Removal of halogenated anaesthetics from a closed circle system with a charcoal filter

Halogenated anaesthetics may be removed from a closed circle system by means of a charcoal filter. With this technique dispersion to the atmosphere and a possibly destructive effect of the halogenated volatiles on the protective layer of ozone is avoided. Removal of halothane or isoflurane with a charcoal filter was studied in a closed circle system connected to an artificial lung. The concentration of anaesthetic (Ct) was recorded in relation to time by an anaesthetic gas monitor interposed between the system and the lung at ventilations of 3, 5, 7 or 9 l/min (v). Based on theoretical considerations, it was expected that Ct = Co.exp (-v/V.t), (V: volume of the system). Analysis of regression demonstrated that the results fitted well to an exponential decrease (R2 greater than 0.94) and the downslope increased with increasing rate of ventilation. However, the slopes deviated significantly from the theoretically predicted slopes, possibly because of adsorption to hoses and bags and unequal distribution of the volatiles in the system. Halothane was eliminated more slowly than isoflurane. This study demonstrates that halogenated volatiles are eliminated in an exponential way following charcoal filtration and the rate depends on the ventilation and type of volatile.