A-a Gradient Calculator

Formula

The formula to calculate the A-a Gradient is:

\[ \text{A-a Gradient} = (FiO2 \times (Patm - PH2O) - \frac{PaCO2}{0.8}) - PaO2 \]

Where:

\( \text{A-a Gradient} \) is the Alveolar-arterial Gradient
\( \text{FiO2} \) is the fraction of inspired oxygen
\( \text{Patm} \) is the atmospheric pressure
\( \text{PH2O} \) is the partial pressure of water
\( \text{PaCO2} \) is the partial pressure of carbon dioxide in arterial blood
\( \text{PaO2} \) is the partial pressure of oxygen in arterial blood

Definition

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.

Example

Let's say the fraction of inspired oxygen (FiO2) is 0.21, the atmospheric pressure (Patm) is 760 mmHg, the partial pressure of water (PH2O) is 47 mmHg, the partial pressure of carbon dioxide in arterial blood (PaCO2) is 40 mmHg, and the partial pressure of oxygen in arterial blood (PaO2) is 100 mmHg. Using the formula:

\[ \text{A-a Gradient} = (0.21 \times (760 - 47) - \frac{40}{0.8}) - 100 \]

We get:

\[ \text{A-a Gradient} \approx -0.27 \]

So, the A-a gradient is approximately -0.27.