With monochromatic infrared gas spectrometers (MIS), the displayed concentration is computed from measured IR absorption and a gain factor specific for the selected volatile agent (VA). As MIS cannot detect which VA is actually present, the displayed concentration can be very different from the actual one. As bottles and vaporizers are very specific for desflurane, it is impossible to misfill a vaporizer; however an erroneous selection of VA on MIS remains possible. The aim of this study was to assess the displayed concentrations after erroneous vapour selection on the monitor. When either desflurane, or isoflurane or enflurane were delivered at constant concentrations, all VA measured by the MIS, namely desflurane, sevoflurane, isoflurane, enflurane and halothane were successively selected and the displayed concentrations compared with the actual vapour concentration using a Capnomac(TM) Ultima(Datex) monitor. Consequences of erroneous selection can be included in three categories: 1) dangerous error, when a displayed concentration is much lower than the actual one, eg desflurane or sevoflurane erroneously selected, 2) evident error, when displayed concentration is much higher than 10 vol%; 3) incomfortable situation, when displayed and actual concentrations are similar, eg isoflurane erroneously selected instead of desflurane. This error can only be detected by a careful checking of the device.
