Capnography is the measurement of the partial pressure of CO₂ during breathing. It is a non-invasive method that provides information on ventilation, perfusion and metabolism, all of which are extremely important for airway management[1].
If you would like to learn more about capnography, you can read our pages
‘What is Capnography?’ and
‘What is Waveform Capnography?’.
Infrared light is absorbed by CO₂ particles during capnography. The amount of CO₂ present determines how much infrared is absorbed, which indicates patients' breathing quality. In order to get EtCO₂ readings, there needs to be an infrared source, a sampling chamber that allows infrared to pass through, and an infrared detector.
The infrared travels into the sampling chamber, where it is absorbed by the CO₂ and the level of infrared left is measured by the infrared detector. The more CO₂ present, the more infrared that is absorbed, and therefore, the amount of infrared detected will be lower. Conversely, the lower the level of CO₂ in the sampling chamber, the higher the amount of infrared that will be detected. Therefore, the lower the amount of infrared detected by the source is an indication that there is a higher amount of CO₂ present.
There are a few factors that can affect the efficacy of EtCO₂ monitoring, which are important to address to optimise patient care.
Water vapour can interfere with CO₂ analysis, which leads to errors, so it’s important that it is avoided. Water should be prevented from entering the sampling site, but an error might also occur because of moisture from the patient’s expired breath condensing onto the analyser.
Transit time is the time it takes for the CO₂ to get to the detector. The longer it takes affects whether the results are in ‘real time’ or if they are slightly delayed.
Historically, there are two types of capnography, sidestream and mainstream, but Microstream™ capnography has recently become more prevalent in healthcare settings. The main difference between the different types is whether the monitor is diverting or non-diverting.
Sidestream capnography is diverting, meaning that it transports a portion of CO₂ to the sampling site, located in the main unit. One option for sidestream is to aspirate the CO₂ via a sampling tube attached to a t-piece adapter located in the endotracheal tube and breathing circuit. Alternatively, some models do not require an endotracheal tube.
Microstream™ is the latest type of sidestream capnography. It uses laser-based molecular correlation spectroscopy as the infrared emission source, creating wavelengths that match CO₂. As a result, it matches EtCO₂ exactly, without the need for a sensor at the airway. The main difference between Microstream™ and the other type of capnography is that it requires less breath to take a reading. Microstream™ capnography works on a minimal breath sample of 15ml per minute; this makes it a good option for children as they have less breath strength.
Mainstream capnography is a non-diverting method of capnography, where the sample is tested at the source, meaning that the sensor is located between the endotracheal tube and breathing circuit[1]. The sensor, consisting of the sample cell and the infrared bench, is placed at the airway, resulting in a ‘crisp’ graphical representation of the varying CO₂ values, known as a capnogram, that reflects the partial pressure of CO₂ in the airway in real time. Mainstream capnography is mostly used by healthcare professionals in the operating theatre.
As the above has demonstrated, capnography can prove to be a complex subject area and there is a lot of information to be gleaned. If you would like to find out more about what capnography is and how capnograms can be used to identify issues in patients, visit our Knowledge Hub, or get in contact with us.
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