METHANE INFLUENCES INFRARED TECHNIQUE ANESTHETIC AGENT MONITORS

Objective, During closed-circuit anesthesia, anesthetic vapor analysis by infrared absorption at 3.3 mu m can be influenced by the concentra tion of accumulated methane, resulting in inaccurate readings of anest hetic concentrations. The current study examined the influence of diff erent known methane concentrations on the analysis of halothane or iso flurane concentrations by the infrared absorption technique. Methods. Three different gas mixtures containing 100, 500 and 1000 ppm methane were given through an experimental sampling bar. Four infrared techniq ue anesthetic agent monitors were examined: (1) the Ultima (Datex), (2 ) the Andros analyzer (Cato anesthesia machine, Drager), (3) the anest hetic gas monitor 1304 (Bruel & Kjaer) and (4) the mainstream analyzer Irina (Drager). All devices, except the Bruel & Kjaer anesthetic gas monitor, function at 3.3 mu m wavelength. The Bruel & Kjaer apparatus functions at 10.3-13 mu m wavelength. The readings were recorded with and without addition of halothane (or isoflurane) at a halothane (or a n isoflurane) dedicated sensitivity after application of methane. Resu lts. At the two highest methane concentrations (500 and 1000 ppm) all studied devices except the Bruel & Kjaer anesthetic gas monitor 1304 d isplayed inaccurate anesthetic concentrations. This was more pronounce d at halothane than at isoflurane sensitivity. Introduction of halotha ne (0.8%) or isoflurane (0.8%) vapor into the experimental sampling ba r resulted in values that were additive to the falsely recorded ones. Conclusions. In closed circuit or low-flow anesthesia, in which methan e can accumulate, infrared measuring techniques for potent inhalation anesthetics that do not use the 3.3 mu m wavelength appear to be prefe rable.