End-tidal CO2 represents which of the following?

End-tidal CO2 represents which of the following?

• A. The CO2 concentration at end of expiration
• B. The CO2 concentration at start of inspiration
• C. The arterial CO2 pressure
• D. The alveolar CO2 plateau

Answer: (A)

End-tidal CO2 is the CO2 concentration in the air exhaled at the very end of expiration. This value comes from the alveolar gas just before the next breath, so it reflects how effectively CO2 is being eliminated from the blood. A capnography waveform reaches its peak at the end of expiration, and the numeric EtCO2 corresponds to that end-point. It closely relates to arterial CO2 because alveolar CO2 equilibrates with the blood, making EtCO2 a useful noninvasive surrogate for PaCO2, though it can be lower than PaCO2 in cases of dead space ventilation or ventilation-perfusion mismatch. It is not the CO2 concentration at the start of inspiration, nor a direct arterial pressure, and while the alveolar CO2 plateau forms the waveform, the reported value is the end-of-expiration CO2.

End-tidal CO2 is the CO2 concentration in the air exhaled at the very end of expiration. This value comes from the alveolar gas just before the next breath, so it reflects how effectively CO2 is being eliminated from the blood. A capnography waveform reaches its peak at the end of expiration, and the numeric EtCO2 corresponds to that end-point. It closely relates to arterial CO2 because alveolar CO2 equilibrates with the blood, making EtCO2 a useful noninvasive surrogate for PaCO2, though it can be lower than PaCO2 in cases of dead space ventilation or ventilation-perfusion mismatch. It is not the CO2 concentration at the start of inspiration, nor a direct arterial pressure, and while the alveolar CO2 plateau forms the waveform, the reported value is the end-of-expiration CO2.