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AA 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 assesses the efficiency of gas exchange in the lungs and helps identify causes of hypoxemia.
The calculator uses the AA Gradient equation:
Where:
Explanation: The AA Gradient reflects the efficiency of oxygen transfer from alveoli to arterial blood. A normal gradient indicates efficient gas exchange, while an increased gradient suggests impaired diffusion or ventilation-perfusion mismatch.
Details: The AA Gradient is crucial for differentiating between causes of hypoxemia. It helps distinguish between pulmonary causes (increased gradient) and extrapulmonary causes like hypoventilation or low FiO2 (normal gradient).
Tips: Enter PAO2 and PaO2 values in mmHg. Both values must be non-negative. The calculator will compute the difference to determine the alveolar-arterial gradient.
Q1: What is a normal AA Gradient value?
A: Normal AA Gradient is typically 5-15 mmHg in young healthy adults breathing room air. It increases with age and higher FiO2.
Q2: What does an increased AA Gradient indicate?
A: An increased gradient suggests ventilation-perfusion mismatch, diffusion impairment, or right-to-left shunt as causes of hypoxemia.
Q3: How is PAO2 calculated?
A: PAO2 = (FiO2 × (Patm - PH2O)) - (PaCO2 / R), where R is the respiratory quotient (typically 0.8).
Q4: When is AA Gradient most useful?
A: Most useful in patients with hypoxemia to differentiate between pulmonary and extrapulmonary causes, and in monitoring patients with respiratory diseases.
Q5: Are there limitations to AA Gradient interpretation?
A: Yes, the gradient increases with age and FiO2. It's less reliable in patients receiving supplemental oxygen and doesn't differentiate between specific types of pulmonary pathology.