An oxygen challenge should be used if PO2 falls between normal and poor levels. After the challenge, the PO2 should be higher by how much?

• A. 10-20 mmHg higher
• B. 30-40 mmHg higher
• C. No increase expected
• D. 5-10 mmHg higher

Answer: (A)

An oxygen challenge tests how arterial PO2 responds to a high inspired oxygen level, helping to distinguish different causes of hypoxemia by showing how well the lungs can transfer oxygen into the blood. When PO2 is between normal and poor, a modest rise is expected after giving 100% oxygen because increasing the inspired oxygen improves diffusion and the oxygen gradient, but not all alveoli participate fully and some blood may bypass oxygenation through shunts. The typical increase is about 10–20 mmHg. A much larger rise would be less common in this scenario, and little to no rise would suggest a more substantial shunt. Hence, a 10–20 mmHg increase is the best expectation.