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The Alveolar–arterial gradient (A-aO2 or A–a gradient) equation can help determine the source of hypoxemia by isolating the location- intrapulmonary (within the lungs) or extrapulmonary (elsewhere in the body). The gradient is the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen.
A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg. Normally, the A–a gradient increases with age. For every decade a person has lived, their A–a gradient is expected to increase by 1 mmHg. A conservative estimate of normal A–a gradient is less than [age in years/4] + 4. Thus, a 40-year-old should have an A–a gradient less than 14.
An abnormally increased A–a gradient suggests a defect in diffusion, V/Q (ventilation/perfusion ratio) mismatch, or right-to-left shunt. An elevated A–a gradient could indicate a patient breathing hard to achieve normal oxygenation, a patient breathing normally and attaining low oxygenation, or a patient breathing hard and still failing to achieve normal oxygenation.
If lack of oxygenation is proportional to low respiratory effort, then the A–a gradient is not increased. A healthy person who hypoventilates would have hypoxia, but a normal A–a gradient.

#### A-a Gradient Calculator

Patient age* :
(*Will be used to calculate normal A-a gradient based on age)
PB (Local barometric pressure) in mmHg: mmHg
PaO2 (partial pressure of arterial O2) from ABG: mmHg
PaCO2 (partial pressure of arterial CO2) from ABG: mmHg
FiO2 (fraction of inspired oxygen):

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#### References

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