SUMMARY Using two types of infrared anaesthetic analysers (Capnomac, Datex, Finland and Anaesthetic Gas Monitor [AGM] 1304, Bruel & Kjaer, Denmark) simultaneous measurements were made of halothane and methane concentrations in the expired gases from nine anaesthetised horses. Methane concentrations varied from 232 to 1770 ppm. The Capnomac gave halothane readings 0.35 to 4.5 per cent higher than those given by the other analyser. A highly significant linear relationship (r = 0.87) was found between this difference and the concentration of methane in the anaesthetic circuit. This linear relationship was confirmed by an in vitro study using methane-air mixtures containing approximately 500 to 2,500 ppm of methane and analysed in both the halothane and isoflurane measuring modes of the analysers. The AGM 1304 was totally unaffected by methane. We concluded that monitors with absorption in the upper end of the infrared spectrum are preferable for measuring the concentration of inhalation agents in low-flow anaesthetic systems.
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Moens et al. (1991) studied this question.
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