Measurement of gas concentrations: oxygen, carbon dioxide, nitrogen, nitrous oxide and volatile anaesthetic agents

The measurement of the gas concentrations used during anaesthesia and in the intensive care environment is standard in modern anaesthesia practice. This is done using different gas analysers. It is important to understand the basic principles of the measurement techniques for proper interpretation of the clinical data. In this article, the basic principles of the gas analysers in most common use during anaesthesia and intensive care practice are described. Some of the instruments used mainly for research purposes are also described. Techniques for measuring oxygen, including the polarographic electrode, the fuel cell and the paramagnetic analyser, are discussed in detail, including their advantages, disadvantages and limitations. For carbon dioxide measurement, the infrared analyser, molecular correlation spectrography and photoacoustic spectrography are discussed. Finally, the measurement of inhalational anaesthetic agents by mass spectrometry, Raman spectroscopy, refractometry and gas chromatography in addition to the infrared analyser are explained. Many less known measurement techniques are mentioned where appropriate.