A ventilated patient has an arterial carbon dioxide tension of 48 mmHg and an end-tidal carbon dioxide of 30 mmHg. Using the Enghoff modification of the Bohr equation, the physiological dead space fraction (Vd/Vt) is approximately:
- A 0.19
- B 0.38 ✓
- C 0.62
- D 1.60
Correct answer: B. 0.38
Explanation
Vd/Vt equals the difference between PaCO2 and end-tidal CO2 divided by PaCO2: (48 minus 30) divided by 48, which is 18 divided by 48, approximately 0.38. The widened PaCO2 minus PETCO2 gradient of 18 mmHg reflects increased alveolar dead space from ventilated but poorly perfused alveoli. Option D results from dividing 48 by 30, inverting the formula, and 1.60 exceeds any physiologically possible dead space fraction.
Reference: Morgan and Mikhail's Clinical Anesthesiology, 7th ed.
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