SERVO-CONTROLLED CLOSED-CIRCUIT ANAESTHESIA

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Abstract
A method of closed-circuit anaesthesia has been developed in which the end-tidal concentration of the volatile anaesthetic agent used is controlled automatically using a closed-loop servo system. End-tidal anaesthetic concentrations, measured by the Engstrom EMMA, were maintained in the closed circuit by direct liquid injection. The system was tested in the laboratory and in clinical use (12 subjects). Accurate estimation of anaesthetic uptake was readily obtained and results for the uptake of halothane agreed closely with those of previous workers. The major sources of error in the method were the result of zero offset in the Engstrom EMMA which in turn was caused by humidity and the intrinsic characteristic of the simple proportional controller used. These errors were easily correctable, and end-tidal halothane concentration could be controlled to within 0.1%. Mean halothane vapour uptake at a constant end-tidal concentration of 0.8% was 114 ml min−1 at 1 min, 36 ml min−1 at 5 min, 29 ml min−1 at 10 min and between 22 and 18 ml min−1 at 20–35 min.

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