1. What is A-a Gradient?

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

2. How Does the Calculator Work?

The calculator uses the A-a Gradient equation:

Where:

• \( F_iO_2 \) — Fraction of inspired oxygen (0.21 for room air)
• \( P_{atm} \) — Atmospheric pressure (760 mmHg at sea level)
• \( P_{H_2O} \) — Water vapor pressure (47 mmHg at 37°C)
• \( P_aCO_2 \) — Arterial carbon dioxide partial pressure
• \( R \) — Respiratory quotient (typically 0.8)
• \( P_aO_2 \) — Arterial oxygen partial pressure

Explanation: The equation calculates the alveolar oxygen tension using the alveolar gas equation and subtracts the measured arterial oxygen tension to determine the gradient.

3. Clinical Significance

Details: Normal A-a gradient is typically 5-15 mmHg in young healthy adults breathing room air. Increased gradient suggests ventilation-perfusion mismatch, diffusion impairment, or right-to-left shunt.

4. Using the Calculator

Tips: Enter all values in mmHg. Use standard values for atmospheric pressure (760 mmHg) and water vapor pressure (47 mmHg) unless specific conditions require adjustment. Respiratory quotient is typically 0.8.

5. Frequently Asked Questions (FAQ)

Q1: What is a normal A-a gradient?
A: Normal is 5-15 mmHg in young adults breathing room air. It increases with age (approximately 3 mmHg per decade) and with higher F_iO2.

Q2: When is A-a gradient increased?
A: Increased in V/Q mismatch, diffusion defects, right-to-left shunts, and some cases of hypoventilation with lung disease.

Q3: What causes normal A-a gradient hypoxemia?
A: Hypoventilation alone (e.g., drug overdose, neuromuscular disease) typically shows normal A-a gradient with elevated P_aCO2.

Q4: How does altitude affect A-a gradient?
A: At altitude, atmospheric pressure decreases, which affects the calculation. Use actual local atmospheric pressure for accurate results.

Q5: What are limitations of A-a gradient?
A: Less reliable at high F_iO2 levels, affected by technical factors in blood gas measurement, and doesn't differentiate between specific causes of increased gradient.