A-a gradient example: PAO2 = 275 mmHg and PaO2 = 90 mmHg. A-a gradient is:

• A. 150 mmHg
• B. 185 mmHg
• C. 275 mmHg
• D. 200 mmHg

Answer: (B)

The A-a gradient measures how effectively oxygen moves from the alveoli into the arterial blood. It’s found by subtracting PaO2 from PAO2: PAO2 minus PaO2. With PAO2 = 275 mmHg and PaO2 = 90 mmHg, the gradient is 275 − 90 = 185 mmHg. This is markedly elevated—normal is about 10–15 mmHg in young adults and rises modestly with age—so it indicates substantial gas-exchange impairment, such as diffusion limitation, ventilation–perfusion mismatch, or a shunt. If the problem were simple hypoventilation, the gradient would typically be normal despite a low PaO2.