Printed on 2/13/2026
For informational purposes only. This is not medical advice.
The alveolar-arterial (A-a) oxygen gradient is the difference between the oxygen concentration in the alveoli and the arterial blood. It helps clinicians determine the cause of hypoxemia: a normal A-a gradient with low PaO₂ suggests hypoventilation, while an elevated gradient points to V/Q mismatch, shunt, or diffusion impairment. The expected normal A-a gradient increases with age.
Formula: A-a gradient = [FiO₂ × (Patm − 47) − PaCO₂/0.8] − PaO₂
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The A-a gradient measures the difference between alveolar oxygen (calculated from the alveolar gas equation) and arterial oxygen (measured from an ABG). A normal gradient is approximately (Age + 10) / 4. On room air, a normal A-a gradient is typically 5–15 mmHg in young adults.
An elevated A-a gradient indicates an impairment in oxygen transfer from the lungs to the blood. Causes include V/Q mismatch (pneumonia, COPD, PE), right-to-left shunt (cardiac shunt, ARDS), and diffusion impairment (pulmonary fibrosis, emphysema).
A normal A-a gradient with hypoxemia suggests the lungs are working normally but receiving insufficient oxygen. This occurs with hypoventilation (CNS depression, neuromuscular disease, obesity hypoventilation) or low FiO₂ (high altitude).
Each liter of nasal cannula adds approximately 3–4% to FiO₂. Room air = 21% (0.21). So: 1L = ~24%, 2L = ~28%, 3L = ~32%, 4L = ~36%, 5L = ~40%, 6L = ~44%. These are estimates that vary with breathing pattern.