ACCUMULATION OF FOREIGN GASES DURING CLOSED-SYSTEM ANESTHESIA

In a previous study, accumulation of methane was found at the end of c losed-system ventilation. As on-line analysis of gas concentrations is now available, we examined the progressive increase in concentrations of methane, carbon monoxide and acetone during modern, closed-system conditions, and their influence on infrared halothane analysis, in 26 non-pregnant, gynaecological patients. A computer-controlled closed-sy stem anaesthesia apparatus (PhysioFlex) was used for ventilation durin g total i.v. anaesthesia (excluding nitrous oxide or potent inhalation anaesthetics) for gynaecological laparoscopy. Methane, carbon monoxid e and acetone concentrations were analysed every 15 min in a photoacou stic infrared monitor and halothane concentrations by built-in infrare d spectrometry. Mean methane concentrations increased progressively af ter 105 min to 941 (SD 1094) ppm, but concentrations of carbon monoxid e and acetone did not increase significantly. In 18 patients, the infr ared measurement falsely indicated 0.79 (0.52)% ''halothane'' after 60 min, but no reading appeared in the other eight patients. We conclude that methane accumulated progressively under strict closed-system con ditions in higher concentrations than reported previously. In two-thir ds of patients it induced false ''halothane'' readings.