A-a Gradient Calculator

The A-a Gradient Calculator estimates the alveolar-arterial oxygen gradient using PaO₂, PaCO₂, FiO₂, and atmospheric pressure. This tool helps medical professionals assess gas exchange efficiency and evaluate causes of hypoxemia. Whether you are diagnosing V/Q mismatch, checking for pulmonary embolism, or monitoring lung disease severity, this resource provides fast, reliable results for better clinical decisions.

Enter arterial partial pressure of oxygen in mmHg
Enter arterial partial pressure of carbon dioxide in mmHg
Select the fraction of inspired oxygen
Enter patient age in years
Enter atmospheric pressure in mmHg (default: 760 mmHg at sea level)

How A-a Gradient Is Calculated

The A-a gradient measures the difference between oxygen concentration in the alveoli and the arterial blood. It helps determine if the lungs are effectively moving oxygen into the bloodstream. The calculation relies on the Alveolar Gas Equation, a standard medical formula used to assess gas exchange efficiency.

PAO₂ = (FiO₂ × (Patm - PH₂O)) - (PaCO₂ / RQ)

Where:

  • Patm = Atmospheric pressure (760 mmHg at sea level)
  • PH₂O = Water vapor pressure (47 mmHg)
  • RQ = Respiratory quotient (0.8)

First, the tool calculates the alveolar oxygen pressure ($PAO_2$) by adjusting for atmospheric pressure and water vapor. Then, it subtracts the measured arterial oxygen ($PaO_2$) from this alveolar value. This result highlights how much oxygen is lost during the transfer process, offering a precise look at lung function integrity.

What Your A-a Gradient Means

Understanding your result helps identify the specific cause of low oxygen levels in a patient. A normal gradient generally means the lungs are moving oxygen well, so hypoxemia is likely due to breathing patterns. An elevated gradient suggests the lung tissue itself is impaired.

Normal Gas Exchange

If the gradient is within the expected range for the patient's age (Age/4 + 4), gas transfer is intact. In this scenario, focus on airway patency and respiratory drive to solve the oxygenation problem.

Elevated Gradient

Values significantly higher than the age-predicted limit indicate issues like V/Q mismatch, shunting, or diffusion defects. Common causes include pulmonary embolism, ARDS, pneumonia, or pulmonary fibrosis. This finding often warrants immediate diagnostic imaging or changes in ventilator support.

Important: Results depend on the accuracy of the arterial blood gas sample and assume the patient is in a steady-state condition.

This calculator is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Repeated measures often provide more insight than a single calculation.