A high A-a gradient most commonly indicates which issue?

A high A-a gradient most commonly indicates which issue?

• A. Hypoventilation
• B. Normal aging
• C. Low inspired oxygen
• D. V/Q mismatch

Answer: (D)

The A-a gradient shows how well oxygen moves from the alveoli into the arterial blood. When this gradient is elevated, it means there’s a defect in gas transfer. The most common cause is ventilation-perfusion mismatch, where some lung units are poorly ventilated relative to their blood flow. In those regions, alveolar oxygen can stay relatively high while the blood leaving the area is under-oxygenated, widening the difference between PAO2 and PaO2. Hypoventilation lowers both PAO2 and PaO2 without widening the gap, low inspired oxygen reduces these values roughly in parallel, and aging only modestly increases the gradient. So a high A-a gradient most strongly points to V/Q mismatch.

The A-a gradient shows how well oxygen moves from the alveoli into the arterial blood. When this gradient is elevated, it means there’s a defect in gas transfer. The most common cause is ventilation-perfusion mismatch, where some lung units are poorly ventilated relative to their blood flow. In those regions, alveolar oxygen can stay relatively high while the blood leaving the area is under-oxygenated, widening the difference between PAO2 and PaO2. Hypoventilation lowers both PAO2 and PaO2 without widening the gap, low inspired oxygen reduces these values roughly in parallel, and aging only modestly increases the gradient. So a high A-a gradient most strongly points to V/Q mismatch.