CARBON-DIOXIDE ANALYZERS - ACCURACY, ALARM LIMITS AND EFFECTS OF INTERFERING GASES

Six mainstream and twelve sidestream infrared carbon dioxide (CO2) ana lysers were tested for accuracy of the CO2-display value, alarm activa tion and the effects of nitrous oxide (N2O), oxygen (O-2) and water va pour according to the ISO Draft International Standard (DIS) #9918. Ma instream analysers (M-type): Novametrix Capnogard 1265; Hewlett Packar d HP M1166A (CO2-module HP M1016A); Datascope Passport; Marquette Tram scope 12; Nellcor Ultra Cap N-6000; Hellige Vicom-sm SMU 611/612 ETC. Sidestream analysers: Bruel and Kjaer Type 1304; Datex Capnomac II; Ma rquette MGA-AS; Datascope Multinex; Ohmeda 4700 OxiCap (all type SI: r espiratory cycles not demanded); Biochem BCI 9000; Bruker BCI 9100; Dr ager Capnodig and PM 8020; Criticare Poet II; Hellige Vicom-sm SMU 611 /612 A-GAS (all type S2: respiratory cycles demanded). The investigati ons were performed with premixed test gases (2.5, 5, 10 vol%, error le ss than or equal to 1% rel.). Humidification (37 degrees C) of gases w ere generated by a Drager Aquapor. Respiratory cycles were simulated b y manually activated valves. All monitors complied with the tolerated accuracy bias in CO2 reading (less than or equal to 12% or 4 mmHg of a ctual test gas value) for wet and dry test gases at all concentrations , except that the Marquette MGA-AS exceeded this accuracy limit with w et gases at 5 and 10 vol% CO2. Water condensed in the metal airway ada pter of the HP M1166A at 37 degrees C gas temperature but not at 30 de grees C. The Servomex 2500 (nonclinical reference monitor), Passport ( M-type), Multinex (S1-type) and Poet II (S2-type) showed the least bia s for dry and wet gases. Nitrous oxide and O-2 had practically no effe ct on the Capnodig and the errors in the others were max. 3.4 mmHg, st ill within the tolerated bias in the DIS (same as above). The differen ce between the display reading at alarm activation and the set point w as in all monitors (except in the Capnodig: bias 1.75 mmHg at 5 vol% C O2) below the tolerated limit of the DIS (difference less than or equa l to 0.2 vol%). The authors conclude that the tested monitors are safe for clinical use (except those failing the DIS limits). The accuracy of the CO2-reading (average of mean absolute bias) is better in the M- type than in the S1- or S2-type analysers although no statistical (nor clinical) significant differences could be detected. Most manufacture rs work with stricter limits than those proposed by the DIS.