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The use of gas and vapour analysers is mandatory in the delivery of safe anaesthesia. An understanding of the principles used and their limitations is essential. In this short review, we describe the principles of the commonly used analysers. Some of the clinical applications are also described.
A Raman spectrometer multiple gas analyzer was used to monitor inspired and expired concentrations of oxygen (O2), nitrogen (N2), carbon dioxide (CO2), nitrous oxide (N2O), halothane, and isoflurane in 10 patients. The Raman spectrometer and a dedicated mass spectrometer were connected to each patient to provide a comparison of the two instruments. Results show that readings from the Raman spectrometer are within 0.62 vol% of known gas standards for O2, N2, N2O; within 0.03 vol% for CO2; and within 0.04 vol% for halothane, enflurane and isoflurane. Clinical results show that Raman spectrometer readings are within 1.36 vol% of the mass spectrometer readings for O2, N2, N2O; within 0.01 vol% for CO2; and within 0.22 vol% for halothane and isoflurane. The clinical and laboratory results indicate the Raman spectrometer monitors airway gases and vapors as accurately as a dedicated mass spectrometer.
Contemporary multigas analyzers determine anesthetic gas concentrations using (near) infrared analysis at either 3.3 or 8-9 microns. Methane also absorbs infrared light at 3.3 microns, but not at 8-9 microns. Consequently, erroneous anesthetic agent readings may result when methane is present in the circuit (e.g. during closed circuit anesthesia), potentially compromising patient safety. We have analyzed in laboratory conditions the influence of different known methane concentrations (100, 500 and 1000 ppm) on the gas-analysis readings provided by some clinical monitoring devices that use infrared absorption for the measurement of inhalation anesthetic concentration. At 3.3 microns wavelength the influence on the measurement of halothane was important, whereas the influence on that of enflurane and isoflurane was less pronounced. For desflurane and sevoflurane measurements, the influence of methane at 3.3 microns wavelength proved to be minimal. At higher wavelengths (8-9 microns) no influence of methane could be demonstrated.