Capno 101: How does capnography work?
Capnography is exceptional both for its utility in the field, and for the ease of its use. In a recent survey taken of EMS1 readers, 95 percent of respondents agreed or strongly agreed that capnography is easy to use and an important monitoring tool for many different situations in the field.
The essential mechanism of capnography is basic. It is grounded on the property that carbon dioxide (CO2) absorbs infrared radiation. When the patient exhales, a beam of infrared light is passed over the gas sample on a sensor. The presence or lack of CO2, is inversely indicated by the amount of light that passes through the sensor. High CO2 levels are indicated by low infrared, and low CO2 levels result in high amounts of light.
The CO2 levels are then displayed on the detector and adjusted for the presence of nitrous oxide, which can also absorb infrared rays and thus can cause some confusion if not accounted for. What is measured by the capnograph is known as the End Tidal CO2 (ETCO2) or the amount of (or partial pressure of) the carbon dioxide released at the end of expiration, an essential component of measuring cardiac output.
A normal capnography waveform looks like *image*. The length of the wave represents the time, while the height of the wave represents the amount of CO2 in the exhaled breath. Thus faster breathing is represented by a relatively short duration of the waveform, whereas slower breathing is shown with longer waveforms. On the other axis, the taller the waveform, the more ETCO2 is in the breath. Inspiration, therefore, is shown on the waveform by a drop of the CO2 levels to zero.
The process is relatively simple, as is the equipment. There are two types of capnography, mainstream and sidestream. Mainstream capnography is characterized as invasive and non-diverting, meaning that the measurement of the ETCO2 is done at the airway, or the sample site, thus providing a real-time measurement of the carbon dioxide pressure.
Sidestream, on the other hand, is non-invasive and diverting. This means that the gas sample is transported from the sample site through a plastic tube and analyzed in a sample cell. This results in a couple seconds delay of the analysis and a possible distortion of the analysis, which can be consequential in an emergency situation.
However, the non-invasive nature of sidestream capnography means that it can be easily used on non-intubated patients, unlike mainstream capnography, which can be difficult to adapt to non-intubated patients. In addition, mainstream capnography can be prone to technological difficulties and fragility, which is less of an issue with sidestream capnography.
Ultimately, capnography is a simple process that is useful both in emergency settings and in the hospital.
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