A-a Oxygen Gradient

Arterial O2 partial pressure - PaO2:
Arterial CO2 partial pressure - PaCO2:
Atmospheric Pressure:
Inspired Oxygen Concentration (FiO2):
Room Air (21%)


A-a Gradient:

Computational notes

A-a gradient is calculated as follows (all units mmHg):

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


  1. Helmholz HF Jr (1979) The abbreviated alveolar air equation. Chest 75:748.

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