End-tidal CO2 measurement is taken at the end of which phase of the respiratory cycle?

• A. End of inspiration
• B. End of expiration
• C. Beginning of inspiration
• D. Mid-expiration

Answer: (B)

End-tidal CO2 is the CO2 level of alveolar gas at the very end of expiration. As you exhale, gas from the dead space (airways) contains little CO2, and gradually the gas from the alveoli—where gas exchange occurs—dominates. The CO2 concentration rises and then levels off during the expiratory phase; the value read at the end of this expiratory plateau reflects the alveolar gas and closely approximates arterial CO2. Measuring at the end of expiration provides a stable, representative snapshot of CO2 in the lungs. If you look at the end of inspiration you’d be sampling fresh, CO2-free air; mid-expiration still mixes dead-space and alveolar gas, so it wouldn’t represent alveolar CO2 as well.