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A-a Gradient Calculator
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A-a Gradient
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About
The alveolar-arterial (A-a) oxygen gradient measures the difference between the oxygen concentration in the alveoli and in arterial blood. It helps differentiate between causes of hypoxemia: a normal A-a gradient suggests hypoventilation or low FiO2, while an elevated gradient suggests V/Q mismatch, shunt, or diffusion impairment.
Formula
A-a Gradient = PAO₂ − PaO₂
PAO₂ (Alveolar O₂) = FiO₂ × (Patm − PH₂O) − PaCO₂ / 0.8
Patm = 760 mmHg (sea level), PH₂O = 47 mmHg
Expected A-a Gradient = (Age / 4) + 4
Interpretation
| A-a Gradient | Interpretation |
|---|---|
| ≤ Expected for age | Normal |
| > Expected for age | Elevated — V/Q mismatch, shunt, or diffusion impairment |
References
- Sarkar M, et al. Mechanisms of hypoxemia. Lung India. 2017;34(1):47-60.
- Mason RJ, et al. Murray and Nadel's Textbook of Respiratory Medicine. 6th ed. Elsevier; 2016.
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Disclaimer
For educational and informational purposes only. Not intended to replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional.