Calculate the physiologic dead-space ventilation ratio (Vd/Vt) using Bohr's equation with PaCO2 45 mm Hg and mixed expired CO2 tension (PECO2) 30 mm Hg.

Calculate the physiologic dead-space ventilation ratio (Vd/Vt) using Bohr's equation with PaCO2 45 mm Hg and mixed expired CO2 tension (PECO2) 30 mm Hg.

• A. 0.1
• B. 0.2
• C. 0.3
• D. 0.4

Answer: (C)

Bohr's equation for physiologic dead-space ventilation relates how much CO2 is rebreathed from areas of the lung that don’t participate in gas exchange. The fraction dead space is Vd/Vt = (PaCO2 − PECO2) / PaCO2. Plugging in the numbers: (45 − 30) / 45 = 15 / 45 = 1/3 ≈ 0.33. This rounds to about 0.3, so the best match is 0.3. In words, a larger difference between PaCO2 and mixed expired CO2 means more CO2 coming from dead space. If there were no dead space, PECO2 would equal PaCO2 and Vd/Vt would be zero.

Bohr's equation for physiologic dead-space ventilation relates how much CO2 is rebreathed from areas of the lung that don’t participate in gas exchange. The fraction dead space is Vd/Vt = (PaCO2 − PECO2) / PaCO2.

Plugging in the numbers: (45 − 30) / 45 = 15 / 45 = 1/3 ≈ 0.33. This rounds to about 0.3, so the best match is 0.3.

In words, a larger difference between PaCO2 and mixed expired CO2 means more CO2 coming from dead space. If there were no dead space, PECO2 would equal PaCO2 and Vd/Vt would be zero.