What does the PaO2/FiO2 ratio indicate?

• A. Cardiac function
• B. Oxygenation impairment severity and ARDS classification
• C. Carbon dioxide clearance
• D. Hemoglobin binding capacity

Answer: (B)

The PaO2/FiO2 ratio measures how effectively the lungs transfer oxygen from the air you breathe into the blood. PaO2 is the amount of oxygen in arterial blood, and FiO2 is the fraction of inspired oxygen. When the lungs oxygenate well, this ratio is high; when gas exchange is impaired, PaO2 falls or you must raise FiO2, lowering the ratio. This ratio is used to gauge how severe the oxygenation problem is and to categorize ARDS severity (lower numbers mean worse oxygenation). It also helps guide treatment decisions and monitor response to therapy. The other options don’t fit because the ratio specifically reflects oxygen transfer efficiency, not cardiac function, carbon dioxide clearance, or hemoglobin’s oxygen-carrying capacity.