measure carbon dioxide production, a measure of metabolism. Increased production is seen during fever and shivering. Reduced production is seen during anesthesia and hypothermia. Working mechanism Capnographs work on the principle that is a polyatomic gas and therefore absorbs infrared radiation. A beam of infrared light is passed across the gas sample to fall on a sensor. The presence of 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 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. Today, technologies have since improved and are able to measure the values of near instantaneously and has become a standard practice in medical settings. There are currently two main types of sensors that are used in clinical practice: main-stream sensors and side-stream sensors. Both effectively serve the same function to quantify the amount of that is being exhaled in each breath. Capnogram model The capnogram waveform provides information about various respiratory and cardiac parameters. The capnogram double-exponential model attempts to quantitatively explain the relationship between respiratory parameters and the exhalatory segment of a capnogram waveform. According to the model, each exhalatory segment of capnogram waveform follows the analytical expression: where represents the partial pressure of carbon dioxide measured by the capnogram as a function of time since the beginning of exhalation. represents the alveolar partial pressure of carbon dioxide. represents the inverse of the dead space fraction (i.e. the ratio of tidal volume to dead space volume). represents the pulmonary time constant (i.e. the product of pulmonary resistance and compliance) In particular, this model explains the rounded "shark-fin" shape of the capnogram observed in patients with obstructive lung disease. See also Integrated pulmonary index Medical equipment Medical test Respiratory monitoring Colorimetric capnography Citations External links CapnoBase.org: Respiratory signal database that contains clinical and simulated capnogram recordings Anesthesia Breath tests Diagnostic emergency medicine Diagnostic intensive care medicine Diagnostic pulmonology