A-a gradient at sea level


pCO₂ :

FiO₂ :

A-a gradient =

This computerized calculation assumes that the current barometric pressure is 760 mm Hg and that the patient is close to sea level.

Modify the FiO₂if the patient is not on room air. There is no satisfactory method for accurately estimating the FiO₂ for patients managed with nasal canulas.

The formula for doing this calculation by hand is (Calculated O₂ - Measured O₂)  or (PAO₂ - PaO₂)

The PAO₂may be calculated with the following formula:     PAO₂ = FiO₂ (760-47) - (PaCO₂ /0.8)

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