A-a gradient calculation: PAO2 = 365 mmHg and PaO2 = 80 mmHg. What is A-a gradient?

• A. 205 mmHg
• B. 315 mmHg
• C. 285 mmHg
• D. 385 mmHg

Answer: (C)

The A-a gradient reflects how well oxygen moves from the alveoli into the arterial blood. It’s found by subtracting the arterial O2 from the alveolar O2: PAO2 minus PaO2.

here PAO2 is 365 mmHg and PaO2 is 80 mmHg, so the gradient is 365 − 80 = 285 mmHg.

This is a very large gradient (normal is roughly 5–15 mmHg in young people on room air and rises with age), indicating impaired oxygen transfer, such as from V/Q mismatch, diffusion limitation, or shunt.