Emma (an instrument made by Engstrom) which is new and unproven. Various methods of determining the concentration of any anaesthetic against a variable concentration of the other anaesthetic circuit gases are examined. Of these gas chromatography, adapted to give a separation in less than 15 seconds and components selected for use in the operating theatre, appears the most promising. A laboratory prototype is built and the selection of the component parts is described. A solenoid operated gas sampling valve and a refractometer detector are developed. This model satisfactorily separates halothane, penthrane or ethrane from the other anaesthetic circuit gases in the required time. Other anaesthetic agents were not tried, but it is expected that they would behave similarly with the possible exception of cyclopropane, the lightest and at normal temperatures and pressures the only gaseous potent anaesthetic agent. The operating conditions of the chromatograph are determined. The modifications that would be required to adapt the laboratory model into a clinical instrument are discussed, and the conclusion drawn that this system could form the basis of a satisfactory instrument.
|Date of Award||1981|