PetCO2 monitoring reflects which of the following?

• A. Arterial CO2
• B. End-tidal CO2
• C. Mixed venous CO2
• D. Inspired CO2

Answer: (B)

Capnography measures end-tidal CO2—the CO2 concentration in the gas at the end of an exhaled breath. This reflects the CO2 content of alveolar gas, which in turn tracks the arterial CO2 under normal conditions, making it a practical, noninvasive proxy for PaCO2. It is not a direct measure of arterial CO2, nor does it represent mixed venous CO2 or inspired CO2. In healthy lungs the PaCO2 is only slightly higher than the end-tidal value (a small gradient), but this gradient can widen with dead space ventilation or ventilation–perfusion mismatch. Clinically, end-tidal CO2 is used to assess ventilation adequacy, detect changes in perfusion, and confirm airway placement among other dynamic monitoring cues.