In Reply:--I agree with Breen that carbon monoxide may be detected and quantitated directly by mass spectrometry in certain situations. However, significant human toxicity may result from less than 1,000 ppm carbon monoxide. This may be as much as 100 times less than the amount administered in the study by Breen et al. The potential sources of interference with the technique of direct measurement of carbon monoxide by mass spectrometry includes changing levels of nitrogen and carbon dioxide as well as the presence of anesthetic agents and other gases. The changes in concentrations of nitrogen and carbon dioxide found in breathing circuits are likely to be many times greater than the amounts of carbon monoxide, which may result from anesthetic breakdown. An increase in indicated nitrogen may be due to the presence of carbon monoxide or the presence of nitrogen from air leaks, patient denitrogenation, or the use of air as part of the respiratory gas mixture. Increased inspired carbon dioxide may be due to the presence of carbon monoxide or the presence of carbon dioxide due to carbon dioxide absorbent exhaustion or leaking valves in a circle system. If 500 ppm (0.05%) carbon monoxide was present in a patient's breathing circuit and was displayed as an increase of either 0.05% nitrogen or 0.05% inspired carbon dioxide, I speculate that this increase would not be distinguishable from innocuous fluctuations of these gases. Therefore, I suggest that, before direct detection of carbon monoxide by mass spectrometry can be used to warn of a patient's exposure to carbon monoxide during anesthetic breakdown, studies must be conducted to show the validity of this technique with clinically relevant concentrations of carbon monoxide.

Harvey J. Woehlck, M.D., Department of Anesthesiology, Froedtert Memorial Lutheran Hospital, 9200 West Wisconsin Avenue, Milwaukee, Wisconsin 53226.

Stewart RD: The effect of carbon monoxide on humans. J Occupational Med 18:304-309, 1976.
Breen PH, Isserles SA, Westley J, Roizen MF, Taitelman UZ: Combined carbon monoxide and cyanide poisoning: A place for treatment? Anesth Analg 80:671-677, 1995.