Capnography

Capnography
Intervention
Typical capnogram. Expiration phase on the left, inspiration on the right.
MeSH	D019296
[]

Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO
2) in the respiratory gases. Its main development has been as a monitoring tool for use during anesthesia and intensive care. It is usually presented as a graph of expiratory CO
2 (measured in millimeters of mercury, "mmHg") plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired CO
2, 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 CO
2, and an indirect monitor of the CO
2 partial pressure in the arterial blood. In healthy individuals, the difference between arterial blood and expired gas CO
2 partial pressures is very small. 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.

Capnographs usually work on the principle that CO
2 absorbs infrared radiation. A beam of infrared light is passed across the gas sample to fall on a sensor. The presence of CO
2 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 infrared absorption via the phenomenon of collision broadening. This must be corrected for measuring the CO
2 in human breath by measuring its infrared absorptive power. This was established as a reliable technique by John Tyndall in 1864, though 19th and early 20th century devices were too cumbersome for everyday clinical use.