A–a Gradient (Alveolar–arterial Oxygen Gradient) Calculator for Hypoxemia Evaluation

Use this A–a (Alveolar–arterial) Oxygen Gradient Calculator to evaluate the cause of hypoxemia in patients. Helpful for clinicians, RRTs, and medical students assessing V/Q mismatch, shunting, or diffusion impairment.

Sea level pressure is ~760 mmHg. Adjust for altitude.

47 mmHg is standard at body temperature (37°C)

Default is 0.8; measured via indirect calorimetry

An elevated A–a gradient (alveolar–arterial oxygen gradient) indicates a problem with oxygen transfer from the alveoli to the bloodstream. It helps distinguish the cause of hypoxemia — whether it’s due to ventilation/perfusion mismatch, diffusion defect, or shunting.