A-a Gradient Calculator — Free Online Calculator
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A-a Gradient
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About This Calculator
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
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.
Frequently Asked Questions
What does a high A-a gradient mean?
An elevated A-a gradient indicates impaired gas exchange in the lungs. Causes include V/Q mismatch (e.g., PE, pneumonia), diffusion impairment (e.g., pulmonary fibrosis), or right-to-left shunt. A normal A-a gradient with hypoxemia suggests hypoventilation or low FiO₂.
How do you calculate the expected A-a gradient?
The expected A-a gradient increases with age. A common approximation is: Expected A-a gradient = (Age ÷ 4) + 4, or alternatively (Age + 10) ÷ 4. Values significantly above the expected gradient suggest pathology.
What is a normal A-a gradient?
In a young, healthy adult breathing room air at sea level, the normal A-a gradient is 5–15 mmHg. It increases by about 1 mmHg for every 4 years of age. An A-a gradient above the age-adjusted expected value is abnormal.
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⚠ Medical Disclaimer
This tool is for educational and informational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for clinical decisions.