['Heme oxygenase produces carbon monoxide (CO) during the breakdown of heme molecules. A variety of stressors upregulate this enzymatic activity and can increase exhaled CO levels. Recently, exhaled CO levels have been reported to increase in critically ill patients and after anesthesia and surgery. To use this measurement during mechanical ventilation, it is important to clarify the effects of factors which interfere with exhaled CO levels. The fraction of inspired oxygen (Fio2) is often changed during artificial ventilation. To investigate the effect of changes of Fio2 on exhaled CO, we measured exhaled CO levels during general anesthesia.', 'Thirty patients who underwent elective operations were enrolled in this study. Anesthesia was maintained with sevoflurane and fentanyl. All patients were tracheally intubated and ventilated with a non-rebreathing ventilator. Exhaled CO levels were measured in gas sampled from the expired limb of the respiration circuit using a CO monitor. The effects of sequential changes of Fio2 on exhaled CO levels, and the effects of long-term inhalation of Fio2 0.75 and Fio2 0.35 on exhaled CO levels and arterial carboxyhemoglobin concentrations were investigated.', 'Exhaled CO levels changed rapidly in response to changes of Fio2. Long-term inhalation of Fio2 0.75 initially increased and then gradually decreased exhaled CO to basal levels, concomitant with a decrease of arterial carboxyhemoglobin. Long-term inhalation of Fio2 0.35 did not elicit any significant change in the observed variables.'] Do exhaled carbon monoxide levels change in relation to inspired oxygen fraction during general anesthesia?