In the Vd/Vt equation, what does PECO2 represent?

In the Vd/Vt equation, what does PECO2 represent?

• A. Partial pressure of end-tidal CO2
• B. Partial pressure of arterial CO2
• C. Alveolar CO2 partial pressure
• D. Inspired CO2

Answer: (A)

PECO2 is the partial pressure of CO2 in the gas exhaled at the end of expiration—the end-tidal CO2. This value comes from the alveolar gas and is what capnography reports as PETCO2. In the Vd/Vt (dead space to tidal volume) calculation, the end-tidal CO2 serves as a practical, noninvasive estimate of alveolar CO2 (PACO2). When ventilation-perfusion is well matched, PECO2 closely approximates PACO2, allowing the dead-space fraction to be estimated from the Bohr relationship. If dead space ventilation is increased, the end-tidal CO2 tends to drop relative to arterial CO2, reflecting more CO2 being delivered to non-gas-exchanging spaces.

PECO2 is the partial pressure of CO2 in the gas exhaled at the end of expiration—the end-tidal CO2. This value comes from the alveolar gas and is what capnography reports as PETCO2. In the Vd/Vt (dead space to tidal volume) calculation, the end-tidal CO2 serves as a practical, noninvasive estimate of alveolar CO2 (PACO2). When ventilation-perfusion is well matched, PECO2 closely approximates PACO2, allowing the dead-space fraction to be estimated from the Bohr relationship. If dead space ventilation is increased, the end-tidal CO2 tends to drop relative to arterial CO2, reflecting more CO2 being delivered to non-gas-exchanging spaces.