What is alveolar Arterial gradient ( A-a)
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"alveolar-arterial gradient" hypoxemia
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Alveolar-Arterial (A-a) Gradient
The A-a gradient is the difference between the partial pressure of oxygen in the alveoli (PAO2) and the partial pressure of oxygen in arterial blood (PaO2):
A-a gradient = PAO2 - PaO2
In a perfectly matched lung both values would be equal, but normal physiological ventilation/perfusion (V/Q) mismatch means arterial PO2 is always somewhat lower than mean alveolar PO2 - Medical Physiology, p. 681.
How it's calculated
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Draw an arterial blood gas (ABG) to get PaO2 and PaCO2.
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Calculate the alveolar PO2 using the alveolar gas equation:PAO2 = FiO2 x (Patm - PH2O) - (PaCO2/R)On room air at sea level this simplifies to roughly:PAO2 ≈ 150 - 1.25 x PaCO2
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Subtract the measured PaO2: A-a gradient = PAO2 - PaO2 - Comprehensive Clinical Nephrology, 7th ed.
Normal value
Normal is roughly 5-15 mmHg on room air, and it rises with age. A commonly used estimate for the expected normal upper limit is:
Normal A-a gradient ≈ (Age + 10) / 4
- Comprehensive Clinical Nephrology, 7th ed.
Why it matters clinically
The A-a gradient is used to distinguish the mechanism of hypoxemia:
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Widened A-a gradient occurs with:
- V/Q mismatch (e.g., COPD, pulmonary embolism, pneumonia)
- Right-to-left shunt (intracardiac or intrapulmonary)
- Diffusion impairment (e.g., interstitial lung disease, early pulmonary edema)
- This is described as "the most common mechanism for hypoxemia" - Morgan and Mikhail's Clinical Anesthesiology, 7e.
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Normal A-a gradient with hypoxemia points instead to:
- Alveolar hypoventilation (e.g., opioid overdose, neuromuscular weakness, CNS depression)
- Low inspired oxygen (e.g., high altitude)
A useful bedside trick: giving 100% O2 helps separate shunt from other V/Q mismatch causes, because pure shunt physiology corrects poorly with supplemental oxygen while non-shunt V/Q mismatch corrects well - Medical Physiology, p. 683.
- Tintinalli's Emergency Medicine also notes the A-a gradient (alongside PaO2/FiO2 ratio) is used to estimate the effectiveness of alveolar oxygenation, especially in ARDS and respiratory failure assessment.
Summary table
| Finding | Interpretation |
|---|---|
| Hypoxemia + widened A-a gradient | V/Q mismatch, shunt, or diffusion defect |
| Hypoxemia + normal A-a gradient | Hypoventilation or low FiO2/altitude |
| Corrects with 100% O2 | Favors V/Q mismatch over shunt |
| Doesn't correct with 100% O2 | Favors true right-to-left shunt |
No conflicting recent systematic reviews were found specific to A-a gradient physiology itself (the PubMed search on this topic returned studies on related but distinct issues like arterial pressure gradients and postoperative hypoxemia prevalence, not changes to the A-a gradient concept), so the physiology above reflects standard, unchanged teaching.
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