[Carbon monoxide concentrations during low flow anesthesia]

Elevated carbon monoxide (CO) concentrations have been reported during general anesthesia using inhaled anesthetics. In order to assess the safety of low flow anesthesia, we examined CO concentrations in anesthesia circuit during low flow anesthesia. Thirty patients were randomly divided into two groups. Anesthesia was induced with intravenous thiopental 5 mg.kg-1 and maintained with oxygen 2 l.min-1, nitrous oxide 4 l.min-1 and isoflurane (group H) or oxygen 0.5 l.min-1, nitrous oxide 0.5 l.min-1 and isoflurane (group L). The concentrations of CO in exhaust gas of anesthetic gas monitor were examined by CO sensor (XC-341, Shinkosumosudenki, Tokyo) and COHb was analyzed. The concentrations of CO in group L were significantly higher than in group H. The highest value, however, was 30 ppm. The COHb levels were slightly higher in group L, but the highest value was 2.1%. We consider that these results show no clinical risk and low flow isoflurane anesthesia may be performed safely. Measuring the CO concentrations is helpful for the safety of low flow anesthesia.