A-a Gradient Calculator (Alveolar-Arterial Oxygen)

Your details

Age in years. The normal A-a gradient rises by roughly 1 mmHg per 4 years of age.
years
Partial pressure of oxygen measured in arterial blood from an ABG sample.
mmHg
Partial pressure of carbon dioxide from the ABG. Normal range is 35-45 mmHg.
mmHg
Fraction of inspired oxygen. Room air = 0.21 (21%). Use 0.40 for a simple face mask, 1.0 for 100% non-rebreather.
Barometric pressure. Sea level = 760 mmHg. Reduces at altitude (Denver ~630 mmHg, Mount Everest base camp ~380 mmHg).
mmHg
Ratio of CO2 produced to O2 consumed. Typically 0.8 on a mixed diet; rises toward 1.0 on a high-carbohydrate diet.
A-a GradientNormal
9.7mmHg

Difference between alveolar and arterial oxygen pressure

PAO₂ (alveolar O₂)99.7mmHg
Age-adjusted normal14mmHg
Above normal by-4.3mmHg
InterpretationNormal A-a gradient: hypoxemia, if present, likely has a non-pulmonary cause such as hypoventilation or reduced FiO2.
-4.3 mmHg vs age-adjusted normal
Normal<0Mildly elevated0-10Moderately elevated10-25Severely elevated25+

A-a gradient is 9.7 mmHg - within the age-adjusted normal of 14.0 mmHg.

  • The calculated alveolar oxygen pressure (PAO2) is 99.7 mmHg; measured arterial PaO2 is 90 mmHg, giving a gradient of 9.7 mmHg.
  • The age-adjusted normal for a 40-year-old is approximately 14.0 mmHg (formula: age/4 + 4).
  • This gradient is within the age-expected range. If hypoxemia is present, consider hypoventilation, low FiO2 (currently 21%), or a non-pulmonary cause.
  • An elevated A-a gradient alone does not pinpoint a diagnosis; correlate with clinical findings, chest imaging, and other ABG parameters.

Next stepIf the patient is hypoxemic with a normal gradient, review ventilation (PaCO2), drug history, and central nervous system causes of hypoventilation.

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