A healthy subject on a partial rebreathing circuit inspires 50 percent oxygen at sea level (PB = 760 mmHg, PH2O = 47 mmHg). Arterial PCO2 is 40 mmHg and respiratory exchange ratio is 0.8. Using the alveolar gas equation, the expected alveolar PO2 is closest to:
- A 156 mmHg
- B 306 mmHg ✓
- C 330 mmHg
- D 357 mmHg
Correct answer: B. 306 mmHg
Explanation
PAO2 = FiO2 x (PB minus PH2O) minus PaCO2 divided by R = 0.5 x 713 minus 40/0.8 = 356.5 minus 50 = approximately 306 mmHg. Option C results from omitting water vapor correction and using full barometric pressure, and option D results from dropping the CO2 term entirely; both are classic calculation traps examined in gas exchange items.
Reference: West's Respiratory Physiology: The Essentials, 11th ed.
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