What is the critical pO2 value threshold?

• A. <= 40 mmHg
• B. <= 60 mmHg
• C. >= 40 mmHg
• D. >= 60 mmHg

Answer: (A)

The main idea here is that arterial oxygen tension (PaO2) marks how much oxygen is available to tissues. As PaO2 falls, oxygen delivery drops because hemoglobin saturation declines along the oxygen–hemoglobin dissociation curve, and the amount of dissolved oxygen in plasma is very small. Around PaO2 of 40 mmHg the curve shows a steep decline in saturation, and tissues are at risk of not getting enough oxygen to support aerobic metabolism. Below this level, the likelihood of cellular hypoxia and metabolic distress increases quickly, which is why 40 mmHg is considered a critical threshold. Values at or above this point are generally safer, whereas dips toward or below 40 mmHg signal severe hypoxemia that needs urgent attention.