Carbon Dioxide Measurement

Carbon dioxide (CO₂) is routinely measured in anaesthetic practice: arterial blood gas analysis and capnography. The CO₂ electrode is a modified pH electrode in contact with sodium bicarbonate solution. It is kept separate from the blood sample to be analysed by a semi-permeable membrane. Capnography has become an integral part of monitoring in anaesthesia. Monitors can use infrared spectrography, mass spectrography, Raman spectrography, photoacoustic analysers or colorimetric devices to measure carbon dioxide in the respiratory gases and then give a numerical reading and a waveform. Most anaesthetists would probably request capnography because of the amount of information that can be gained from capnogram analysis: confirmation of endotracheal intubation; detection of rebreathing; detection of obstructive expiratory airflow; detection of inter-breathing in a ventilated patient Sudden fall in end-tidal CO₂ may indicate low systemic blood pressure, circulatory arrest or pulmonary embolism; end-tidal CO₂ provides an estimation of arterial PaCO₂; and detection of malignant hyperpyrexia.