In patients with increased dead space, end-tidal CO2 (EtCO2) tends to do what relative to arterial CO2 (PaCO2)?

In patients with increased dead space, end-tidal CO2 (EtCO2) tends to do what relative to arterial CO2 (PaCO2)?

• A. Overestimate PaCO2
• B. Equals PaCO2
• C. Underestimate PaCO2
• D. Unrelated to PaCO2

Answer: (C)

The main idea is that end-tidal CO2 reflects the CO2 of alveolar gas from well‑perfused units, while arterial CO2 represents the overall blood CO2 that accumulates when elimination is imperfect. When dead space increases, a larger fraction of each breath does not participate in gas exchange. The air coming from dead space dilutes the CO2-rich gas from functional alveoli, so the CO2 measured at the end of expiration (EtCO2) is lower than the arterial CO2 (PaCO2). As dead space grows, this discrepancy widens, so EtCO2 underestimates PaCO2. This is why capnography shows a smaller EtCO2 relative to PaCO2 in conditions with high dead space, such as pulmonary embolism or significant V/Q mismatch.

The main idea is that end-tidal CO2 reflects the CO2 of alveolar gas from well‑perfused units, while arterial CO2 represents the overall blood CO2 that accumulates when elimination is imperfect. When dead space increases, a larger fraction of each breath does not participate in gas exchange. The air coming from dead space dilutes the CO2-rich gas from functional alveoli, so the CO2 measured at the end of expiration (EtCO2) is lower than the arterial CO2 (PaCO2). As dead space grows, this discrepancy widens, so EtCO2 underestimates PaCO2. This is why capnography shows a smaller EtCO2 relative to PaCO2 in conditions with high dead space, such as pulmonary embolism or significant V/Q mismatch.