Background and Goal of Study:
Xenon (Xe) is increasingly used in anaesthesia with O2 as a balance gas and typically measured using a thermal mass meter (TMM) and fuel cell respectively.[1] Speed of sound (SoS) in a binary gas mixture is related to temperature, pressure and molar masses of the component gases.[2] We adapted an ultrasonic time-of-flight (ToF) flow meter (UFM) to measure Xe in O2 (XeU) with the goal of making a single non-contact device to perform real-time flowmetry and binary gas concentration measurement.

Materials and Methods: 
A bespoke microcontroller based datalogger obtained raw ToF data from the UFM (SpiroCell, UK), from which SoS was calculated and combined with ambient temperature measurements using a polynomial function to derive XeU. XeU was recorded at 0.5 Hz simultaneously with the gold standard of a TMM (GKM03, Russia) (XeT). The apparatus (Fig 1) was flushed with 100% Xe. 20 measurements of XeU and XeT were collected at 100 kPa. The Xe was incrementally diluted and mixed with aliquots of O2 and measurements repeated until XeT ~5%. XeU and XeT were compared using scatter- and Bland-Altman (BA) plots.

Results and Discussion: 
2029 data pairs of XeU and XeT were collected. XeU correlated closely with XeT across the range 5% to 95% (r = 0.989; linear fit equation: XeU = 1.1081 * XeT - 3.528). BA analysis (Fig 2) showed that variability of individual readings (+/-~10%) was too great for clinical use due to a 1.28 Hz bimodal sawtooth oscillation in gain inherent in the UFM. Future studies will investigate and compensate for this.

Conclusion(s): 
Ultrasound ToF can be used to estimate real-time Xe% in O2. With further work it may be possible to make a single device to perform both flowmetry and binary gas concentration measurement to a clinically acceptable degree of accuracy.

References:
[1]  Anesthesiol. 2000;92(3):865-8
[2]  Sensors 2014, 14:11260-76

Real-time measurement of Xenon in a binary gas mixture using ultrasound time-of-flight: a feasibility study