What is the correct formula for the alveolar-arterial oxygen gradient (A-a)?

What is the correct formula for the alveolar-arterial oxygen gradient (A-a)?

• A. A-a = PaO2 − PAO2
• B. A-a = PAO2 − PaO2
• C. A-a = PaO2 / PAO2
• D. A-a = PAO2 + PaO2

Answer: (B)

The A-a gradient shows how much oxygen is lost between the alveoli and the arterial blood, reflecting the efficiency of gas exchange. It is calculated as the difference between alveolar oxygen tension (PAO2) and arterial oxygen tension (PaO2). So the correct expression is A-a = PAO2 − PaO2. PAO2 represents the oxygen level in the alveoli (estimated by the alveolar gas equation), while PaO2 is the oxygen level in arterial blood. In healthy individuals, this gradient is small and may widen with age; a larger gradient suggests issues like diffusion limitation, V/Q mismatch, or intrapulmonary shunt. The other forms—PaO2 minus PAO2, a ratio, or a sum—do not measure the actual difference in oxygen between the alveoli and the blood.

The A-a gradient shows how much oxygen is lost between the alveoli and the arterial blood, reflecting the efficiency of gas exchange. It is calculated as the difference between alveolar oxygen tension (PAO2) and arterial oxygen tension (PaO2). So the correct expression is A-a = PAO2 − PaO2. PAO2 represents the oxygen level in the alveoli (estimated by the alveolar gas equation), while PaO2 is the oxygen level in arterial blood. In healthy individuals, this gradient is small and may widen with age; a larger gradient suggests issues like diffusion limitation, V/Q mismatch, or intrapulmonary shunt. The other forms—PaO2 minus PAO2, a ratio, or a sum—do not measure the actual difference in oxygen between the alveoli and the blood.