End-tidal carbon dioxide (CO2) monitors measure the pCO2 of which gas?

• A. Exhaled gas at end expiration
• B. Gas mid-respiratory cycle
• C. Inspired gas
• D. Exhaled gas throughout the respiratory cycle

Answer: (A)

End-tidal CO2 monitors measure the partial pressure of CO2 in the gas at the end of expiration. At that moment, the gas exhaled is predominantly alveolar gas that has participated in gas exchange, so its CO2 content closely reflects arterial CO2. This is why the end-expiratory portion is used as the representative value. Gas sampled mid-expiration is diluted by dead space air and isn’t as representative, and inspired gas has little to no CO2. Sampling CO2 throughout the entire exhalation would mix early dead-space gas with alveolar gas, giving a less accurate read.