The invention relates to a method and apparatus for determination of the partial pressure of carbon dioxide in a patient's arterial blood. The method and apparatus are particularly useful when the patient is under anaesthesia. The carbon dioxide concentration is measured at the end of the patient's expiration. The nitrous oxide concentration is also measured, so that the measured values of carbon dioxide concentration can be corrected in accordance with the measured values for nitrous oxide concentration. Special detecting wavelengths are described together with other parameters of the apparatus for achieving a high level of precision. IMAGE
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