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

Abstract

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.

Applications and Perspectives of Ultrasonic Multi-Gas Analysis with Simultaneous Flowmetry

“…Piezoelectric transducers can have significant emission delays (10-100 µs) relative to their electric excitation signals, making the multigas calibration protocol of Equations (10) and (11) indispensable. Preliminary ultrasonic studies in Xe/O 2 blends were recently presented [8]. New custom ultrasonic instrumentation is under development to provide simultaneous flowmetry and Monitoring of xenon concentration in anesthesia and to reduce wastage in the post-operative environment, by allowing xenon to be extracted from recovered breathing gas mixtures for recycling.…”