A-aDO2 measures the gradient between?

A-aDO2 measures the gradient between?

• A. Measures the gradient between arterial PO2 and venous PO2
• B. Measures the gradient between alveolar PO2 and arterial PCO2
• C. Measures the gradient between alveolar PO2 and arterial PO2
• D. Measures the gradient between alveolar PO2 and arterial PO2 (best done after 100% O2 for 20 minutes)

Answer: (D)

The main idea is that the A–a gradient shows how well oxygen moves from the alveoli into the blood. It is the difference between the alveolar oxygen tension (PAO2) and the arterial oxygen tension (PaO2). You estimate PAO2 using the alveolar gas equation and then subtract PaO2 to get the gradient. A normal gradient increases with age; a larger-than-normal gradient indicates a gas-exchange problem such as V/Q mismatch, diffusion limitation, or a true shunt. The statement describing the gradient as between alveolar PO2 and arterial PO2 is the correct conceptual definition. The note about performing the assessment after 100% oxygen for 20 minutes is a clinical step used to help differentiate causes of hypoxemia (shunt tends to resist improvement with high O2, while V/Q mismatch or diffusion limitation may improve).

The main idea is that the A–a gradient shows how well oxygen moves from the alveoli into the blood. It is the difference between the alveolar oxygen tension (PAO2) and the arterial oxygen tension (PaO2). You estimate PAO2 using the alveolar gas equation and then subtract PaO2 to get the gradient. A normal gradient increases with age; a larger-than-normal gradient indicates a gas-exchange problem such as V/Q mismatch, diffusion limitation, or a true shunt.

The statement describing the gradient as between alveolar PO2 and arterial PO2 is the correct conceptual definition. The note about performing the assessment after 100% oxygen for 20 minutes is a clinical step used to help differentiate causes of hypoxemia (shunt tends to resist improvement with high O2, while V/Q mismatch or diffusion limitation may improve).