Alveolar-Arterial (A-a) Gradient
Calculate the A-a oxygen gradient to evaluate gas exchange and localise hypoxemia causes
Clinical Interpretation
An elevated A-a gradient indicates an oxygenation defect due to V/Q mismatch (e.g. PE, pneumonia), shunt, or diffusion barrier. A normal gradient in a hypoxemic patient suggests hypoventilation or low inspired oxygen (e.g. altitude).
Mathematical Metric
Evidence & Lit
References: West JB. Respiratory Physiology: The Essentials.
West JB. Respiratory Physiology: The Essentials (10th Ed).Frequently Asked Questions
What is the Alveolar-Arterial (A-a) oxygen gradient?
The A-a gradient is the difference between the partial pressure of oxygen in the alveoli (PAO2) and in arterial blood (PaO2). It evaluates the integrity of the alveolar-capillary membrane.
How is the expected normal A-a gradient calculated?
A normal A-a gradient increases with age due to physiological lung senescence. It can be estimated using the formula: Expected Normal Gradient = (Age / 4) + 4.
What does a normal A-a gradient with hypoxemia mean?
If a patient is hypoxemic (low PaO2) but has a normal A-a gradient, the cause of hypoxemia is extra-pulmonary: either alveolar hypoventilation (e.g. opioid overdose, neuromuscular disease) or a low fraction of inspired oxygen (e.g. high altitude).
What does an elevated A-a gradient mean?
An elevated A-a gradient (> expected age-adjusted value) indicates a pulmonary cause of hypoxemia. This is typically due to ventilation-perfusion (V/Q) mismatch (e.g. pulmonary embolism, COPD, pneumonia), right-to-left shunt (e.g. atelectasis, AVMs), or diffusion limitations.