ACCURACY AND CROSS-SENSITIVITY OF 10 DIFFERENT ANESTHETIC GAS MONITORS

Objective. The objective of this study was to test, the accuracy and c ross-sensitivity of commercially available anesthetic gas monitors. Me thods. Using gas chromatography (GC) as a reference method, the accura cy, cross-sensitivity, and ability to recognize an erroneously selecte d agent were determined in the following 10 monitors for volatile anes thetics: Datex Capnomac Ultima-S, Datex Capnomac, Ohmeda 5330 agent mo nitor, Iris Drager, Andros Drager PM 8020 (all monochromatic, infrared analyzers), Nellcor N-2500E, Criticare POET II, Irina Drager (all pol ychromatic, infrared analyzers), Siemens Servo Gas Monitor 120 (a piez oelectric analyzer), and Bruel & Kjaer Type 1304 (a photoacoustic anal yzer). Accuracy was determined at 0.5, 1, 2, and 4 times the minimal a lveolar concentration (MAC) of either halothane or isoflurane in oxyge n (O2). The cross-sensitivity tests were performed with 70 vol% nitrou s oxide in O2, 5 vol% carbon dioxide in O2, 0.032 vol% alcohol in O2, and 70% water vapor in O2. The photoacoustic analyzer showed a higher accuracy for isoflurane than the polychromatic infrared monitors. The greatest inaccuracy with isoflurane was found in the Iris Drager monit or, which had a maximal bias percentage by volume (vol%) of 0.09 at 0. 5 MAC. (This bias was within the manufacturer's specified tolerance of +/-0.1 vol% or 10% relative difference of reading, whichever is great er.) Irina Drager was the most accurate analyzer with halothane (mean % bias [relative %] +/- SD, 0.9 +/- 2.0%). The greatest bias with halo thane was found in the monochromatic infrared analyzers, with a maxima l % bias at 0.5 MAC of 50.3% of the GC reading (12.4% with a new inner Nafion tube) found in the Datex Ultima monitor. The Siemens gas monit or showed a cross-sensitivity for water vapor (-0.248 vol%). The monoc hromatic infrared analyzers showed a small sensitivity to alcohol (add itional deviation of 0.011 to 0.147 vol% at 2 MAC isoflurane) but no s ensitivity to nitrous oxide. No cross-sensitivity was found in the pol ychromatic infrared and photoacoustic analyzers. An incorrect selectio n of anesthetic agent when using a monochromatic infrared analyzer can be fatal; for example, when using halothane and selecting isoflurane the values measured by the Datex Capnomac monitor were nearly 6 times below the actual value (i.e., 1 vol% ''isoflurane'' on the display = 6 vol% halothane in reality). Conclusions. The photoacoustic measuremen t principle is more accurate than the other methods, although the poly cbromatic infrared analyzers are safer because they detect erroneously selected agents.