What is the correct formula for Vd/Vt expressed as a percentage?

What is the correct formula for Vd/Vt expressed as a percentage?

• A. [(PaCO2 - PECO2) / PaO2] x 100
• B. [(PaCO2 - PECO2) / PaCO2] x 100
• C. [(PaCO2 - PECO2) / PaCO2] x 10
• D. [(PaCO2 - PECO2) / PaCO2] x 1000

Answer: (B)

This is about the dead space fraction of each breath—the portion of tidal volume that does not participate in gas exchange. The correct form uses the difference between the CO2 tension in the arterial (PaCO2) and the CO2 tension in the expired air (PECO2) in the numerator. If there were no dead space, expired CO2 would match arterial CO2 and the difference would be zero. If some air bypasses the gas-exchange surfaces, the expired CO2 is lower, increasing the difference and the dead space fraction. Dividing that difference by PaCO2 and multiplying by 100 expresses this as a percentage of tidal volume: [(PaCO2 - PECO2) / PaCO2] × 100. Using PaO2 would not reflect CO2 elimination, and multiplying by 10 or 1000 would distort the scale.

This is about the dead space fraction of each breath—the portion of tidal volume that does not participate in gas exchange. The correct form uses the difference between the CO2 tension in the arterial (PaCO2) and the CO2 tension in the expired air (PECO2) in the numerator. If there were no dead space, expired CO2 would match arterial CO2 and the difference would be zero. If some air bypasses the gas-exchange surfaces, the expired CO2 is lower, increasing the difference and the dead space fraction. Dividing that difference by PaCO2 and multiplying by 100 expresses this as a percentage of tidal volume: [(PaCO2 - PECO2) / PaCO2] × 100. Using PaO2 would not reflect CO2 elimination, and multiplying by 10 or 1000 would distort the scale.