The A-a gradient is the difference between the oxygen level your alveoli calculate they should have, and the oxygen level your arterial blood actually has. A wide gap means oxygen isn't crossing from lung to blood the way it should.
Low blood oxygen has two very different root causes, and the A-a gradient is how you tell them apart. If someone is hypoxemic but their gradient is normal, the lungs themselves are transferring oxygen fine — the problem is they simply aren't moving enough air (hypoventilation, opioid overdose, high altitude). If the gradient is elevated, something is interfering with oxygen crossing into the blood — V/Q mismatch, a shunt, or a diffusion problem at the alveolar-capillary membrane.
| Term | Meaning | Typical |
|---|---|---|
| FiO₂ | Fraction inspired O₂ | 0.21 room air |
| Patm | Atmospheric pressure | 760 mmHg sea level |
| PH₂O | Airway water vapor | 47 mmHg constant |
| PaCO₂ / PaO₂ | From an ABG | measured |
At or below expected for age. Gas exchange is working as expected.
Up to ~15 mmHg above expected — early/mild V/Q mismatch (early COPD, mild asthma, small PE).
~15–30 mmHg above expected — more significant V/Q mismatch or an early shunt component.
More than 30 mmHg above expected — substantial shunt or severe V/Q mismatch (ARDS, severe pneumonia, large PE).
Given: room air, PaCO₂ 40 mmHg, PaO₂ 90 mmHg, sea level.
149.7 − 50 = 99.7 mmHgLungs transfer oxygen fine — the patient simply isn't breathing enough. Opioid overdose, CNS depression, neuromuscular weakness, severe obesity, high altitude.
Some alveoli ventilated but poorly perfused, or vice versa. Most common cause — COPD, asthma, pulmonary embolism.
Deoxygenated blood bypasses ventilated alveoli entirely — severe pneumonia, ARDS, structural cardiac shunts.
Alveolar-capillary membrane thickened/damaged — pulmonary fibrosis, interstitial lung disease.
Oxygen crosses from the alveolus into the capillary — the gap is what's left over.
The difference between the oxygen level calculated in the alveoli and the oxygen level actually measured in arterial blood — it shows whether oxygen is failing to cross from lungs into bloodstream properly.
Roughly 5–15 mmHg in a healthy young adult on room air, rising gradually with age. A common estimate is (age ÷ 4) + 4.
Small, normal changes in lung elasticity and V/Q matching accumulate over a lifetime, so an older adult's normal gradient is naturally wider.
Yes — the gradient normally widens on higher FiO₂ even in healthy lungs, so it's most reliable on room air.
No. Elevated in ~85% of PE cases, but ~15% of patients — especially younger patients with a small PE — have a normal gradient. Never use a normal result alone to exclude PE.