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LETTERS TO THE EDITOR

Carbon Monoxide Rebreathing during Low Flow Anesthesia

Department of Anesthesiology, Froedtert Memorial Lutheran Hospital, Milwaukee, WI

To the editor:

Tang et al. (1) studied intraoperative carbon monoxide (CO) concentrations, confirming that many smokers exhale more than 50 ppm CO (2). Competitive binding of CO and oxygen may produce breathing circuit CO concentrations that are directly proportional to FIO₂. Tang et al. (1) demonstrated that their CO did not originate from anesthetic degradation. Therefore, their data should not create a false sense of security for small patients because the severity of CO exposures caused by anesthetic breakdown is inversely related to patient size (3,4).

Unlike environmental exposures in which CO is absorbed via the lungs, increased intraoperative CO concentrations during low-flow anesthesia result from rebreathing the CO displaced from a smoker’s own carboxyhemoglobin. Total CO quantities (gas phase, dissolved and bound to carboxyhemoglobin) should increase only from metabolism. Data suggest that after a CO exposure, the slow dissociation of CO from carboxyhemoglobin may allow the arterial pCO (dissolved CO) to remain below equilibrium values (5). This dissolved CO may be the prime determinant of toxicity. If true, the equilibration of inspiratory, expiratory, and breathing circuit CO concentrations during low-flow anesthesia may indeed be the equivalent of an active exposure because increasing pCO may enhance mitochondrial poisoning. Although further study is needed, Tang et al.’s data (1) support increased use of intraoperative CO monitoring.

References

1. Tang CS, Fan SZ, Chen CC. Smoking status and body size increase carbon monoxide concentrations in the breathing circuit during low-flow anesthesia. Anesth Analg 2001; 92: 542–7.[Abstract/Free Full Text]
2. Woehlck HJ, Connolly LA, Cinquegrani MP, et al. Acute smoking increases ST depression in humans during general anesthesia. Anesth Analg 1999; 89: 856–60.[Abstract/Free Full Text]
3. Woehlck HJ, Dunning MB III, Raza T, et al. Physical factors affecting the production of carbon monoxide from anesthetic breakdown. Anesthesiology 2001; 94: 453–6.[Medline]
4. Woehlck HJ, Mei D, Dunning MB III, Ruiz F. Mathematical modeling of carbon monoxide exposures from anesthetic breakdown: effect of subject size, hematocrit, fraction of inspired oxygen, and quantity of carbon monoxide. Anesthesiology 2001; 94: 457–60.[Medline]
5. Goldbaum LR, Ramirez RG, Absalon KB. What is the mechanism of carbon monoxide toxicity? Aviat Space Environ Med 1975; 46: 1289–91.[Medline]

Institute of Occupational Medicine and Industrial Hygiene, College of Public Health, National Taiwan University, Taipei, Taiwan

In Response:

We basically agree with Dr. Woehlck’s comments on our paper. Our findings do support that carbon monoxide (CO) concentration in the breathing circuits was increased by patient’s smoking status and body weights (1). The value of our study is that our data reflect patients’ real CO exposure situation under a common clinical practice of low-flow anesthesia. Our current measurements show that the water content of soda lime was 7.7%–21.3% during the low gas flow rates of 1.4 ± 0.9 L/min reported in our paper. Production of CO from anesthetic breakdown is minimal under such condition from laboratory testing results (2). However, we agree that the need to monitor intraoperative CO monitoring cannot be overemphasized. Special attention should be directed to susceptible subjects, including patients with smaller size (as Dr. Woehlck suggests). The possible equilibrium mechanism among COHb, pCO, and CO in breathing circuit suggested by Dr. Woehlck is definitely a worthwhile subject for further studies. This is also a research topic that we are currently undertaking. We will share our findings on this important subject with the readers of Anesthesia & Analgesia as soon as possible once we complete our study.

References

1. Tang CS, Fan SZ, Chan CC. Smoking status and body size increase carbon monoxide concentrations in the breathing circuit during low-flow anesthesia. Anesth Analg 2001; 92: 542–7.
2. Woehlck HJ, Dunning MB III, Raza T, et al. Physical factors affecting the production of CO from anesthetic breakdown. Anesthesiology 2001; 94: 453–6.

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