Why is end-tidal CO2 (EtCO2) typically lower than arterial CO2 (PaCO2)?

Why is end-tidal CO2 (EtCO2) typically lower than arterial CO2 (PaCO2)?

• A. Ion-specific electrode error
• B. Alveolar-capillary gradient
• C. One-way values
• D. Alveolar dead space

Answer: (B)

End-tidal CO2 is the CO2 concentration of the last portion of expired gas, reflecting gas in the alveoli at the end of expiration. Arterial CO2 (PaCO2) reflects CO2 carried in the blood after systemic tissue gas exchange. In a perfectly uniform lung, EtCO2 would equal PaCO2, but in reality there is an alveolar-capillary gradient. This arises because not all ventilated alveoli are well perfused and because some gas comes from dead space that isn’t actively exchanging CO2. The end-expired gas is a mixture, dominated by CO2-rich gas from well-perfused units but diluted by CO2-poor gas from dead space. Because CO2 diffuses readily, the gradient is usually small (a few mmHg) but persists, so EtCO2 is typically a bit lower than PaCO2. Larger gradients occur with V/Q mismatch or certain lung pathologies.

End-tidal CO2 is the CO2 concentration of the last portion of expired gas, reflecting gas in the alveoli at the end of expiration. Arterial CO2 (PaCO2) reflects CO2 carried in the blood after systemic tissue gas exchange. In a perfectly uniform lung, EtCO2 would equal PaCO2, but in reality there is an alveolar-capillary gradient. This arises because not all ventilated alveoli are well perfused and because some gas comes from dead space that isn’t actively exchanging CO2. The end-expired gas is a mixture, dominated by CO2-rich gas from well-perfused units but diluted by CO2-poor gas from dead space. Because CO2 diffuses readily, the gradient is usually small (a few mmHg) but persists, so EtCO2 is typically a bit lower than PaCO2. Larger gradients occur with V/Q mismatch or certain lung pathologies.