A-a Gradient Calculator
Calculate the Alveolar-arterial (A-a) oxygen gradient to determine the cause of hypoxemia. Normal A-a gradient is (Age/4) + 4 mmHg. An elevated gradient indicates V/Q mismatch, intrapulmonary shunt, or diffusion impairment, while a normal gradient with hypoxia points to hypoventilation.
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Formula
Calculates the difference between the oxygen in the alveoli (lungs) and the oxygen in the arteries. A high gradient suggests a problem with the lung tissue itself.
Worked Example
Interpreting the A-a Gradient
- Normal A-a gradient = (Age / 4) + 4 mmHg, approximately 5-15 mmHg in young adults
- Normal gradient with hypoxia: hypoventilation (opioids, neuromuscular disease) or high altitude
- Elevated gradient: V/Q mismatch (PE, pneumonia), shunt (ARDS, ASD), or diffusion impairment (fibrosis)
- The gradient is most accurate on room air (FiO2 21%) and less reliable at high FiO2
- Always interpret alongside PaCO2, pH, and the clinical picture for a complete assessment
The A-a gradient requires an arterial blood gas (ABG) sample. It should be used alongside imaging, clinical exam, and other lab values to guide diagnosis and treatment.
You can also calculate changes using our PaO2/FiO2 Ratio Calculator or Alveolar Gas Equation Calculator.
Frequently Asked Questions
What is a normal A-a gradient?
A rough estimate of normal is (Age / 4) + 4 mmHg. For a 20-year-old, that is about 9 mmHg. For a 60-year-old, normal is approximately 19 mmHg.
What does a high gradient mean?
It suggests a problem with gas exchange across the lung membrane, common in pneumonia, pulmonary edema, or pulmonary embolism.
What does a normal gradient with low oxygen mean?
It suggests the patient is hypoventilating (not breathing enough) or at a high altitude, but the lungs themselves are healthy.
How is the A-a gradient calculated step by step?
First calculate PAO2 using the alveolar gas equation: PAO2 = FiO2 x (760 - 47) - (PaCO2 / 0.8). Then subtract the measured PaO2 from the ABG. For example, on room air with PaCO2 of 40: PAO2 = 0.21 x 713 - 50 = about 100 mmHg.
What is the difference between PaO2 and PAO2?
PaO2 (lowercase a) is the measured arterial oxygen from a blood gas sample. PAO2 (capital A) is the calculated alveolar oxygen, representing the theoretical maximum oxygen in the lung air sacs.
Can medications affect the A-a gradient?
Medications that cause hypoventilation (like opioids or sedatives) lower PaO2 but typically maintain a normal A-a gradient. Drugs that cause pulmonary toxicity (like bleomycin or amiodarone) can elevate the gradient by damaging lung tissue.
Medical Disclaimer
This calculator is provided for informational and educational purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any decisions based on these results. Do not disregard or delay seeking medical advice because of information obtained from this tool.
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Our AI assistant provides general estimates and advice only. Please consult a qualified healthcare professional before making any health-related decisions.
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