Approach Considerations
End-tidal carbon dioxide (EtCO2) can be monitored in several ways, each of which has its own advantages. Options to be considered include the following:
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Qualitative vs quantitative measurement
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Mainstream vs sidestream capnometry
Monitoring of End-Tidal Carbon Dioxide
Colorimetry
Carbon dioxide colorimetry uses acidic changes in expired air containing carbon dioxide to change colors. Often, these devices are used in the emergency department (ED) or by an emergency medical technician (EMT) to confirm endotracheal tube placement.
Mainstream capnometry
Mainstream capnometry refers to the use of a nondiverting sampling device. This form of capnography is newer to clinical use than sidestream capnography is, with technologic advances allowing the development of smaller and lighter sampling devices. The advantage of this approach is that a real-time display is available during exhalation because the device is part of the breathing circuit and samples at the distal end of the ventilatory circuit. [15]
Sidestream capnometry
Sidestream capnometry refers to the use of a diverting sampling device. This form of capnography has been in use for some time and probably is still the most commonly used approach. During exhalation, a small portion of air is diverted at a T-piece to an external EtCO2 analyzer.
Several disadvantages are noted, including the following [15] :
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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 readout on capnometry
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The air sampled is removed from the circuit and can require increased flow in closed-circuit ventilation
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The T-piece used at the point of sampling is rather small and can be occluded with patient secretions
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Because dead space is added in the circuit, some dead space air mixes with exhaled air; however, the significance is negligible
Portable capnometry
Portable capnometry [18, 19, 20] is newer than the methods previously mentioned and has not yet been as widely used, largely because of limitations imposed by cost and operative parameters.
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Normal breathing.
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Normal mechanical ventilation.
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Hyperventilation.
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Hypoventilation.
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Bronchospasm.
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Exhausted carbon dioxide absorbant.
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Esophageal intubation.
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Capnogram breathing. Courtesy of Pedro Tanaka, MD.
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Curare cleft.
