END-TIDAL CARBON-DIOXIDE MEASUREMENT AND BREATHING SYSTEM FILTERS

Breathing system filters are recommended for use during general anaest hesia. Manufacturers of gas sampling equipment recommend that gas is s ampled from the 'machine side' of these filters, thereby avoiding cont amination of the sampling line and waterlogging in the case of heat an d moisture exchangers. The aim of this study was to investigate differ ences between the measured end-tidal carbon dioxide values at either s ide of the filter. Fifteen adults were studied during mechanical venti lation and 15 during spontaneous ventilation under general anaesthesia . End-tidal carbon dioxide values were significantly lower at the mach ine side of the filter in both groups (p = 0.00001). The measurement e rror induced by the inclusion of the breathing system filter was signi ficantly greater in the spontaneously breathing group (p = 0.0004).