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TECHNOLOGY, COMPUTING, AND SIMULATION

Measurement of Anesthetics in Blood Using a Conventional Infrared Clinical Gas Analyzer

Philip J. Peyton, MD, MBBS, FANZCA^*, Michael Chong, MBBS, FANZCA^*, Christopher Stuart-Andrews, MEng, Gavin J. B. Robinson, MBBS, FANZCA, Robert Pierce, MBBS, MD, and Bruce R. Thompson, PhD^||

From the *Department of Anaesthesia, Austin Hospital, and University of Melbourne, Melbourne; Department of Electrical and Computer Systems Engineering, Monash University; Department of Anaesthesia and Perioperative Medicine, The Alfred; Institute for Breathing and Sleep, Austin Hospital and University of Melbourne; and ||Department of Allergy, Immunology and Respiratory Medicine, The Alfred Hospital and Monash University, Melbourne, Australia.

Address correspondence and reprint requests to A. Philip Peyton, MD, MBBS, FANZCA, Department of Anaesthesia, Austin Hospital, Heidelberg 3084, Melbourne, Australia. Address e-mail to phil.peyton{at}austin.org.au.

BACKGROUND: Measurement of the partial pressure of volatile anesthetics in blood is usually done using a "headspace equilibration" method with gas chromatography. However, it is not often performed in clinical studies because of the technical, equipment, and logistic requirements. To improve the accessibility of this measurement, we tested the use of a common infrared clinical gas analyzer, the Datex-Ohmeda Capnomac, for this purpose.

METHODS: After characterization of the linearity of the device in measuring the volatile anesthetic concentration in the presence of nitrous oxide, carbon dioxide, and water vapor, blood was tonometered with known concentrations of sevoflurane (actual value between 0.5% and 5.0%) in oxygen and oxygen/nitrous oxide mixtures, as well as mixtures of isoflurane and desflurane in oxygen.

RESULTS: Mean bias (standard deviation) overall for sevoflurane in oxygen relative to the tonometered reference partial pressure was –4.5 (4.8%) of the actual concentration. This was not altered significantly by measurement in 40% oxygen/60% nitrous oxide. For isoflurane and desflurane it was –3.9 (3.3%) and –4.6 (3.8%), respectively, of the actual concentration.

CONCLUSIONS: The accuracy and precision of measurement of volatile anesthetic gas partial pressures in blood by a double headspace equilibration technique, using a clinical infrared gas analyzer, were comparable to that achieved by previous studies using gas chromatography.

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