Which ABG pattern is typically seen in PCP-related respiratory decompensation?

• A. PaO2 around 90 with normal A-a gradient
• B. PaO2 around 64 with high CO2
• C. PaO2 around 70 with A-a gradient >35
• D. PaO2 around 100 with normal A-a gradient

Answer: (C)

In PCP-related respiratory decompensation the main issue is impaired oxygen transfer from the alveoli to the blood due to alveolar filling and inflammation, which causes ventilation-perfusion mismatch. This leads to hypoxemia with a markedly increased A-a gradient, reflecting a gas exchange problem rather than simple alveolar hypoventilation. You typically see a PaO2 in the 60s–70s range together with an A-a gradient greater than about 35. The CO2 level is often not elevated early, because patients tend to hyperventilate; hypercapnia suggests fatigue and worsening ventilation.

So the pattern of a PaO2 around 70 with a high A-a gradient (>35) best fits PCP-related gas-exchange impairment. Choices with normal oxygenation or normal gradient, or with isolated hypercapnia, don’t match this typical PCP pattern.