Clinical Tool
A-a Gradient
Alveolar-arterial oxygen difference
PAO₂
99.7
mmHg
A-a Gradient
9.7
mmHg
Formula
PAO₂ = FiO₂ × 713 − (PaCO₂ ÷ 0.8)
A-a = PAO₂ − PaO₂
Normal ≈ age ÷ 4 + 4 (on room air)
About the A-a gradient calculator
The alveolar–arterial oxygen gradient is the difference between the oxygen tension calculated for the alveolus and the one measured in arterial blood. It separates hypoxaemia caused by the lung itself from hypoxaemia caused by everything else.
What the number separates
A hypoxaemic patient with a normal gradient is not failing at gas transfer. That points at hypoventilation or a low inspired oxygen tension — an opioid overdose, a neuromuscular problem, altitude — where the alveolar oxygen is low to begin with and the blood is faithfully reflecting it.
A widened gradient means the alveolar oxygen is not reaching the blood, which narrows the cause to V/Q mismatch, shunt, or a diffusion problem. Shunt is distinguished from the others by its poor response to supplemental oxygen.
The expected value rises with age
A gradient of 20 mmHg is unremarkable in a 76-year-old and abnormal in a 20-year-old. The usual bedside estimate of the expected value on room air is age divided by four, plus four.
The expected gradient also rises with FiO₂, so a value measured on supplemental oxygen cannot be compared against a room-air expectation. This is a common source of misreading.
The alveolar gas equation behind it
The alveolar oxygen tension is calculated from the inspired oxygen fraction, the barometric pressure less water vapour pressure, and the arterial CO₂ divided by the respiratory quotient. Because PaCO₂ appears in it, a patient who is hypoventilating has a genuinely lower alveolar oxygen — which is exactly why their gradient stays normal while their PaO₂ falls.
Common questions
- What is a normal A-a gradient?
- On room air, roughly age ÷ 4 + 4 mmHg. About 10 mmHg in a young adult, rising with age.
- What causes a widened A-a gradient?
- V/Q mismatch, right-to-left shunt, and diffusion limitation. Pulmonary embolism, pneumonia, ARDS and pulmonary oedema all widen it.
- Can the A-a gradient be normal in a hypoxaemic patient?
- Yes, and it is diagnostically useful when it is. Hypoventilation and a low inspired oxygen tension lower the alveolar oxygen itself, so the blood is in equilibrium with a low alveolar value and the gradient stays normal.
- Does supplemental oxygen change the expected gradient?
- Yes. The expected gradient widens as FiO₂ rises, so a value taken on oxygen should not be judged against a room-air normal.
Related
Educational reference for respiratory therapy students and clinicians preparing for the NBRC examinations. It supports clinical reasoning and does not replace institutional protocol, a device's instructions for use, or the judgement of the team caring for the patient.