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TECHNOLOGY, COMPUTING, AND SIMULATION

Exhaled Carbon Monoxide Levels Change in Relation to Inspired Oxygen Fraction During General Anesthesia

Takehiko Adachi, MD, PhD^*, Kiichi Hirota, MD, PhD^*, Tomoko Hara, MD^*, Yukiko Sasaki, MD, PhD^*, and Yasufumi Hara, MD^*

From the *Department of Anesthesiology, Kitano Hospital, The Tazuke Kofukai Medical Research Institute; and Department of Anesthesia, Kyoto University Hospital, Osaka, Japan.

Address correspondence to Takehiko Adachi, MD, PhD, Department of Anesthesiology, Kitano Hospital, The Tazuke Kofukai Medical Research Institute, 2-4-20 Ohgimachi, Kita-ku, Osaka 530-8480, Japan. Address e-mail to t-adachi{at}kitano-hp.or.jp.

BACKGROUND: 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.

METHODS: 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.

RESULTS: 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.

CONCLUSION: When monitoring exhaled CO levels during mechanical ventilation, it is important to consider the effects of Fio₂.