A patient's mixed expired air has a PCO2 of 28 mmHg while his arterial PCO2 is 40 mmHg and alveolar PCO2 approximates the arterial value. Using the Bohr equation, what is his physiologic dead space as a fraction of tidal volume?
- A 0.15
- B 0.30 ✓
- C 0.45
- D 0.60
Correct answer: B. 0.30
Explanation
The Bohr equation gives Vd/Vt = (PaCO2 minus PECO2) divided by PaCO2. Substituting, (40 minus 28) / 40 = 12/40 = 0.30. Normal physiologic dead space fraction is about 0.2 to 0.35, so this patient lies at the upper normal range. A value of 0.45 or higher would suggest significant dead space pathology such as pulmonary embolism, and 0.15 falls below the normal adult range.
Reference: West's Respiratory Physiology: The Essentials, 11th ed.
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