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A-a Gradient Equation:
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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 to differentiate causes of hypoxemia.
The calculator uses the A-a gradient equation:
Where:
Explanation: The A-a gradient represents the pressure difference between oxygen in the alveoli and oxygen in arterial blood, indicating how effectively oxygen is transferred across the alveolar-capillary membrane.
Details: A-a gradient is crucial for diagnosing and differentiating types of respiratory failure. Normal A-a gradient increases with age and helps identify ventilation-perfusion mismatch, diffusion defects, and shunts.
Tips: Enter PAO₂ and PaO₂ values in mmHg. Both values must be valid (≥0). The calculator will compute the A-a gradient difference.
Q1: What is a normal A-a gradient?
A: Normal A-a gradient is typically 5-15 mmHg in young healthy adults breathing room air, and increases with age (approximately 1 mmHg per decade over 20 years).
Q2: What causes elevated A-a gradient?
A: Elevated gradient indicates impaired gas exchange due to conditions like pneumonia, pulmonary embolism, COPD, pulmonary edema, or interstitial lung disease.
Q3: When is A-a gradient normal in hypoxemia?
A: Normal A-a gradient with hypoxemia suggests hypoventilation (e.g., drug overdose, neuromuscular disorders) rather than parenchymal lung disease.
Q4: How does FiO₂ affect A-a gradient?
A: A-a gradient normally increases with higher FiO₂. Interpretation should consider the inspired oxygen concentration.
Q5: What are the limitations of A-a gradient?
A: It doesn't differentiate between specific causes of impaired gas exchange and requires accurate measurement of arterial blood gases and inspired oxygen concentration.