We all know that oxygen is necessary for life and that oxygen goes through a series of steps before reaching cells, which use oxygen and produce carbon dioxide (CO2) as a waste product. Carbon dioxide, like oxygen, goes through a series of steps before it is excreted. Capnography monitors the CO2 emitted. This is a very useful measurement because it can help detect problems along the CO2 path. For example, if the patient stops breathing, the CO2 will not be able to be "exhausted". This problem will cause the capnometer to display a low CO2 reading and trigger an alarm, alerting medical personnel to the problem.
Capnography monitors use infrared waves to measure carbon dioxide. Infrared rays are waves that are invisible to the naked eye and have lower frequencies than visible light. The frequency is lower than that of red light, which is why it is called "infrared". Infrared light is absorbed by gases with "two or more different atoms". Oxygen has two atoms that are indistinguishable. Therefore, oxygen does not absorb infrared rays. Unlike oxygen, carbon dioxide gas has different atoms. Therefore, since carbon dioxide gas has different atoms, it absorbs infrared waves. In fact, in addition to CO2, infrared absorption can also be used to measure other gases with different atoms in their structure. For example, infrared can be used to measure nitrous oxide and isoflurane.
The basic principle of capnography is based on infrared and Beer Lambert's law. This is a combination of two laws, Beer's Law and Lambert's Law. The amount of infrared absorption is proportional to the concentration of infrared absorbing substances. capnography monitors work on this principle. The more carbon dioxide present, the more infrared light is absorbed. The basic analyzer system consists of an infrared source, a sample chamber, and a detector. The more carbon dioxide there is, the more infrared waves are absorbed.
Capnography is main stream or side stream. In a "mainstream" analyzer, the analyzer is placed directly in close proximity to the carbon dioxide exhaled by the patient. Mainstream analyzers are "attached" to the patient. The analyzer is connected to the monitor by a long wire. In a "side stream" analyzer, the analyzer is located away from the CO2 exhaled by the patient. An elongated tube is attached to the patient end.