Authors: Karel Roubík, Jan Filip


Roubik, K., Filip J.: Reliability and source of errors in end-tidal gas concentration evaluation algorithms during avalanche snow and rebreathing experiments. Lekar a Technika 2017, 47(3), pp.72-80.

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Published in Lékař a Technika – Clinician and Technology.


During breathing experiments in avalanche snow, measurement of CO2 is often conducted in order to monitor the volunteers or as an endpoint of the trial. From the measured CO2 signal, monitors calculate end-tidal CO2 concentrations (EtCO2). The aim of the study was to investigate several related points: to determine if the Datex-Ohmeda S/5 anesthesia monitor evaluates EtCO2 and other parameters of breathing gas correctly, to characterize the frequency and magnitude of error and to determine the possible cause of the error. Data from a previous experiment aimed at investigation of work of breathing into snow in the presence and absence of an artificial air pocket were used to study accuracy of the monitor. The analysis found that an error of EtCO2 occurred in 39% and in 30% of the total experimental time of breathing, with and without the air pocket respectively (range from 13% to 93% of time). Breathing experiments with simulated snow were conducted in order to find the cause of the error. We determined the error occurs immediately after a significant increase of CO2 in the breathing circuit as a consequence of expired gas rebreathing and is independent of other breathing parameters. The study confirmed that a newer model monitor (CARESCAPE B650) is prone to this error as well. The last experiment conducted with a standard anesthesia machine confirmed, that the error occurs even in a standard clinical setup in the presence of rebreathing. This problem might result in improper actions and could potentially result in harm to a volunteer or a patient.


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