Home
Back
A-a Gradient Equation:
| From: | To: |
The Alveolar-arterial (A-a) gradient measures the difference between alveolar oxygen partial pressure (PAO₂) and arterial oxygen partial pressure (PaO₂). It helps assess the efficiency of oxygen transfer from alveoli to blood and is used in evaluating pulmonary function and gas exchange abnormalities.
The calculator uses the A-a gradient equation:
Where:
Explanation: The A-a gradient represents the difference between the oxygen pressure in the alveoli and the oxygen pressure measured in arterial blood, indicating the efficiency of pulmonary gas exchange.
Details: A normal A-a gradient is typically 5-15 mmHg in young healthy adults breathing room air. Elevated values suggest ventilation-perfusion mismatch, diffusion defects, or right-to-left shunting. The gradient normally increases with age and higher FiO₂.
Tips: Enter both PAO₂ and PaO₂ values in mmHg. Ensure measurements are taken under the same conditions (same FiO₂, same time). Values must be non-negative numbers.
Q1: What is a normal A-a gradient range?
A: Normal is approximately 5-15 mmHg in young adults breathing room air. It increases by about 1 mmHg per decade of life after age 30.
Q2: What causes an elevated A-a gradient?
A: Common causes include pneumonia, pulmonary embolism, COPD, asthma, pulmonary fibrosis, congestive heart failure, and right-to-left cardiac shunts.
Q3: When is A-a gradient normal in hypoxemia?
A: When hypoxemia is due to hypoventilation (e.g., drug overdose, neuromuscular disease) without significant lung pathology.
Q4: How does FiO₂ affect A-a gradient?
A: The gradient increases with higher FiO₂. Interpretation should consider the inspired oxygen concentration used during measurement.
Q5: What are limitations of A-a gradient?
A: It doesn't differentiate between causes of impaired gas exchange and requires accurate measurement of both PAO₂ and PaO₂ under stable conditions.