What is Capnography?

Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO₂) in the respiratory gases. Its main development has been as a monitoring tool for use during anaesthesia and intensive care. It is usually presented as a graph of expiratory CO₂ plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired CO₂, which is of interest when rebreathing systems are being used.

The capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of CO₂, and an indirect monitor of the CO₂ partial pressure in the arterial blood. In healthy individuals, the difference between arterial blood and expired gas CO₂ partial pressures is very small, and is probably zero in children. In the presence of most forms of lung disease, and some forms of congenital heart disease (the cyanotic lesions) the difference between arterial blood and expired gas increases and can exceed 1 kPa.

During anaesthesia, there is interplay between two components: the patient and the anaesthesia administration device (which is usually a breathing circuit and a ventilator or respirator). The critical connection between the two components is either an endotracheal tube or a mask, and CO₂ is typically monitored at this junction. Capnography directly reflects the elimination of CO₂ by the lungs to the anaesthesia device. Indirectly, it reflects the production of CO₂ by tissues and the circulatory transport of CO₂ to the lungs.

When expired CO₂ is related to expired volume rather than time, the area beneath the curve represents the volume of CO₂ in the breath, and thus over the course of a minute, this method can yield the CO₂ minute elimination, an important measure of metabolism. Sudden changes in CO₂ elimination during lung or heart surgery usually imply important changes in cardiorespiratory function.

Capnographs usually work on the principle that CO₂ absorbs infra-red radiation. A beam of infra-red light is passed across the gas sample to fall on to a sensor. The presence of CO₂ in the gas leads to a reduction in the amount of light falling on the sensor, which changes the voltage in a circuit. The analysis is rapid and accurate, but the presence of nitrous oxide in the gas mix changes the infra-red absorption via the phenomenon of collision broadening. This must be corrected for.