RELIABILITY AND SOURCE OF ERRORS IN END-TIDAL GAS CONCENTRATION EVALUATION ALGORITHMS DURING AVALANCHE SNOW AND REBREATHING EXPERIMENTS

Abstract

During breathing experiments in avalanche snow, measurement of CO₂ is often conducted in order to monitor the volunteers or as an endpoint of the trials. From the measured CO₂ curves, monitors calculate end-tidal CO₂ concentrations (EtCO₂). The aim of the study is to investigate, whether Datex-Ohmeda S/5 anesthesia monitor evaluates EtCO₂ and other parameters of breathing gas correctly, otherwise to characterize the occurrence of the error and to find out a possible cause of error. Data from a real experiment aimed at investigation of work of breathing into snow in the presence and absence of an artificial air pocket were used to study precision of the monitor. The data were evaluated in Matlab environment. The analysis found that the average error of EtCO₂ evaluation 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. The error occurs immediately after a significant increase of CO₂ in the breathing circuit as a consequence of expired gas rebreathing and is independent on other breathing parameters related to a change in breathing pattern. The experiment confirmed that a newer model CARESCAPE B650 monitor is prone to this error as well. The last experiment conducted with a standard anesthesia machine but with removed CO₂ absorbing soda lime confirmed, that the error occurs even in a standard clinical setup, which might cause a wrong diagnoses or might harm a volunteer or a patient.