To the Editor:
It has come to our attention that the use of desflurane with the Marquette MGA Advantage 1100 mass spectrometer system (Marquette Electronics Inc., Milwaukee, WI) not modified to detect desflurane can lead to unneeded changes in patient management. During a recent desflurane anesthetic, we noticed a capnogram suggestive of rebreathing. Examination of the numeric data obtained by the mass spectrometer revealed the presence of inspired carbon dioxide. We had known for some time that the MGA system detects the presence of desflurane as enflurane at a concentration different from the concentration in the circuit. In an attempt to determine the source of the inspired CO2, we switched from the circle system to an Ambu bag supplied with fresh gas from the anesthesia machine. The change in apparatus did not eliminate the presence of inspired CO2. The desflurane was discontinued and isoflurane substituted. We then noted the disappearance of the inspired CO2, and the capnogram reverted to a normal tracing.
To confirm this finding, we later created an "artificial patient" by using the circle system with a breathing bag placed at the patient connection of the "Y" piece and then continuously added carbon dioxide into the end of the breathing bag through an intravenous cannula. Using this system we compared mass spectrometer data and data from an infrared gas analyzer. In this system the MGA shows the presence of inspired CO2 when desflurane is added to the circuit; the infrared analyzer does not.
The Marquette MGA has targets (Farraday collectors) at precise locations in its dispersion chamber that detect ionized particles from the gases being analyzed. Each target detects particles with a specific mass/charge ratio. Many gases that pass through the ionization chamber of the mass spectrometer are fragmented or "cracked" into several ionized species. This phenomenon allows the mass spectrometer to differentiate two molecules of identical mass/charge ratios from each other. Detection of the C (+) fragment of CO2 allows it to be separated from N2 O as they both have the same mass of 44 d. A similar process is used to differentiate isoflurane and enflurane, which are isomers . It appears that one of the ionized particles from desflurane has a mass/charge ratio close to that of C+, which the MGA uses to identify CO2. Another fragment strikes the enflurane target. The quantity of fragments produced by the system is proportional to the concentration of the gas being measured. The amount of inspired CO2 measured by the mass spectrometer will be proportional to the amount of desflurane present in the sampled gas.
When a capnogram suggestive of rebreathing is noted during the use of desflurane with an unmodified MGA system, a change to isoflurane can resolve the question of whether the inspired CO2 is real or artifact. In systems configured with the Life Watch Trademark option (an infrared system), looking at the real time data on the patient display unit will also show the absence of inspired CO2.
While Marquette recommends that the monitor be upgraded to detect desflurane, it may be more cost-effective to wait for the release of sevoflurane and have the system upgraded one time to measure both of the new anesthetics.
Joel A. Bennett, DDS, MD
David Seitman, MD
Department of Anesthesiology, Hahnemann University, Philadelphia, PA 19102
1. Ehrenwerth J, Eisenkraft JB. Anesthesia equipment principles and applications. St. Louis: Mosby, 1994:203-7.