Kapnometrie im Luftrettungsdienst - Experimentelle Untersuchungen zur Genauigkeit von drei CO2-Analysatoren in der Unterdruckkammer

AIM OF THE STUDY: The atmospheric pressure (pB) changes are much more prominently in an airplane than on the ground. Since CO2 measurements are influenced by pB changes we must ask whether accurate results are at all possible under such conditions. In our study we aimed at exposing three selected CO2 analysers to flight-specific pB changes in a hypobaric chamber while determining their accuracy and precision during such changes. METHODS: We tested Normocap CD 200 (Datex); 5250 RGM (Ohmeda) and Capnodig (Drager). Precision was tested by means of test gases having defined CO2 concentrations. The CO2 values showing up on the monitors in mmHg and vol% were compared with the relevant set values and the deviations dF (vol%) and dP (mmHg) determined. During the first flight simulation (profile I) the pressure value changed with 500 ft/min. Measurements were performed at ground level, at 4000, 6000 and 8000 ft pressure levels and again at ground level, the capnometers running continuously all the time. During a second simulation (profile II) the instruments were switched off after the 8000 ft pressure level had been attained and were then re-started. RESULTS: During simulation according to profile I, Capnodig showed dF increases with increasing pressure levels, whereas with CD 200 both dF and dP increased (dF and dP at 8000 ft with test gas containing 5.0% CO2: -0.9 vol% and +1.0 mmHg [Capnodig] or -0.3 vol% and +6mmHg [CD 200], respectively). 5250 RGM failed to indicate any numeric values after leaving the ground level. Profile II: after restarting at 8000 ft both 5250 RGM and CD 200 gave readings of satisfactory precision. Directly after the ground level pressure had been reattained, CD 200 did not yield any numeric values, whereas 5250 RGM had a dP of -6.5 mmHg (test gas with 5 vol% CO2). The results show that CO2 values indicated by such instruments should not be accepted indiscriminately. Only one of the 3 instruments tested by us was capable of long-term reproducibility. CONCLUSION: It appears that the compensating mechanisms with which capnometers are provided to balance out weather-conditioned pB variations are not directly capable of coping with rapid and pronounced pB changes. Hence, it is recommended to at least roughly test the flight characteristics of an analyser by means of a test gas and by taking the cockpit pressure into consideration before using the instrument within the framework of air rescue service flights.