End-Tidal Capnography Technique

Updated: Dec 15, 2015
  • Author: Christopher D Press, MD; Chief Editor: Zab Mosenifar, MD, FACP, FCCP  more...
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Technique

Approach Considerations

End-tidal carbon dioxide can be monitored in several ways, with different advantages. These include the following:

  • Qualitative versus quantitative measurement
  • Mainstream versus sidestream capnometry
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Calorimetry

Carbon dioxide colorimetry uses acidic changes in expired air containing carbon dioxide to change colors. Often, these devices are used in the emergency room or by an EMT to confirm endotracheal tube placement.

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Mainstream Capnometry

Mainstream capnometry refers to a nondiverting sampling device. This form of capnography is relatively new in clinical use, with recent technological advances allowing smaller and lighter sampling devices. The advantage of this mechanism is a real-time display during exhalation because the device is part of the breathing circuit and samples at the distal end of the ventilatory circuit. [8]

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Sidestream Capnometry

Sidestream capnometry refers to a diverting sampling device. This form of capnography has been in use for some time. During exhalation, a small portion of air is diverted at a T-piece to an external end-tidal carbon dioxide analyzer. Sidestream capnometry seems to be the most commonly used because manageable mainstream devices are relatively new to the market.

Several disadvantages are noted. The diversion of air to a distant external device creates extra dead space between the patient and the sampler, causing a delay in time between exhalation and read out on capnometry. The air sampled is removed from the circuit and can require increased flow in closed circuit ventilation. The T-piece used at the point of sampling is rather small and can be occluded with patient secretions. Additionally, because dead space is added in the circuit, some dead space air mixes with exhaled air; however, the significance is negligible. [8]

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Portable Capnometers

This technology is relatively new and not widely used because of limitations in cost and operative parameters.

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