Alveolar-arterial (A-a gradient) Calculator

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The Alveolar-arterial gradient  is used to evaluate causes of hypoxemia. Enter values and press 'calculate' button to calculate the gradient between the alveolar and arterial oxygen tensions.
Patient's Age
Fraction of Inspired Oxygen (FiO2): Percent
Atmospheric Pressure: mm Hg (760)
pH2O (mmHg) mmHg (47)
RQ 0.8
PaCO2 (partial pressure of arterial CO2): mmHg
PaO2 (partial pressure of arterial O2): mmHg
A-a Gradient = mmHg
Estimated normal gradient= (Age/4) + 4 mmHg

A-a (O2) = (FiO2%/100) * (Patm - 47 mmHg) - (PaCO2/0.8) - PaO2

     FiO2 Room Air = 21 %
     Atmospheric Pressure= 760 mm Hg at sea level
     Water vapor pressure pH2O (mmHg) = 47 mm Hg at 37 degrees Celsius
     Respiratory quotient RQ (VCO2/VO2) = 0.8 (usual)

Normal range increases with age.  5 to 20 is normal up to middle age

Hypoxemia with a normal gradient suggests:

  • Hypoventilation (decreased respiratory drive or neuromuscular disease)
  • Low FiO2

Hypoxemia with an increased gradient suggests:

  • Ventilation-perfusion imbalance -also known as V/Q mismatch (asthma, COPD)

  • Shunt : Cardiac right to left shunt such as patent foramen ovale, alveolar collapse, (atelectasis), intraalveolar filling (pneumonia, pulmonary edema), or intrapulmonary shunt.

Supplemental O2 will help to correct  the hypoxemia in hypoventilation and V/Q mismatch but not hypoxemia resulting from a shunt.

All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgment. Neither nor any other party involved in the preparation or publication of this site shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user's use of or reliance upon this material.

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