The EtCO2 value is best defined as:

• A. The measurement of CO2 at the end of a tidal volume exhalation
• B. The average arterial CO2 pressure
• C. The amount of CO2 inhaled per breath
• D. The inspiratory CO2 level

Answer: (A)

End-tidal CO2 is the CO2 level measured at the end of a exhaled breath, capturing the gas that comes from the alveoli just before inhalation begins again. This moment provides the most representative sample of alveolar CO2, since it reflects gas that has participated in gas exchange rather than gas in dead space. Capnography reads this value, giving a partial pressure (often in mmHg) or a percent concentration that clinicians use to assess ventilation. In healthy lungs, EtCO2 tracks the arterial CO2 pressure (PaCO2) but is usually a few mmHg lower because some exhaled gas comes from poorly perfused or dead-space pockets. The other options describe different things: arterial CO2 pressure is measured in blood, the amount of CO2 inhaled per breath refers to inspired CO2, and inspiratory CO2 level is typically near zero in room air or fresh gas mixtures. Practically, EtCO2 is a key indicator of how well ventilation and perfusion are working and is used for monitoring during anesthesia, critical care, and CPR.