Real-Time Measurement of Xenon Concentration in a Binary Gas Mixture Using a Modified Ultrasonic Time-of-Flight Anesthesia Gas Flowmeter: A Technical Feasibility Study

BACKGROUND: Xenon (Xe) is an anesthetic gas licensed for use in some countries. Fractional concentrations (%) of gases in a Xe:oxygen (O2) mixture are typically measured using a thermal conductivity meter and fuel cell, respectively. Speed of sound in such a binary gas mixture is related to fractional concentration, temperature, pressure, and molar masses of the component gases. We therefore performed a study to assess the feasibility of developing a novel single sterilizable device that uses ultrasound time-of-flight to measure both real-time flowmetry and fractional gas concentration of Xe in O2. METHODS: For the purposes of the feasibility study, we adapted an ultrasonic time-of-flight flowmeter from a conventional anesthetic machine to additionally measure real-time fractional concentration of Xe in O2. A total of 5095 readings of Xe % were taken in the range 5%–95%, and compared with simultaneous measurements from the gold standard of a commercially available thermal conductivity Xe analyzer. RESULTS: Ultrasonic measurements of Xe (%) showed agreement with thermal conductivity meter measurements, but there was marked discontinuity in the middle of the measurement range. Bland-Altman analysis (95% confidence interval in parentheses) yielded: mean difference (bias) 3.1% (2.9%–3.2%); lower 95% limit of agreement −4.6% (−4.8% to −4.4%); and upper 95% limit of agreement 10.8% (10.5%–11.0%). CONCLUSIONS: The adapted ultrasonic flowmeter estimated Xe (%), but the level of accuracy is insufficient for clinical use. With further work, it may be possible to develop a device to perform both flowmetry and binary gas concentration measurement to a clinically acceptable degree of accuracy. Accepted for publication August 17, 2018. Funding: None. Conflicts of Interest: See Disclosures at the end of the article. Some preliminary data from the first part of this study were presented as a poster at the European Society of Anaesthesia Annual Conference, Geneva, Switzerland, June 5, 2017. Reprints will not be available from the authors. Address correspondence to David J. Williams, FRCA, Department of Anaesthetics, Morriston Hospital, Swansea SA6 6NL, United Kingdom. Address e-mail to davidjwilliams@doctors.org.uk. © 2018 International Anesthesia Research Society

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