Capnography Technology

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The technology for detecting and measuring the concentration of carbon dioxide (CO₂) in the breathing tube is spectroscopy as illustrated in Figure 1. The measured CO₂ is then displayed on a capnography monitor as shown in Figure 2.

Figure 1. Infrared Spectroscopy

Image Source: S. J. Sundar (2020), Principle of Capnography

According to the illustration, EtCO₂ monitoring determines the concentration of carbon dioxide in exhaled gas using a technology called infrared spectroscopy (Jaffe & O’Donnell, 2020; McArdle & Finlay, 2011). A photodetector measures the amount of infrared light absorbed by gas in the airway during inhalation and exhalation. Because CO₂ molecules absorb specific wavelengths of infrared light, the amount of light absorbed directly reflects the CO₂ concentration in the sampled air.

Let’s review the process again with more detail. End-tidal carbon dioxide (EtCO₂) monitoring measures the concentration of carbon dioxide in exhaled air. This is done using a photodetector that senses how much infrared light is absorbed by airway gases during both inspiration and expiration (Jaffe & O’Donnell, 2020; McArdle & Finlay, 2011.

When CO₂ in the breathing tube passes through the infrared light in the capnograph, the carbon dioxide molecules absorb specific wavelengths of infrared light. The device compares the amount of light sent from the infrared light source to the amount received by the infrared detector. From this, it calculates the CO₂ pressure.

The greater the CO₂ concentration, the more infrared light has been absorbed. The monitor uses this information to calculate a numeric CO₂ value measured in millimeters of mercury and to generate a corresponding capnography waveform, a capnogram.

Figure 2. Capnographs

Figure 2 shows different types of capnography monitors called capnographs. As shown, capnographs may vary in how and where the waveform or capnogram is displayed. On many monitors, you can adjust the location of the EtCO₂ waveform alongside other parameters like the electrocardiogram (ECG) and oxygen saturation (SpO₂). It’s essential to familiarize yourself with your specific monitor before using capnography in the field.

Audio Transcript

Now that we have a base knowledge of capnography, how does it actually work?

The technology for detecting and measuring the concentration of carbon dioxide in the breathing tube is spectroscopy as illustrated in Figure 1. The measured CO₂ is then displayed on a capnography monitor as shown in Figure 2.

Let’s review Figure 1. According to the illustration, EtCO₂ monitoring determines the concentration of carbon dioxide in exhaled gas using a technology called infrared spectroscopy. A photodetector measures the amount of infrared light absorbed by gas in the airway during inhalation and exhalation. Because CO₂ molecules absorb specific wavelengths of infrared light, the amount of light absorbed directly reflects the CO₂ concentration in the sampled air.

Let’s review the process again with more detail.

End-tidal carbon dioxide (EtCO₂) monitoring measures the concentration of carbon dioxide in exhaled air. This is done using a photodetector that senses how much infrared light is absorbed by airway gases during both inspiration and expiration.

When CO₂ in the breathing tube passes through the infrared light in the capnograph, the carbon dioxide molecules absorb specific wavelengths of infrared light. The device compares the amount of light sent from the infrared light source to the amount received by the infrared detector. From this, it calculates the CO₂ pressure. The greater the CO₂ concentration, the more infrared light has been absorbed. The monitor uses this information to calculate a numeric CO₂ value measured in millimeters of mercury and to generate a corresponding capnography waveform, a capnogram.

Figure 2 shows different types of capnography monitors called capnographs. As shown, capnographs may vary in how and where the waveform or capnogram is displayed. On many monitors, you can adjust the location of the EtCO₂ waveform alongside other parameters like the electrocardiogram (ECG) and oxygen saturation (SpO₂). It’s essential to familiarize yourself with your specific monitor before using capnography in the field.

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