A patient's mixed expired PCO2 is 28 mmHg and arterial PCO2 is 40 mmHg with a tidal volume of 500 mL. Using the Bohr equation, the physiologic dead space is approximately:
- A 75 mL
- B 150 mL ✓
- C 250 mL
- D 350 mL
Correct answer: B. 150 mL
Explanation
Bohr equation: VD/VT = (PaCO2 minus PeCO2) divided by PaCO2 = (40 minus 28)/40 = 0.30. Dead space = 0.30 x 500 mL = 150 mL. Normal physiologic dead space is about 150 mL in an adult, roughly a third of tidal volume, comprising anatomic plus alveolar dead space. Values approaching half the tidal volume suggest significant wasted ventilation, as in embolic disease.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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