Respiratory Therapy – Formulas Practice Test

In a scenario with PAO2 = 365 mmHg and PaO2 = 100 mmHg, what is the A-a gradient?

285 mmHg 265 mmHg 305 mmHg 345 mmHg

Correct Answer:

265 mmHg

Explanation:

The A-a gradient measures how well oxygen moves from the alveoli into the arterial blood. It’s simply the difference between alveolar oxygen tension (PAO2) and arterial oxygen tension (PaO2). Here, PAO2 is 365 mmHg and PaO2 is 100 mmHg. Subtracting gives 365 − 100 = 265 mmHg. So, the A-a gradient is 265 mmHg. That’s a markedly elevated gradient (normal is roughly 5–15 mmHg in young adults, rising a bit with age), indicating impaired transfer of oxygen from the alveoli to the blood, which can reflect diffusion limitations, V/Q mismatch, or shunt, especially in the context of high PAO2.

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