Medical aerosol propellant interference with infrared anaesthetic gas monitors

Background. 1,1,1,2 Tetrafluoroethane is a hydrofluoroalkane(HFA) that is replacing chlorofluorocarbons (CFC) as a medicalaerosol propellant in an attempt to reduce damage to the ozonelayer. This study compared the effects of HFA- and CFC-basedinhalers on four anaesthetic gas monitoring systems. Methods. The HFA- and CFC-based inhalers were activated in closeproximity to the sample line of two Datex Ohmeda, an Agilentand a Siemens infrared anaesthetic agent monitoring systems.The effects were recorded on each system for five common anaestheticagents. Results. The HFA inhaler caused either maximal false positivereadings (with the exception of desflurane) or transient measurementfailure on all systems. The Datex Ohmeda AS/3 system misidentifiedthe HFA inhaler as carbon dioxide at low concentration (2 ±0 mm Hg). The CFC-based inhaler caused a minor false-positivereading (0.4 ± 0%) for halothane only on the Datex OhmedaAS/3 system only and was misidentified as carbon dioxide at33.3 (SD 2.1) mm Hg and 22.4 (8.9) mm Hg by the Agilent andSiemens systems. Conclusions. The HFA inhaler adversely affected all equipmenttested. The infrared spectra of HFA and the common anaestheticgases have considerable overlap at the 8–12 µm rangethat is not shared by the CFCs. The differences in spectraloverlap explain the different effects of the HFA and CFC propellants.Anaesthetic gas concentration data may be erroneous using theHFA-based inhalers. Br J Anaesth 2004; 92: 865–9