Pulmonology
Alveolar-arterial (A-a) gradient
The A-a gradient is the difference between the oxygen in the alveolus and the oxygen in the artery, and it separates causes of hypoxemia. A normal gradient points at hypoventilation or altitude; a raised one at shunt, V/Q mismatch or a diffusion problem.
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Sea level is 760 mmHg. Reduce it at altitude.
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The formula
- PAO₂ = FiO₂ × (Patm − 47) − PaCO₂ ÷ 0.8
- A-a gradient = PAO₂ − PaO₂
- Expected gradient ≈ (age ÷ 4) + 4
Worked example
Room air (21%) at sea level, PaO₂ 80, PaCO₂ 40, age 40 → PAO₂ = 0.21 × 713 − 50 = 99.7; gradient = 19.7 mmHg against an expected 14
When to use it
- Working up hypoxemia on a blood gas
And when not to
- The 0.8 respiratory quotient is an assumption that fails in some metabolic states.
- At altitude, correct the atmospheric pressure or the gradient reads falsely.
- On supplemental oxygen the gradient loses precision; the P/F ratio is often used instead.
References
- Clinical application of the alveolar-arterial oxygen gradient — Standard respiratory physiology texts
Last reviewed 07/09/2026.
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