The influence of breathing system filters on paediatric capnography

Breathing system filters are in common use during paediatric anaesthesia. E xpired gas sampling from the patient side of these filters may contaminate and saturate the sampling line, while sampling from the machine side may ca use underestimation of end-tidal carbon dioxide (PECO2). The aim of this in vestigation was to elucidate the degree of underestimation of PECO2 induced by sampling from the machine side of a breathing system filter. Ten sponta neously breathing children and ten children receiving mechanical ventilatio n under general anaesthesia were studied. PECO2 was higher at the patient s ide of the filter in both ventilated and spontaneously breathing groups (P< 0.002 for each). The bias in measuring at the machine side of the filter wa s significantly greater in the spontaneously breathing children as compared with the mechanically ventilated children (-1.8 vs -0.7kPa; P<0.004).