Which expression is the Bohr equation for physiologic dead space?

Which expression is the Bohr equation for physiologic dead space?

• A. VD/VT = (PECO2 - PaCO2) / PaCO2
• B. VD/VT = (PaCO2 - PECO2) / PaCO2
• C. VD/VT = PaCO2 / PECO2
• D. VD/VT = (PaCO2 - PECO2) / PaCO2

Answer: (D)

Physiologic dead space is the portion of tidal volume that does not participate in gas exchange. The Bohr equation expresses this as a fraction by comparing the CO2 content of alveolar gas with the CO2 content of the expired gas. The amount of CO2 that remains in dead space air is represented by the difference between alveolar CO2 and expired CO2. Therefore, the dead-space fraction is VD/VT = (PACO2 − PECO2) / PACO2. In practice, PACO2 is often approximated by PaCO2, giving VD/VT ≈ (PaCO2 − PECO2) / PaCO2. This form makes sense because the numerator measures CO2 that didn’t equilibrate with perfused alveoli (dead space), while the denominator provides the alveolar CO2 level to normalize the fraction. The other expressions either reverse the subtraction, yield negative values, or use the wrong CO2 terms, which would not reflect the true dead-space fraction.

Physiologic dead space is the portion of tidal volume that does not participate in gas exchange. The Bohr equation expresses this as a fraction by comparing the CO2 content of alveolar gas with the CO2 content of the expired gas. The amount of CO2 that remains in dead space air is represented by the difference between alveolar CO2 and expired CO2. Therefore, the dead-space fraction is VD/VT = (PACO2 − PECO2) / PACO2. In practice, PACO2 is often approximated by PaCO2, giving VD/VT ≈ (PaCO2 − PECO2) / PaCO2. This form makes sense because the numerator measures CO2 that didn’t equilibrate with perfused alveoli (dead space), while the denominator provides the alveolar CO2 level to normalize the fraction. The other expressions either reverse the subtraction, yield negative values, or use the wrong CO2 terms, which would not reflect the true dead-space fraction.