A-a Gradient Calculator
Calculate the alveolar-arterial oxygen gradient to assess the cause of hypoxemia.
How to Use the A-a Gradient Calculator
- Enter FiO2 as a decimal (e.g., 0.21 for room air, 1.0 for 100% O2).
- Enter atmospheric pressure (760 mmHg at sea level).
- Enter PaCO2 from arterial blood gas in mmHg.
- Enter PaO2 from arterial blood gas in mmHg.
- Click Calculate to see A-a gradient, alveolar PO2, and interpretation.
使用例
- •Differentiating causes of hypoxemia in the ICU or emergency department.
- •Evaluating pulmonary embolism workup alongside clinical probability scores.
- •Teaching respiratory physiology and oxygenation concepts.
計算式
PAO2 = FiO2 × (Patm − 47) − PaCO2/0.8. A-a gradient = PAO2 − PaO2. Normal A-a < 10 mmHg on room air.
よくある質問
What does an elevated A-a gradient indicate?
An elevated A-a gradient (> 10–15 mmHg on room air) suggests a problem with gas exchange within the lung such as V/Q mismatch (pneumonia, PE), intracardiac shunt, or diffusion defect (pulmonary fibrosis). A normal A-a gradient with hypoxemia suggests hypoventilation.
How does age affect the normal A-a gradient?
The normal A-a gradient increases with age. A rough estimate: normal A-a = (age / 4) + 4 mmHg. For practical purposes, values under 10 mmHg in young adults and under 25 mmHg in elderly patients are generally considered normal.
Disclaimer
For educational purposes only. A-a gradient interpretation requires arterial blood gas values obtained under carefully documented conditions (FiO2, altitude/Patm). Not a substitute for clinical assessment.