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LETTER TO THE EDITOR

Exhaled CO After Surgery: A Consequence of Postoperative Narcotics?

Department of Anesthesiology and Resuscitology; Okayama University Medical School; Okayama, Japan; takatoru{at}cc.okayama-u.ac.jp

In Response:

We appreciate Drs. Dunning and Woehlck’s arguments regarding our paper. It is true that many confounding factors, including Fio₂, rebreathing, and alveolar ventilation, might affect our observations. However, what we have found in our study was that exhaled CO concentration and arterial carboxy hemoglobin (COHb) did not increase in recovery room postoperatively, but did increase in day 1 postoperatively (1). We cannot believe only respiratory status can explain our findings due to the following reasons.

First, it is true that transient increase of Fio₂ could increase exhaled carbon monoxide (CO) concentrations (2). However, we observed that, even in spinal patients (who did not receive oxygen therapy), exhaled CO concentrations increased postoperatively. Furthermore, most of our general anesthesia patients receive oxygen therapy only in recovery room, but not in postoperative day 1. Therefore, it is unlikely that only increased Fio₂ is attributable to our observations.

Second, it is true that decreased alveolar ventilation could increase exhaled CO and COHb concentrations. However, we observed that exhaled CO and COHb concentrations did not increase just after surgery and did increase postoperative day 1. It is most likely that patients’ alveolar ventilation decreased after anesthesia and they were recovering on postoperative day 1. Therefore, only decreased alveolar ventilation could not explain our observations.

Finally, it is extraordinarily difficult to measure Fio₂, rebreathing, and alveolar ventilation in awake, spontaneously breathing patients. Furthermore, in this study, we used the deep-breath and exhalation method to collect exhaled samples. This method could not allow us to consider the effects of respiratory status on exhaled CO and COHb concentrations. Currently, we are addressing these issues using newly developed continuous side-stream sampling method to measure exhaled CO concentration in mechanically ventilated patients, taking into account patients’ respiratory variables.

In summary, we agree with Drs. Dunning and Woehlck’s suggestions that respiratory variables could affect our results. However, it was too difficult to measure these variables in this study setting. Our findings suggested that not only respiratory status, but also increased CO production, which might be caused by surgical stress and heme degradation, increased exhaled CO, and arterial CO-Hb concentrations postoperatively.

References

1. Hayashi M, Takahashi T, Morimatsu H, et al. Increased carbon monoxide concentration in exhaled air after surgery and anesthesia. Anesth Analg 2004;99:444–8.[Abstract/Free Full Text]
2. Zegdi R, Caid R, Van De Louw A, et al. Exhaled carbon monoxide in mechanically ventilated patients: influence of inspired oxygen fraction. Intensive Care Med 2000;26:1228–36.[Web of Science][Medline]

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