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The measurement of carbon dioxide (CO2) was first developed in the early 1900s; however, it was complex and of limited clinical use.[1] On the most basic level, end-tidal CO2 (ETCO2) detectors measure the exhaled concentration of carbon dioxide. The concentration of exhaled CO2 correlates to the arterial concentration of CO2, although multiple factors can impact the relationship. Technological advances have allowed CO2 to become a common measurement in healthcare with many clinical applications. Although CO2 detectors ultimately measure exhaled CO2, some units have design differences. Detectors can be divided into quantitative and qualitative types. Qualitative detectors demonstrate the presence or absence of CO2. The most basic unit of this type is the colorimetric detector, producing a color change as the gas flows through a litmus paper film in the airway circuit. Qualitative detectors are primarily used to initially confirm the proper placement of an endotracheal tube by demonstrating the presence (connoting successful placement in the trachea) or lack (esophageal placement) of exhaled CO2. These detectors are usually elementary, light, and portable devices that do not require power. Quantitative detectors measure the level of CO2 that is exhaled and, therefore, provide more detailed information. Qualitative detection can be further subdivided into capnography and capnometry. Capnography produces a waveform in a graphic format, while capnometry provides a numeric display of the CO2 concentration. Waveform capnography provides more information to the clinician. Alterations of the waveform morphology can be interpreted to give clues to the clinical situation. Their location in the airway circuit is also used to categorize detectors. These are typically divided into mainstream and sidestream devices. Mainstream detectors utilize an IR sensor positioned directly within the gas flow path and produce real-time graphs of the CO2 waveform. These tend to be more costly units used primarily for intubated patients. Sidestream detectors are located off the main path of flow in the airway circuit. Sidestream detectors often use small-diameter tubing leading to a detector that allows gas sampling. Nasal monitoring of end-tidal CO2 typically makes use of this configuration.

Their location in the airway circuit is also used to categorize detectors. These are typically divided into mainstream and sidestream devices. Mainstream detectors utilize an IR sensor positioned directly within the gas flow path and produce real-time graphs of the CO2 waveform. These tend to be more costly units used primarily for intubated patients. Sidestream detectors are located off the main path of flow in the airway circuit. Sidestream detectors often use small-diameter tubing leading to a detector that allows gas sampling. Nasal monitoring of end-tidal CO2 typically makes use of this configuration. There are some pitfalls and considerations when using CO2 detectors. Fluids such as vomitus, frothy secretions from a patient experiencing acute pulmonary edema, sputum from pneumonia, or moisture can render CO2 detectors ineffective. The detector paper in colorimetric units is particularly sensitive to fluids. Mainstream detectors are located directly in the path of the airway circuit and, therefore, increase the volume of undesirable dead space. On the other hand, Sidestream detectors may experience a delay compared to mainstream detectors because of the remote location of the sensor.

Nurses, respiratory therapists, and clinicians must be aware of the pitfalls and considerations when using CO2 detectors. an interprofessional team approach to monitoring patients using these devices produces the best patient outcomes.