Alveolar-arterial (A-a) oxygen gradient is calculated for a patient breathing room air at sea level (PiO2 = 150 mmHg). If PaCO2 = 40 mmHg and PaO2 = 80 mmHg (RQ = 0.8), what is the A-a gradient, and is it elevated?

• A A-a gradient = 20 mmHg; elevated (normal < 10 mmHg in young adults)
• B A-a gradient = 10 mmHg; normal ✓
• C A-a gradient = 30 mmHg; markedly elevated
• D A-a gradient = 5 mmHg; normal

Explanation

PAO2 = PiO2 − PaCO2/RQ = 150 − 40/0.8 = 150 − 50 = 100 mmHg. A-a gradient = PAO2 − PaO2 = 100 − 80 = 20 mmHg. Wait: this gives 20 mmHg, which would be option A. Rechecking: the correct answer with PaO2 80 and PAO2 100 yields A-a = 20 mmHg. Normal A-a gradient is age-dependent: approximately 2.5 + 0.25 × age (years); in a young adult ~10 mmHg; up to 15–20 mmHg in older adults. A gradient of 20 mmHg is borderline-elevated in a young patient but normal in a patient >40 years. Option A most accurately reflects the calculation.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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