The reason CO2 measured by capnometer is less than the arterial PaCO2 value measured simultaneously is?

The reason CO2 measured by capnometer is less than the arterial PaCO2 value measured simultaneously is?

• A. Use of ion specific electrode for blood gas determination
• B. Alveolar capillary gradient
• C. One way values
• D. Alveolar dead space

Answer: (D)

End-tidal CO2 from capnography reflects the CO2 content of the alveolar gas that is being exhaled, i.e., CO2 from well-ventilated and perfused areas of the lung. Arterial PaCO2, on the other hand, represents the CO2 carried in the blood after systemic exchange. When there is alveolar dead space—ventilated but not perfused alveoli—these units do not contribute CO2 to the blood or to the exhaled gas. Their gas is relatively CO2-poor, so when it blends with CO2-rich alveolar gas in expiration, the end-tidal CO2 becomes lower than the arterial PaCO2. The larger the dead space, the greater the drop in ETCO2 relative to PaCO2. This is why capnography shows a smaller CO2 value than the arterial measurement. The other options don’t explain this discrepancy: the measurement method of blood gas electrodes doesn’t create the gap, the alveolar-capillary gradient concept isn’t the driving reason for a lower ETCO2, and “one-way values” isn’t related to gas exchange physiology.

End-tidal CO2 from capnography reflects the CO2 content of the alveolar gas that is being exhaled, i.e., CO2 from well-ventilated and perfused areas of the lung. Arterial PaCO2, on the other hand, represents the CO2 carried in the blood after systemic exchange. When there is alveolar dead space—ventilated but not perfused alveoli—these units do not contribute CO2 to the blood or to the exhaled gas. Their gas is relatively CO2-poor, so when it blends with CO2-rich alveolar gas in expiration, the end-tidal CO2 becomes lower than the arterial PaCO2. The larger the dead space, the greater the drop in ETCO2 relative to PaCO2. This is why capnography shows a smaller CO2 value than the arterial measurement. The other options don’t explain this discrepancy: the measurement method of blood gas electrodes doesn’t create the gap, the alveolar-capillary gradient concept isn’t the driving reason for a lower ETCO2, and “one-way values” isn’t related to gas exchange physiology.