Respiratory Gas Analysis—Technical Aspects

A technology-focused review of respiratory gas analysis, with an emphasis on carbon dioxide analysis, is presented. The measurement technologies deployed commercially are highlighted, and the basic principles and technical concerns of infrared spectroscopy and mainstream versus sidestream gas sampling are discussed. The specifications of particular interest to the clinician, accuracy and response time, and the related standard, with typical values for a capnometer, are presented. Representative time and volumetric capnograms are shown with the clinically relevant parameters described. Aspects of the terminology in present-day use and the need for clarity in defining what is a breath and an end-tidal value are reviewed. The applications of capnography of particular interest to the anesthesiologist are noted, and key references are provided. Ongoing developments with respect to respiratory gas analysis, and those that will impact it, are noted.