In a patient with PaCO2 48 mmHg and PECO2 32 mmHg, what is the physiologic dead space to tidal volume ratio (VD/VT)?

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Multiple Choice

In a patient with PaCO2 48 mmHg and PECO2 32 mmHg, what is the physiologic dead space to tidal volume ratio (VD/VT)?

The quantity that links CO2 in the blood to CO2 in the air we exhale is the physiologic dead space fraction. It’s described by the Bohr equation, which uses the difference between alveolar (or arterial) CO2 and end-tidal CO2: VD/VT = (PACO2 − PECO2) / Paco2. In practice, PaCO2 is used as an approximation for Paco2. Here, PaCO2 is 48 mmHg and PECO2 is 32 mmHg. The difference is 16 mmHg. Divide by the arterial value: 16 / 48 = 0.333. So VD/VT is about 0.333 (33%). This means about one-third of each breath does not participate in gas exchange. A value around 0.2–0.4 is typical, so 0.333 sits within the normal range. The larger the difference between Paco2 and Peco2, the greater the dead space relative to tidal volume, reflecting more ventilation that doesn’t reach perfused alveoli.

The quantity that links CO2 in the blood to CO2 in the air we exhale is the physiologic dead space fraction. It’s described by the Bohr equation, which uses the difference between alveolar (or arterial) CO2 and end-tidal CO2: VD/VT = (PACO2 − PECO2) / Paco2. In practice, PaCO2 is used as an approximation for Paco2.

Here, PaCO2 is 48 mmHg and PECO2 is 32 mmHg. The difference is 16 mmHg. Divide by the arterial value: 16 / 48 = 0.333. So VD/VT is about 0.333 (33%).

This means about one-third of each breath does not participate in gas exchange. A value around 0.2–0.4 is typical, so 0.333 sits within the normal range. The larger the difference between Paco2 and Peco2, the greater the dead space relative to tidal volume, reflecting more ventilation that doesn’t reach perfused alveoli.

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