Enter the fraction of inspired oxygen, atmospheric pressure, partial pressure of water, partial pressure of carbon dioxide in arterial blood, and partial pressure of oxygen in arterial blood into the calculator to determine the A-a Gradient.

A-A Gradient Formula

The following formula is used to calculate the A-A Gradient.

A-a Gradient = (FiO2 * (Patm – PH2O) – PaCO2/0.8) – PaO2

Variables:

  • A-a Gradient is the Alveolar-arterial Gradient
  • FiO2 is the fraction of inspired oxygen Patm is the atmospheric pressure
  • PH2O is the partial pressure of water
  • PaCO2 is the partial pressure of carbon dioxide in arterial blood
  • PaO2 is the partial pressure of oxygen in arterial blood

To calculate the A-a Gradient, multiply the fraction of inspired oxygen by the difference between the atmospheric pressure and the partial pressure of water. Subtract the quotient of the partial pressure of carbon dioxide in arterial blood and 0.8 from the result. Finally, subtract the partial pressure of oxygen in arterial blood from the result.

What is a A-A Gradient?

The A-a gradient, or Alveolar-arterial gradient, is a measure used in pulmonology to assess the efficiency of gas exchange in the lungs. It compares the amount of oxygen in the alveoli (air sacs in the lungs) with the amount of oxygen in the arteries. A normal A-a gradient indicates efficient oxygen transfer, while an increased A-a gradient suggests a problem with oxygen diffusion or ventilation-perfusion mismatch in the lungs.

How to Calculate A-A Gradient?

The following steps outline how to calculate the A-a Gradient:

  1. First, determine the value of FiO2 (Fraction of inspired oxygen).
  2. Next, determine the value of Patm (Atmospheric pressure).
  3. Next, determine the value of PH2O (Partial pressure of water).
  4. Next, determine the value of PaCO2 (Partial pressure of carbon dioxide in arterial blood).
  5. Finally, determine the value of PaO2 (Partial pressure of oxygen in arterial blood).
  6. After obtaining the values for all the variables, substitute them into the formula: A-a Gradient = (FiO2 * (Patm – PH2O) – PaCO2/0.8) – PaO2.
  7. Calculate the A-a Gradient using the formula.
  8. After calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge:

FiO2 = 0.4

Patm = 760 mmHg

PH2O = 47 mmHg

PaCO2 = 40 mmHg

PaO2 = 90 mmHg