1. What is A-a Gradient?

The Alveolar-arterial (A-a) gradient measures the difference between alveolar oxygen tension and arterial oxygen tension. It helps differentiate causes of hypoxemia and assess gas exchange efficiency in the lungs.

2. How Does the Calculator Work?

The calculator uses the simplified A-a gradient formula:

Where:

• \( PaCO_2 \) — Partial pressure of carbon dioxide in arterial blood (mmHg)
• \( PaO_2 \) — Partial pressure of oxygen in arterial blood (mmHg)
• 150 — Estimated alveolar oxygen pressure at sea level
• 1.25 — Respiratory quotient factor

Explanation: This simplified formula provides a quick estimation of the A-a gradient without requiring FiO₂ measurement.

3. Clinical Significance

Details: Normal A-a gradient is typically ≤10 mmHg in young adults and increases with age. Elevated gradient suggests ventilation-perfusion mismatch, diffusion defects, or right-to-left shunting.

4. Using the Calculator

Tips: Enter PaCO₂ and PaO₂ values in mmHg from arterial blood gas analysis. Ensure values are from the same blood sample for accurate calculation.

5. Frequently Asked Questions (FAQ)

Q1: What is a normal A-a gradient?
A: Normal is typically 5-10 mmHg in young adults, increasing by about 1 mmHg per decade of life after age 30.

Q2: When is A-a gradient elevated?
A: Elevated in conditions like pulmonary embolism, pneumonia, ARDS, pulmonary fibrosis, and right-to-left cardiac shunts.

Q3: What factors affect A-a gradient?
A: Age, altitude, FiO₂, cardiac output, and body position can all influence the gradient measurement.

Q4: How does age affect normal values?
A: Normal A-a gradient ≈ (Age/4) + 4. So a 60-year-old would have normal gradient up to 19 mmHg.

Q5: When is this calculation most useful?
A: Most valuable when evaluating hypoxemia in patients breathing room air, helping differentiate pulmonary from extrapulmonary causes.